Bee-001 Block 2

Indira Gandhi National Open University School of Engineering and Technology

BEE-001 POWER DISTRIBUTION SECTOR

Block

2 OPERATION AND MAINTENANCE UNIT 4 Introduction to the Power Distribution System

7

UNIT 5 Substation Equipment and Distribution Lines

49

UNIT 6 Distribution Transformer

77

Course Design Committee Shri R.V. Shahi Former Secretary, Ministry of Power Govt. of India Shri Arvind Jadhav Former Joint Secretary, Distribution Ministry of Power, Govt. of India Shri V.S. Saxena Director, Power Finance Corporation Shri Gaurav Bhatiani Project Manager, USAID, India Shri Vinod Behari GM(HRD), Power Finance Corporation Dr. D. Ray Addl. GM (HRD) Power Finance Corporation Shri Sudhir Vadehra Chief, DRUM Project Secretariat

Prof. S.C. Garg School of Sciences IGNOU Prof. Gayatri Kansal School of Engineering and Technology IGNOU Prof. Vijayshri School of Sciences IGNOU Dr. Ajit Kumar School of Engineering and Technology IGNOU Sh. R.K. Chaudhry NPTI, Delhi Ms. Indu Maheshwari NPTI, Delhi

Project Coordinator: Prof. S.C. Garg Programme Coordinator: Mrs. Rakhi Sharma

Block Preparation Team Shri Pankaj Prakash, Editor Ms. Anjuli Chandra Director, CEA Director (Finance) Uttaranchal Electricity Regulatory Commission Dehradun Mr. N. R. Halder Prof. Vijayshri NPTI, Delhi School of Sciences, IGNOU Mrs. Rakhi Sharma School of Engineering and Technology, IGNOU Course Coordinator: Mrs. Rakhi Sharma Acknowledgements: Prof. S.C. Garg for valuable comments and suggestions on the units. The contribution of Shri Pankaj Batra, Director, CEA to the section on Grid Management, Load Scheduling and Load Balancing is thankfully acknowledged. Some material contained in the units has been sourced from the courses developed under the DRUM project, and is thankfully acknowledged.

Production Shri Y.N.Sharma, SO(P) School of Engineering and Technology, IGNOU

Shri Aditya Gupta

This programme has been developed by the School of Engineering and Technology, IGNOU in collaboration with the Ministry of Power, USAID-India and the Power Finance Corporation under the Distribution Reform Upgrades and Management (DRUM) Project. March, 2007 © Indira Gandhi National Open University, 2007 ISBN: All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from the Indira Gandhi National Open University. Further information on the Indira Gandhi National Open University courses may be obtained from the University’s office at Maidan Garhi, New Delhi-110 068. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by Director, School of Engineering & Technology. Printed at

Contents Operation and Maintenance

Unit 4

5

Introduction to the Power Distribution System 7 4.1 Introduction 4.2 Description of the Power Distribution System

8 8

Voltage Levels Conductors High Voltage Distribution System (HVDS)

10 10 11

4.3 Components of the Distribution System

13

Substation Transformer Feeders Meters for Measurement of Energy and Other Electrical Quantities

4.4 Distribution System Planning Planning Horizon Principal Areas of Activity

4.5 Operation and Maintenance Objectives and Activities Operation and Maintenance Objectives Activities Involved in Operation and Maintenance Renovation and Modernisation (R&M) and Life Extension Schemes

4.6 Grid Management, Load Scheduling and Load Balancing

13 14 16 19

20 21 22

26 26 28 28

30

Grid Management Load Scheduling and Dispatch Load Balancing

31 35 39

4.7 Summary 4.8 Terminal Questions Appendix 1 Reactive Power Control in Distribution Systems Appendix 2 Functions of O&M

41 42

Unit 5

43 45

Substation Equipment and Distribution Lines 5.1 5.2 5.3 5.4

49

Introduction 66-33/11 kV Substation Equipment 11/0.4 kV Substation Equipment Distribution Line Equipment

50 50 57 60

Overhead Lines Underground Power Cables

60 65

5.5 O&M Practices for Substation Equipment and Distribution Lines General Maintenance Practices Maintenance of Lines Operation and Maintenance of Capacitors Hot Line Maintenance

68 68 70 71 72

5.6 Length of LT Lines, HT:LT Ratio and Impact on Losses and Voltage Impact of Increasing HT Lines

5.7 Summary 5.8 Terminal Questions

Unit 6

73 74

74 75

Distribution Transformer 6.1 Introduction 6.2 Distribution Transformers: Selection and Placement Classification of Transformers Criteria for Transformer Selection Placement of Transformers

6.3 Reasons for Transformer Failures Ageing Manufacturing Defects Improper Structure of Distribution Transformer Impact of Natural Calamities Improper Operation and Maintenance (O&M)

6.4 Transformer Testing Testing of Windings−Insulation and Mechanical Strength Testing of Insulating Transformer Oil Other Tests

6.5 Enhancing Transformer Life and Efficiency Transformer Operation Maintenance Methods

6.6 Summary 6.7 Terminal Questions Appendix 1 Case Studies on Averting Distribution Transformer Failure Appendix 2 A Checklist for Preventive Maintenance of Distribution Transformers

77 78 78 78 80 81

81 82 83 85 85 86

87 87 88 90

90 91 93

96 96 98 103

OPERATION AND MAINTENANCE In Block 1, you have learnt about the challenges that the power distribution sector in India is faced with such as huge transmission and distribution losses, unreliable power supply, poor quality, lack of concern for consumers and a highly skewed tariff structure. You have also studied about the salient features of the Energy Conservation Act, 2001, Electricity Act, 2003, the National Electricity Policy and the National Tariff Policy. We have discussed the distribution reforms being ushered in the otherwise monopolistic service sector. The overarching aim of these reforms is to help the power sector overcome its weaknesses. You will agree that distribution is the cutting-edge of the power industry and it needs to get back on the right track. This Block is dedicated to the most critical part of the power system viz. the Power Distribution System or simply the Distribution System. The Units in the block describe the components of the Distribution System and the philosophy and practices of their operation and maintenance. Unit 4 (entitled Introduction to the Power Distribution System) is devoted to the general description of the Distribution System and spells out the operation and maintenance philosophy, objectives and activities. It also touches upon an overview of the planning process and the concept of grid management, load scheduling and load balancing in the context of the Distribution System. Units 5 and 6 go deeper into the components of the Distribution System describing their operation and maintenance practices. Unit 5 as is evident from its title, Substation Equipment and Distribution Lines, deals with the O&M of the substation equipment and distribution lines and Unit 6 (entitled Distribution Transformer) covers the working principle, operation and maintenance of Distribution Transformers in detail. We hope that the information about the power distribution system and the O&M practices presented here would help you in improving the performance of the power distribution system and deliver reliable quality power to the consumer within optimum fixed and operating costs. We wish you all the very best!

Unit 4 Learning Objectives After studying this unit, you should be able to:
describe the important features of the power distribution system;
outline the advantages of high voltage distribution system (HVDS);
describe various components of the power distribution system;
explain various activities involved in distribution system planning;
discuss the operation and maintenance principles and practices for the power distribution system; and


explain the fundamental features of grid management, load scheduling and load balancing.

Introduction to the Power Distribution System

Operation and Maintenance

4.1 INTRODUCTION In Unit 1 of Block 1, you have been very briefly introduced to the power supply system. You have also learnt in Unit 1 that the demand for electrical power in India is enormous and growing steadily. Units 2 and 3 have provided you an overview of the power sector with a special focus on the power distribution sector, which is responsible for covering the last mile in reaching power to the consumers. In this Unit, we give a description of the power distribution system and its components. We acquaint you with the concept of distribution system planning, which forms the basis for the smooth operation of the power distribution system. We also present the general principles and practices underlying the operation and maintenance of the system. In the next Unit, we deal specifically with the operation of substation equipment, distribution lines and their maintenance requirements.

4.2 DESCRIPTION OF THE POWER DISTRIBUTION SYSTEM You are familiar with the power supply system. You know that electricity is generated at 11 kV by electrical generators which utilise the energy from thermal, hydro, nuclear, and renewable energy resources. To transmit electricity over long distances, the supply voltage is stepped up to 132/220/ 400/800 kV, as required. Electricity is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometres and deliver the power into a common power pool called the grid. The grid is connected to load centres (cities) through a sub-transmission network of usually 33 kV (or sometimes 66 kV) lines. These lines terminate into a 33 kV (or 66 kV) substation, where the voltage is stepped-down to 11 kV for power distribution to load points through a distribution network of lines at 11 kV and lower. The power network of concern to the end-user is the distribution network of 11 kV lines or feeders downstream of the 33 kV substations. Each 11 kV feeder which emanates from the 33 kV substation branches further into several subsidiary 11 kV feeders to carry power close to the load points (localities, industrial areas, villages, etc.). At these load points, a transformer further reduces the voltage from 11 kV to 415 V to provide the last-mile connection through 415 V feeders (also called Low Tension (LT) feeders) to individual customers, either at 240 V (as single-phase supply) or at 415 V (as three-phase supply). The utility voltage of 415 V, 3-phase is used for running the motors for industry and agricultural pump sets and 240 V, single phase is used for lighting in houses, schools, hospitals and for running industries, commercial establishments, etc.

8

A feeder could be either an overhead line or an underground cable. In urban areas, owing to the density of customers, the length of an 11 kV feeder is generally up to 3 km. On the other hand, in rural areas, the feeder length is

much larger (up to 20 km). A 415 V feeder should normally be restricted to about 0.5 −1.0 km. Unduly long feeders lead to low voltage at the consumer end. The power supply system, including the distribution network, is depicted in Fig. 4.1.

Introduction to the Power Distribution System

Fig. 4.1: Typical Electric Power Supply System with Distribution Network

The main components of the power distribution system and their brief descriptions are given in Table 4.1. Table 4.1: Components of the Power Distribution System Component

Description

Grid Substation (GSS)

Power from transmission network is delivered to sub-transmission network after stepping down the voltage to 66 kV or 33 kV through 220/132/66/33kV Grid substations.

Sub-transmission Network

Power is carried at 66 or 33 kV by overhead lines or underground cables.

Power Sub-Transmission (PSS)

Power is stepped down by 66-33/11 kV to 11 kV for distribution.

Primary Distribution Feeders

Power is delivered from PSS through primary feeders at 11 or 6.6 kV to various distribution transformers.

Distribution Substation (DSS)

Power is further stepped down by 11/0.4 kV transformers to utilisation voltage of 415 V.

Secondary Distribution Network

It carries power from DSS at 415 V (240 V single phase) to various consumers through service lines and cables.

9

Operation and Maintenance

4.2.1 Voltage Levels You have just learnt that the voltage range varies widely in various parts of the power supply system. We give these voltages in Table 4.2. Table 4.2: Voltages at Different Segments in the Power Distribution System Power System Segment

Voltages

Generation voltages

415 V, 6.6 kV, 10.5 kV, 11 kV 13.8 kV, 15.75 kV, 21 kV and 33 kV

Transmission voltages

33 kV, 66 kV, 132 kV, 220 kV, 400 kV

High voltage primary distribution or sub-transmission

3.3 kV, 6.6 kV, 11 kV, 22 kV, 33 kV, 66 kV

Low voltage distribution phase

415 V (3 phase) and 240 V (1 phase)

Higher voltages are used for 3-phase, 3-wire supply to large consumers. Low voltage distribution of generally 415 V, 3-phase 4-wire system and 240 V single phase, two wire, phase to neutral system is used for small and medium consumers. The size and, hence, voltage of supply to a consumer is decided by the load of the consumer.

4.2.2 Conductors The 11 kV feeders carry comparatively bulk power from secondary substation (33/11 kV) to distribution substation transformers (DTRs). Distributors (or secondary network) carry power from DTRs through service lines (or LT feeders) which deliver power from the supplier’s nearest support to consumer’s premises up to the energy meter, through a weather-proof service wire. All lines have inherent resistances, inductances and capacitances, resulting in a voltage drop in the line. Thus, to minimise voltage drop in a line, the values of these parameters should be carefully selected. For LT supply, the declared voltages at the consumer premises are 415/240 V. All appliances and motors give good performance for long duration if this voltage is maintained. The following factors should be considered for the proper selection of conductor size: •

current carrying capacity; and

•

tensile strength of the conductor.

The size of conductor for a distributor is determined in the following manner:

10

•

The current that the distributor has to carry is calculated on the basis of the load incident on the conductor (including anticipated load growth).

•

The conductor size capable of carrying this current at the ambient temperature of the area is selected from standard tables.

•

The voltage drop is calculated taking products of loads and their distances.

The following types of conductors are available: •

All Aluminium (Standard) Conductor (AAC);

•

Aluminium Conductor Steel Reinforced (ACSR Conductors);

•

All Aluminium Alloy Conductors (AAAC).

Introduction to the Power Distribution System

ACSR and AAAC conductors are used for secondary distribution systems. ACSR conductors are preferred to AAC conductors for long spans owing to their greater tensile strength. The current carrying capacity of ACSR conductors is as follows: Squirrel (7/2.11)

115 A

Weasel (7/2.59)

150 A

Rabbit (7/3.35)

208 A

The numbers in bracket indicate the number of strands/diameter in mm.

4.2.3 High Voltage Distribution System (HVDS) You have learnt in Unit 1 that significantly high losses take place in the secondary distribution system. This is due to higher current densities and ease of pilferage at low voltages. One of the latest innovations in efforts to reduce technical and commercial losses is the use of High Voltage Distribution System (HVDS) or LT-less system.

Fig. 4.2: Typical High Voltage Distribution System

In this system, the secondary distribution system with long LT feeders running up to consumer premises from the distribution substation is totally absent. The primary distribution system at HT level (11 or 33 kV) is used to reach the nearest point for a group of small number of consumers. The consumers are then connected to the HT Distribution System at these points through small pole mounted transformers used for supplying power to them through LT service lines.

11

Operation and Maintenance

We now describe the advantages of HT distribution compared to conventional LT distribution system. v Low Losses and Improved Voltage Profile The comparison of current, losses and voltage drop for the distribution of the same power through HT and LT systems is presented in Table 4.3. We have considered 100 as the base value for LT system. From the table, you can see that for the distribution of the same power, technical losses and voltage drop are much less in HT distribution system when compared to LT distribution systems. Table 4.3: Comparison of Current, Voltage Drop and Power Losses for Power Distribution through HT and LT Distribution Systems

Single phase 6.35 kV HT distribution system

3 phase 4 wire 415 V LT Distribution system

Current (Amps)

11.0

100.0

Losses (kW)

8.5

100.0

Voltage drop

12.7

100.0

LT distribution systems are easily accessible and prone to pilferage and the use of HVDS reduces the chances of theft of electricity to a very low level. Now-a-days, utilities are installing meters at the HT transformer itself to ascertain commercial losses on that particular transformer. In sum, the HT distribution system has the following advantages: •

use of small size ACSR or aluminium alloy conductor or high conductivity steel wire;

•

better voltage profile;

•

reduced line losses; and

•

reduced commercial losses.

v Improved Reliability and Security of Supply The use of HT distribution leads to improved reliability and security of supply for the following reasons:

12

•

The faults on HT lines are far less compared to those of LT lines.

•

In order to avoid theft in LT lines from transformer to consumer premises, usually Aerial Bunched Cables (AB Cables) are used to supply power at LT to consumer from the distribution transformer. With AB Cables, the faults on LT lines are eliminated. This, in turn,

reduces the failure of distribution transformers and enhances reliability of supply. •

Introduction to the Power Distribution System

Since the number of small distribution transformers is high in HVDS, the failure of one transformer does not affect supply to other consumers connected to other transformers. In the event of failure of distribution transformers, only a small number of consumers (2 to 3 power consumers or 10 to 15 domestic consumers) would be affected. On the other hand, a large distribution transformer supplies power through LV distribution lines to even remotely located consumers in LVDS. Hence, the failure of an existing large size distribution transformer would affect a group of 40 to 50 power consumers and/or 100 to 200 domestic consumers.

You may like to consolidate these ideas before studying further.

SAQ 1: Power distribution system a) Compare the distribution system of your utility with another utility in respect of voltage levels and conductors used for each component of the distribution system. ………………………………………………………………………………. ………………………………………………………………………………. b) What is HVDS? Outline its advantages over the LT system. ………………………………………………………………………………. ……………………………………………………………………………….

4.3 COMPONENTS OF THE DISTRIBUTION SYSTEM In this section, we describe various components of the power distribution system, viz. substations, transformers, feeders, lines and metering arrangements.

4.3.1 Substation A substation is the meeting point between the transmission grid and the distribution feeder system. This is where a fundamental change takes place within most T&D systems. The transmission and sub-transmission systems above the substation level usually form a network (about which you will study in the next section). But arranging a network configuration from the substation to the customer would simply be prohibitively expensive. Hence, most distribution systems are radial (also described in the next section), i.e., there is only one path through the other levels of the system. Typically, a substation consists of high and low voltage racks and buses for

13

Operation and Maintenance

power flow, circuit breakers at the transmission and distribution level, metering equipment and the control house, where the relaying, measurement and control equipment is located. But the most important piece of equipment that gives the substation its capacity rating is the substation transformer. It converts the incoming power from transmission voltage levels to the lower primary voltage for distribution. Very often, a substation has more than one transformer.

Fig. 4.3: Power Distribution Substations

Apart from the transformer, a substation has other equipment such as lightning arrestors, isolators, etc. You will learn about the substation equipment in detail in Unit 5 and the distribution transformers in Unit 6. Here we give a brief introduction of the most critical component of a substation, the transformer.

4.3.2 Transformer A transformer is an electrical device that transfers power from one circuit to another without change in frequency. The purpose of a transformer is to convert one AC voltage to another AC voltage. A transformer comprises two or more coupled conducting coils (windings), which are wound on common laminated core of a magnetic material such as iron or iron-nickel alloy (Fig. 4.4). These are called primary and secondary windings. The alternating current in the primary winding creates an alternating magnetic field in the core just as it would in an electromagnet. The secondary winding is wrapped around the same core. The changing magnetic flux (magnetic field per unit area per unit time) in the primary winding induces alternating current of the same frequency in the secondary winding. The voltage in the secondary winding is controlled by the ratio of the number of turns in the two windings.

14

If the primary and secondary windings have the same number of turns, the primary and secondary voltages will be the same. For step-down

Introduction to the Power Distribution System

transformers, the secondary winding has lesser number of turns than the primary. For example, to step-down voltages from 240 V at the mains to 6 V, there needs to be 40 times more turns in the primary than in the secondary. In case of step-up transformers, the number of turns in the secondary winding is more than those in the primary winding. The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved every day.

NOTE The relation between the voltages, currents and number of turns in the primary and secondary coils is given by V2

I N = 1 = 2 V1 I 2 N1

Fig. 4.4: Principle Underlying a Transformer

For an ideal transformer, it is assumed that the entire magnetic flux linked with the primary winding is also linked to the secondary winding. However, in practice it is impossible to realize this condition. While a large portion of the flux called common or mutual magnetic flux links with both the coils, a small portion called the leakage flux links only with the primary winding. This leakage flux is responsible for the inductive reactance of a transformer.

Here V1, I1 and N1 represent the voltage, current and the number of turns, respectively, in the primary coil and V2, I2 and N2 represent the voltage, current and the number of turns, respectively, in the secondary coil.

Specifications of Transformer A transformer should be provided with more than one primary winding or with taps on the winding if it is to be used for several nominal voltages. The Rated Power of the transformer is the sum of the VA (Volts x Amps) for all the secondary windings. The important specifications for a transformer are: primary frequency of incoming voltage (50 Hz), maximum primary voltage rating, maximum secondary voltage rating, maximum secondary current rating, maximum power rating, efficiency, voltage regulation and output type (3 wire or 4 wire). Transformers in a distribution system can be configured as either single-phase primary configuration (with three single-phase transformers) or a three-phase configuration (one three-phase transformer). Three-phase transformers are connected in delta (∆) or wye (Y) configurations. While delta configuration is used for three wire transmission and sub-transmission Fig. 4.5: 11 kV/415 V - 240V system, wye (or star) configuration is suitable for 4 wire distribution systems. Pole Mounted Transformer A wye-delta (Y - ∆) transformer has its primary winding connected in a wye and its secondary winding connected in a delta. A delta-wye transformer has 15

Operation and Maintenance

its primary winding connected in delta and its secondary winding connected in a wye. Types of Transformer

Efficiency is the ratio of output power (kW or MW) and input power (kW or MW), whereas energy efficiency is the ratio of energy delivered (kWh or MWh) and energy injected (kWh or MWh) in a system expressed in percentage terms. Efficiency of transformer is maximum at a loading (as a fraction of full load current) when its iron losses equal ohmic or copper losses (due to current flowing in the windings). The voltage regulation of transformer is the ratio of voltage drop in a transformer from no-load to full load and the no-load voltage expressed in percentage terms.

Transformers can be categorised based on the type of core used, type of cooling used, the method of mounting the transformer or the intended use for which it is designed. We shall deal with the first three categories of transformers in Unit 6. Here we give a brief introduction to the categorisation of transformers on the basis of their use as power transformers and distribution transformers. Power substations use power transformers while the distribution substations employ distribution transformers. While the underlying principle of operation is the same for both the transformers, they differ in their design since they are required to operate under different conditions at power and distribution substations. Table 4.4 gives a comparison of these two types of transformers. Table 4.4: Comparison of Power Transformers and Distribution Transformers Power Transformers

Distribution Transformers

Convert power-level voltages from one level to another in a Grid Substation (GSS) or Power Substation (PSS) with voltages above 33 kV.

Step down the primary distribution voltage of 11 kV or 22 kV to secondary distribution of 400 V between phases and 230 V between phase and neutral through delta-star winding.

Since it is fed through the grid network, a power transformer is usually not a critical component for supply to consumers as alternative paths for flow of power are available through the grid. Accordingly, it is generally possible to cut it out of circuit without affecting supply to consumers.

Distribution transformers, being connected to the consumers through radial feeders, which have only one path, have to be continuously energised for maintaining uninterrupted supply to consumers.

Power transformers are most of the time loaded to levels just below the rated power and, accordingly, they are designed to operate at maximum possible flux density level with maximum efficiency at near full load.

Distribution transformers are most of the time lightly loaded and in order to have maximum all day efficiency, they are designed to work at low flux levels with maximum efficiency occurring at lower loading.

4.3.3 Feeders

16

Feeders route the power from the substation throughout the service area. They are typically either overhead distribution lines mounted on wooden poles, or underground buried or ducted cable sets. Feeders operate at the primary distribution voltage in primary distribution system and secondary distribution voltage in the secondary distribution system.

Introduction to the Power Distribution System

Fig. 4.6: Distribution Feeders

Definition of a feeder By definition, the feeder consists of all primary or secondary voltage level segments of distribution lines between two substations or between a substation and an open point (switch).

The most common primary distribution voltages in use are 11 kV, 22 kV and 33 kV. The main feeder, which consists of three phases, may branch into several main routes.

Fig. 4.7: Typical layout of feeders in a primary distribution system (numbers indicate transformer capacities)

The main branches end at open points where the feeder meets the ends of other feeders – points at which a normally open switch serves as an emergency tie between two feeders.

17

Operation and Maintenance

Feeders are connected in a configuration, which depends on the type of network required in the distribution system. Three types of network are normally available in the electrical distribution system: •

radial;

•

loop; and

•

cross-loop network.

Since the radial feeder emanates from one point and ends at the other in the radial network, load transfer in the case of breakdown is not possible. Although a radial feeder can be loaded to its maximum capacity, in the case of breakdown, quite a large area may remain in dark until the fault is detected and repaired. In loop arrangement, two feeders are connected to each other so that in the case of breakdown, the faulty section can be isolated and the rest of the portion can be switched on. In this type of system, the feeder is normally loaded to 70% of its capacity so that in the event of breakdown it can share the load of other feeders also. A cross-loop network provides multiple paths and the flexibility further increases. In case of breakdown in any line, the faulty system can be isolated and supply can be resumed very quickly. In this type of network, feeders should normally be loaded to 70% of their current carrying capacity. This system is highly reliable, but very expensive.

Fig. 4.8: Alternative Layouts for Primary and Secondary Network, 33 and 11 kV

18

In big cities, the concept of 33 kV ring main is very popular and two ring mains are laid: one outer and one inner. The outer ring main is laid using the panther conductor and the inner ring main is laid using the dog conductor. The use of these two types of ring mains provides excellent flexibility to the system and at the time of breakdown, supply can be immediately switched on from another 132 kV substation. While making any distribution planning (discussed in Sec. 4.4) for metros, the aspect of outer and inner 33 kV ring

mains is extremely essential and should be included for providing uninterrupted supply.

Introduction to the Power Distribution System

Table 4.5 gives a comparison of the three types of network configurations. Table 4.5: Comparison of radial, loop and cross-loop network Loop Distribution System

Radial Distribution System

Cross-loop Network Distribution System

•

Single path to each group of customers

•

Double path to each group of customers

•

Multiple paths to each group of customers

•

Lowest construction cost system

•

Medium cost system

•

High cost system

•

Simple to plan, design and operate

Moderately simple to plan, design and operate

•

•

Complex to plan, design and operate Highest reliability

No reserve – loss of feeder implies loss of supply

•

•

•

Loss of feeder results only in temporary loss of supply

•

Used in large cities and for critical loads

USED IN RURAL AREAS

USED IN URBAN AREAS

USED IN URBAN AREAS

4.3.4 Meters for Measurement of Energy and Other Electrical Quantities Meters are required to be installed at various points of the Distribution System including the substation equipment and the consumer end. They are required for correct recording of electrical quantities for operational and safety purposes as well as energy accounting. The meters installed at the interface points of generation-transmission and transmission-distribution are called interface meters. Meters installed at consumer premises by the utility are called consumer meters. The Central Electricity Authority regulations on Installation and Operation of Meters provide for the type, standards, ownership, location, accuracy class, installation, operation, testing and maintenance, access, sealing, safety, meter reading and recording, meter failure or discrepancies, anti tampering features, quality assurance, calibration and periodical testing of meters, additional meters and adoption of new technologies in respect of the following meters for correct accounting, billing and audit of electricity: §

Interface meter,

§

Consumer meter, and

§ Energy accounting and audit meter. It is important to note that these regulations make the use of static meters mandatory for new consumers.

19

Operation and Maintenance

SAQ 2: Components of power distribution system a) Make a list of the substation equipment. ………………………………………………………………………………. ………………………………………………………………………………. b) What is a feeder? Compare the radial, loop and cross-loop network configurations in an electrical distribution system. ……………………………………………………………………………… ………………………………………………………………………………

4.4 DISTRIBUTION SYSTEM PLANNING The need for electrical power is growing at a rapid pace on account of rapid growth of population, industrialisation and urbanisation resulting in high load density pockets with multi storied complexes. This is coupled with manifold increase of deep tube wells on account of low ground water level and huge number of electric pumps connected to the system during the agricultural season in rural areas. In order to meet the future power needs of the nation, it is essential to upgrade the existing distribution system and increase its efficiency and at the same time reduce the technical losses. This requires proper planning: Utilities have to plan much ahead to meet the present as well as the projected future demand for quality power supply. In the context of the current chronic power shortage, the shooting prices of fuel and the need for conservation of available fossil fuel resources, you can well understand the urgency of eliminating high losses in the transmission and distribution system. The high percentage of losses in our country is a matter of national concern. The main cause of these high losses is laying of unplanned distribution system in the country. Proper distribution system planning, financial support and implementation of the plans should be able to bring down the losses and provide uninterrupted quality supply to the consumers. Distribution Planning requires an analysis of various factors such as load growth, funds, ecological consideration, availability of land, etc. Distribution planning in a utility involves

20

•

ascertaining the time horizon for which it is envisaged,

•

spelling out the specific activities required in the planning process, and

•

implementation of plans.

We take up each of these in the following sections.

4.4.1 Planning Horizon Distribution planning studies can be carried out in different manners, each with different objectives and requirements. Planning can be done for different time horizons and accordingly it is called medium/long-term planning or short-term planning.

Introduction to the Power Distribution System

v Medium/Long-term Planning Medium/long-term planning is normally carried out as a part of a master plan for the distribution system as a whole. It normally considers a 5 to 15 years time frame and is based on the state/national as well as local load forecasts, industrialisation plan and agricultural load forecasts. The main objectives of this type of plan are to: •

verify the present capacity of lines and substations;

•

verify additional capacity and investments required to meet the load growth for putting up new 33/11 kV substations, new 33 kV lines, 11 kV feeders, etc.;

•

arrange tie-ups for additional power-purchase agreements;

•

upgrade existing transmission capacity;

•

upgrade existing networks;

•

develop strategies for reduction of technical losses;

•

estimate the funds required; and

•

arrange tie-ups with the financial agencies for funds.

v Short-term Planning For proper distribution planning, we first need to study the existing system, ascertain loss level and decide on immediate action to be taken to meet the requirement of consumers and provide them uninterrupted quality power supply. In the present scenario, it has been found that 11 kV and 33 kV feeders are loaded more than 100% of their rated current carrying capacity. This results in very high technical losses and needs to be immediately relieved through short-term planning. Thus, the objectives of short-term planning are to: •

develop specific case studies and projects in a systematic manner;

•

adjust capacity of 33 kV,11 kV feeders, power transformers, distribution transformers and LT lines;

•

take immediate action to bifurcate heavily loaded 33 kV, 11 kV feeders;

•

augment conductor with the properly sized conductors;

•

reduce length of LT lines (maximum 0.5 km per transformer);

•

implement projects for proper maintenance;

•

calculate required investments; and

•

arrange tie-ups with financial institutions for funds.

21

Operation and Maintenance

SAQ 3: Planning horizon Identify the short-term, medium/long-term planning objectives for your utility. ………………………………………………………………………………....... ………………………………………………………………………………....... ………………………………………………………………………………....... ………………………………………………………………………………....... ……………………………………………………………………………….......

4.4.2 Principal Areas of Activity The principal areas of activities associated with distribution planning are as follows: •

•

Existing Load Data Study: The study of the existing system forms a critical input to distribution planning and includes activities such as −

updating all distribution system statistics;

−

evaluating changes in technic and economic planning criteria; and

−

evaluating and updating load forecasts, voltages and consumers category-wise with a time horizon of 10 to 15 years.

Future Load Growth Study: Load forecasts are extremely important in Distribution Planning. These are mainly used for: −

power purchases;

−

reinforcement of distribution system expansion planning;

−

demand side management;

−

tariff application; and

−

monitoring of loss reduction programme.

The forecasts may be done on a short-term, medium-term or long-term basis. These have to be carried out systematically and rigorously to be of any help in distribution planning. Otherwise, they can lead to wrong estimates and the planning can go awry. The steps involved in the load forecasting process are:

22

−

data collection;

−

data validation;

−

selection of methodologies;

−

development of assumptions;

−

development of energy and demand forecasts; and

−

comparison with the historical load growth data.

•

Power Factor/Reactive Load Study: A study of loading pattern and voltage drops needs to be carried out to ascertain the reactive power compensation, which is required to be provided at different points of the distribution system so as to maintain voltages within specified limits. You can study about power factor and reactive compensation in Appendix 1 given at the end of this unit.

•

Study of Thermal Capability of Conductors (Capacity of Feeders/ Circuits): The thermal capacity of line circuit is dependent on the size of the conductor and type of environmental factors, i.e., ambient temperature, wind speed and solar radiation. A study of peak loadings for the conductors needs to be carried out to figure out overloaded feeders with the help of standard tables giving rated currents for each type of conductor. Accordingly, a proper action plan for bifurcation or replacement of feeder needs to be chalked out.

•

Economic Impact Study: A study of the economic impact of the implementation of the distribution system plan needs to be carried out. Cost-benefit analysis needs to be done to ascertain whether the investments in implementation of distribution plan lead to long term savings and improvement in supply quality.

Introduction to the Power Distribution System

We now present a case study to illustrate how utilities can take advantage of distribution planning.

DISTRIBUTION SYSTEM PLANNING: A CASE STUDY OF MP MADHYA KSHETRA VIDYUT VITARAN COMPANY LIMITED MP Madhya Kshetra Vidyut Vitaran Company Limited, Bhopal (the Central Discom) is one of the leading utilities, to have established a Distribution System Planning Cell. In the first phase, the Central Discom took up short term system planning and established the equipment and network data base. Field data such as the length of feeders, size of conductors, configuration of poles, present loading, annual input and single line diagrams was collected for all 33 kV and 11 kV feeders. Load forecasts were made for the next five years on the basis of the historical load growth data. The following information was also collected: •

details of existing capacitors installed at 33/11 kV substations; and

•

details of existing 33/11 kV substations along with data on the capacity of transformers, number of feeders, loading, etc.

The number of 33 kV rural feeders in the Discom is 282. These have been strung with Racoon conductors having current carrying capacity of 200 A. The Discom selected all the 33 kV feeders having loading more than150 A for study and analysis. As the National Tariff Policy has made it mandatory for power utilities to segregate technical and commercial losses within one year, a detailed study

23

Operation and Maintenance

was conducted through CYMDIST software, which is a proven tool for finding technical losses at each voltage level of the distribution system. This software provides for two types of studies: •

voltage drop analysis; and

•

short circuit analysis.

On the basis of the study and the data acquired, the DISCOM took measures such as: •

capacitor placement;

•

augmentation of the conductor of feeder; and

•

bifurcation of the feeder.

These were followed by further systemic analysis. It was concluded that losses could be reduced substantially by adopting all the three measures or combinations of two or only one of these, depending upon loading conditions and voltage regulation. The study was initially conducted on 20 select feeders but later it was extended to the heavily loaded 224 rural feeders. It was found that 49 feeders had losses amounting to more than 10% and 52 feeders had losses between 5 -10%. An analysis of the data revealed that for some feeders, voltage regulation could not be brought within permissible limits even after the placement of Capacitor Bank, proposing a new 33 kV feeder and augmenting the conductor size. Further studies were carried out and locations were identified for putting up 132/33 kV substations. Through short term studies, the Discom identified 5 such locations. A similar study conducted for 11 kV feeders led to the following conclusions: i)

additional power transformer would need to be put up in the existing substation;

ii) a new 33/11 kV substation would need to be constructed, thereby reducing the length of 11 kV lines; iii) the existing 11 kV feeders would need to be bifurcated; and iv) the conductors of existing feeders would need to be augmented. The details of planning activities undertaken by the Central Discom are presented below. •

24

Existing Load Data Study: The number of 33 kV feeders in the Central Discom is 347, of which 65 are urban feeders and 282 rural. The CYMDIST software was used to study all the parameters of existing feeders in urban areas. Various steps, such as augmentation of existing Racoon conductor by Dog conductor, bifurcation of feeder and installation of Capacitor Bank in 33/11 kV substations, were taken. After the implementation of these measures, voltage regulation was found to be within permissible limits. The loading of the 33 kV rural feeders is given ahead.

Table 4.6: Loading of 33 kV Rural Feeders Load

Number of 33 kV Feeders

More than 300A

31

−300 A Between 250−

25

−250 A Between 200−

56

−200 A Between 150−

28

Less than 150 A

142

Introduction to the Power Distribution System

The number of 11 kV feeders in the Central Discom is 1749, of which 358 are urban feeders and 1391 rural. In the first phase, a study of all 11 kV urban feeders was conducted through the CYMDIST software and action was taken for bifurcation of feeders, augmentation of conductor capacity and putting up new 33/11 kV substations. All 11 kV feeders of urban areas now have Racoon conductor and the load of each feeder is within 100 A. The voltage regulations are also within permissible limits. In the second phase, a study of 11 KV rural feeders is being carried out. All the existing feeders are laid on ACSR weasel conductor. The length of the feeders ranges from 4 km − 100 km and voltage regulation varies from 3% to 24%. The break-up of the rural feeders on the basis of load is given in Table 4.7. Table 4.7: Break-up of the Rural Feeders Load

Number of 11 kV Feeders

More than 200A

180

−200 A Between 150−

170

−150 A Between 100−

790

−100 A Between 75−

172

Less than 75 A

79

The number of 11 kV feeders having load more than 200 A is 180 and a study is being conducted on them in the first phase through the CYMDIST software. •

Future Load Growth Study: Historical load growth data was the major basis in anticipating the future load growth.

•

Power Factor and Reactive Load Study: A thorough study was made on the existing power factor and existing load on the system and as per the data obtained from CYMDIST software study, necessary compensation was provided by installation of 11 kV capacitor bank on 33/11 kV substations.

25

Operation and Maintenance

•

Thermal Capability of Conductor: Operating data was used to check whether the conductors were being operated within thermal capability limits.

•

Economic Impact: On analysis of the data derived from the CYMDIST study, it was concluded that the payback period is between 24 to 30 months. The results obtained from CYMDIST software have been used for preparing the loss reduction model of the company.

You may like to review this information in your own context. Attempt the following SAQ!

SAQ 4: Distribution planning Suggest ways in which your utility can benefit from distribution system planning. ………………………………………………………………………………...... ………………………………………………………………………………......

4.5 OPERATION AND MAINTENANCE OBJECTIVES AND ACTIVITIES The distribution system constitutes the interface of a utility with consumers who judge the performance of the utility by the performance of its distribution system. Therefore, proper operation and maintenance of the power distribution system is essential. Any failure on this account may deprive the user of electric supply and lead to chaotic conditions. There are two types of maintenance: Preventive Maintenance and Breakdown Maintenance. •

Preventive Maintenance is maintenance done prior to the onset of Monsoon and after the end of Monsoon.

•

Breakdown Maintenance is done on breakdown in the installation.

In this section, we discuss the general O&M objectives and activities for the power distribution system.

4.5.1 Operation and Maintenance Objectives

26

You will agree that the prime goal of a power utility, like any other business, is to achieve consumer satisfaction with optimum effort and costs while maintaining reasonable profit levels. The operation and maintenance (O&M) practices of a utility contribute significantly in attaining this goal. These activities should help in improving the reliability and maintenance of plant and equipment, maximising capacity utilisation, increasing operating efficiency, and reducing operating and maintenance costs.

The objectives of O&M for distribution systems may thus be spelt out as follows.

Introduction to the Power Distribution System

OBJECTIVES OF OPERATION AND MAINTENANCE v Ensuring quality and reliability of supply to consumers. v Reducing equipment operating and maintenance costs through effective utilisation of capacity and resources. v Increasing the availability and reliability of plant and equipment with effective maintenance planning. v Improving spares planning and reducing spares inventories. v Standardising work procedures. v Ensuring the safety of maintenance personnel. v Providing a mechanism for making estimates and controlling maintenance expenses.

NOTE Source: Special report on CEA website “Guidelines for Project Management and Performance Evaluation of Sub-transmission and Distribution Project”.

v Generating MIS reports for better decision-making and control. v Bringing down technical and commercial losses to an optimum minimum level. v Avoiding any bottleneck in capacity by matching expansion with the growing demand.

The O&M strategy adopted by a utility can be evaluated in terms of certain parameters, which are given below. PARAMETERS FOR EVALUATION OF O&M STRATEGY v Reduction in

− T&D losses, − overloading of feeders and transformers, − consumer interruption − cost per consumer − number of trippings due to overloading. v Degree of improvement in voltage profile vis-à-vis voltage regulation. v Increase in revenue. v Enhancement of peak demand and energy supplied. v Number of consumers supplied. v Improvement in level of service and collections.

The specific functions of the O&M System are described in detail in Appendix 2 to this unit.

27

Operation and Maintenance

4.5.2 Activities Involved in Operation and Maintenance The following activities are involved in the operation and maintenance of the Distribution System: •

continuity of service;

•

technical operation and maintenance;

•

training and retaining of operational staff;

•

renewal of maintenance contract;

•

upkeep of spare parts inventory;

•

record keeping of faults in the network/equipment problems, solutions, modifications and enhancements;

•

close monitoring of budgeted expenditure;

•

preparation, continuous updating and proper maintenance of operational and network data;

•

record of protective and isolating devices installed and their relay settings;

•

record of schedule of maintenance and preventive and routine maintenance of network elements;

•

development of spare parts;

•

development of maintenance practices, tools and procedures for trouble free operation; and

•

record of transformer/switchgear oil testing and its parameters.

Utilities should have manuals for O&M to ensure efficient and trouble free operation of the system/equipment. These manuals should contain the following information: •

factory and site test certificates for each item of the system with reference to relevant design calculation and quality assurance standards;

•

maintenance instructions for all plants and other preventive and corrective maintenance procedures;

•

maintenance and inspection schedules for all items/equipment giving type of works required on a weekly, monthly, annual basis; and

•

proforma of the required maintenance record sheets for all the component/equipment.

We now outline the modern approach to operation and maintenance of power systems.

4.5.3 Renovation and Modernisation (R&M) and Life Extension Schemes

28

Basically, the deterioration of electrical components in the distribution system is related to electric, thermal, mechanical, chemical, environmental and combined stresses. Hence, failure of equipment could be due to insulation failure, thermal failure, mechanical failure or any combinations thereof.

The concept of simple replacement of power equipment in the system, considering it as weak or a potential source of trouble, is no longer valid in the present scenario of financial constraints. Renovation, modernization and life extension of existing substations, sub-transmission and distribution network and field equipment outside the substations is one of the cost effective options for maintaining continuity and reliability of the power supply to the consumers. Renovation and modernisation (R&M) is primarily needed to arrest the poor performance of the substation equipment (mainly transformers and switchgears), which are under severe stress due to poor grid conditions, poor and inadequate maintenance and polluting environment. In this changed paradigm, efforts today are being directed to explore new approaches/techniques of monitoring, diagnosis, life assessment and condition evaluation, and possibility of extending the life of existing assets, i.e., generator, circuit breaker, surge arrestor, oil filled equipment like transformers, load tap changer, etc., which constitute a significant portion of assets for generation, transmission and distribution system.

Introduction to the Power Distribution System

NOTE There are no established guidelines for the time interval during which R&M and life extension studies must be carried out. The R&M and life extension studies must be done when the performance of the equipment is noticed as deteriorating but not later than two years from the previous such study.

Assessing the condition of the equipment is the key to improving reliability. The knowledge of equipment condition helps to target the maintenance efforts to reduce equipment failures. Reduction of failures of equipment improves reliability and effectively extends the life of equipment. Hence, utilities are continuously in search of ways and means other than conventional methods/techniques to assess the condition of equipment in service. Thus, remedial measures can be taken in advance to avoid disastrous consequences thereby saving valuable resources. For assessing normal operation, strategic planning and scheduling, three major tasks need to be identified: •

incipient failure detection and prevention − supervisory function, monitoring;

•

identification of malfunction or fault state − offered by diagnostic techniques; and

•

planning for repair, replacement and upgrading − life assessment and condition evaluation techniques.

Researchers and manufactures have come out with various condition assessment, diagnostic monitoring, preventive maintenance, predictive maintenance (PDM) techniques for the equipment to reduce the risk of failure and extend their effective life and thereby help utilities overcome the challenges they face. Various condition assessment tools are used to establish the health of equipment using latest on line and off line diagnostic testing techniques/technologies. Predictive maintenance is gaining popularity as it helps eliminate unscheduled downtime of expensive equipment and reduces the overall cost of maintenance. This approach, sometimes called ‘condition-based

29

Operation and Maintenance

maintenance’, relies on planned inspections, testing, analysing and trending of the relevant equipment parameters. In most cases, these parameters can determine the equipment’s health and must be followed up by proactive actions that change the way the equipment is operated to reach the goals set out. In other words, the performance of the equipment is analysed to determine its condition and predict when it will need attention. The techniques so developed are grouped under Residual Life Assessment (RLA) techniques. The potential of such techniques is tremendous and their benefits are so many that utilities cannot ignore their importance in the present scenario. The main objective of RLA is to determine the condition of a set of equipment (e.g., transformers) in order to identify the most vulnerable component/equipment. Based on the evaluation, utilities can develop a strategic replacement plan for a particular population of equipment. The aim should be to maximize the availability and utilization by avoiding unexpected failures and at the same time minimizing risk. Strategies for life assessment are quite complex and involve many aspects (both user-oriented and manufacturer-oriented). Their details are beyond the scope of this course. In the next section, we introduce the concepts of grid management, load scheduling and load balancing. However, you may first like to revise the ideas presented in this section.

SAQ 5: O&M objectives and activities Outline the O&M objectives of your distribution utility. What activities are undertaken by it to fulfil these objectives? ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… ………………………………………………………………………………

4.6 GRID MANAGEMENT, LOAD SCHEDULING AND LOAD BALANCING

30

In this section, we consider the aspects related to grid management, load scheduling and load balancing in a power distribution system.

4.6.1 Grid Management Let us consider the following questions:

Introduction to the Power Distribution System

q What is a grid? q What is grid management? q What does grid management involve? GENERATION (POWER PLANTS)

q What is a grid? You know that a power system has a generating unit to generate electrical energy, which is consumed at the load. This energy cannot be stored and has to be consumed at the same instant. But since the load is not concentrated at one place and it is not possible to have a generator very close to the load centre at all times, we go for transmission lines, which facilitate transmission of power from generator to load. Thus all generation units and load centres are connected and a grid is formed (Fig. 4.9). The grid is basically a connection of generating stations, substations and loads through transmission lines, at a voltage level above the distribution voltage. The distribution voltage, however, is not strictly defined. It is different for different areas. In some distribution systems, power is taken from the grid at 33 kV, in some it is taken at 66 kV and in some, it may even be taken at 220 kV. Therefore, the grid covers the above mentioned high voltage system down to the level of connection point of the distribution system.

TRANSMISSION NETWORKS (GRID)

LOCAL DISTRIBUTION SYSTEM

Definition of a grid Grid is defined in the Electricity Act, 2003, as : “the high voltage backbone system of inter-connected transmission lines, substations and generating plants.” CUSTOMERS

There are many advantages of having a grid. ADVANTAGES OF A GRID v RELIABILITY: The system is more reliable since we can serve the load in more than one ways. As a result, even if one generation unit fails the rest can share its load. v STABILITY: The system becomes more stable as the chances of a fault disturbing the whole system become less. v ECONOMY: In a grid, the cost required is lesser than a dedicated

Fig 4.9: Grid as an Intermediary Between Generation and Load

system since lesser installed capacity is required as well as lesser spinning reserve is involved.

31

Operation and Maintenance

Regional and State Grid in India In the 1960s, India was demarcated into 5 electrical regions (NR, SR, ER, WR, NER) for planning, development and operation of the power system with regional self sufficiency. As on date, we have three synchronous power systems: Northern, Central (WR-ER-NER) and Southern. Bulk power transfer is possible among the regional grids through the inter-regional links. The Northern and Southern Systems are connected to Central System through separate HVDC links and, hence, each of the three systems can operate at different frequencies. The State Grid of each State is connected to Regional Grid for inter-State power exchanges. q What is Grid Management? Grid management, as the term implies, is managing the grid. This consists of on-line real-time operation of the grid as well as off-line operational planning. The real time operation of the grid is looked after by the Load Dispatch Centre, which is basically a round-the-clock control room manned by grid operators or load dispatchers, who operate the grid by giving instructions to the personnel of the concerned generators and substations. Load Dispatching, as the name implies, involves dispatching of load (or power) from the generator to the load. This is done through the transmission system. Load Dispatch Centre constantly observes the grid parameters and tries to ensure good grid operation. Operational planning is the planning done in advance, in order to ensure that ♦ generation matches the load at all points of time; ♦ the voltage profile at all points of the grid remains within acceptable limits; ♦ none of the transmission lines or inter-connecting transformers get over-loaded; and ♦ the grid operates in a stable manner, i.e., there are no power swings. Operational Planning also involves coordination of protection of the grid so that only the faulted element gets isolated and the remaining grid continues to operate in a satisfactory manner. The Grid Management in our country is done by the Regional Load Dispatch Centre (RLDC) at the Regional Level and by the State Load Dispatch Centre (SLDC) at the State Level. Each State, Central Generating Stations and Independent Power Producers (IPPs) are treated as constituents of the Region.

REGIONAL LOAD DISPATCH CENTRE

32

STATE LOAD DISPATCH CENTRE

Fig. 4.10: Two Key Players in Grid Management

OBJECTIVES OF GRID MANAGEMENT

Introduction to the Power Distribution System

v Reliability, v Grid security, v Economy, and v Quality in electric supply. v RELIABILITY comes with integrated grid operation for smooth evacuation of power from generating stations and its delivery at the states’ periphery. v SECURITY comes by maintaining the system parameters like frequency, bus voltages, line loadings and transformer loadings within permissible limits. It involves stable and smooth operation of the grid, i.e. minimum interruptions of power, either through tripping of single grid elements (like generator, transmission line, interconnecting transformer, HVDC back-to-back pole) or grid disturbances involving tripping of a large number of grid elements simultaneously or even a total blackout. v ECONOMY comes by merit order generation, optimization of hydro resources, minimization of losses and judicious inter-regional exchanges. It envisages getting the cheapest power to the customers through minimization of transmission losses and ensuring that the cheapest generation is used first, then the next costly generation and so on. v QUALITY in electric supply is now gaining importance. The parameters of quality are frequency, voltage and harmonics.

Let us explain further the quality parameters of electric supply. Frequency is a global phenomenon, i.e., it is the same at all points of a grid which is operating in synchronous operation. Frequency is an indication of the balance between generation and load in a grid. If the generation exactly matches the load, the frequency would be the nominal frequency, i.e., 50 Hz. If generation is more than the load in a grid as a whole, the system frequency would be greater than 50 Hz. If generation is less than the load, the system frequency would be less than 50 Hz. Voltage is a local phenomenon, i.e., it can be different at different points of the grid. Therefore, the grid operator has to ensure that the proper voltage profile is maintained at all points of the grid. For ensuring proper voltage profile, capacitors or reactors are installed at different points in the grid. If it is observed that the voltage is low at a particular point in the grid, then capacitors are installed at that point. Similarly, if voltage is observed to be high, as per the studies, then reactors are installed at that point. The basic

33

Operation and Maintenance

purpose of these elements is to ensure that the reactive power requirement of the load or transmission lines is met. Besides this, there are also other voltage phenomena like unbalanced voltage in the three phases, voltage dip, etc. Voltage unbalance in the grid could be caused due to the tripping of one of the phases of a transmission line or due to unbalanced load in the three phases emanating from the distribution systems or bulk loads. Voltage dip, on the other hand, is a transient phenomenon caused by a transient fault or tripping of an element at a remote location of the grid. Stormy weather could also cause flashover between arcing horns, resulting in voltage dip. Harmonics is recently becoming an issue in the modern world, due to a number of electronic devices connected in the grid as well as in the distribution system, which converts AC to DC through rectifiers or which chop an AC wave for voltage or current control. In the grid, harmonics are caused by HVDC stations, which convert AC to DC and back from DC to AC. In the distribution system, harmonics are caused by power supplies and inverters through which power is supplied to computers and all household appliances using digital technology, which have permeated our lives. For this, standards have been laid down in the Regulations for Technical Standards for Connectivity to the Grid. As per the provisions of these Standards, the limits for individual and total harmonics distortion have been given. q What Does Grid Management Involve? Grid management involves •

forecasting (demand pattern);

•

planning (outages, unit commitment, resource scheduling);

•

coordination (between stakeholders);

•

supervision (grid parameters);

•

real time operation and control for optimal utilization of available resources in the grid, which involves − scheduling, − monitoring, and − restoration of grid;

•

off-line operational planning involving grid security issues, restoration of grid and commercial issues or billing.

We shall talk about these aspects in detail in the next section.

34

The load dispatch centre is primarily responsible for management of the grid. Its various functions: ex-ante (a Latin term meaning before-hand), real-time and post-facto (meaning after the fact) are given in Table 4.8.

Table 4.8: Grid Management Functions EX-ANTE

POST-FACTO

REAL-TIME

•

Forecasting demand for the forthcoming period

• • Resource re-scheduling as and when required

Reporting events occurring in a grid operation

•

Scheduling of resources at disposal

• Implementation of proper contract of service as entitled

•

Analyzing the events that occurred

•

Planning the grid element outages like generator maintenance, etc.

• Supervising and controlling grid parameters

•

Collecting the energy meter data

•

Processing the data collected

•

Energy accounting

•

Operating the pool account, unscheduled interchange account and the reactive energy account.

•

Providing for open access transmission corridors.

• Ensuring real-time balancing of resources • Ensuring grid security and reliability • Coordinating the outages and load shedding

Introduction to the Power Distribution System

• Ensuring proper power quality • Minimizing losses and optimizing resource utilization • Tackling emergencies effectively and efficiently.

4.6.2 Load Scheduling and Dispatch Load scheduling means fixing the schedules of generation of power for generating stations and the schedules for drawal of power by the States taking into account drawal schedules from shared power sector projects and schedules of power purchased from buyers to sellers. Scheduling is done for the day ahead by the Regional Load Dispatch Centre to ensure balance between load and generation in the grid with the aim of achieving an operating grid frequency of 50 Hz. Since power cannot be stored to a large extent, power generated has to be used at that instant of time. Therefore, it has to be ensured that the generation matches the load at each point of time. Schedules are prepared on a 15 minute basis, to see to it that the average generation of electrical energy over 15 minutes matches the load over those 15 minutes. Scheduling is done one day before for the day ahead, as per a time schedule specified in the Indian Electricity Grid Code (named simply as

35

Operation and Maintenance

“Grid Code” as per the Electricity Act, 2003), so that the State Power Utilities plan for load management for the next day. For example, suppose a State finds that after taking into account its own expected generation for the next day and the net drawal schedule for the next day, it would fall short of meeting its anticipated requirement for the next day by 200 MW during peak time. Then it would have to plan a load shedding of 200 MW during peak time. Schedules can also be changed on the same day due to major load variations experienced by a State due to abnormal weather conditions. For example, rains in summer could cause reduction of agricultural load and AC load; heavy rains could cause disruptions in the transmission system and hence loss of load. The rescheduling would be valid after a time gap of about one and a half hours so as to enable implementation of the new schedules. Rescheduling can also be done by the Regional Load Dispatcher in cases of transmission bottleneck and grid disturbance. Scheduling is important because it is meant to ensure the desired operating frequency of the grid. There are financial penalties for violating these schedules if these violations burden the grid and financial incentives if the violations help the grid, through a component of the tariff, known as unscheduled interchanges. Monitoring the parameters of the grid is the prime real time function of the Load Dispatch Centre. These parameters include operating frequency, voltage levels at all points of the grid, status of line and transformer loading throughout the grid, especially at crucial points. In order to help the grid operator monitor these parameters over the large number of points in the grid, the Load Dispatch Centre is equipped with SCADA (Supervisory Control and Data Acquisition) System. You will study about it in Block 3 of the course BEE-002. Based on the alarms generated by the SCADA System, action is taken by the grid operator by giving instructions to all concerned. All instructions of the grid operator have to be followed. All instructions are also recorded on a sound recorder, to be replayed at the time of analysis of a grid incident or any other contingency. The grid needs to be constantly monitored to observe whether it is operating within its limit. This work is being done in the RLDCs and SLDCs. The LDCs coordinate between the Central Generating Stations and States (through SLDCs). Restoration of grid involves restoring the grid after a grid disturbance. Grid disturbance normally takes place in a matter of milliseconds and there is no time for the grid operator to react. Therefore, the operational procedures for restoration of a grid are planned well in advance and come under the scope of off-line operational planning. The grid operator just has to follow the procedure for restoring the grid. We now describe the load scheduling process as it takes place. The Load Scheduling Process

36

The process starts with the Central Generating Stations (CGS) in the region declaring their expected output capability (in MW) for 96 slots of 15 minutes duration during the next day to the Regional Load Dispatch Centre (RLDC).

The RLDC breaks up and tabulates these output capability declarations as per the beneficiaries’ plant-wise shares and conveys their entitlements to State Load Dispatch Centres (SLDCs). The latter then carry out an exercise to see how best they can meet the load of their consumers over the day, from their own generating stations, along with their entitlement in the Central stations. They also take into account the irrigation release requirements, distribution utilities’ load schedules for next day and load curtailment, etc. that they propose in their respective areas.

Introduction to the Power Distribution System

The SLDCs then convey to the RLDC their schedule of power drawal from the Central stations (limited to their entitlement for the day). The RLDC aggregates these requisitions and determines the dispatch schedules for the Central generating stations and the drawal schedules for the beneficiaries (State as a whole) duly incorporating any bilateral agreements and adjusting for transmission losses. These schedules are then issued by the RLDC to all concerned and become the operational as well as commercial datum for inter-State and CGS transactions. However, in case of contingencies, Central stations can prospectively revise the output capability declaration, beneficiaries can prospectively revise requisitions, and the schedules are correspondingly revised by RLDC. It is for the SLDCs to further break-up these State entitlements into Discom entitlements and State Generation Schedules. While the schedules so finalized become the operational datum, and the regional constituents are expected to regulate their generation and consumer load in a way that the actual generation and drawals generally follow these schedules, deviations are allowed as long as they do not endanger the system security. Load Shedding During the normal operation of a grid, it is possible that the load exceeds the generation. If this happens the frequency of the system goes down. The standard frequency is 50 Hz. But the frequency can go down to about 49.0 Hz. After this value, it is not advisable to allow it to reduce it any further since it can cause the system to lose synchronism and lead to ultimate collapse of the system. As a result, we go for purposeful shedding of load, known as load shedding. The load shedding is a process of reducing load on the grid so as to save the grid as a whole. Load shedding can be done in two ways: 1. Automatic: For this purpose automatic under-frequency relays are installed. These relays carry out automatic shedding of load if the frequency falls below a certain level. 2. Manual: Special guidelines have been provided by RLDCs/SLDCs for the load shedding at different frequency levels. These guidelines depend upon the grid parameters at the particular instance as well as

37

Operation and Maintenance

some fixed guidelines for frequency falling below a particular limit or area-wise/consumer category-wise shedding. Off-Line Operational Planning You have studied in Sec. 4.6.1 that off-line operational planning involves •

grid security issues;

•

restoration of grid; and

•

commercial issues or billing.

We discuss these briefly. Grid security issues involve •

load generation balance planning in respect of active power for the next year, which is reviewed on a quarterly and then monthly basis, in order to ensure that the frequency stays at the nominal level;

•

installation of capacitors or reactors to obtain a proper voltage profile in the grid;

•

line and transformer loading;

•

protection coordination;

•

monitoring of the grid; and

•

proper analysis of tripping of lines as well as of grid disturbance and taking corrective measures thereof.

Under-frequency and rate-of-change of frequency relays are installed as security measures to cut off load in case of gradual or sudden drop in generation, respectively, to ensure nominal frequency in the grid. Islanding schemes of important generators and loads are also planned as a last resort to isolate or island them in case of a blackout so that the important power stations and loads keep functional. Therefore, these islands are made in such a way that the generation and load in these islands approximately match. Under operational planning, procedures are also formed for restoration of grid in case of tripping of some or more elements of the grid or for total blackout. This is done by the Regional Load Dispatch Centre responsible for real time operation of the regional grids, in consultation with all the players involved in grid operation. One of the points involved in grid restoration is the “black start”, which means starting of a generating unit after a blackout. Since hydro generators require the least power for starting, they are normally started first or, in other words, used for black start.

38

Commercial billing by the various generators is done in accordance with the Availability Based Tariff approved by the Central Electricity Regulatory Commission. Under the Availability Based Tariff (ABT), the beneficiaries are required to pay charges in three components, viz., annual fixed charges, energy charges and Unsheduled interchanges (UI) charges.

Annual fixed charges are required to be paid by the beneficiaries irrespective of actual drawals or schedules. The implemented schedules, as described earlier, are used for determination of the amounts payable as energy charges. Deviations from schedules are determined in 15-minute time blocks through special metering, and these deviations are priced depending on frequency. These deviations are called unscheduled interchanges (UI).

Introduction to the Power Distribution System

The pricing for UI is linked to system frequency such that the constituent causing the grid frequency to improve/worsen in worst conditions gets rewarded/penalised at higher price and vice versa. Further, the UI pool account is zero sum account, i.e., the amounts received from constituents are distributed amongst the other constituents. As long as the actual generation/ drawal is equal to the given schedule, UI is zero and the payment on account of the third component of Availability Tariff is zero. In case of under-drawal, a beneficiary is paid back to that extent according to the frequency dependent rate specified for deviations from schedule.

4.6.3 Load Balancing Load Balancing is the process of achieving and maintaining equal load on each phase of a distribution transformer. The loadings on primary and secondary side of a DTR are shown in Fig. 4.11.

Fig. 4.11: Balancing of Load in a DTR

If load on each phase of the distribution transformer is not equal, it is called unbalanced loading of transformer. Practically speaking, balanced load cannot be maintained on the transformer due to the inherently varying nature of load. Each transformer supplies power to resistive loads (bulbs, heaters, etc.) and inductive loads (motors, etc.). These loads can be either single phase, distributed separately on the three phases, or three-phase in

39

Operation and Maintenance

nature. If the distribution transformer is supplying power to only three phase loads, then achieving and maintaining balanced load on transformer could be an easier task. But in practice, this happens very rarely, because each installation possesses either three phase or single phase or both the loads, which keep changing at different points of time. Apart from natural unbalancing, unbalanced load may also result from load shedding of one phase in each of the LT feeders emanating from a distribution substation. Even though the system may have been balanced initially, it is difficult to have similarly loaded outgoing feeders and achieve equal load shedding in the three phases. In some cases, due to constraints on availability of proper switching facility on each feeder, it is difficult to shed equal load from each phase. Thus, it is really a difficult task for a distribution utility to maintain balanced load on the distribution transformer. However, it is important for many reasons. IMPORTANCE OF LOAD BALANCING

NOTE Diversity factor is defined as the ratio of the sum of the individual maximum demands of various parts of a power distribution system to the maximum demand of the whole system. It measures the staggering of different hours of the day and indicates flatness of load curve. That is, it denotes MVA vs hours of the day curve.

40

•

Extended life of the transformer, which remains in service for longer period of time.

•

Improved quality of supply.

•

Lesser maintenance cost on distribution/power transformer leading to increase in Utility’s Operational Profit.

•

Consumer Satisfaction.

Difficulties in Maintaining Balanced Load on a Transformer The distribution transformer supplies power to the domestic and/or commercial consumers. In practice, it is seen that all consumers will not switch on their entire connected load at the same instant of time. The switching of load will vary with time and also with the requirement of the consumer. This is expressed in terms of the diversity factor. If it is equal to 1, it means that all the consumers are in need of their entire connected load at the same instant of time. In practice, the diversity factor ranges between 2 and 3. If diversity factor is more, then there is a greater possibility that the load on each phase of the distribution transformer will vary and not be equal. Now, consider an Electric Utility which is supplying power to small industrial consumers. In this case, the transformer will be supplying to more three phase loads than single phase loads. So, the diversity factor for small industrial consumers will be less compared to the domestic/commercial consumers. For large industrial consumers, the diversity factor will be even less. But some degree of unbalanced loading will still remain on the transformer. The task of distribution utility is to reduce this degree of unbalanced loading.

We offer some tips in this regard. Some Tips to Operate Transformers Near Balanced Loading •

Connect single phase load on each phase of distribution transformer, so that at the end, current in each phase of transformer will be almost equal. It has been seen that, linemen or wiremen connect the single phase load to the lower phase of a pole. It may be due to illiteracy and/or hesitation to connect the single phase load on the top phase of pole. The distribution utility must ensure supervision of the job at the time of connecting new single phase load to avoid such practices.

•

In some distribution utilities, transformers do not have current measuring instruments and, hence, continuous surveillance cannot be done to check whether distribution transformer is equally loaded (Balanced). In this case, the current must be measured for all phases by using Clamp-on-Meters, at least once during peak hours and a record should be kept. By analysing the past trend, the average current can be calculated. But due consideration must be given to changing weather conditions and/or extra loads due to festivals, etc.

•

Providing Solid Earthing to the neutral of transformer.

•

Using proper size of Blow-out-Fuses.

Introduction to the Power Distribution System

On this note, we bring the discussion in this unit to an end. In this unit, you have learnt about the Power Distribution System, and the general goals and practices for its maintenance. We now summarise the contents of the unit.

4.7 SUMMARY •

The Distribution System contains: −

Sub-transmission system in voltage ranges from 33 kV to 220 kV. The energy goes from power substations to distribution substations through primary system and then from distribution substations to secondary distribution system for local voltage distribution.

−

Primary circuits of feeders, usually operating in the range of 11 kV to 33 kV, supply the load in well defined geographical areas.

−

The distribution transformers, usually installed on poles or near the consumer sites, transform the primary voltage to the secondary voltage, which is usually 240/415 V.

−

Secondary circuits at service voltage which carry energy from the distribution transformers along the streets, etc.

•

The components of Distribution System include substations, transformers, feeders and metering system, etc.

•

The O&M objectives and general practices for distribution system focus on improving the reliability and maintenance of plant and equipment, maximising capacity utilisation, increasing operating efficiency, and

41

Operation and Maintenance

reducing operating and maintenance costs. •

Grid management, load scheduling and load balancing are important for the smooth functioning of the power distribution system.

4.8 TERMINAL QUESTIONS 1. Which component of the distribution system can be a critical bottleneck in supplying uninterrupted power to consumers and why? 2. What configurations of feeder networks can be used in a distribution system? Discuss their suitability in different circumstances. 3. Compare the distribution system planning criteria used in your utility with those given in the unit and describe them. 4. What do you understand by load scheduling and Unscheduled Interchanges? 5. Does your utility have written O&M practices? If yes, study those and suggest improvements in the same. If not, what practices are followed in the field? 6. What is load balancing? How can it be achieved? 7. What is the significance of Grid Management? Who has the responsibility of Grid Management in your State and your utility? 8. What are the activities involved in Distribution System Planning?

42

APPENDIX 1: REACTIVE POWER CONTROL IN DISTRIBUTION SYSTEMS

Introduction to the Power Distribution System

Reactive power (kVAR) control represents an efficient method of reducing the cost of utility operation. The savings brought about by kVAR/voltage control are not confined to the monetary value of the energy saved; the released system capacity can serve to delay a costly expansion and reduce the ageing of components. kVAR control provides appropriate placement of compensation devices to ensure a satisfactory voltage profile while minimizing the power losses and the cost of compensation. Other ancillary benefits gained by correcting the power factor are: •

lower energy losses,

•

better voltage regulation, and

•

released system capacity.

All electric equipment requires “vars”, a term used by electric power engineers to describe the reactive or magnetizing power required by the inductive characteristics of electrical equipment. These inductive characteristics are more pronounced in motors and transformers, and therefore, can be quite significant in industrial facilities. The flow of vars, or reactive power, through a watt-hour meter will not affect the meter reading, but the flow of vars through the power system will result in energy losses on both the utility and the industrial facility. Some utilities charge for these vars in the form of a penalty, or kVA demand charge, to justify the cost for lost energy and the additional conductor and transformer capacity required to carry the vars. In addition to energy losses, var flow can also cause excessive voltage drop, which may have to be corrected by either the application of shunt capacitors, or other more expensive equipment, such as load-tap changing transformers, synchronous motors, and synchronous condensers. The power factor triangle shown in Fig. 1 is the simplest way to understand the effects of reactive power. The longest leg of the triangle (on the upper or lower triangle), labelled total power, represents the vector sum of the reactive power and real power components. Mathematically, TOTAL POWER =

(REAL POWER)2 + (REACTIVE POWER)2

The angle Φ in the power triangle is called the power factor angle and is mathematically equal to: cos Φ =

REAL POWER (kW) TOTAL POWER (kVA)

43

Operation and Maintenance

Fig. 1: Power Factor Triangle

The ratio of the real power to the total power in the equation above (or the cos of Φ) is called the power factor. The advantages of PF improvement by capacitor addition are as follows: a)

Reactive component of the network and total current in the system from the source end are reduced.

b)

I2R power losses are reduced in the system because of reduction in current.

c)

Voltage level at the load end is increased.

d)

kVA loading on the source generators as also on the transformers and line up to the capacitors reduces giving relief. A high power factor can help in utilizing the full capacity of your electrical system.

Cost Benefits of PF Improvement While cost of PF improvement is in terms of investment needs for capacitor addition, the benefits to be quantified for feasibility analysis are: a)

Reduced kVA (maximum demand) charges in utility bill,

b)

Reduced distribution losses (kWh) within the plant network,

c)

Better voltage at motor terminals and improved performance of motors.

A high power factor eliminates penalty charges imposed when operating with a low power factor. Investment on system facilities such as transformers, cables, switchgears, etc. for delivering load is reduced.

44

It is the power distribution engineer’s responsibility to manage the operating system at an optimum power factor.

APPENDIX 2: FUNCTIONS OF O&M

Introduction to the Power Distribution System

Material and Equipment Information −

Classification of maintenance material (class / sub-class).

−

Material identification with a material code.

−

Equipment identification and details.

−

Updating equipment details.

−

Maintaining commercial details.

−

Maintaining equipment hierarchy for all equipment and assemblies.

−

Maintenance planning and scheduling.

−

Maintaining bills of material / sub-assembly / component.

Operational Location Information −

Identification of the operational location.

−

Maintaining corresponding location of equipment.

− Maintaining details of locations. Work Specification −

Standardising work specifications.

−

Maintaining notes on work specifications.

Preventive Maintenance and Overhaul −

Generate preventive maintenance plans.

−

Listing of locations, equipment and tasks to be included in the plan.

−

Allocation of tasks to specific groups.

−

Recording the material required.

−

Generating work orders.

−

Overhauling equipment as per requirement.

Breakdown Maintenance −

Noting the time of fault and the priority status.

−

Processing the request.

−

Enabling maintenance work.

Condition-based Maintenance −

Predicting potential machine failures.

−

Recording data about malfunctioning equipment.

Signature Analysis −

Identifying parameters to be monitored for each class of equipment.

45

Operation and Maintenance

−

Identifying test point for measuring the above.

−

Entering a target or optimum value, and limits for the parameter.

−

Determining the frequency of measurements to be made at each test point.

−

Generating schedules for taking readings.

−

Capturing readings at each test point.

−

Analysing readings and generating alarm / warning signals.

−

Displaying message to generate work order if required.

Maintenance Requests −

Issuing maintenance requests.

−

Closing maintenance requests.

Maintenance Work Orders −

Generating work orders.

−

Informing the concerned departments.

−

Assigning tasks to maintenance personnel or vendors.

−

Issuing material for doing the same.

−

Storing the details of material issued / purchased.

−

Monitoring status of tasks.

Safety Procedures and Permit To Work −

Sending permit-to-work (PTW) requests to operations.

−

Receiving the necessary permits from operations.

Spares Planning −

Identifying spares required for equipment.

−

Viewing the updated stock positions.

−

Generating lists of spares featuring quantity required and criticality.

Maintenance History −

Recording detailed maintenance history.

−

Recording breakdown details / cause of failure / action taken / downtime.

−

Generating reports for equipment failures.

Document Management −

Maintaining document master.

−

Generating document register.

Maintaining Contract Details. 46

Maintenance Personnel and Workgroup

−

Maintaining work group details.

−

Viewing work schedules.

Introduction to the Power Distribution System

Capturing Operations Data −

Maintaining operations logs.

−

Maintaining generation and transmission schedules.

−

Maintaining parameter values.

−

Maintaining fuel details.

Estimates and Expenditure −

Generating estimates.

−

Capturing estimates.

MIS Reports −

Generation reports.

−

Plant-availability reports.

−

Outage reports.

−

Fuel reports.

−

Generation schedules.

−

Equipment registers.

−

Equipment-performance reports.

−

Equipment-history cards.

−

Fault-analysis reports.

−

Maintenance plans.

−

Maintenance requests.

−

Permit to work.

−

Work-order permits.

−

Condition-analysis reports.

−

Expenditure reports.

−

Maintenance schedule miss reports.

−

Material-consumption reports.

−

Document register.

−

Accident reports.

47

Operation and Maintenance

48

Substation Equipment and Distribution Lines

Unit 5

Learning Objectives After studying this unit, you should be able to:

Substation Equipment and Distribution Lines


describe the main equipment required for the construction of a 66-33/11kV substation;
classify the distribution line equipment;
describe the main equipment for overhead lines;
discuss the important features of underground power cables in the distribution system network;
enunciate the general operation and maintenance practices for substation equipment, distribution lines and capacitors;
explain hotline maintenance techniques and tools; and
explain the effect of HT/LT ratio on line losses and voltage.

49

Operation and Maintenance

5.1 INTRODUCTION In Unit 4, you have studied about the power distribution system and its components. You have also learnt about distribution system planning and the general O & M objectives and practices. You will agree that the smooth operation of the power distribution system depends on how well it is maintained. This includes the operation and maintenance of all its components. We begin this unit with a discussion of the substation equipment and distribution lines so that you know the standards prescribed for the equipment. Adhering to these standards would ensure the smooth operation of the equipment. We next discuss the operation and maintenance of equipment used in the 66-33/11 kV substations, 11/0.4 kV substations, overhead lines, underground cables and capacitors. Finally, we take up hot line maintenance tools and techniques and the impact of LT/HT ratio on losses. In the next unit, we deal with the O & M of distribution transformers separately.

5.2 66 - 33/11 kV SUBSTATION EQUIPMENT Equipment in a substation can broadly be categorized as follows:

50

•

structures;

•

power transformers;

•

bus-bars;

•

circuit breakers (33 kV and 11 kV);

•

isolators or isolating switches (33 kV and 11 kV);

•

earthing switches;

•

insulators;

•

power and control cables;

•

control panel;

•

lightning protection − surge arrestors;

•

instrument transformers (current and power transformers, i.e., CTs and PTs);

•

earthing arrangements;

•

reactive compensation;

•

DC supply arrangement;

•

auxiliary supply transformer; and

•

fire-fighting system.

The design of the substation equipment must comply with the requirement of relevant Indian Standards.

We now briefly describe each one of these. •

Structures

Substation Equipment and Distribution Lines

Structures are required to provide entry from the overhead line to the substation and to extend out required number of feeders. The numbers of structures should be kept to a minimum as large number of structures would not only be uneconomical but give an ugly look to the substation and may prove to be obstructions in extending bus-bar, lines, etc. The main structures required for 33/11 kV substations are: − incoming and outgoing gantries; − support structures for breaker, isolators, fuses, insulators, CTs and PTs; and − bus-bars. Switchyard structures can be made of fabricated steel, RCC or PSCC, Rail or RS Joist. •

Power Transformers You have learnt about the underlying principle and design of a power transformer in Unit 4. The general operation and maintenance practices of power transformers are similar to those of distribution transformers, which are discussed in detail in Unit 6.

•

Bus-bars A bus-bar in electrical power distribution refers to thick strips of copper or aluminum that conduct electricity within the substation (Fig. 5.1). The size of the bus-bar is important in determining the maximum amount of current that can be safely carried. The bus-bar should be able to carry the expected maximum load current without exceeding the temperature limit. The capacity of bus should also be checked for maximum temperature under short circuit conditions. Different types of bus-bars, namely, single bus-bar, single bus-bar with bus sectionalizer, main and transfer bus, double bus-bar, double bus-bar with double breaker scheme and mesh scheme are used in a substation in accordance with the safety and reliability considerations.

•

Circuit Breakers

Fig. 5.1: Bus-bars

A circuit breaker is a switching device built ruggedly to enable it to interrupt/ make not only the load current but also the much larger fault current, which may occur on a circuit. A circuit breaker contains both fixed contacts and moving contacts. The purpose of circuit breakers is to eliminate a short-circuit that occurs on a line. Circuit breakers are found at the arrivals and departures of all lines incident on a substation. When the circuit breaker is closed these contacts are held together. The mode of action of all circuit breakers consists in the breaking of the fault current by the separation of the moving contacts away from the fixed ones. An arc is immediately established on

51

Operation and Maintenance

separation of the contacts. Interruption of the current occurs after the arc at these contacts is extinguished and current becomes zero. Elements of a Circuit Breaker Circuit breakers contain the following elements, irrespective of the medium for arc quenching and insulation: −

main contact at system voltage;

−

insulation, such as porcelain, oil or gas, between the main contacts and ground potential;

−

operating and supervisory accessories, of which tripping facilities are most important.

A wide variety of closing and tripping arrangements (using relays with variable time delay) and a number of operating mechanisms (based on solenoids, charged springs or pneumatic arrangements) are available now-a-days. The types of breakers used in a distribution system are: −

air break type;

−

oil break type;

−

vacuum type; and

−

SF6 gas breaker.

(a)

(b)

Fig. 5.2: Circuit Breakers: a) Oil Break Type Breaker; and b) SF6 Gas Breaker

52

The rated voltage of circuit breakers for 33 kV level is 36 kV, and for 11 kV, it is 12 kV. The short circuit current rating is 25 kA. The 11 kV switchgear is generally metal enclosed indoor type.

•

Isolators Isolators are mechanical switching devices capable of opening or closing a circuit −

when a negligible current is broken or made, or

−

only a small charging current is to be interrupted, or

−

when no significant voltage difference exists across the terminals of each pole.

Substation Equipment and Distribution Lines

Fig. 5.3: Isolators

Isolators are capable of carrying current under normal conditions and short circuit currents for a specified time. In open position, the isolator should provide an isolating distance between the terminals. The standard value of rated duration of short time current capacity withstand for isolator and earthing switch is normally 1 second. A value of 3 seconds is also sometimes specified. For 33 kV, horizontal type isolating switches are used. The rated normal current is 630 A at 36 kV. For 11 kV, both horizontal and vertical mounting isolating switches of 400 Amps at 12 kV are used. •

Earthing Switches Earthing switches are provided at various locations to facilitate maintenance. Main blades and earth blades are interlocked with both electrical and mechanical means. The earthing switch has to be capable of withstanding short circuit current for short duration as applicable to the isolator.

•

Insulators An electrical insulator resists the flow of electricity. Application of voltage difference across a good insulator results in negligible electrical current. Adequate insulation is of prime importance for obvious reasons of reliability of supply, safety of personnel and equipment, etc. The insulators in use at substations are post insulators of pedestal type. The station design should be such that the number of insulators is kept at a minimum at the same time ensuring security of supply. In the areas where the problem of insulator pollution is expected (such as near the sea or thermal station, railway station, industrial area, etc.) special insulators with higher leakage resistance should be used.

53

Operation and Maintenance

•

Power and Control Cables Power and control cables of adequate current carrying capacity and voltage rating are provided at the substation. Power cables are used for 33kV,11 kV or LT system to carry load current. The control cables are required for operating and protection system connections. The cables are segregated by running in separate trenches or on separate racks.

•

Control Panels Control panels installed within the control building of a switchyard provide mounting for mimic bus, relays, meters, indicating instruments, indicating lights, control switches, test switches and other control devices. The panel contains compartments for incoming lines, outgoing lines, bus-bars with provision for sectionalizing, relays, measuring instruments, etc. The panel is provided with:

•

−

suitable over-current and earth fault relays to protect the equipment against short circuit and earth faults; and

−

measuring instruments such as ammeter, voltmeter and energy meter for 33kV and 11 kV systems.

Lightning Protection− −Surge Arrestors Large over voltages that develop suddenly on electric transmission and distribution system are referred to as “surges” or “transients”. These are caused by lightning strikes or by circuit switching operations. Surge arrestor is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current. The surge arrestor which responds to over-voltages without any time delay is installed for protection of 33 kV switchgear, transformers, associated equipment and 11 kV and 33 kV lines. The rated voltage of arrestors for 33 kV should be 30 kV for use on 33 kV systems and with nominal discharge current rating of 10 kA. The rated voltage of lightning arrestors should be 9 kV (r.m.s.) for effectively earthed 11 kV system (coefficient of earth not exceeding 80 % as per IS: 4004) with all the transformer neutrals directly earthed. The nominal discharge current rating should be 5 kA.

Fig. 5.4: Surge Arrestors

•

Instrument Transformers (Current and Voltage Transformers) The substations have current and voltage transformers designed to isolate electrically the high voltage primary circuit from the low voltage secondary circuit and, thus, provide a safe means of supply for indicating instruments, meters and relays. Ø Current Transformer (CT)

54

Current transformers are used in power installations for supplying the current circuits of indicating instruments

(ammeter, wattmeter, etc.), meters (energy meter, etc.) and protective relays. These transformers are designed to provide a standard secondary current output of 1 or 5 A, when rated current flows through the primary. A fundamental characteristic of CT is its transformation ratio, expressed as the ratio of the rated primary to rated secondary current. Current transformers have two inherent errors: the current ratio and phase displacement. These two errors serve as a basis on which current transformers are classified for accuracy.

(a)

(b) (b)

Substation Equipment and Distribution Lines

NOTE A current transformer is an instrument transformer in which the current ratio is within the specified limit. The primary winding is connected in series with the load and carries the load current to be measured. The secondary winding is connected to the measuring instrument or relay, which together with the winding impedance of the transformer and lead resistance constitute the burden of the transformer.

Fig. 5.5: a) Current Transformers; and b) Voltage Transformer

Ø Voltage Transformer or Potential Transformer (PT) These instrument transformers are used for supplying the voltage circuit of indicating instruments, integrating meters, other measuring apparatus and protective relays or trip coils. These may be of single phase or three phase design and of the dry or oil immersed types. A voltage transformer or PT is rated in terms of the maximum burden (VA output) it will deliver without exceeding specified limits of error. On the other hand, a power transformer is rated by the secondary output it will deliver without exceeding specified temperature rise. All voltage transformers are designed for a standard secondary voltage of 110 V or 110 / 3 V. •

Earthing Arrangements Earthing has to be provided for −

safety of personnel,

−

prevention of and minimizing damage to equipment as a result of flow of heavy fault currents, and

−

improved reliability of power supply.

The basic grounding system is in the form of an earth mat with risers.

NOTE Voltage transformer is an instrument transformer in which the secondary voltage is substantially proportional to the primary voltage and phase angle near to zero for an appropriate direction of connection.

55

Operation and Maintenance

Risers of MS flat are generally provided. Earth mat is provided within the substation area. The earth rods are connected to the station earth mat. The earthing must be designed so as to keep the earth resistance as low as possible. Earthing practices have been discussed in Unit 6 of the course BEE-002. •

Reactive Compensation Reactive compensation (as indicated by system studies of the network) has to be provided. It is always a good idea to ensure a power factor correction for transformers, since even when they are operating on low load (e.g., during the night) they absorb reactive power, which must be compensated to avoid unnecessary loadings and losses. You can recall this aspect from Appendix 1 to Unit 4. Shunt capacitors (Fig. 5.6) are connected on the secondary side (11 kV side) of the 33/11 kV power transformers. The capacitors are generally of automatic switched type. The automatic system of the capacitor bank has the task of switching in the necessary capacitance according to the load requirements at each given moment.

• Fig. 5.6: Shunt Capacitors

Station Battery/DC Supply Arrangement Station batteries supply energy to operate protection equipment such as breakers and other control, alarm and indicating equipment (Fig. 5.7). The station batteries are a source for operating DC control system equipment during system disturbances and outages. During normal conditions the rectifier provides the required DC supply. However, to take care of rectifier failure, a storage battery of adequate capacity is provided to meet the DC requirements. Normally, in a 33/11 kV substation, the DC system is of 30 cells consisting of 15 lead acid storage batteries or Nickel-Cadmium batteries. The battery is connected in parallel with a constant voltage charger and critical load circuits. The charger maintains the required voltage at battery terminal and supplies the normally connected loads. This sustains the battery in fully charged condition. The correct size battery charger has to be selected for the intended application.

Fig. 5.7: Battery Bank

•

An Auxiliary Supply Transformer of adequate capacity is required to be provided for internal use for lighting loads, battery charging, oil filtration plant, etc. The supply should be reliable. In a substation it is normally provided from a station transformer connected on 33 or 11 kV bus bar. •

56

Auxiliary Supply Transformer

Fire Fighting System In view of the presence of oil filled equipment in a substation, it is important that proper attention is given to isolation, limitation and extinguishing of fire so as to avoid damage to costly equipment and reduce chances of serious dislocation of power supply as well as ensure safety of personnel. The layout of the substation itself should be such that the fire should not spread to other equipment as far as possible. Fire

extinguishers of the following type must be provided: −

Carbon dioxide extinguisher, and

−

Dry chemical powder extinguisher.

Substation Equipment and Distribution Lines

Carbon dioxide (CO2 type) extinguisher and Dry chemical powder type extinguisher should conform to IS: 2878 and IS:2171, respectively. For oil fire, foam type extinguishers are used (see Unit 7, BEE-002 also). The fire fighting equipment should be maintained and kept in top condition for instant use as per IS: 1948-1961 “Fire Fighting Equipment and its Maintenance including Construction and Installation of Fire Proof DoorsFire Safety of Buildings (General)”. So far we have described the equipment in a 66-33kV/11kV substation. You may like to review the information before studying further.

SAQ 1: Equipment at 66-33/11kV substation List the equipment being used in your utility for the construction of 33/11 kV substation along with their typical ratings. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ………………………………………………………………………………………

5.3 11/0.4 kV SUBSTATION EQUIPMENT The main equipment at an 11/0.4 kV distribution substation comprises: •

distribution transformers;

•

transformer mounting structure;

•

protection system;

•

earthing system;

•

lightning arrestors;

•

LT distribution box; and

•

reactive compensation.

We shall be discussing the distribution transformers in detail in the next unit. Here, we briefly describe the remaining components. •

Transformer Mounting Structure Transformers can be mounted outdoors (Figs. 5.8 and 5.9) in one of the

57

Operation and Maintenance

following ways: Plinth mounting, H-pole mounting and direct mounting. We describe these mountings, in brief.

(a)

(b)

Fig. 5.8: Transformer Mountings: a) Plinth Mounting; and b) H-Pole Mounting

Ø Plinth mounting: The transformer is mounted on a plinth made of concrete. The plinth has to be higher than the surroundings. The method can be used for all sizes of transformers. Where the distribution substations are plinth mounted, they are efficiently protected by fencing so as to prevent access to the apparatus by unauthorized persons. Ø H-pole mounting: The transformer can be mounted on crossarms, fixed between two poles, which are rigidly fastened to the poles. The transformer has two base channels, which rest on the transformer mounting structure. Ø Direct mounting: The transformer is clamped directly to the pole by suitable clamps and bolts. This method is used for transformers up to 25 kVA only. •

Fig.5.9: Direct Mounting

58

Protection System The HT side of all transformers is normally protected by drop out expulsion type fuse. Three 11 kV drop out fuse units comprising a set are installed on mounting cross-arm. The fuse element is soldered on both ends between woven wires, which are sufficiently strong to withstand tension when fixed to the terminals on both ends. The element is housed in an insulated tube of paper or insulating material. Horn gap fuses are also used on distribution transformers on HT side. The fuse wire is fixed between arcing horns. The advantage is that ordinary fuse wire of rated capacity can be used for replacement while for drop out fuses, fuse elements are required to be stocked for replacement.

•

Earthing Pipe earthing or rod earthing is provided for the distribution substation. Three electrodes forming an equilateral triangle are provided so that adequate earth buffer is available.

•

Substation Equipment and Distribution Lines

Lightning Arrestors 11 kV lightning arrestors 9 (kV) of outdoor type are used for diverting the lightning surges to earth resistance of earth. The lightning arrestor should be installed on the HT side and its lead should be kept at a minimum.

•

LT Distribution Box For transformers of 100 kVA and above, sheet metal LT distribution box consisting of LT breaker and fuse cut-outs is provided from where distribution feeders are to be taken out. The size of the box has to be suitable for accommodating MCCB, fuse cut-outs, cable connectors, bus-bars, etc.

•

Reactive Compensation The load incident on the distribution system is mostly inductive, requiring large reactive power. The best method is to compensate the reactive power at the load end itself but it is difficult to implement in practice. Hence, providing compensation on the distribution system is essential. So wherever the power factor is low, reactive compensation may be provided on the distribution transformers. The shunt capacitor supplies constant reactive power at its location, independent of the load. Fixed or automatic switched type capacitors of adequate rating are to be provided on the LT bus of the distribution transformers. In the switched capacitor system, the capacitors are switched on and off along with the load to avoid over-voltage during low load operation.

SAQ 2: Equipment at distribution substation List the equipment and their typical ratings, being used in the distribution substations of your utility. Are all the protection equipment listed above being used? ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… 59

Operation and Maintenance

5.4 DISTRIBUTION LINE EQUIPMENT The distribution lines can be either overhead or underground. These are usually overhead, though for higher load densities in cities or metropolitan areas, these are underground. The choice between overhead and underground depends upon a number of widely differing factors such as the importance of service continuity, improvement in appearance of the area, feasibility in congested areas, comparative annual maintenance cost, capital cost and useful life of the system.

5.4.1 Overhead Lines An overhead power line is intended for transmission of electric power by a bare or covered overhead conductor supported by insulators, generally mounted on cross-arms near the top of poles. The overhead line may be 66, 33, 11 kV or LT line. The basic equipment used for the line remains the same. The main equipment required for an overhead line is as follows:

(a)

(b) Fig. 5.10: a) Overhead Lines Mounted on Cross-arm Poles; b) Close-up

•

supports,

•

cross-arms,

•

insulators,

•

earthing knob,

•

earthing coil,

•

strain hardware set,

•

conductors,

•

line accessories,

•

guard wires, and

•

LT line spacers.

We describe each one of these, in brief. •

Supports A support is a column of wood, concrete, steel or some other material supporting overhead conductors by means of arms or brackets. The supports used for overhead line construction vary in design and the purpose they have to perform. The different types of supports for overhead lines are: wood poles, concrete poles, steel poles and lattice type towers.

60

Ø Wood poles: Chemically treated wood poles are used for distribution lines. The advantage of using wood poles is that they are low in cost. However, they are susceptible to decay. The specifications for wood poles are covered by IS:876 and IS:5978. According to this standard, the timber suitable for poles has been classified into three groups

depending upon its strength. For example, IS 6056 for jointed wood poles for overhead lines specifies that sal, deodar, chir, kail, wood be used. Jointed wood poles with wire bound lap joint are considerably less expensive and found to be very suitable for LT and HT lines in rural areas.

Substation Equipment and Distribution Lines

Ø Concrete poles: Concrete poles are more expensive than wood poles but cheaper than steel tubular poles. Concrete poles are of three types: −

Pre-cast cement concrete poles (PCC) made of cement concrete;

−

Reinforced cement concrete poles (RCC);

−

Pre-stressed cement concrete poles (PSCC).

The low maintenance, competitive price and aesthetic appearance of PCC poles makes them superior to steel or wood for use in electric lines. Ease and speed of installation means faster project completion and lower installation cost. RCC poles have an extremely long life and need little maintenance but they are bulky in size and comparatively heavy. They have shattering tendency when hit by a vehicle. PSCC poles take care of these shortcomings to some extent. However, the handling, transportation and erection of these poles is more difficult because of their heavy weight. Ø Steel poles: The steel poles are of the following types: −

Steel tubular poles whose specifications are covered by IS:2713-1967. Due to their light weight, high strength to weight ratio and long life, they possess distinct advantages over other types of poles. The use of a pole cap at the top, concrete muff in the ground and regular coating of paint prolongs their life.

−

Old and second hand rails and Rolled Steel (RS) joists are frequently used as supports for overhead lines. The portion embedded in the ground should be protected by concrete muff and the remaining portion by regular paint unless galvanised steel is used.

Ø Lattice type supports: These are fabricated from narrow base steel structures. They are light in weight and economical and can be assembled at site if bolted construction is used. Normally both welded and bolted types are used. •

Cross-arms The shape and length of the cross-arms depend upon the desired configuration of conductors. The following types of cross-arms and brackets are used: −

V cross-arms for tangent locations with clamps;

61

Operation and Maintenance

−

double channel cross-arms for tension or cut point locations where double poles are used; and

−

top clamps.

Cross-arms of hand wood (sisso, sal), or creosoted soft wood (chir) or fibre glass are mostly used. Steel cross-arms are stronger and last much longer. MS angle iron and channel iron sections are generally used for this purpose. Smaller sections are used for communication circuits. •

Insulators You have learnt that an electrical insulator resists the flow of electricity. Application of a voltage difference across a good insulator results in negligible electrical current. Insulators made of glazed porcelain, tough glass and polymers are used for supporting the conductors. Porcelain insulators prevent the electrical current from energizing the power pole. The principal types of insulator are described below: −

Pin insulators are manufactured for voltages up to 33 kV and are cheaper than the other types. IS:1445 and 731 cover detailed specifications for these. The pins for the insulators are fixed in the holes provided in the cross-arms and pole top brackets. The insulators are mounted over the pins and tightened. The cost of pin insulators increases very rapidly as the working voltage is increased. For high voltages these insulators are uneconomical. Moreover, replacements are expensive.

−

Disc insulators are made of glazed porcelain or tough glass. They are used as insulators on high voltage lines for suspension and dead ending.The line conductor is suspended below the point of support by means of the insulator or a string of insulators. A disc insulator consists of a single disc-shaped piece of porcelain, grooved on the under-surface to increase the surface leakage path between the metal cap at the top and the metal pin underneath. The cap is recessed so as to take the pin of another unit, and in this way a string of any required number of units can be built up. The cap is secured to the insulator by means of cement. Disc insulators are “ball and socket” or “tongue and clevis” type. A suspension clamp is used to support the conductor, if suspension configuration of the line is chosen.

−

Shackle insulators are used for distribution lines dead ending and supporting conductors laid in vertical formation. IS:1445-1977 covers shackle insulators for voltages below 1000 V. The two standard sizes listed in this specification are 90 mm dia x 75 mm height and 115 mm dia x 100 mm height. A shackle insulator is supported by either two straps and two MS bolts or one U clamp or D strap and two MS bolts as per IS:7935.

−

Stay insulator/Guy strain insulators of egg type porcelain are used for insulating stay wire, guard wires, etc. wherever it is not

Fig. 5.11: Pin Type Insulator

Fig. 5.12: Disc Type Insulator 11 kV

62

proposed to earth them. As per IS: 5300, two strength sizes (ultimate tensile strength) are used: 44 kN and 88 kN, respectively, for LT and HT lines. −

•

Substation Equipment and Distribution Lines

Stays/Guys and staying arrangement: Guys of stranded steel wire are used on all terminal, angle and other such poles where the conductors have a tendency to pull the pole away from its true vertical position. The guys are fastened to the poles near the load centre point with the help of pole clamps. The other end of the guy/stay is secured to a stay rod embedded in the ground. The stay rod should be located as far away as possible.

Earthing Knob The earthing knob is used for supporting the neutral-cum-earth wire used for earthing of metal parts of supporting structures of low-tension lines, i.e., 400/230 V lines. The knob is generally made of cast iron 52x42 mm and its electrical resistance is not to exceed 200 mega ohms. Moreover, the breaking strength at the neck of the knob is not to be less than 11,500 kg when force is applied.

•

Earthing Coil Two types of earthing arrangements are used. One is with GI pipe and the other is with GI wire. In case of GI pipe earthing, 40 mm dia and 2500 mm long pipe is used for earthing of supports and fittings. GI wire is used for earthing of lines. Generally 8 SWG wire with 115 turns, 50 mm dia and 1500 mm length is used.

•

Strain Hardware Set The conductor is strung between sections through a strain hardware set. It is fixed with the last disc of the string of disc insulator. It is made from malleable iron or aluminium alloy. Alloy hardware is preferable as the losses are less.

•

Conductors Conductor represents 30 − 50% of the installed cost of the line. All aluminium conductors (AAC), all aluminium alloy conductors (AAAC) and aluminium conductor steel reinforced (ACSR) are generally used. Technical specifications of conductors are covered in IS: 398. These conductors are of standard construction and the ultimate tensile strength of the whole conductor is based on the total strand strength.

•

Line Accessories This is the associated equipment required for fastening the conductors to supports and taking off the power or supply points such as joints material, clamps and compounds. For lines up to 33 kV, the following fittings are used: ♦ Conductor dead-end fittings −

LT conductor dead-end grips,

63

Operation and Maintenance

−

guy grips dead-end,

−

service grips,

−

full tension splices,

−

distribution ties,

−

side ties,

−

spool ties,

−

tee connectors,

−

lashing rods, and

−

line guards.

Preformed fittings made of aluminium alloy are used as it saves cost, labour and time. It also eliminates chances of error of judgment. No tools are required. These fittings are fast and simple to apply and assure uniformity of application every time. ♦ Joints should conform to IE Rule 75. For conductors up to 50mm2, crimped joints are made with simple hand crimping tools and for higher sizes, compression type or hydraulic type crimping tools are used. Joints are of the following types: −

uni-joints/ compression joints,

−

twisting joints,

−

two part compression joints, and

−

dead-end joints.

♦ Insulator ties secure the conductor to the insulator. In general, the tie wire should be the same kind of wire as the line wire, i.e., for tying aluminium conductors on insulators, aluminium wire should be used. The tie should be made of soft annealed wire so that it is not brittle and does not injure the line conductor. ♦ Taps and jumpers are made by various accessories, which are not subjected to mechanical tension. Tapping should be taken off only at a point of line support.

•

Guard Wires Guard wires are to be used at all points where a line crosses a street, road or railway line, other power lines, telecommunication lines, canals, rivers, along the road and public places. As per IE Rule 88, guard wires of galvanized steel of minimum 4 mm dia having breaking strength not less than 635 kg should be used.

•

LT Line Spacers Very often clashing of LT conductors in the mid span takes place due to sag, wind and longer spans. This results in faults and interruptions. Spacers are provided to overcome this problem.

64

You may like to revise the information given in this section before studying further.

SAQ 3: Overhead lines

Substation Equipment and Distribution Lines

List the equipment being used for construction of overhead distribution lines in your utility. Describe the types of supports and insulators being used for construction of lines. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… ………………………………………………………………………………………

5.4.2 Underground Power Cables Due to the fast growth in load densities in major towns and cities, 33 kV, 11 kV and LT underground cables are being used to meet the ever growing demand of electric power. The underground cable system has attained considerable importance in distribution networks. This is because in towns and cities, almost all roads are already occupied by LT, HT overhead lines, telephone lines, street lights, advertising boards, etc., on either side of the roads. Further, high-rise buildings make it difficult to go for overhead systems for sub-transmission or distribution. Moreover, the overhead system with bare conductors is prone to frequent breakdowns causing interruption in power supply. Uninterrupted power supply can be maintained by employing underground cable ring system. The underground cabling system is particularly important for metropolitan cities, city centres, airports and defence services. Underground distribution costs are between 2 to 10 times that of the overhead system. Yet, it is preferred due to elimination of outages caused by abnormal weather conditions such as snow, rain, storms, lightning, fires, stress, accidents, etc. Moreover, this system is environment-friendly and has a long life. In addition, improved cable technology has reduced the maintenance cost of the underground system compared to the overhead system. We summarise the main reasons for underground cable systems in Box 5.1. Box 5.1: Reasons for Having Underground Cables

Ø

The right of way for erecting overhead systems is no longer available;

Ø

It is possible to extend the supply from source to load centres on any route profile;

Ø

Fairly uninterrupted and reliable power supply can be maintained; and

Ø

Aesthetic beauty of the town/city as a whole can be ensured.

65

Operation and Maintenance

We now describe, in brief, the selection criteria, sizing, jointing and terminating of underground cables. •

Selection of Cable The following factors influence the selection of cable: −

load;

−

system voltage;

−

type of insulation;

−

short circuit rating;

−

mode of installation; and

−

economy and safety.

For the same conductor size, the maximum continuous current carrying capacity depends on the depth of laying, ground temperature, silicon oil resistivity, ambient temperature, proximity of other cables, type of ducts used. Paper Insulated Lead Covered, PVC and XLPE cables are being used. Depending upon the voltage at which the power is transmitted or distributed, the cables are designed as follows: 1. 2. 3. •

EHV Cables Medium and HV Cables LT Cables

66 kV and above 3.3 kV to 33 KV up to 1100 V

Sizing of Cables The sizing of cables depends on the following factors: − current carrying capacity, − short circuit current, − voltage drop, and − losses.

•

Jointing and Terminations Cables are laid in lengths supplied over reels. Cable extensions are made through joints and terminated at the ends to connect them to the system for use. Since the cable consists of many items right from the conductors to the outer sheath, all joints are to be made as straight through joints so that each joint has the same features/characteristics of the original cable. Straight jointing is ensured by providing: − core continuity; − stress controlling screens; − insulation;

66

− continuity of earth potential parts of the cable by clamping and running the earth lead;

− mechanical protection by installing the brass or aluminium covers; and − finishing over the mechanical protective cover. Cable ends are terminated by providing: −

stress control screens;

−

the earthing clamp lead, etc.;

−

insulation;

−

lugs; and

−

rain sheds.

Substation Equipment and Distribution Lines

While making joints and terminations, it is essential to know the size and type of the cable in order to select appropriate kits for joints and terminations. The kits contain the accessories required along with instruction sheets for step-by-step procedure for making joints and terminations. The cable and end terminations should be prepared as per the dimensional drawing and procedure given in the instruction sheet. Types of Joints and Terminations The joint is considered to be the weakest link in the system but the overall reliability of a distribution system depends on it. Therefore, jointing accessories and techniques have an important and critical role despite their comparative low value in the overall investment. The following types of joints and terminations are used: −

cast iron moulded,

−

epoxy resin type,

−

heat shrinkable,

−

cold shrinkable, and

−

‘push on’ type.

The heat shrinkable, cold shrinkable and ‘push on’ type joints and terminations do not need any setting time and can be taken into service immediately.

SAQ 4: Underground cables List the reasons for using underground cables. State the selection criteria, sizing, jointing and terminating of underground cables. ……………………………………………………………………………………… ……………………………………………………………………………………… ……………………………………………………………………………………… So far, we have discussed the construction of the substation equipment and distribution lines. In the next two sections, you will learn about the general O&M practices for these components of the power distribution system.

67

Operation and Maintenance

5.5 O&M PRACTICES FOR SUBSTATION EQUIPMENT AND DISTRIBUTION LINES Planned maintenance schedules for various components of the power distribution systems are carefully drawn up by the power distribution utility even before the installation work is completed. During plant shut-downs for overall maintenance and before re-energisation, the sub-transmission and distribution plants are subjected to certain inspection and testing procedures. This also applies to the cable route that has been de-energised for a long period of time. Such planned shut down of the plant to be tested and network reconfiguration ensure continuity of supply to consumers while the testing takes place.

NOTE Source: Special report on CEA website “Guidelines for Project Management and Performance Evaluation of Sub-transmission and Distribution Project”.

The power distribution utility must formulate such planned outage schemes at different times of the year (depending upon the load demands) for different maintenance periods in such a fashion that consumer supply is least affected. This also involves putting in place a system for handling customer complaints about power supply breakdowns. Customer Relationship Management System A trouble call management facility should be provided to attend to the power supply interruptions promptly and to improve the reliability of power supply as well as minimise the down time. It should also attend to fuse off calls promptly as well as the complaints of the customer on quality of supply. A computer based facility provided in the substation/complaint attending centre would certainly improve this aspect of O&M. We now describe some general maintenance practices for the substation equipment and distribution lines.

5.5.1 General Maintenance Practices There are two aspects of general maintenance: v Firstly, replacement of the parts that are worn out during the normal operation must be carried out from time to time. v Secondly, preventive maintenance should be carried out for detecting deterioration and mal-operation of the system components. In the daily operation of the substation it is the duty of the attendant to inspect the equipment externally and remedy any abnormality that does not require disconnection of the apparatus. During this inspection, a watch is required to be kept for deposits of dust and dirt on the equipment, heating of contacts, joint or some part, low oil level and oil leakages, etc. Checks should also be made to ensure that

68

•

the locks and doors of the switch house are in good condition,

•

no leaks have developed in the roof,

•

the ventilating and heating systems are operating normally,

•

the prescribed safety aids are in place and in good order,

•

the earthing connections remain unbroken,

•

the packing of the cables entering or leaving a cable trench or tunnel within the premises are intact,

•

the ventilating louvers are not damaged, and

•

the access roads leading to the oil filled apparatus are unobstructed, and will allow the approach of the fire engines in the event of an oil fire during an emergency.

Substation Equipment and Distribution Lines

On-line inspection and testing is normally limited to visual, external and physical examination in order to ensure that the plant is in a safe condition. Infra-red detectors must be used periodically for inspection of overhead lines and open terminal, substation bus-bars for hot spots caused by faulty terminals. In addition, live line washing techniques are also available for cleaning overhead lines or open terminal substation insulators. Purified water with a high resistance value is used in a fine spray fitted from well-earthed nozzle. Functional testing and trip schemes require special switching arrangements initially to reconfigure the power system network. Switchgear site tests during operational maintenance stage vary from utility to utility depending upon the quality of upkeep of the equipment and environmental conditions of the site. These generally involve the following checks and tests: • General checks include inspection and checking of −

the tightness of terminal connection, piping junctions and bolted joints;

−

painting and corrosion protection;

−

cleanliness;

−

cracking and chipping of bushings;

−

foundation bolts; and

−

lubrication of contacts and moving parts of the circuit breakers.

• Electrical circuit checks include checking of

•

−

insulation check;

−

dielectric strength of the insulating oil;

−

level of the oil;

−

quality of SF6 gas/ insulating medium such as humidity content, filling pressure or density except for sealed apparatus;

−

leakage of oil, etc.

Mechanical tests include −

inspection of operating circuits (hydraulic, pneumatic, spring charged) and consumption during operation;

69

Operation and Maintenance

− •

•

verification of correct rated operating sequence (recharging, etc).

Time checks include checking and adjustment of −

track alignment and interlocking mechanism;

−

closing and opening times;

−

operation and control of auxiliary circuits;

−

recharging time of operating mechanism after specified sequence; and

−

other specific operations.

Electric tests include −

dielectric tests; and

−

testing of the resistance of main circuit.

If the substation is constantly attended, the rounds of switchgear are usually planned for each shift so that all the equipment will be looked at least once a day. Equipment is also inspected immediately after a trip out. Substation switchgear requires regular cleaning in accordance with its design, type of insulation, the degree of pollution of the atmosphere or ambient air, etc. The frequency of cleaning depends upon the type of layout of the apparatus and insulators. However, cleaning must be done during each preventive maintenance activity. Even though the vacuum switchgear does not require elaborate maintenance like the oil insulated switchgear, it is still necessary to make periodic routine inspection. The absence of ionized gas and carbon during interruption removes the major source of insulation contamination.

5.5.2 Maintenance of Lines Pre-monsoon inspection of all 33 kV lines should be completed between January and March every year after obtaining due approvals for pre-arranged shut downs for the entire programme. The staff responsible for the pre-monsoon inspection should carry all the necessary equipment such as ropes, petroleum jelly, cotton waste and sufficient O&M materials like insulators, discs, nuts for the pins, binding wire, etc. In the routine maintenance practices, all the tree clearances are done and all the minor defects like damaged insulators, improper pin binding, loose jumpering and loose stays are rectified during the inspection itself. All the insulators are cleaned, all AB switches are lubricated and defective blades replaced. The defects that may take considerable time for rectification are noted down and attended within the next one week. Examples are insertion of poles, replacement of damaged conductors, replacement of damaged supports, etc. 70

Periodical patrolling of 33 kV lines has to be done on a monthly basis. The

patrolling is also done and suspected defects rectified, whenever the line trips on fault. One of the major precautions to be kept in mind by the maintenance staff is to take the permit to work or line clear to work on distribution lines.

Substation Equipment and Distribution Lines

Procedure for Permit to Work (Line Clear) A line clear or a permit to work (PTW) on any electrical equipment or line is issued by an authorised person to another authorised person. If there are more than one gangs working under the same supervisor, each gang takes sub-line clears from the supervisor who has taken the line clear. In case, the line clear has to be issued for the supervisor, s/he takes self line clear. In this case also, all the precautions that are to be followed in issue and return of line clear are followed. Line clear books are very important records. Pages in these books are serially numbered and no paper from this book is used for any other purpose. If any page is to be destroyed, the custodian specifically mentions the reasons for doing so. It is attested by his/her dated signature. The line clear books are reviewed periodically by the Competent Authority. Line clear can be issued/received over telephone. It is desirable that the issuer/receiver recognise each other’s voice. The requisition for line clear and the line clear issue messages are repeated by both the parties to ensure that line clears are issued/received on the equipment on which it is intended. A secret code number is followed in such cases. You may like to revisit Units 6 and 7 of the course BEE-002 for the details.

5.5.3 Operation and Maintenance of Capacitors A routine check of the capacitor performance is made by measuring current with the help of Ammeter/Tong tester once in two months and the record is maintained. If any reduction in current /failure of capacitor is noticed, supplier/ manufacturers must be contacted immediately and replacement of capacitor initiated. The status of the capacitor is determined by the voltage at the highest voltage bus available at the substation. It is subject to the maximum permissible voltage at the bus on which the capacitor bank is connected and the loading factor. The loading factor is the ratio of the total MVA load on the bus at which the capacitor is installed to the MVAR rating of the capacitor. Accordingly, the switching on/off of the capacitor bank is done as per Table 5.1. Table 5.1: Voltage of Highest Level at the Substation System Voltage

Voltage of Highest Level at the Substation (kV) Above

Below

Between

For 220 kV level

230

230 - 220

220- 215

215- 205

205

For 132 kV level

140

140 - 130

132 - 128 128 - 122

122

For 66 kV level

70

70 - 68

68 - 65

65 - 60

60

For 33 kV level

30

35 - 34

34 - 32

32 - 30

30

71

Operation and Maintenance

The loading factor and the status of capacitor switch are given in Table 5.2. Table 5.2: Loading Factor and the Status of Capacitor Switch Status of Capacitor Switch

Loading Factor Above 2

Off

Status-Quo

On

On

On

Between 1 to 2

Off

Off

Status-Quo

On

On

Below 1

Off

Off

Off

Status-Quo

On

LV bus voltage is controlled by changing transformer taps. Notwithstanding the above, if the voltage at the bus on which capacitor is connected is 1.1 per unit or higher, the capacitor is switched off.

5.5.4 Hot Line Maintenance Work performed on transmission and distribution lines while they are energized and in service is called hot line maintenance. Hot line tools are all types of tools mounted on insulated poles used to maintain energized high voltage lines and other safety equipment. Insulated disconnect stick, wire-holding stick, auxiliary arm, cross-arm mount, pole mount, wire tong, saddles,flexible line hose and hoist link stick are some of the hot line tools in use. When working with energized power lines, linemen must use protection to eliminate any contact with the energized line. Some distribution-level voltages can be worked using rubber gloves. The limit of how high a voltage can be worked using rubber gloves varies from company to company according to different safety standards and local laws. You may like to refer to Units 6 and 7, Block 2 (BEE-002) for more information.

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Fig. 5.13: Hot Line Maintenance

Voltages higher than those (which can be worked using gloves) are worked with special sticks known as hot-line tools, with which power lines can be safely handled from a distance. Linemen must also wear special rubber insulating gear when working with live wires to protect against any accidental contact with the wire. The buckets from which linemen sometimes work are also insulated using rubber.

Substation Equipment and Distribution Lines

For high voltage and extra-high voltage transmission lines, specially trained personnel use so-called “live-line” techniques to allow hands-on contact with energized equipment. In this case, the worker is electrically connected to the high voltage line so that he is at the same electrical potential. The lineman wears special conductive clothing which is connected to the live power line, at an instant such that the line and the lineman are at the same potential allowing the lineman to handle the wire safely. Since training for such operations is lengthy, and still presents a danger to personnel, only very important transmission lines are the objects of live-line maintenance practices.

SAQ 5: Hot line maintenance a)

At which line voltages do personnel in your company carry out the maintenance work using i) rubber gloves, and ii) hot-line tools?

b)

Explain the live-line maintenance technique. ………………………………………………………………………………… ………………………………………………………………………………….. …………………………………………………………………………………..

5.6 LENGTH OF LT LINES, HT:LT RATIO AND IMPACT ON LOSSES AND VOLTAGE The ratio of primary line length to its concerned secondary distribution line length is one of the important factors that influence the performance of primary distribution. Over the years, large scale expansion of the urban system and rural electrification programme in the country has resulted in considerable expansion of Low Tension (LT) distribution network. The size of the distribution transformers has been constantly increasing to meet the increasing demand due to load growth. As a result, the length of LT lines/circuits is also increasing resulting in high losses in LT lines, excessive voltage drops, frequent faults on LT network and higher rate of failure of distribution transformers. This has also resulted in very large length of LT lines as compared to High Tension (HT) lines resulting in high LT/ HT ratios. The ratio of LT to HT lines in our country has been of the order of 3. This results in high losses and low voltages at the consumer end.

73

Operation and Maintenance

5.6.1 Impact of Increasing HT Lines Increasing HT lines can help in reducing both line losses and voltage drops. Reduction in Line Losses In the low voltage distribution system, supply at low voltages with long LT lines using smaller conductor sizes causes high line losses. However, the loss in HV system for the distribution of the same power is less than 1% of the LV system. Hence, with HV system the total energy losses are considerably reduced. Reduction in Voltage Drops The voltage drop in LV lines is very high as the lines are long and have smaller conductor sizes. In HV distribution systems, the voltage drop for the distribution of same quantum of power is less than 1% as against that in low voltage distribution system. This ensures proper voltage profile at the consumer end. All other parameters, like load factor, power factor, etc., remaining the same, the percentage losses in a system having higher LT/HT ratio will be higher than in a system having lower LT/HT ratio. A ratio of 1 to 1.2 would be very beneficial to power distribution. As this measure is a must to improve efficiency and voltage regulation of distribution, additional capital investment should not come in the way. With this discussion on the impact of increasing HT lines on reduction in line losses and voltage drops, we now end the unit and summarise its contents.

5.7 SUMMARY

74

•

In the overall power development scenario, the Transmission and Distribution system constitutes the essential link between power generating sources and the ultimate consumers and substations and lines have to be erected for providing quality power supply.

•

The main equipment used in a substation comprises structures, transformers, bus-bars, circuit breakers, isolators, earthing switches, lightning arrestors, substation batteries, fire extinguishing equipment, etc. Overhead distribution lines and underground cables (in urban areas) carry power to the end-user.

•

There are two aspects of general maintenance: replacement of parts that are worn out from time to time and preventive maintenance for detecting deterioration and mal-operation of the system components. Periodic checks and tests should be carried as per specified procedures, which may vary from utility to utility depending upon the site conditions.

•

Special hot line maintenance techniques and tools are required for maintaining live lines.

•

Due to increasing LT lines in the distribution system, losses, excessive voltage drops and frequent faults have resulted in the LT network leading to

a higher rate of failure of distribution transformers. The high LT/HT ratios result in high losses and low voltages at the consumer end. An LT/HT ratio of 1 or 1.2 is preferable.

Substation Equipment and Distribution Lines

5.8 TERMINAL QUESTIONS 1. Describe the equipment required for the construction of a 66-33/11 kV substation. 2. Describe the equipment required for the construction of a 11/0.4 kV distribution substation. 3. What equipment is required for the construction of an overhead distribution line? 4. Distinguish between current and voltage transformers. 5. List the different types of underground cables in use today. What criteria are used for the selection of these cables? 6. State the precautions that need to be taken in jointing and terminating underground cables. 7. Give reasons why underground cabling is being opted for in urban areas. What are its advantages? 8. Explain hot line maintenance techniques and tools. 9. Explain the impact of LT/HT ratio on losses and voltage.

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Operation and Maintenance

76

Distribution Transformer

Unit 6

Learning Objectives

Distribution Transformer

After studying this unit, you should be able to:
classify distribution transformers;
explain the criteria for distribution transformer selection and placement;
identify the causes for failure of distribution transformers;
discuss the various methods of testing a transformer; and


describe the different ways of enhancing transformer life and efficiency.

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Operation and Maintenance

6.1 INTRODUCTION In Unit 4, you have learnt that the transformer is an electrical device used for stepping down or stepping up the supply voltage. You know that the distribution transformer (DTR) steps down the primary distribution voltage of 11 kV or 33 kV to secondary distribution of 415V between phases and 240V between phase and neutral. In this unit, you will study about distribution transformers in detail covering their selection criteria, causes of failure, tests on DTRs and ways of improving their life and efficiency.

6.2 DISTRIBUTION TRANSFORMERS: SELECTION 6.2 DISTRIBUTION TRANSFORMERS: SELECTION AND PLACEMENT AND PLACEMENT Distribution transformers are used both in electrical power distribution and transmission systems. The power rating of a transformer is normally determined by the cooling method and the coolant used. Oil or some such other heat conducting material is commonly used as coolant. Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings; voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer. The criteria for selection of a transformer and its technological details depend on its intended use or purpose, working conditions and operating requirements. For example, a power transformer is selected in the sub-transmission level because it is consistently loaded up to full rating in order to obtain maximum efficiency at full load. On the other hand, distribution transformers (in the power distribution system) are under-loaded most of the time in order to ensure maximum all day efficiency at the expense of lower efficiency during peak hours. In order to understand these criteria, you need to know about transformer classification.

6.2.1 Classification of Transformers Apart from classification on the basis of purpose (as power transformers and distribution transformers) transformers are also classified on the basis of: v Type of Core Used; and v Type of Cooling Used. v Type of Core Used In laminated-steel-core transformers, two main types of cores are used: core type and shell type. • 78

Core type transformers have cores with a hollow square through the centre (Fig. 6.1). Note that the core is made up of many laminations of steel.

Distribution Transformer

Fig. 6.1: Core Type Transformer

•

Shell type transformers are the most popular and efficient transformers and they have a shell core (Fig.6.2). Note that each layer of the core consists of E- and I-shaped metallic sections, which are butted together to form the laminations. The laminations are insulated from each other before being pressed together to form the core.

Fig. 6.2: Shell Type Transformer

v Type of Cooling Used There are two types of transformers in this category: Dry type and oil-filled. •

Dry type transformers use natural air cooling and are usually of very small ratings. They are rugged and simple in construction and are not plagued with the failures related with oil cooling.

•

Oil filled transformers are of two types: One type uses self-cooling and the other type uses forced cooling. Self-cooling oil filled transformers have natural circulation of insulating oil within which the entire transformer is immersed. These are of moderate ratings and are suitable for outdoor duty as these

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Operation and Maintenance

require no housing other than their own and thereby save on cost. For higher ratings, either the smooth surface of tank is corrugated or is provided with radiators/pipes to get greater heat radiation area. Large transformers require forced oil/water cooling. The coolant is circulated by a pump to radiate high quantity of heat generated and also to minimise the size of the transformer.

6.2.2 Criteria for Transformer Selection The criteria used for selection of proper rating/size of DTRs are described below: A.

NOTE Diversity factor is defined as the ratio of the sum of the individual maximum demands of various parts of a power distribution system to the maximum demand of the whole system. It measures the staggering of different hours of the day and indicates flatness of load curve. That is, it denotes MVA vs hours of the day curve.

Voltage Rating

While the secondary side voltage rating of the transformer is fixed as 400 V, 3 phase with neutral available along with the three phase wires in star configuration, the primary side voltage is decided by the voltage of incoming feeder(s). If there are more than one incoming feeders, the feeder for fixing the criterion for transformer selection can be decided on the basis of its proximity with the substation, load delivering capability and availability of suitable voltage transformer. The number, steps and type of tap changer, i.e., on load or off load, is decided by operating requirements of voltage and current. B.

Size/Capacity/kVA Rating

kVA rating of transformer(s) is decided on the basis of the following factors: •

existing load to be catered;

•

future load growth to be absorbed;

•

diversity factor (DF) of load for different categories of consumers. Diversity factor is greater than or equal to one and can be used to get the required rating by dividing the sum of maximum demand of individual consumer categories by DF;

•

margin for future load growth;

•

safety factor for avoiding overloading;

•

level of all day efficiency to be achieved; and

•

selection of an optimum loading of transformer(s), usually around 60%, used to calculate the required capacity of the transformer(s).

C.

Number of Transformers

Distribution substations are seldom provided with a single transformer of required rating (except pole mounted transformers/substations). However, reliability of supply can be severely affected if only one transformer is used especially when it fails or when it is under maintenance. Reliability of supply increases with increase in the number of transformers.

80

It is important to note that capacity requirement and hence cost per transformer reduces with increase in the number of transformers, as the load gets divided, but the total cost increases as the relative benefit of reduction in

the cost of each transformer is lower than the reduction in capacity. Moreover, there is additional cost for associated equipment for each additional unit of transformer. Thus, in order to optimise the reliability and the cost involved, the number of transformers is usually kept between two to four. Transformers are required to comply with the latest edition of IS 2026. The size specifications for this purpose are given in Table 6.1.

Distribution Transformer

Table 6.1: Standard Sizes of Transformers 11kV / 0.433 kV

160, 200, 250, 315, 500, 630, 1000, 1600, 2000 kVA

33kV / 0.433 kV

630, 1000, 1600, 2000 kVA

6.2.3 Placement of Transformers You have learnt in Unit 4 that in the High Voltage Distribution System (HVDS), transformers are usually of very small rating (5 to 25 kVA) and provide supply to 10 − 25 consumers. Such transformers, being large in number and very small in size are mounted on a pole at an optimum location and are provided with metering and isolating equipment. Similarly, medium sized self-oil cooled transformers are also placed in open space and may be pole mounted. Large sized transformers are fixed on the ground with proper foundation and are enclosed within premises housing the associated equipment of transformers. So far we have discussed the selection criteria and their placement. We will now describe causes of transformer failures. But, before proceeding further, you may like to answer an SAQ.

SAQ 1: Transformer selection and placement SAQ 1: Transformer selection and placement a) Describe the salient features of a distribution transformer. …………………………………………………………………………………. …………………………………………………………………………………. b) Classify the different types of transformer in your utility. What parameters are used in the selection of transformers in your utility? …………………………………………………………………………………. ………………………………………………………………………………….

6.3 REASONS FOR TRANSFORMER FAILURES 6.3 REASONS FOR TRANSFORMER FAILURES As you are aware, the distribution sector has a large number of distribution transformers of various capacities. Any failure of these transformers is bound to cause great inconvenience to the consumers and huge financial losses to the utilities. It is therefore extremely important to avoid transformer failure.

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Operation and Maintenance

We list below some important reasons for distribution transformer failure. •

Poor Performance This could be due to

•

−

low oil level;

−

draining of oil due to leakage/theft of oil;

−

improper earthing;

−

frequent faults on LT lines due to loose spans leading to short circuit;

−

mechanical failure of winding;

−

improper tree clearance of LT lines;

−

defective breather and consequent ingress of moisture;

−

low electric strength of oil/winding insulation; and

−

corrosion of core laminations.

Improper Protection This could be the result of

•

−

using defective or over rated fuses;

−

consistent overloading; and

−

not providing Lightning Arrestors (LAs).

Manufacturing Defects These result from −

improper / inadequate design;

−

poor quality of material;

−

bad workmanship; and

−

poor short circuit withstand capacity.

All these reasons for transformer failure can be classified under the following heads (Fig. 6.4): v Ageing, v Manufacturing defects, v Improper structure/erection of distribution transformer, v Improper operation and maintenance, and v Natural calamities. We now discuss each one of these briefly.

6.3.1 Ageing

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The expected life span of the distribution transformers above 100 kVA capacity is about 35 years and that of up to 100 kVA capacity is about 25 years. But experience shows that most of transformer failures begin to occur even before 20 years of its life.

Until a few years ago, distribution transformer manufacturers incorporated many more safety factors in design. In recent years, manufactures have adopted the cost-benefit approach in the design of transformers, which just about manages to satisfy the requirements of IS specification. The result is that they compromise on both quality and reliability requirements of IS specification. While the transformers so manufactured meet the requisite standards when tests are conducted before and immediately after installation, they fail to do so after a few years of being in operation due to ageing.

Distribution Transformer

Thus, many transformers are unable to serve the expected full life period and even if they are in service, they are quite likely to fail before the expected full life due to lower reliability. Hence, giving top priority to the replacement of those in-service transformers that have served their full-life period will reduce transformer failure.

6.3.2 Manufacturing Defects In the past, distribution transformers served for more than 60 years, which is double the life expectancy. But now many distribution transformers fail after a few years of service and have to be repaired twice or thrice during their life time. Many reasons for pre-mature failure of the distribution transformer are related to manufacturing defects. We describe them, in brief. A. INADEQUATE/POOR DESIGN The trend of design is now towards lowering the manufacturing costs per unit even if it is at the expense of quality. Moreover, the tender appraisal is mostly confined to the initial cost, with no consideration for maintenance of the transformer up to the end of its fair life period. Consequently the safety factor is affected adversely. The following aspects of transformer design impact transformer failure: •

Transformer tank size: Inadequate clearance for free circulation of oil can lead to abnormal temperature rise, causing great damage to the HV winding insulation and, consequently, premature failure of transformers.

•

Percentage impedance (mechanical strength of coil): Most of the distribution transformers are located in remote areas and many a times it is not possible to give special attention to the operating conditions. Harsh conditions can also lead to failure. The solution to this problem lies in designing transformers with large impedance so as to increase theirshort circuit withstand capacity.

NOTE The cost of repairs during the fair life period plus the initial cost is called the estimated cost of the transformer. It is at present a few times more than the initial cost at which the distribution transformer is procured.

Percentage impedance depends upon the following factors. −

Size of wire used in HV coils − Economical size of coil yields lower size gauge wire, but this reduces the mechanical capability of coils. As a result, the coils may not be able to withstand higher current densities which occur during the short circuit conditions.

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Operation and Maintenance

−

Radial distance between HV and LV coils − Increasing the radial distance between HV and LV coils increases the percentage impedance. It also leads to better mechanical strength of the coil to withstand higher short circuit stresses developed during short circuit conditions. But this will lead to higher cost.

−

Effect of impedance on the short circuit stresses −The short circuit stresses are proportional to the square of the short circuit current. If the impedance is increased from 4.5 % to 5 % - 5.5%, the effect on the short circuit stresses developed in the transformer is reduced considerably.

•

Improper use of aluminium wires: Improper use of aluminium wires leads to HV coil failure. The use of aluminium conductors has been recommended for windings up to 200 kVA transformers. However, the super enamel covering the aluminium wire tends to crack during asymmetrical conditions and leads to coil failure. Hence, the use of higher cross-section conductors with double paper covering would be desirable.

•

Improper use of interlayer papers: Coil failure is usually seen as an electrical failure. This generally occurs when interlayer insulation breaks down at the end of the turn and creeps to the next layer. This type of insulation failure can be avoided by using folding papers and reinforcing the end turn insulation with proper sleevings. Uniform separation of HV coil along with the inner coil, using spacers helps to avoid pressing of end turns as well as any further shrinkage during service.

•

Use of inferior quality materials: Use of inferior quality wires for coils, poor quality of oil and other insulation material, etc., to bring down the cost of the transformer also increases the probability of failure of the transformer before full life of the transformer. The design calculation of the tenderers should conform to the quantity and grade of input materials of core and windings furnished in the tender. For ensuring this, the transformer should be subjected to strip test. This will make sure that the losses and impedance furnished in the tender have been actually achieved by the transformer.

B. IMPROPER WORKMANSHIP Apart from poor design, sub-standard execution of a good design also becomes a reason for transformer failure. We now briefly describe some such reasons. •

84

Improper alignment of HV windings: When a transformer is loaded, the primary and secondary ampere-turns act in magnetic opposition but are in complete alignment with respect to the core and coils. When current flows through the coils, magnetic field is set up around them, which has an associated magnetic flux. Even a small error in the alignment of either coils, i.e., an asymmetrical ampere-turn balancing,

leads to production of cross magnetic fluxes. This results in lower impedance and hence mechanical failure of the coils. •

Improper clamping arrangement: Inadequate clamping arrangements of the HV coils lead to vibrations and movement of the coils during short circuit conditions resulting in failure of HV Coils.

•

Improper connections: In many cases, the connecting delta leads to the bushing are not properly supported on the framework, resulting in breakage during trans-shipment or at the time of the first charge of transformer. Moreover, improper soldering of leads will result in open circuit even at normal full load conditions. Also, such transformers may fail while encountering the first fault or after a few faults.

•

Inadequate tightening of core: Even with proper fuse protection on the HV side, inadequate tightening will result in failure of transformer due to collapse of the windings. The transformer can fail due to this fault even under minor fault condition in the LT distribution due to mechanical vibration in the core and windings.

Distribution Transformer

6.3.3 Improper Structure of Distribution Transformer IE Rules, 1956 specify various standard clearances to be maintained when distribution transformers are to be erected. The standard clearances adopted for transformer structures will avert its failure (Table 6.2). Table 6.2: Clearances for Transformer Structures Transformer Structure

Clearance

HT bushing to ground

13 feet

ABS switch fixed contact to HG fuse

7 feet

Guy Shackle to ground

10 feet

If the length of the jumper is more than 5 inches

LT/HT pin insulators are used to fix the jumper

Non-adherence to these standards makes DTRs prone to failures.

6.3.4 Impact of Natural Calamities v Heavy lightning: If the HTLAs (HT Lightning Arrestors) fail to divert direct lightning strikes or surges due to discontinuity in the earthing system, the HV winding can fail due to surge voltage or the HT Lightning Arrestor itself may burst. v Bushing flashover: Dust and chemicals carried with air and deposited on the bushings reduce the electric leakage distance and cause flashover. To avoid this, the bushings (both HT and LT) should be cleaned properly at regular intervals. However, cotton waste should not be used for cleaning, as this may cause scratches in the bushing and subsequently lead to flashover of bushing.

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Operation and Maintenance

v Failure due to contact with birds and other animals: To avoid failure of the distribution transformer due to a squirrel crossing it or due to birds sitting on it, the HT/LT bushing and HT/LT jumper leads from the bushing should be covered with yellow tape insulation. This yellow tape insulation will also indicate the overloaded operation of the transformer by the change of colour of the tape from Yellow to Black.

6.3.5 Improper Operation and Maintenance (O&M) Transformer failure can also stem from poor O&M practices. For example, in addition to normal full load, continuous over-heating and higher no-load losses may reduce the life of the transformer due to reduction in the life of insulating papers, oil, etc. Other improper O&M practices leading to transformer failure are discussed later in the section on enhancing transformer life and efficiency. In Table 6.3, we summarise some reasons for transformer failure. Table 6.3: Failure of Distribution Transformers Reason for DTR Failure Damage to LV Coils

Causes

Failure Rate 65 %

• Compressed windings; • Open Circuit Insulation failure; • Dislodged spacers; and • Broken support/inadequate bolts.

Damage to HV Coils

5%

• Overloading; • Defective termination of coils; and • Inadequate size of fuses.

Damage to Both

10 %

• Any of the above causes.

Other Reasons

20 %

• Poor construction of transformer tank; • Defective Joints; • Oil oozing out; • Punctured radiators and bushing gaskets; • Damaged tap changers, etc.

SAQ 2: Failure of distribution transformers Collect data on the annual failure rate of DTRs in your utility for the past three years. Identify the reasons for their failure and classify them among the above mentioned five aspects described from Sec. 6.3.1 to Sec. 6.3.5. …………………………………………………………………………………. …………………………………………………………………………………. 86

Thus far, you have studied about the selection criteria of transformers, their placement and the reasons for transformer failure. If you are able to prevent these causes of transformer failures, the battle is more than half won. The rest is taken care of by transformer testing prior to its installation.

Distribution Transformer

6.4 TRANSFORMER TESTING 6.4 TRANSFORMER TESTING Transformer testing needs to be carried out to ensure that the distribution transformers are built with • adequate electrical strength to withstand over voltage (due to switching surges) impinging on the winding without causing flashover; and • adequate mechanical strength to bear the mechanical stresses developed on the winding during short circuits. The following tests must ideally be conducted on the units before their acceptance: •

testing of windings − insulation and mechanical strength;

•

testing of insulating transformer oil; and

•

other tests.

NOTE In practice, transformer testing is usually dispensed with just for want of testing facility at the utility’s laboratories.

We now describe these tests in brief.

6.4.1 Testing of Windings − Insulation and Mechanical Strength This involves four types of tests, which we have described briefly in Table 6.4. Table 6.4: Testing of Windings Test

Description

High Voltage Test

This is done to check the dielectric strength of the insulation between the windings operating at different voltages (HV and LV) and between each of these windings, core and earthed parts of the transformer. It is also called the Major Insulation Test for the transformers.

Induced Voltage Test

This is conducted to test the dielectric strength of the inter-turn, inter-layer, inter-disc and inter-phase insulation. This test is also called the Minor Insulation Test.

Short Circuit Test

This is conducted for testing the mechanical strength of the transformer in terms of its impedance parameters.

Ratio Test

This test can be carried out for testing the transformer ratio (for example, shorted turns can cause improper transformer ratio). This ratio is measured with a high accuracy portable digital turns ratio tester only after ensuring shutdown and complete isolation of the transformer from the system.

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Operation and Maintenance

We give below the findings of the Central Power Research Institute (CPRI) about the short circuit test. Box 6.1: Findings of Central Power Research Institute on Short Circuit Test From the available data on short circuit tests, it seems that the failure rate of transformers manufactured by small and medium scale industries is at par with those of large scale industries. But in practice the rate of failure is very high. The reason could be that the materials used for bulk manufacture of transformers are not the same as those used for the transformer produced for testing purposes. This is a distinct possibility because the materials cost contributes a major share to transformer cost. Manufacturers use inferior quality materials to bring down the cost to compete in the highly competitive market. Hence, there is a need for proper quality assurance at the manufacturing stage even though the prototype has successfully passed the Short Circuit Test.

6.4.2 Testing of Insulating Transformer Oil At present, transformer oil is subjected to Breakdown Voltage (BDV) test to ensure its electrical strength. Other tests to confirm important transformer characteristics such as acidity, resistivity, etc., are not carried out before accepting bulk supply. Thus, there is every possibility that manufacturers use inferior quality of oil. This can lead to poor insulation resistance between High Voltage to Earth, Low Voltage to Earth and High and Low Voltages and reduced cooling rate. Moreover, it can give rise to abnormal temperature increases even before loading the transformer to its rated capacity. Thus, it is important to check whether the oil used is a new one or a reconditioned one or a reclaimed one before the transformer is installed. Both water and water saturated oils are heavier than clean and dry oil, and sink to the bottom of the container. The following tests are usually conducted on the transformer oil: •

inspection of samples;

•

acidity test;

•

analysis of dissolved gases; and

•

electric strength test.

We now briefly describe these tests. •

Inspection of Samples: Colour and odour of the oil provide useful information on the quality of oil and its fitness for use. In Table 6.5, we list the factors that should be watched out for during inspection. Table 6.5: Inspection of Oil Samples Reason

Presentation

88

Cloudiness

Suspended solid matter such as iron oxide/sludge

Muddy colour

Moisture

Dark brown

Presence of dissolved asphaltenes

Green colour

Presence of dissolved copper compounds

Acid smell

Presence of volatile acids which can cause corrosion

•

Acidity Test: Transformer oil deteriorates gradually while in service due to oxidation. The acidity in the oil causes rusting of ironing work inside the tank above the oil level and the attached varnish on the windings. The recommended limits for acidity test and the action required are given in Table 6.6.

Distribution Transformer

Table 6.6: Action Required for Various Acidity Levels of Transformer Oil Acidity Level

•

Action

Below 0.5 mg KOH/g

No action needs to be taken provided the condition of oil is satisfactory in all other respects.

Between 0.5 and 1.0 mg KOH/g

Oil should be kept under observation.

Above 1.0 mg KOH/g

Oil should be treated or discharged.

Analysis of Dissolved Gases: The permissible concentrations of dissolved gases in the oil of a healthy transformer are given in Table 6.7. Table 6.7: Permissible Concentrations of Dissolved Gases in Transformer Oil Gas

4-10 Years in Service

More than 10 Years in Service

Hydrogen

100 / 150 ppm

200 / 300 ppm

200 / 300 ppm

Methane

50 / 70 ppm

100 / 150 ppm

200 / 300 ppm

Acetylene

20 / 30 ppm

30 / 50 ppm

100 / 150 ppm

100 / 150 ppm

150 / 200 ppm

200 / 400 ppm

30 / 50 ppm

100 / 150 ppm

800 / 1000 ppm

200 / 300 ppm

400 / 500 ppm

600 / 700 ppm

3000 / 3500 ppm

400 / 500 ppm

600 / 700 ppm

Ethylene Ethane Carbon monoxide Carbon dioxide

•

Less than 4 Years in Service G

Electric Strength Test (Breakdown Voltage Test): This is the most commonly known test applicable to mineral insulating oils, and it was originally developed to test the breakdown voltage of the oil. We describe the method of its measurement in Box 6.2. Box 6.2: Method of Measurement of Breakdown Voltage An oil test cell is used in which an alternating voltage is applied between two metal spheres 12.5 mm in diameter with a gap of 2.5 mm between them. The voltage is increased until breakdown occurs. The flashover must be quickly stopped to allow six successive measurements of the rupture voltage on the same sample. The value of the electric strength of the sample tested is the average of the six measurements.

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Operation and Maintenance

6.4.3 Other Tests We now briefly describe some other tests, which are usually conducted on transformers. These include temperature rise test, drying out of transformer, and open circuit and Sumpner’s test. •

Temperature rise test is performed to measure the temperature rise of the main and conservator tanks. The transformer passes the test if the rise is within the specified limits given by the manufacturer.

•

Drying out of transformer is necessary if −

tests indicate the presence of moisture in transformer oil;

−

the oil does not withstand the dielectric strength test; and

−

the insulation resistance readings are not satisfactory. Box 6.3: Method of Drying Out the Transformer

Normally HOT OIL CIRCULATION method should be used for drying out the distribution transformer. In special circumstances, where this method does not give satisfactory results, SHORT CIRCUIT WITH HOT OIL CIRCULATION should be used. In this method, both core and winding inside the tank are simultaneously dried out/streamlined with filter. The moisture dries out from the windings into the oil and is removed from the oil by evaporation and filtering.

•

Open circuit and Sumpner’s tests: The open circuit test is carried out on a transformer for calculating no load parameters and core losses. Sumpner’s back to back test is done for testing performance on full load for two similar transformers.

You may like to review the information presented so far before studying further.

SAQ 3: Transformer testing State the purpose of tranformer testing. Draw a list of the tests to be carried out ona transformer defore it is accepted by a utility …………………………………………………………………………………. ………………………………………………………………………………….

6.5 ENHANCING TRANSFORMER LIFE AND EFFICIENCY

90

In this section, we describe the O&M practices that can enhance the life of a transformer.

6.5.1 Transformer Operation

Distribution Transformer

The following factors need to be kept in mind while operating a transformer: A. Overloading of distribution transformer should be avoided. •

•

Inadvertent burden of extra load due to unauthorized connections or loads should be identified by periodic testing of current in the distribution transformer. The Tong Tester may be used for this purpose at peak hours and other times of the day. Alternatively, maximum demand ammeter may be connected to exactly determine the maximum load current drawn from the transformer. In the event of extra load, transformer failure can be prevented and its life enhanced by −

transferring the load to the nearby transformer;

−

enhancement of the existing transformer capacity; or

−

installing a new transformer.

Unequal loading in three phases may also cause overloading in one phase. In such a case, redistribution of the loads, as far as possible equally, among the 3 phases, will prevent transformer failure.

B. Fuse wires (HG fuse and feeder fuses) should be of proper size. •

HG fuse is the only reliable protection for distribution transformers under conditions of fault in LT distribution. The LT fuses should be of a heavy size since these are meant for high currents on the LT side. Then the chances of LT fuses blowing in short time decrease.

•

Higher size HG fuses are sometimes used in distribution transformers due to non-availability of proper size HG fuse wire and also because the seriousness of the consequences is not realized. THIS SHOULD NEVER BE DONE because if the HG fuse is of higher size, the fault will sustain for a longer period until the heavy size fuse blows. This can result in increased chances of transformer failure. Even if the fault lies within the transformer, using HG fuse of proper size will minimize the damage to the transformer.

•

Notwithstanding the use of HG fuse protection, the use of proper size of fuses on LT feeder depending upon the load has to be ensured to avert transformer failure.

C. Two phasing in rural areas should be prevented. •

Often, two phase supply is maintained on rural distribution feeders to prevent operation of three phase motors for staggering the peak hour loads. However, agriculturists/consumers have invented many methods to start the motor and run it under 2-phase conditions. The total power intake of the motor under the 2-phase condition will be approximately the same as under the 3-phase, contributingunbalanced

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Operation and Maintenance

overload in 2 phases. In turn, the distribution transformer will also be subjected to overload in these 2 phases and the core of the transformer will have unbalanced magnetic field in the region of saturation point. This will also cause transformer failure and needs to be checked. •

Similarly, in all the areas covered by the distribution lines with 2-phase arrangements, all the single phase lighting loads are dumped in these 2 phases so that supply is available under all conditions. This leads to heavy unbalanced current through the neutral conductor and the transformer is likely to be overloaded in these 2 phases. Further, the flow of unbalanced current in the neutral conductor will raise the potential of the neutral with respect to the earth which is dangerous to the consumers. In both these cases, adopting 3-phase balance load scheme should enhance both the life and efficiency of the transformer as it avoids unnecessary overloading of 2 phases.

D. Thin breakable diaphragm should be used in the explosion vent. Use of metallic diaphragm will result in the explosion of the transformer itself due to the development of high pressure. To avoid this, a thin breakable diaphragm should be used in the explosion vent. This will cause the explosion of only the vent under high pressure conditions. It will avert transformer failure on this account. E. The correct diversity factor for loads should be adopted. The value of diversity factor (DF) is assumed for different categories of load to decide the capacity of the transformers. Due to reduced hours of supply in the hours of critical power generation, the actual DF is less than the assumed value. This leads to overloading of the transformers and results in their failure. Transferring load from an overloaded transformer to another transformer or replacing it with a new higher capacity transformer will avert such situations and enhance the life and efficiency of the transformer. F. Non-standard methods should be avoided. Avoiding the use of non-standard methods can avert transformer failure. We now describe some of these.

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•

Use of ACSR conductor, bare or enclosed in PVC pipe from the transformer bushing as against insulated PVC cables.

•

Use of open type fuse for the secondary control of the transformer as against the standard porcelain fuses.

•

Use of single fuse to control more than one feeder.

•

Use of Aluminium strands of ACSR conductors as fuses in LT open type fuses and HG fuses as against tinned copper.

Having learnt about the correct ways of transformer operation, you may like to know: What methods are used for transformer maintenance?

6.5.2 Maintenance Methods

Distribution Transformer

We describe these methods, in brief. •

Prevention of Tree Fouling on LT Lines: Sustained tree fouling with LT conductors may result in conductor snapping or cracked LT Pin Insulators. This causes heavy earth fault current, which may lead to transformer failure. Regular tree cutting and trimming will avoid such failures.

•

Prevention of Tree Fouling on HT Lines: Tree fouling on HT lines may cause failure of transformers due to flow of earth fault current. This is because the primary of all the transformers are delta-connected and all the 3 windings from cluster of transformers connected to this HT distribution line will feed the fault apart from the source of EHT substation (EHTSS). Monitoring and regular tree cutting is the solution to this problem for enhancing the life of the transformer.

•

Maintenance of Breather: Non-provision of fly-nuts for the breather container will create a gap through which moisturised air will enter into the transformer tank. To arrest this gap, neophrine gaskets should be provided instead of rubber gaskets. Rubber gets damaged if it comes in contact with transformer oil. If oil is not filled in the breather, then the dust particles will not be absorbed from the air entering the transformer tank, causing transformer failure. Hence, proper maintenance of breather will prevent such failures.

•

Removal of Water Condensate in the Transformer: Due to absorption of moisture from atmosphere over a long period of time, a large quantity of water may collect in the transformers. Since water has higher density it gets collected at the bottom of the transformer. Indeed water level can even reach the bottom level of the windings, resulting in failure of transformers. Sometimes it leads to bursting as well. This is because of silt formation at the bottom of oil, which prevents escape of gas formed, resulting in bursting of the bottom of the tank. Occasional draining of oil from the bottom of the transformer will check collection of such large quantity of water. The conservator tank also acts as the collector of water-condensate of moisture entering through the breather. This should be removed before it contaminates the oil and causes transformer failure.

•

Prevention of Oil Leakage in Bushings or Any Other Weak Part of the Transformer: Oil leakage may be due to excessive heating or pressure that may develop in the bushing. This will bring down the oil level in the tank. Moreover, moisture will find its way into the tank through the aperture from which oil is oozing. This can result in the contamination of the oil in the tank and hence in the deterioration of HV/LV insulation and ultimately lead to transformer failure. To avoid this, bimetallic clamps with proper size of bolt and nuts connected to the LT bushing may be used, which will reduce excessive heating and damage to bushing rods. Reduction in heating will lead to higher life and efficiency of the transformer.

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Operation and Maintenance

•

Avoiding Low Oil Level: Poor visibility of the oil level in the glass level gauge due to accumulated dust, etc., may not show the exact level of oil in the tank. Oil is likely to go below the core level and the jumper wire from core winding assembly to the bushing rod will not be covered with oil. This leads to excessive temperature rise and the failure of inter-turn insulation as well as flashover of the windings. Transformer life and efficiency can be improved by avoiding such excessive temperature rises.

•

Avoiding High Oil Level: Oil should be filled upto the marking in the conservator tank. There should be space in the conservator tank for expansion of oil when the transformer is loaded. If the conservator tank is completely filled with oil, the transformer may fail due to high pressure resulting in explosion of vent pipe. Maintaining correct oil level will, thus, enhance transformer life by avoiding such failure.

•

Prevention of Low BDV of the Oil: The Breakdown Value/Voltage (BDV) of the transformer oil may become very low due to oil contamination. This results in the increase of the carbon content and decrease of resistance in the oil. Since the temperature of the oil remains the same, acidity of the oil increases resulting in deterioration of insulation of the windings and transformer failure. To prevent this, oil has to be filtered in order to remove the dust and processed through the reclamation plant to reduce the acidity and improve the BDV value of the oil. The minimum BDVs for different voltage ratings of transformers are given in Table 6.8. Table 6.8: Minimum Breakdown Voltage Ratings for Transformers of Different Voltage Ratings Rated Voltage of the Transformer

Minimum Breakdown Voltage

Up to 66 kV

30 kV

Above 66 kV and Up to 110 kV

40 kV

Above 110 kV and Up to 230 kV

50 kV

Higher electric strength of oil will, therefore, reduce chances of failure during abnormal conditions of operation such as lightning/surge voltages in the system and increase the life of transformer. •

94

Preventing Low Insulation Resistance (IR) Value: The insulation resistance may be lowered due to moisture content in the oil and in the winding insulation, and may cause transformer failure. To prevent this, the entire transformer core with windings should be placed in hot air chamber until the moisture content is removed from the core and winding insulation resistance can be measured with the help of a Megger. The minimum safe insulation resistance for different voltage ratings of windings is given in Table 6.9.

Table 6.9: Minimum Safe Insulation Resistance in MΩ Ω (Mega-ohm) Rated Voltage of the Winding

Distribution Transformer

Minimum Safe Insulation Resistance in MΩ Ω at Different Temperatures 30°°C

40°°C

50°°C

60°°C

66 kV and above

600

300

150

75

22 kV and 33 kV

500

250

125

65

6.6 kV and 11 kV

400

200

100

50

Below 6.6 kV

200

100

50

25

Ensuring appropriate insulation resistance enhances the life of transformer. •

Preventing Loose LT Lines: If the LT lines are very loose or they sag, shorting of LT lines could occur and cause frequent blowing of feeder fuses. It could also cause conductor snapping if proper size of fuses are not used or if the structure is not properly earthed. To prevent this from happening, phase separators should be used or lines should be restaged. Loose LT lines should be checked to reduce short circuit stresses on transformer and hence enhance its life.

•

Proper Maintenance of Fuse Gaps in HT/LT Side of the Transformer: Improper maintenance of the fuse gap setting results either in frequent blowing of fuses or non-blowing of fuses when required. The desirable fuse gap setting is given in Table 6.10. Adequate gap setting will avert transformer failure and hence enhance its life. Table 6.10: Appropriate Fuse Gap Setting Type of Fuse Gap

Fuse Gap Setting

11 kV HG fuse gap

8 inches

22 kV HG fuse gap

10 inches

LT open type feeder fuse gap

6 inches

In this section, we have discussed major O&M measures required to prevent transformer failure and enhance its life. It is your job to ensure that these measures are implemented on a regular basis. At this point, you may like to pause and review these measures.

SAQ 4: O&M of transformers List the reasons for distribution transformer failure in your area. Which

SAQ O&M described of transformers O&M4: measures above could have prevented these? …………………………………………………………………………………. …………………………………………………………………………………. 95

Operation and Maintenance

With this discussion on various ways of enhancing transformer life by paying proper attention to its operation and maintenance, we end this unit. In this unit you have studied important aspects related to distribution transformer, reasons of transformer failure, transformer testing and ways of enhancing life and efficiency of transformer by paying attention to the operation and maintenance aspects of these transformers. Let us now present the summary of its contents.

6.6 SUMMARY 6.6 SUMMARY •

Distribution transformers are used both in electrical power distribution and transmission systems. Their power ratings as well as continuous voltage rating are the highest. The criteria for selection of a transformer and its technological details depend on its intended use or purpose, working conditions and operating requirements.

•

Transformers are classified on the basis of type of core used and type of cooling used.

•

The criteria used for selection of proper rating/size of DTRs are based on voltage rating, size/capacity/kva rating, number of transformers, etc.

•

Transformers may be mounted on a pole placed in open space or fixed on the ground with proper foundation depending upon their sizes and uses.

•

Some important reasons for distribution transformer failure are poor performance due to low oil level, draining of oil due to leakage/theft of oil, improper earthing, frequent faults on LT lines due to loose spans leading to short circuit, mechanical failure of winding, improper protection, manufacturing defects, etc. These reasons for transformer failure can be classified under the heads of ageing, manufacturing defects, improper structure/erection of distribution transformer, improper operation and maintenance and natural calamities.

•

Transformer failure can also stem from poor O&M practices such as continuous over-heating and higher no-load losses.

•

Transformer testing needs to be carried out to ensure that the distribution transformers are built with adequate electrical and mechanical strength.

•

Tests such as testing of windings − insulation and mechanical strength, testing of insulating transformer oil, must ideally be conducted on the units before their acceptance.

•

Transformer life can be enhanced by following proper O&M practices.

6.7 TERMINAL QUESTIONS 6.7 TERMINAL QUESTIONS 1. Classify the distribution transformers being used in your utility as per the categories given in Sec. 6.2. 96

2. Examine the parameters of at least two substations of your utility and write

your assessment on whether the transformers installed are of correct number and sizes. Give reasons for your answer.

Distribution Transformer

3. Discuss measures to avoid DTR failures at the Operation and Maintenance level. 4. Which tests can be performed on DTRs before and immediately after installation? 5. How many consumer complaints in a year are related to DTR failure in your utility? 6. Describe the kind of DTR failures that take place in your utility. 7. What is the normal correction time for complaints related to DTR failures in your utility? How can this time be reduced to a level acceptable to consumers? Explain giving reasons. 8. Analyse the reasons for transformer failure in your utility. 9. Which of the Operation and Maintenance Methods are applied in your utility to ensure improved life and efficiency of transformers? 10. Suggest ways to reduce DTR failure rate in your utility.

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Operation and Maintenance

APPENDIX 1: CASE STUDIES ON AVERT ING DISTRIBUTION TRANSFORMER FAILURE Classification of Failures Minor

Major 1. Insulation Failure

1. Oil Sample not satisfactory

2. Damage to HT Coil

2. Lead connections cut off

3. Damage to LT Coil

3. Wornout bushing rods

4. Damage to Core and Laminations

4. Broken bushings

5. Failure of Tap switch and Tap arrangement

6. Welding leakage

5. Gasket leakage

7. Leakage through valves 8. Broken gauge glass 9. Broken vent diaphragm 10. Worn out breather

Care to be Exercised to Avoid Failure In Manufacturing Stage 1. Proper insulation arrangement 2. Mechanical rigidity to withstand heavy forces 3. Adequate cooling arrangement 4. Adequate quantity of oil for insulation and cooling 5. Maintaining atmospheric pressure inside with pure air 6. Rigid fixing of core-coil unit inside main tank

98

In Transport 1. Safe handling during transport and erection 2. Adoption of standards for erection of transformer structure 3. Standard construction of LT lines

In Working Conditions 1. Maintenance of oil level 2. Maintenance of breather with silica gel and oil seal 3. Periodical testing of IR values 4. Periodical tests in transformer 5. Earth resistance values and Earth maintenance 6. Keeping standard voltage and frequency at load terminals 7. Maintaining LAs to prevent

damage due to surges

7. Pucca earthing of core and other metallic parts

Distribution Transformer

8. Maintaining LT System 9. Keeping the loading within the limits

Examination of Failed Distribution Transformers External Check up

1. Oil level and quantity available 2. Places of oil leakage

Internal Physical Verification 1. Conditions of HT coils in all the three phases 2. Checkup of lead connection from coil (Delta and Star points)

3. Condition of breather and silica gel

3. Condition of core

4. Condition of bushing and bushing rods 5. Condition of vent diaphragm 6. Condition of valves

4. Condition of tap switch and connections 5. Condition of core earthing 6. Presence of sludge and moisture in oil and physical condition of oil

7. IR value and continuity

Transformer Health Test 1. By injecting 15 V on LV side and measuring stack voltage of HT coil 2. By injecting 15/Ö3 volt in between one LV phase and Neutral and measuring voltage of stack on corresponding HT phase coils and on other phase HT coils 3. Short circuit test by injecting 400V on HV side

8. BDV test on oil

Case Studies on Averting Distribution Transformer Failures Details of Defects Noticed at Field

Observation at Lab

Probable Cause for the Defects

Rectification Done and Suggestions to Avoid Recurrence in Future

Case 1

Case 1

Case 1

Case 1

All 3 HG Fuses blow out slowly after 1 hour (Examined at Field).

HT leads insulation near top of bushing found charred.

Conservator oil level below bushing rod. Absence of oil around caused heating of leads.

Insulation sleeves changed. Higher oil level was

99

Operation and Maintenance

100

asked to be maintained. This is a manufacturing defect and company was addressed. Case 2

Case 2

Case 2

Case 2

Oil spurt out (Similar happening on previous transformers also).

B phase end windings (Top and Bottom found shattered).

Lightning surges entered and caused shattering. (Area prone to lightning).

Coil rewound and sent. The previous failure was also in same “B” Phase. Asked to examine HT LAS of “B” Phase. All three LAS changed and no such failure thereafter.

Case 3

Case 3

Case 3

Case 3

Undue heating in LT Rod. “R” phase Rod worn out and Insulation tape charred.

“R” phase rod inside connection loosened.

Connection loosened due to improper handling of bushing during jumper connections.

Connections tightened and sent for use. Advised to use checknuts and proper handling.

Case 4

Case 4

Case 4

Case 4

L.T.voltage measured and found alright.But found voltage drop when load is connected.

Neutral bushing connection loosened inside.

Improper handling of neutral bushing during meggering.

Neutral Connection set right and sent for use. Advised to handle bushing connections properly.

Case 5

Case 5

Case 5

Case 5

LT voltage found alright. But load in “R” Phase could not be loaded.

LT & HT connection found alright. No visual defect. SC test revealed no current in R Phase, full current in neutral.

Examined the soldered connections in “R” phase and found “R” phase bottom delta connection improper. (not completely cut).

Defective connections resoldered and found alright. Improper soldering gave way during use.

Case 6

Case 6

Case 6

Case 6

HG Fuse in all 3 phases blown out.

Inter turn short in R Phase − 3rd stack. Y Phase − 4th stack B Phase − 2nd and 3rd stack.

Suspected heavy absorption of moisture. Oil sample not satisfactory. Crackle test proved positive.

All good stacks removed. Core with LT coil placed in Hot Air Chamber and dried. Failed coils replaced. Put into use

after circulation and test. Case 7

Case 7

Case 7

Case 7

HG Fuse frequently blows out in “B” phase.

Insulation of lead inside “B” phase bushing found charred. All coils in good condition.

Insulation of lead inside “B” phase bushing found charred. All coils in good condition.

Advised the field to release air monthly.

Case 8

Case 8

Case 8

Case 8

Informed that a) No oil could be taken from sampling valve oil not flowing out. b) Oil level in conservator is full (attended at field).

Examined at spot and observed that the Field Report is correct.

Examined at spot and observed that the Field Report is correct.

Air trapped under the bottom portion and pushed the oil up. Advised to release air frequently through air plug and also through top lid also.

Case 9

Case 9

Case 9

Case 9

HG in “R” Phase blown out. (Similar failure in 5 months).

“R” phase HT coils failed with symmetry giving way. LT “R” phase also failed.

Due to Heavy Short Circuit force because of intermittent feeding of fault current.

LT R Phase conductor frequently touched nearby neutral. Fuse not blown. Earth value high : 30 Ohms. Asked to rectify earthing system and adopt proper LT fuse.

Case 10

Case 10

Case 10

Case 10

HG fuse blowing out on load. (attended at site).

Oil level found upto core level only. Gauge glass showing OK level. HT lead insulation charred due to heat. Coil alright.

Gauge glass indication misleading. Actually oil level is low.

Gauge glass cleaned. Block in air hole removed. Insulation of leads strengthened. Transformer put back in service.

Case 11

Case 11

Case 11

Case 11

HG fuse blown out. LT IR Value zero.

All LT leads removed. Now LT Megger value is 30 M Ohms.

“Zero” IR value is on LT Leads and not in transformer.

The LT lines and cables were asked to be inspected and defects rectified.

Case 12

Case 12

Case 12

Case 12

HG fuse is blown out.

Core and channel short circuited with “C” phase coil top stack and leads.

Core not kept earthed (not provided after repairs). Stray

Advised the field to revamp earthing system. Transformer

Distribution Transformer

101

Operation and Maintenance

102

Earthing of core not found.

voltage caused short circuit.

condemned since laminations got charred.

Case 13

Case 13

Case 13

Case 13

Transformer changed for two times due to unequal voltage.

No fault in the two transformers brought to Lab.

Advised to inspect the line for any jumper cut.

Reported that one phase line cut on loadside with incoming three phase intact in pin insulator (Location in the midst of a lake full of water).

Case 14

Case 14

Case 14

Case 14

Removed as unequal voltage. IR value and continuity OK.

Delta connection cut in bottom of “B” Phase.

May be due to ageing and wear and tear.

Continuity test is OK since it is connected in Delta. Soldered and sent for use.

Case 15

Case 15

Case 15

Case 15

HG fuse blown out. IR value and continuity OK.

“B” phase coil interturn short in 2nd stack. No shattering of coils, contacting with Earth.

Insulation failure due to ageing. Since no earth contact of winding, I.R. values are OK.

Failed stack replaced and sent for use. Flexibility of connection increased and soldered with rod.

Case 16

Case 16

Case 16

Case 16

Unequal voltage. continuity not found with the “B” phase HT.

HT lead in “B” bushing rod, came out.

The lead was tight. Hence came out from rod.

Flexibility of connection increased and soldered with rod.

Case 17

Case 17

Case 17

Case 17

Oil sample not satisfactory for 3 consecutive tests.

Found heavy sludges and moisture absorption. IR value low. Found vent pipe diaphragm broken. Gauge glass broken. Breather all right.

Sludging due to ageing, water entry through vent pipe and gauge glass.

Oil completely discharged. Core and coil cleaned. Dried in chamber. Put hot oil circulation with new oil and tested OK.

Case 18

Case 18

Case 18

Case 18

Frequent failure of certain make transformer.

Casual examination revealed low quantity of oil than that at name plate − 260 litres. Available 30 litres.

Non availability of sufficient oil − very small spacing between tank and core and top cover, leads to failure.

Taken up with company by Purchase Wing. (Ineffective Cooling System).

APPENDIX 2: A CHECKLIST FOR PREVENTIVE MAINTENANCE OF DISTRIBUTION TRANSFORMERS Sl. No.

What to Inspect

Distribution Transformer

What Maintenance to Do? Who has to do? Under Why? And When? whose Supervision? MONTHLY The ground and the space just immediately under the transformer and the structure should be free from bushes, grass and temporary sheds.

Area wireman/Line Inspector (or) Foreman

Earthpit

The three earthpits provided for the transformer should be free from bushes and should be visible from the ground and should be watered so as to have low resistance value. Low ground resistance is important for satisfactory lightning arrestors operation.

Area wireman/Line Inspector (or) Foreman

3.

Transformer Tank

4.

Oil level

Area wireman/Line Aircooling is used almost Inspector (or) Foreman exclusively for distribution transformers, the surface of the tank being artificially enlarged by radiators, tubing and the like. The rate at which the transformer oil deteriorates increases rapidly with rising temperature. Hence due attention must be paid for the provision of adequate ventilation. One immediate step is to clean the entire transformer tank. Lineman/Line Inspector The transformer core and (or) Foreman the winding should be completely immersed in the oil. Low level of oil will cause burning of H.T. coil. Hence the oil level in the gauge glass has to be checked and topped up whenever necessary. Oil leakages from drain Area wireman/Line valve, gaskets and tank Inspector (or) Foreman have to be checked and rectified at the earliest.

1.

Transformer Yard

2.

Oil leakage

103

Operation and Maintenance

104

Earth

Safety to personnel can be Lineman/Line Inspector assured by adopting an (or) Foreman earthing system so designed that under both normal and abnormal condition no dangerous voltage can appear on the equipment to which personnel have access. Damage to equipment can be minimised by the provision of paths of low impedance from the equipment to the earth system. The earthing system will be of no value unless the connections are tight, and there is continuity from the equipment to the earth and the earth exists.

6.

Breather

Moisture entering the oil as a result of the so called breathing action greatly reduces its dielectric strength so that breakdowns from coils or terminal leads to tank or core structure may take place. To prevent moisture entering into transformer oil, sillicagel dehydrating breathers are fitted to the transformer with a sight glass so that the colour of the sillicagel crystals may be seen. The colour changes from blue to pink as the crystal absorbs moisture. When the crystals get saturated with moisture they become predominantly pink and should therefore be reactivated. For reactivation the crystals should be baked at a temperature of about 200oC until the whole mass is at this temperature and the blue colour has been restored. Alternatively change the sillicagel.

Area wireman/Line Inspector (or) Foreman

7.

L.T. Fuses

In the maintenance card provided to the area wireman the size of the fuse wires for the L.T. main different capacities of the distribution transformers are furnished. Only the correct

Area wireman/Line Inspector (or) Foreman

5.

Distribution Transformer

size of fuse wires have to be provided. QUARTERLY 1.

H.G. Fuses

In the maintenance card provided to the area wireman, the size of the H.G. fuses wires for the different capacities of the distribution transformer is furnished. Only the correct size of the fuse wire has to be provided. In today’s dynamic situation, the distribution transformers are subjected to intermittent overloads resulting in the glow H.G. fuses and subsequent breakdown. Appropriate sizes of these fuses have to be kept in ready stock and replaced timely.

Lineman/Line Inspector (or) Foreman

2.

Insulation resistance

Some of the materials used in insulation are organic in nature and are therefore susceptible to deterioration by heat, oxygen, moisture and corrosive liquids. Even the best are vulnerable if incorrectly used or applied. Insulation breakdowns are most often the result of internal or external heat and moisture. The heating causes chemical changes in the insulation that are aggravated by the presence of moisture. The insulation resistance value between H.V. to earth, L.V. to earth and H.V. to L.V. and main L.T. leads have to be recorded and the trend has to be observed so that corrective steps could be taken before the total failure of the insulation. The minimum safe insulation resistance in meg ohms at 30oC for 11 KV is 400 and for L.V. it is 100. For every 10oC increase in temperature the I.R. value will get halved for both 11 kV and L.V.

Line Inspector (or) Foreman/Junior Engineer (or) Assistant Engineer

3.

Load current

Sustained heavy overloads produce high temperatures throughout

Assistant Engineer or Junior Engineer/Ass. Exe. Engineer

105

Operation and Maintenance

the transformer. The oil insulation becomes brittle and in time probably flakes off the conductors in places on producing short circuits between turns. Transformers with a high ratio of copper loss to iron loss are less able to withstand overload and are therefore more liable to fail on account of overloading. The load current has to be measured at different times especially at peak load hours to know the overload situation and to enhance the transformer capacity or to bifurcate the loads at the appropriate time. 4.

Voltage

The voltage should be checked to make sure that the transformer has the proper tap position. Over voltage produces excessive noload loss. It is also necessary to ensure proper voltage to the consumers as per the Indian Electricity Rules, 1956. For this purpose the voltages at the transformer end and tail end have to be measured and necessary improvements have to be made either by changing the taps provided or by strengthening of conductors, bifurcation of loads, etc.

Assistant Engineer or Junior Engineer/Ass. Exe. Engineer

ANNUAL 1.

106

AB Switch

This component provided on the transformer structure helps to break or make the electric circuit to the transformer through a system of contacts. These have to be lubricated for ease in operation and to avoid accidents to departmental persons due to non-operation of the blades in any of the phases or all.

Lineman/Line inspector (or) Foreman

2.

Line and earth connections

The line and earth connection of the AB switches, H.T. and L.T. lightning arrestors and H.T. and L.T. bushings have to be checked and any loose connections have to be made tight. When the line connections loose, the current is allowed to pass in the form of an arc which creates enormous heat resulting in melting of contacts or breakdown of connections and energisation of the structure. When the earth connections are loose and when the fault occurs, the entire fault current cannot be earthed properly resulting in failure of equipment or causing accidents.

Lineman/Line inspector (or) Foreman

3.

Transformer oil

The dielectric strength of the transformer oil mainly provides an indication of the physical condition of the oil. The impurities most likely to influence the dielectric strength of the oil, in practice, are moisture and fibres. The oil is said to have passed the test if two out of three samples successfully stand the test at 40 KV for one minute.

Line inspector (or) Foreman/A.E. (or) J.E.

4.

Earth resistance

Safety to personnel can be assured by adopting an earthing system so designed that under both normal and abnormal conditions no dangerous voltage can appear on the equipment to which personnel have access. Damage to equipment can be minimised by the provision of paths of low impedance from the equipment to the earth system.

A.E. (or) J.E./Asst. Exe. Engineer

Distribution Transformer

107

Operation and Maintenance

MAINTENANCE SCHEDULE OF DISTRIBUTION TRANSFORMERS Sl. No.

Person responsible for the work

Work to be carried out

Person responsible for the completion of the work

I. Weekly / Fortnightly

1.

Breather oil, silica gel, checking, replenishing

Area Wireman

Line Inspector / Foreman

II. Monthly 1a

Maintaining the transformer yard and the earth-pits neat and tidy and watering the earth-pits.

b

Cleaning the entire transformer including the bushings.

c.

Checking that oil level is below the mark.

d.

Checking for oil leaks and reporting if any noticed.

e.

Checking the earth connections.

f.

Reconditioning the breather. (by reactive silica gel, or replacing if necessary).

g.

Checking the LT fuses and renewing them if necessary.

2.

Topping up oil where necessary.

Area Wireman

Line Inspector / Foreman

Line Inspector

Foreman

III. Quarterly

108

1.

Renewing the H.G. fuses.

Line Inspector

Foreman

2.

Measuring the insulation resistance.

Line Inspector / Foreman

Foreman

3a.

Measuring the load current.

b.

Measuring the voltage at the transformer and at the tailends of the feeders.

Section Officer

Assistant Executive Engineer

Distribution Transformer

IV. Annual 1a.

Lubricating line AB Switches and checking their operation.

b.

Checking the line and earth connections of the HT/LT lightning arrestors.

c.

Checking the HV and LV bushing connections.

2.

Line Inspector

Foreman

Getting the oil samples tested for dielectric strength.

Line Inspector / Foreman

Section Officer

3.

Measuring the earth resistance.

Section Officer

Assistant Executive Engineer

Note

1.The Assistant Executive Engineer should inspect every distribution transformer in the Sub-division once in every year and ensure that maintenance works are carried out as per this schedule. 2. The Section Officer should inspect every distribution transformer in the section once every quarter and ensure that maintenance is carried out as per schedule. 3.The Line Inspector/Foreman should inspect every distribution transformer in his/her jurisdiction once every month and ensure thatb maintenance works are carried out as per schedule.

109

Operation and Maintenance

PREVENTIVE MAINTENANCE SCHEDULE OF DISTRIBUTION TRANSFORMER STRUCTURES Sl. No.

What to inspect

What Maintenance to Do? Why? And When?

Who has to do? Under whose Supervision?

MONTHLY 1.

Yard and Earth Pits

Maintaining the Yard (underneath the Structures) and earth pits neat and tidy and watering the earth pits. Otherwise the site becomes untidy and it becomes difficult to carry out operation on the structure especially during breakdown. By watering the earth pits, the earth resistance will be less, facilitating easy and quick earthing of fault current/voltage.

Wireman/Line inspector, Foreman

2.

Earth Connections

Checking earth connections both at earth pits and also the metal parts. By keeping the earth connections proper, fault current can easily pass to earth during fault condition, thus facilitating quick tripping of the feeders.

Wireman/Line Inspector, Foreman

QUARTERLY 1.

110

Dividing/ Termination

Checking earth condition of dividing/termination to see that no cracks are being developed and it is properly divided/clamped. Also checking that connections from the termination to the bus are proper. Checking once in a quarter, even if a crack is developed, or compound is leaking or fixing is getting loosened or the connections are becoming loose, it may be found out in time and timely action may be taken. Otherwise a breakdown may happen and it is difficult to work in a structure, as L.C. from different source of supply is necessary.

Lineman/Line Inspector, Foreman

Distribution Transformer

ANNUAL 1.

Concreting/ Coping of the Supports

Checking the condition of the concreting/Coping of the supports of the structures to see that the coping and concreting is intact. If there are cracks or the coping or concreting is coming off, preventive action may be taken to concrete or coping. The supports fixing to earth become weak and during the time of heavy rains, cyclone or flooding, the structure may fall, leading to major breakdown. If coping is not done in the case of metal they may get corroded due to urination by dogs.

Wireman/Line Inspector, Foreman

2.

Supports

Checking the condition of the supports and if cracks or damages are noticed, action can be taken to replace or rectify the defects. If not done, the structure itself may fall due to cyclone, floods, etc.

Wireman/Line Inspector, Foreman

3.

Earth Resistance

Measuring the earth resistance to check if it is within permissible limits. If it is beyond permissible limits, action may be taken to reduce the earth resistance so that earth faults are cleared quickly and accidents are avoided.

Junior Engineer or Asst. Engineer/Asst. Exec. Engineer

4.

Earth Connection of Metal Parts

Checking the earth connections of metal parts to ensure that the metal parts are properly connected to the earth so that any earth fault of the metal parts are cleared quickly and efficiently. If not, accidents may happen.

Wireman/Line Inspector, Foreman

5.

Operation of AB Switches

Lubricating the AB Switches and checking their operation so that the AB Switches be operated at ease and correctly, at times of emergency.

Wireman/Line Inspector, Foreman

111

Otherwise during emergencies, the AB Switches cannot be opened leading to unavoidable delay in attending to a switch leading to accidents also.

Operation and Maintenance

112

6.

Line and Earth Connections of AB Switches

Checking that Line and Earth connections of AB Switches are properly done. If line connection is not proper, it may lead to loose contact resulting in breakdown. If earth connection is not proper, it may result in high resistance and faults may not be cleared in time. It may lead to accidents also.

Lineman/Line Inspector, Foreman

7.

Line and earth Connections of HT Lightning Arrestors

To check if line and earth connections of HT lightning arrestors are properly made; if line or earth connection of the lightning arrestor is not made properly, lightning surges will not be cleared leading to breakdown or accident.

Lineman/Line Inspector, Foreman

8.

Connections From and To Busbars

Checking the tightening the connections from the busbars and the connections of the busbars to the lines. If the connections are not tight enough and proper, it may lead to loose contact resulting in breakdown.

Lineman/Line Inspector, Foreman

9.

Provision of Tubular Busbar

Checking the provision of tubular busbar and changing the conductor strung busbars to tubular busbars, if not provided. If rigid tubular busbars are not provided loose connections may develop in strung busbars leading to breakdowns.

Foreman or Lineman/ Junior Engineer or Assistant Engineer

10.

Insulators

Checking all insulators to see that there are no cracks or damages are developing. If they are developing they may be changed in time to avoid accidents.

Lineman/Line Inspector, Foreman

11.

Jumpers

Checking the condition and adequacy of all jumpers to

Foreman or Line Inspector/Junior

see that they are proper. If the condition is bad, they have to be replaced. If they are inadequate also, they have to be replaced. If not done, it may lead to loose contact resulting in breakdown. If the jumper is not adequate, the current carrying capacity becomes less leading to overloading, etc.

Engineer or Assistant Engineer

12.

Guides for AB Switches

Check if the guides for AB Switches are proper. Otherwise, it becomes difficult to operate the AB Switches leading to difficulties at times of emergencies. Sometimes it leads to accidents also.

Wireman/Line Inspector, Foreman

13.

Provision of Danger Boards

Checking the provision of “Danger Board”. If not provided, provide the same. It may, to a certain extent, prevent unauthorised persons to meddle with the structure. It may also caution authorised persons when they have to work on the structure especially at double and multiple feed locations.

Wireman/Line Inspector, Foreman

14.

Provision of Caution Board

Check provision of “Caution Board” to caution that care should be taken to work on the structure. If not available, the same should be provided.

Foreman or Line Inspector/Junior Engineer or Assistant Engineer

15.

Painting of Details

Painting the details of the supply and if not available, paint the same. The details will help to identify the feeder and to work. If not painted, it may lead to mal-operation and may lead to accidents.

Foreman or Line Inspector/Junior Engineer or Assistant Engineer

Distribution Transformer

Once in Three Years 1.

Painting of Metal Parts

Painting all metal parts will avoid corrosion.

Wireman/Assistant Engineer or Junior Engineer

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Operation and Maintenance

MAINTENANCE SCHEDULE OF DISTRIBUTION TRANSFORMER STRUCTURES Person responsible to Sl. No.

Details of the maintenance work to be carried out

Do the work

Check and ensure the completion of the work

MONTHLY 1.

Maintaining the yard (underneath the structure) and the earth pits neat and tidy and watering earth pits.

Wireman

Line Inspector / Foreman

2.

Checking the earth connection.

Wireman

Line Inspector / Foreman

QUARTERLY 1.

Checking the condition of dividing / termination.

Wireman

Line Inspector / Foreman

ANNUAL

114

1

Checking the condition of concreting / coping of the supports of the structures.

Wireman

Line Inspector / Foreman

2.

Checking the condition of the supports of the structures.

Wireman

Line Inspector / Foreman

3.

Measuring the earth resistance.

Junior Engineer / Asst. Engineer

Asst. / Executive Engineer

4.

Checking the earth connection of metal parts.

Wireman

Line Inspector / Foreman

5.

Lubricating the AB switches and checking their operation.

Lineman

Line Inspector / Foreman

6.

Checking the line and earth connection of AB switches.

Lineman

Line Inspector / Foreman

7.

Checking the line and earth connection of HT lightning arrestors.

Lineman

Line Inspector / Foreman

8.

Checking and tightening the connection from bus bars to the lines.

Lineman

Line Inspector / Foreman

9.

Checking for provision of tubular bus bars and changing the conductor strung bus bars to tubular bus bars.

Foreman / Line Inspector

Junior Engineer / Asst. Engineer

10.

Checking all insulators.

Lineman

Line Inspector / Foreman

11.

Checking the conditions and adequacy of all jumpers.

Foreman / Line Inspector

Foreman

12.

Checking guides for AB switches.

Wireman

Asst. Engineer / Junior Engineer

13.

Checking the provision of ‘Danger’ boards and if not provided, providing the same.

Wireman

Foreman / Line Inspector

14.

Checking the provision of caution boards and if not provided, providing the same.

Foreman / Line Inspector

Asst. Engineer / Junior Engineer

15.

Checking the painting of details of HT supply and if not provided, providing the same.

Foreman

Asst. Engineer / Junior Engineer

Distribution Transformer

Once in 3 years 1.

Painting of all metal parts.

Wireman

Asst. Engineer / Junior Engineer

115