Ntpc Project Report.docx

NTPC LIMITED (A Govt. of India Enterprise) Ramagundam Super Thermal Power Station A PROJECT REPORT ON STUDY OF TRANSFORMERS AND SWITCHYARD In partial fulfilment for the award of degree of Bachelor of Technology In Electrical and Electronics Engineering Submitted By: K.SAICHARAN

(B16EE046)

MD.NEHAZ HUSSAIN

(B16EE033)

K.ADITHYA

(B16EE049)

K.SAIKIRAN

(B16EE017)

Under the estimated guidance of SRI.M.VENUGOPAL REDDY SR.MANAGER (EMD)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING KAKATIYA INSTITUTE OF TECHNOLOGY AND SCIENCE WARANGAL-506015 2016-2020

RAMAGUNDAM CERTIFICATE This is to certify that the project entitled “STUDY OF TRANSFORMERS AND SWITCHYARD” on it –NTPC Ramagundam has been successfully carried out, in the partial fulfilment for the award of “Bachelor of Technology in Electrical and Electronics Engineering” from K.SAICHARAN

(B16EE046)

MD.NEHAZ HUSSAIN

(B16EE033)

K.ADITHYA

(B16EE049)

K.SAIKIRAN

(B16EE017)

KAKATIYA INSTITUTE OF TECHNOLOGY AND SCIENCE WARANGAL-506015 This bona fide work has been carried out by students of KAKATIYA INSTITUTE OF TECHNOLOGY AND SCIENCE, WARANGAL carried out, project work under our guidance and supervision at “NTPC Limited-Ramagundam”, during the academic year 2016-2020.

PROJECT GUIDE

PROJECT CO-ORDINATOR

ACKNOWLEDGEMENT We are grateful to SRI.V.RAMAIAH, HOD (EEE) in KAKATIYA INSTITUTE OF TECHNOLOGY AND SCIENCE, WARANGAL for their valuable suggesting, which helped us during the project. We acknowledge our sincere gratitude to SRI.M.VENUGOPAL REDDY, SR.MANAGER (EM) for providing the opportunity to work in NTPC –Limited, RAMAGUNDAM and giving us a rich experience in completing the project. We express our thanks to SRI.CH.VENKATESHWARA RAO, DGM (EM) AND SRI.B.V.SUBRAMANYAM, AGM (EM) I/C for their valuable and timely suggestions during my project. We would like to thanks SRI.G.PRAVEEN KUMAR, MANAGER (HR-EDC), SRI.JYOTHI PRASAD, Engr., SRI.L.HARIDAS, Engr., &SRI.G.BHIMESHWAR RAO, Sub Engr. for providing their help. We sincerely express our gratitude and respect to all those who guided, inspired and helped me in the completion of the project. We are grateful to them who are generous and cooperative during our project.

K.SAICHARAN MD.NEHAZ HUSSAIN K.ADITHYA K.SAIKIRAN

CONTENTS 1. NTPC INTRODUCTION 2. INTRODUCTION OF NTPC Limited, RAMAGUNDAM & PROFILE 3. IMPORTANCE OF TRANSFORMERS IN THERMAL POWER PLANT 4. DIFFERENT TRANSFORMERS IN A POWER PLANT 4.1 Generator Transformer (GT) 4.2 Unit Auxiliary Transformer (UAT) 4.3 Station Transformer (ST) 4.4 Tie Transformer 4.5 Auto Transformer 5. EQUIPMENT AND ACCESSORIES OF TRANSFORMER 5.1 Tank 5.2 Transformer core 5.3 Winding 5.4 Winding Insulation 5.5 Bushing 5.6 Conservator 5.7 Radiator 5.8 Air Cell 5.9 The Dehydrating Breather 5.10 Transformer Oil 5.11 Types of oil 5.12 Properties of transformer insulating oil 5.13 Cooling of Transformers

6. PROTECTION DEVICES EMPLOYED FOR TRANSFORMERS 6.1 Buchholz’s Relay 6.2 Temperature Indicators 6.3 Pressure Relief Device/Expansion Vent 6.4 Sudden Relay 7. TRANSFORMER PROTECTION 8. SWITCH YARD 8.1 About RSTPS 400KV Switch Yard 8.2 Distribution of Electricity 8.3 Switch Yard operation activities 8.4 Switch Yard Equipment 8.5 Switch Yard Control Room Panel

1. INTRODUCTION Vision: To be the world’s leading Power Company, energizing India’s growth. Mission: Provide reliable power and related solutions in an economical, efficient and environment friendly manner, driven by innovation and agility. Overview: NTPC is India’s largest energy conglomerate with roots planted way back in 1975 to accelerate power development in India. Since then it has established itself as the dominant power major with presence in the entire value chain of the power generation business. From fossil fuels it has forayed into generating electricity via hydro, nuclear and renewable energy sources. This foray will play a major role in lowering its carbon footprint by reducing green house gas emissions. To strengthen its core business, the corporation has diversified into the fields of consultancy, power trading, training of power professionals, rural electrification, ash utilisation and coal mining as well. NTPC became a Maharatna company in May 2010, one of the only four companies to be awarded this status. NTPC was ranked 512th in the ‘2018, Forbes Global 2000’ ranking of the World’s biggest companies. The total installed capacity of the company is 52,946 MW (including jvs) with 20 coal based, 7 gas based stations, and 1 Hydro based station and 1 Wind based station. 9 Joint Venture stations are coal based and 11 Solar PV projects. The capacity will have a diversified fuel mix and by 2032, non fossil fuel based generation capacity shall make up nearly 30% of NTPC’s portfolio. NTPC has been operating its plants at high efficiency levels. Although the company has 15.56% of the total national capacity, it contributes 22.74% of total power generation due to its focus on high efficiency.

2. INTRODUCTION TO NTPC LTD., RAMAGUNDAM NTPC Ramagundam, a part of National Thermal Power Corporation, is a 2600 MW Power station situated at Ramagundam in Peddapalli district in the Indian state of Telangana, India. It is the current largest power station in South India. It is the first ISO 14001 certified "Super Thermal Power Station" in India. The TG Hall or the Turbo-Generator hall or the Turbine-Generator Hall is the hall or space where the turbine-generator sets are present. NTPC Ltd., Ramagundam has two TG Halls one for STAGE-I and the other common for STAGE-II and STAGE-III. These TG halls are equipped with heavy overhead cranes that assist in transportation of material within the TG hall. These cranes find their use greatly during overhauls. PROFILE: The whole plant is divided into 3 stages, each stage being planned at one time. STAGE 1 (3×200 MW): This stage consists of three units (Unit-1, Unit-2, Unit-3) each with a generation capacity of 200 MW. The turbines for these three units were manufactured by The Ansaldo Energy Ltd. The construction began in the late 1970s and these units have performed well over a long period setting many records regarding maintenance and generation over the other two stages. But stage (1, 2, and 3) cwp motors are manufactured by BHEL. And all motors are manufactured by Amado. S-I coal mill motors are 240 KW and PA fans are 400 KW. All equipment is very important in plant. STAGE 2 (3×500 MW): This stage again consists of three units (Unit-4, Unit-5, Unit-6) each with a generation capacity of 500MW. The turbines for these three units were manufactured by Bharat Heavy Electricals Limited (BHEL). Stage 3 (1×500 MW): This stage comprises only one unit (Unit - 7). This is a first of its kind in South India being a computer operated unit. A wide disparity may be seen between the control rooms of the other two stages and this computerised unit. To this day, many Power plant engineers train in this unit to upgrade themselves to this new mode of operation. This unit also has the tallest chimney in India (height: 275 metres).

Overhauls: Once in two years, these units are stopped and overhauled, one unit at a time. The overhauls are usually taken up during the months June to September as the monsoons activate hydel power generation which substitute the power generation lost due to the overhaul of the unit. The same practice is followed all through the country. The overhauls usually take 15 to 20 days per unit provided there is no major repair involved. Major repairs include turbine casing, turbine rotor damage and other damages that require transporting the equipment to

another location (usually the manufacturer). The overhauls are the dissipaters of the annual PLF of any power plant. GENERATION DISTRIBUTION STATES: States: As NTPC Ltd. Is a Public Sector Undertaking (PSU), the generation is almost uniformly distributed to 5–6 states all of them sharing about 20–25 percent of the Generation. The States include:      

Andhra Pradesh Telangana Tamil Nadu Kerala Karnataka Maharashtra

INPUTS: Water: The power station gets its water periodically released from the SRSP- Sriram Sagar project. This water is stored in the balance reservoir. The water level in the balance reservoir is monitored daily. Coal: NTPC Ramagundam is a Thermal Power Station and hence uses coal. This coal is available at a large scale from the Singareni Coal mining company nearby and is transported using the MGR(Merry-go-round) system wherein, a train comes on one rail route, delivers coal and returns on another route. The wagons arriving by this route are taken for coal collection wherein a mechanism provided underneath the wagons opens on application of air pressure and drops the coal it is carrying. A separate department (MGR Dept.) Handles this process. Coal also arrives by the Indian Railways. The wagons are routed via Ramagundam railway station to the separate plant line and these coaches arrive at the wagon tippler. The wagons arriving in this manner must be tilted at the wagon tippler to obtain the coal as they do not have the drop mechanism underneath. Other petroleum products required: The station also requires various oils for the following purposes:    

Turbine oil (SP-46)for turbine lubrication HFO, Heavy fuel oil for boiler start-up Diesel for DG sets (Power backup) Other oils for various hydraulic controls and circuits

These are periodically purchased as per requirement from the Indian oil corporation IOCL establishment nearby. Total area of plant: 10,000 acres Land in use: 1. Plant area

: 1200

2. Ash dump area

: 1650

3. Township area

: 700

4. Power Canal area

: 4836

5. Mgr& Rly sliding

: 680

6. Hearting soil area

: 720

7. Others

: 455

Height of chimneys: Stage 1: 210 m Stage 2: 225 m Stage 3: 275 m Electrical Maintenance: This is the largest department under the Maintenance section. This department takes care of all the electrical aspects of the plant. It takes care of the following sections. 

Switchyard  Generator  Generator Transformer  Conveyor motors and other motors  All power transmissions The Switchyard: The switchyard is the place where the station last takes care of the power it produces. The switchyard links the power generated to the southern power grid. The major transmission points are:    

Nagarjunasagar Chandrapur Hyderabad Khammam

DISTRIBUTION OF ELECTRICITY: Total capacity of RSTPS is 2600 MW. NTPC is distributing the electricity to the following:

1. 2. 3. 4. 5. 6.

STATE AP&TS TAMILNADU KARNATAKA KERALA GOA PONDICHERRY

MEGAWATT 610 MW 470 MW 345 MW 245 MW 100 MW 50 MW

PERCENTAGE 29% 22% 16% 12% 5% 2%

3. IMPORTANCE OF TRANSFORMERS IN THERMAL POWER PLANT A transformer is a static electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force (emf) or "voltage" across a second coil wound around the same core. Electric power can be transferred between the two coils, without a metallic connection between the two circuits. The use of Transformers has become inevitable in any thermal power plant rather in any industry today. In 500 MW generation at RSTPS stage-II, stage-III the electricity is generated at 21KV and current is approximately 16000A.If we transmit the same electrical energy to the grid then to transmit a current of 16000A, we need very heavy conductors, very strong transmission towers. Moreover, the lines loss shall be extremely high because current is very high and loss is proportional to the square of the current. 4. DIFFERENT TRANSFORMERS IN THERMAL POWERPLANT In Thermal and Nuclear Power Plants different types of power transformers are employed: Types of Transformers: 1. Generator Transformer 2. Unit Auxiliary Transformer 3. Station Transformer 4. Tie Transformer 5. Auto Transformer 4.1. GENERATOR TRANSFORMER: This is the main power transformer employed in the power plant. It steps the voltage from 21kv to 230 or 400kv and delivers the power. Stepping up the voltage reduces the

transmission losses which occur during the power transmission to long distances. The rating of this transformer (MVA rating) will be almost equal to the alternator or generator rating. SPECIFICATIONS:

1. Manufacturer 2. No. of units 3. Rated HV&LV 4. 5. 6. 7. 8. 9.

Rated Voltage, HV Rated current, LV Rated voltage, LV Rated current, HV Type of cooling Connection symbol

Mitsubishi 3 200MVA for stage 2&3 240 MVA for stage 1 420/3KV 9520 A 21KV 825A OFAF YD 11

4.2. UNIT AUXILIARY TRANSFORMERS: These transformers are connected to the Generator Transformer bus. These transformers steps down the voltage from 230kv or 400kv to 6.6kv (230/6.6kv or 400kv/6.6kv) and supply the power to the electrical auxiliaries present in the plant (motors, drives, lighting and other plant loads). SPECIFICATIONS: 1. 2. 3. 4.

Manufacturer Voltage Ratio Vector Group Cooling

BHEL 21/6.9KV Dynl ONAN/ANAF

4.3. STATION TRANSFORMER: This transformer provides electrical power to the plant during start up when no supply is available to the plant (generator is not operating). It also steps down the voltage like unit auxiliary transformers and supply power the plant auxiliaries. Station Transformer and Unit Auxiliary Transformers are connected to the grid, so that they can get power when Turbo-Generator is not in operation and supply power to the plant auxiliaries. SPECIFICATIONS: 1. Manufacturer 2. No. of units 3. Capacity

BHEL 1 33/11.5 76.9 KV

4.4. TIE TRANSFORMERS:  

 

The TIE Transformers are installed to tie between GRID and the station auxiliaries through station transformers. Generally it is used for two purposes one for it takes 400KV voltage from 400KV bus bar and step down to 33KV and it is used for station supply system having 33KV switch gear. The capacity of TIE Transformer should be at least the sum of connecting station Transformers plus miscellaneous requirements. The no. of TIE Transformers are four that improves reliability.

SPECIFICATIONS:

1. 2. 3. 4. 5.

Manufacturer No. Capacity Ratio Voltage Ratio Cooling

BHEL 1 100MVA 400/34.5 KV ONAN/ONAF

4.5. AUTOTRANSFORMER: Autotransformers are often used to step up or step down voltages in the 110-115120 V range and voltages in the 220-230-240 V range - for example, providing 110 V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or 120 V to be used with a 230 V supply. In all cases the supply and the autotransformer must be correctly rated to supply the required power. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 132 kV to 66 kV for transmission. Another application in industry is to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100 V–130 V and 200 V–250 V). SPECIFICATIONS:

1. 2. 3. 4. 5.

Capacity Voltage HV/IV/LV Current HV/IV/LV Cooling Connection symbol

315 400/220/33 KV 454.7/826.3/1837 A ONAN/ONAF/OFAF YNa0D11

5. EQUIPMENT AND ACCESSORIES OF TRANSFORMER: 5.1. TANK: This is a cylindrical tank mounted on supporting structure on the roof the transformer main tank. The main function tank of transformer is to provide adequate space for expansion of oil inside the transformer. Function: When transformer is loaded and when ambient temperature rises, the volume of oil inside transformer increases. A tank of transformer provides adequate space to this expanded transformer oil. It also acts as a reservoir for transformer insulating oil.

5.2. TRANSFORMER CORE: Closed-core transformers are constructed in 'core form' or 'shell form'. When windings surround the core, the transformer is core form; when windings are surrounded by the core, the transformer is shell form. Shell form design may be more prevalent than core form design for distribution transformer applications due to the relative ease in stacking the core around winding coils. Core form design tends to, as a general rule, be more economical, and therefore more prevalent, than shell form design for high voltage power transformer applications at the lower end of their voltage and power rating ranges (less than or equal to, nominally, 230 kv or 75 MVA).

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space and the core thus serves to greatly reduce the magnetizing current and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs

mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. 5.3. WINDING: High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses. Power-frequency transformers may have taps at intermediate points on the winding, usually on the higher voltage winding side, for voltage adjustment. Taps may be manually reconnected, or a manual or automatic switch may be provided for changing taps.

5.4. WINDING INSULATION:  

Cellulose Fibre Wood pulp is made

5.5. BUSHING: Larger transformers are provided with high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil. Functions:  

Electrical insulation to the conductor for the working voltage and for various over voltages, which occur in service. Mechanical support against various Mechanical forces & carry full load current.

5.6. CONSERVATOR: It is a cylindrical tank mounted on supporting structure on the roof of the transformer's main tank.

Function: When transformer is loaded and when ambient temperature rises, the volume of oil inside transformer increases. A conservator tank of transformer provides adequate space to this expanded transformer oil. It also acts as a reservoir for transformer insulating oil.

5.7. RADIATORS: Because of flow of electric current through the winding of Transformer and due to core losses, heat is produced in the windings and core. Because of this heat the temperature of Transformer oil increases. Thus cooling of Transformer Oil is must as we know that the rating of any electrical equipment depends upon its allowable temperature rise limit. Therefore, if the temperature rise of the Transformer insulating oil is controlled, the rating of Transformer can be extended up to significant limit. The Radiator of Transformer accelerates the cooling rate of Transformer. Thus, it plays a vital role in increasing loading capacity of Transformer. Cooling of Transformer Oil is the basic and main purpose of Radiator. As we know that increasing the surface area increases the rate of cooling. Therefore by any mean if we can increase the surface area of Transformer Oil then cooling of Transformer Oil can be accelerated. Radiator of Transformer serves this purpose of increasing the surface area of Oil. Cooling in Radiator is due to natural convection in the Transformer Oil

.

5.8. AIR CELL: The Air Cell (Flexible Separator) is fitted inside a conservator tank, isolating insulating oil from the atmosphere, and thus preventing contamination of gas and/or moisture from coming in to contact with the transformer oil due to oxidation and hydrolysis. The bag is vented o the transformer through flange type mounting such that it inflates or deflates to accommodate oil volume displacements due to changes in the transformer temperature.

Advantages:     

Simple design with no expandable parts Needs negligible man hours for inspection Extended service life Economic Prevents corrosion.

5.9. THE DEHYDRATING BREATHER: Dehydrating breathers are used to prevent the normal moisture in the air from coming in contact with the oil in electrical equipment. They are frequently used on the oil compartment of a load tap changer or on the air side of a power transformer conservator. The breather contains silica gel which has the ability to scrub the moisture from the air as it passes through the breather. Some breathers are designed for sealed tank transformers and breathe only at preset pressure levels. The dehydrating breathers are filled with Silicage1 that can absorb 20 percent of its own weight in moisture. The breathers are also provided with an oil trap preventing continuous contact between the moist air and the Silicage1. Dehydrating breathers are rated by the amount of oil that the breather can protect - the smallest breather can protect an oil volume of 300 gallons - the largest breather can protect an oil volume of 9600 gallons. Multiple breathers can be applied to the same tank or conservator to protect larger volumes. The smallest breather contains about one-half pound of silica gel and the largest breather contains about 18 pounds of silica gel.

DEHYDRATING BREATHER

5.10. TRANSFORMER OIL: Insulating oil in an electrical power transformer is commonly known as transformer oil. It is normally obtained by fractional distillation and subsequent treatment of crude petroleum. That is why this oil is also known as mineral insulating oil. Transformer oil serves mainly two purposes one it is liquid insulation in electrical power transformer and two it dissipates heat of the transformer i.e., Acts as a coolant. In addition to these, this oil serves other two purposes, it helps to preserve the core and winding as these are fully immersed inside oil, and another important purpose of this oil is, it prevents direct contact of atmospheric oxygen with cellulose made paper insulation of windings, which is susceptible to oxidation. 5.11. TYPES OF TRANSFORMER OIL: Generally there are two types of transformer Oil used in transformer,  Paraffin based transformer oil  Naphtha based transformer oil Naphtha oil gets more easily oxidized than Paraffin oil. But oxidation product, i.e., sludge in the naphtha oil is more soluble than Paraffin oil. Thus sludge of naphtha-based oil is not precipitated in the bottom of the transformer. Hence it does not obstruct convection circulation of the oil, means it does not disturb the transformer cooling system. But in the case of Paraffin oil although oxidation rate is lower than that of Naphtha oil the oxidation product or sludge is insoluble and precipitated at the bottom of the tank and obstruct the transformer cooling system. Although Paraffin-based oil has the disadvantage as mentioned earlier but still in our country, we use it because of its easy availability. Another problem with paraffin-based oil is its high pour point due to the wax content, but this does not affect its use due to warm climate condition of India. 5.12. PROPERTIES OF TRANSFORMER INSULATING OIL: The parameters of transformer oil are categorized as, 1. Electrical parameters: Dielectric strength, specific resistance, dielectric dissipation factor.

2. Chemical parameter: Water content, acidity, sludge content. 3. Physical parameters: Inter facial tension, viscosity, flash point, pour point. 5.13. COOLING OF TRANSFORMERS: For oil immersed transformers. Different cooling methods of transformers are:  Oil natural air natural (ONAN)  Oil natural air forced (ONAF)  Oil forced air forced (OFAF)  Oil forced water forced (OFWF) Oil natural air natural (ONAN):

This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction. This method can be used for transformers up to about 30 MVA.

Oil Natural Air Forced (ONAF):

The heat dissipation can be improved further by applying forced air on the dissipating surface. Forced air provides faster heat dissipation than natural air flow. In this method, fans are mounted near the radiator and may be provided with an automatic starting arrangement, which turns on when temperature increases beyond certain value. This transformer cooling method is generally used for large transformers up to about 60 MVA. Oil forced air forced (OFAF): In this method, oil is circulated with the help of a pump. The oil circulation is forced through the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of fans. The heat exchangers may be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher rating transformers at substations or power stations.

Oil Forced Water Forced (OFWF): This method is similar to OFAF method, but here forced water flow is used to dissipate hear from the heat exchangers. The oil is forced to flow through the heat exchanger with the help

of a pump, where the heat is dissipated in the water which is also forced to flow. The heated water is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several hundreds MVA.

6. PROTECTION DEVICES EMPLOYED FOR TRANSFORMERS: 6.1. BUCHHOLZ’S RELAY: Buchholz relay in transformer is an oil container housed the connecting pipe from main tank to conservator tank. It has mainly two elements. The upper element consists of a float. The float is attached to a hinge in such a way that it can move up and down depending upon the oil level in the Buchholz relay Container. One mercury switch is fixed on the float. The alignment of the mercury switch hence depends upon the position of the float. The lower element consists of a baffle plate and mercury switch. This plate is fitted on a hinge just in front of the inlet (main tank side) of Buchholz relay in transformer in such a way that when oil enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it, will change. PRINCIPLE: The Buchholz relay working principle of is very simple. Buchholz relay function is based on very simple mechanical phenomenon. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz container which causes fall of oil level in it. BUCHHOLZ RELAY OPERATION: The Buchholz relay operation may be actuated without any fault in the transformer. For instance, when oil is added to a transformer, air may get in together with oil, accumulated under the relay cover and thus cause a false Buchholz relay operation.

That is why the mechanical lock is provided in that relay so that one can lock the movement of mercury switches when oil is topping up in the transformer. This mechanical locking also helps to prevent unnecessary movement of breakable glass bulb of mercury switches during transportation of the Buchholz relays. The lower float may also falsely operate if the oil velocity in the connection pipe through, not due to an internal fault, is sufficient to trip over the float. This can occur in the event of the external short circuit when over currents flowing through the winding cause overheated the copper and the oil and cause the oil to expand. 6.2. TEMPARATURE INDICATORS: Temperature indicators available are: 1. Oil Temperature Indicator (OTI) 2. Winding Temperature Indicator (WTI) OIL TEMPERATURE INDICATOR (OTI): The (OTI) oil temperature indicator consists of a sensor bulb, capacity tube, and a dial thermometer, the sensor bulb is fitted at the location of hottest oil. The sensor bulb and capacity tube are fitted with evaporation liquid. The vapour pressure varies with temperature and is transmitted to a bourdon tube inside the dial thermometer, which moves in accordance with the changes in pressure, which is proportional to the temperature. In OTI, there are 2 (two) nos. Of mercury switch i.e. (S1 and S2). S1 is used for Alarm and the S2 switch is used for Trip. OTI ALARM 80°C

OTI TRIP 90°C

WINDING TEMPERATURE INDICATOR (WTI): Winding temperature indicator (WTI) consists of a sensor bulb placed in the oil filled pocket in the transformer tank top cover. The bulb is connected to the instrument housing by

means of two flexible capillary tubes. One capillary is connected to the measuring bellow of the instrument and the other to a compensation bellow. The measuring system is filled with a liquid, which changes its volume with rising temperature. Inside the instrument is fitted with a heating resistance which is fed by a current proportionate to the current flowing through the transformer winding. The instrument is provided with a maximum temperature indicator. The heating resistance is fed by a current transformer associated with the loaded winding of the transformer. (The heating resistance is made out of the same materials as that of the winding) The increase in the temperature of the resistance is proportionate to that of the winding. The sensor bulb of the instrument is located in the hottest oil of the transformer; therefore, the winding temperature indicates (WTI) a temperature of hottest oil plus the winding temperature above hot oil i.e. the hot spot temperature. In the WTI, there are four nos. Of the mercury switch. Two of them is used for Fan and motor pump control and another two nos. The switch is used for high-temperature warning alarm and trip circuit contact. Fan control Fan on: 64°C Fan off: 58°C

Pump Control Pump on: 72°C Pump off: 68°C

WTI Alarm 85°C

WTI Trip 95°C

6.3. PRESSURE RELEIF DEVICE/EXPANSION VENT: Many power transformers with an on-tank-type tap changer have a pressure protection for the separate tap changer oil compartment. This protection detects a sudden rate-of-increase of pressure inside the tap changer oil enclosure. When the pressure in front of the piston exceeds the counter force of the spring, the piston will move operating the switching contacts. The micro switch inside the switching unit is hermetically sealed and pressurized with nitrogen gas. The simplest form of pressure relief device is the widely used frangible disk. The surge of oil caused by a heavy internal fault bursts the disk and allows the oil to discharge rapidly. Relieving and limiting the pressure rise prevent explosive rupture of the tank and consequent fire. Also, if used, the separate tap changer oil enclosure can be fitted with a pressure relief device. 7. TRANSFORMER PROTECTION The electrical equipment and circuits in a substation must be protected in order to limit the damages due to abnormal currents and over voltages. All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage. The role of the

protections is to ensure that these withstand limits can never be exceeded, therefore clearing the faults as fast as possible. In addition to this first requirement a system of protection must be selective. Selectivity means that any fault must be cleared by the device of current interruption (circuit breaker or fuses) being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices. As an example for a short circuit occurring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker installed on the primary side must remain closed. For a transformer protected with MV fuses, the fuses must not blow. They are typically two main devices able to interrupt fault currents, circuit breakers and fuses: 

The circuit breakers must be associated with a protection relay having three main functions:  Measurement of the currents  Detection of the faults  Emission of a tripping order to the breaker  The fuses blow under certain fault conditions. Stresses generated by the supply Two types of over voltages may stress and even destroy a transformer:  

The lightning over voltages due to lightning stroke falling on or near an overhead line supplying the installation where the transformer is installed The switching over voltages generated by the opening of a circuit breaker or a load break switch for instance.

Depending of the application, protection against these two types of voltage surges may be necessary and are often ensured by means of zno surge arrestors preferably connected on the MV bushing of the transformer. Stresses due to the load A transformer overload is always due to an increase of the apparent power demand (kva) of the installation. This increase of the demand can be the consequence of either a progressive adjunction of loads or an extension of the installation itself. The effect of any overload is an increase of the temperature of oil and windings of the transformer with a reduction of its life time. The protection of a transformer against the overloads is performed by a dedicated protection usually called thermal overload relay. This type of protection simulates the temperature of the transformer’s windings. The simulation is based on the measure of the current and on the thermal time constant of the transformer. Some relays are able to take into account the effect of harmonics of the current due to non-linear loads such as rectifiers, computers, variable speed drives etc. This type of relay is also able to evaluate the remaining

time before the emission of the tripping order and the time delay before re-energizing the transformer. In addition, oil-filled transformers are equipped with thermostats controlling the temperature of the oil. Dry-type transformers use heat sensors embedded in the hottest part of the windings insulation. Each of these devices (thermal relay, thermostat, heat sensors) generally provides two levels of detection:  A low level used to generate an alarm to advise the maintenance staff,  A high level to de-energize the transformer. Internal faults in oil filled transformers In oil filled transformers, internal faults may be classified as follow:       

Faults generating production of gases, mainly: Micro arcs resulting from incipient faults in the winding insulation Slow degradation of insulation materials Inter turns short circuit Faults generating internal over pressures with simultaneously high level of line over currents: Phase to earth short circuit Phase to Phase short circuit.

These faults may be the consequence of external lightning or switching over voltage. Earth fault protection An earth fault usually involves a partial breakdown of winding insulation to earth under this circumstance it is profitable to employ an earth fault relay one method of protection is core balance protection. Differential protection Merz price circulating current principle or differential protection is commonly employed for power transformer protection from internal fault.

1.the different of current in primary and secondary must be equalized in diff relay by using appropriate turns on ct’s connected with both primary and secondary 2. The phase difference of current still generates a relay current. To neutralize it is case of healthy condition it is required to connect the cts in phase opposition with respect to ad joint.

Gas Analysis: Dissolved gas analysis (DGA) is the study of dissolved gases in transformer oil. Insulating materials within transformers and electrical equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical fault, and the rate of gas generation can indicate the severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any preventative maintenance program. The collection and analysis of gases in an oil-insulated transformer was discussed. As of 2018, many years of empirical and theoretical study have gone into the analysis of transformer fault gases. DGA usually consists of sampling the oil and sending the sample to a laboratory for analysis. Mobile DGA units can be transported and used on site as well; some units can be directly connected to a transformer. Online monitoring of electrical equipment is an integral part of the smart grid. When gassing occurs in transformers there are several gases that are created. Enough useful information can be derived from nine gases so the additional gases are usually not examined. The nine gases examined are:    

Atmospheric gases: nitrogen and oxygen Oxides of carbon: carbon monoxide and carbon dioxide Hydrocarbons: acetylene, ethylene, methane and ethane Hydrogen

The gases extracted from the sample oil are injected into a gas chromatograph where the columns separate gases. The gases are injected into the chromatograph and transported through a column. The column selectively retards the sample gases and they are identified as they travel past a detector at different times. A plot of detector signal versus time is called the chromatogram. The separated gases are detected by thermal conductivity detector for atmospheric gases, by flame ionization detector for hydrocarbons and oxides of carbon. A methanator is used to detect oxides of carbon by reducing them to methane, when they are in very low concentration. Types of faults Thermal faults are detected by the presence of by-products of solid insulation decomposition. The solid insulation is commonly constructed of cellulose material. The solid insulation breaks down naturally but the rate increases as the temperature of the insulation increases. When an electrical fault occurs it releases energy which breaks the chemical bonds of the insulating fluid. Once the bonds are broken these elements quickly reform the fault gases. The energies and rates at which the gases are formed are different for each of the gases which allow the gas data to be examined to determine the kind of faulting activity taking place within the electrical equipment.



  

Overheating windings typically lead to thermal decomposition of the cellulose insulation. In this case DGA results show high concentrations of carbon oxides (monoxide and dioxide). In extreme cases methane and ethylene are detected at higher levels. Oil overheating results in breakdown of liquid by heat and formation of methane, ethane and ethylene. Corona is a partial discharge and detected in a DGA by elevated hydrogen. Arcing is the most severe condition in a transformer and indicated by even low levels of acetylene.

8. SWITCH YARD It is a switching station which has the following credits:  Main link between generating plant and transmission system, which has a large influence on the security of the supply.  Step-up and/or Step-down the voltage levels depending upon the Network Node.  Switching ON/OFF Reactive Power Control devices, which has effect on Quality of power. 8.1. About RSTPS 400KV Switchyard: 400 KV Switchyard of Ramagundam Super Thermal Power Station is the most vital switching station in the southern Grid 2600 MW of Bulk Power generated by three 200 MW Units and four 500 MW Units of NTPC Ramagundam is evacuated for supplying to the southern states. Switchyard consists of two 400 KV bus bar systems fed by 7 Nos. Of generator feeders, 9 No’s of 400 KV feeders, 3 No’s of 220 KV feeders and two nos. Of 132 Kv feeders as shown in the single line diagram of 400 Kv switch yard. In addition to above four no. Of TIE Transformers, five no. Of Auto Transformers and two shunt reactors are provided in switchyard diagram.

400KV TRANSMISSION LINES: 1. Ramagundam 2. Ramagundam 3. Ramagundam

Nagarjunasagar Circuit -1 Nagarjunasagar Circuit -2 Hyderabad Circuit -1

4. 5. 6. 7. 8.

Ramagundam Ramagundam Ramagundam Ramagundam Ramagundam

Hyderabad Circuit Hyderabad Circuit Hyderabad Circuit Khammam Circuit Chandrapur Circuit

-2 -3 -4 -1 -1

9. Ramagundam

Chandrapur Circuit

-2

Double circuit lines (267Km) Independent (189 Km)

Circuit

lines

Single line (202 Km) HVDC back to back inter grid connecting double circuit lines (180 Km)

220 KV TRANSMISSION LINES: NTPC

AP TRANSCO Line-1

NTPC NTPC

AP TRANSCO Line-2 AP TRANSCO Line-3

Through 400/220 KV 250 MVA At #3 & 4 Through 400/220 KV 315 MVA At #5

132 KV TRANSMISSION LINES: NTPC

AP TRANSCO Line-1

NTPC

AP TRANSCO Line-2

Through 400KV/132 200MVA At #1

KV

AUTO TRANSFORMERS: Five Auto Transformers with no load Tap Changers are provided to interconnect the 400KV system of NTPC and 220/132KV system of AP TRANSCO, situated at 1.8km away from RSTPS switch yard. 400/132 KV 200 MVA (TELK make) 400/220 KV 250 MVA (TELK make) 400/220 KV 315 MVA (Crompton Greevs Ltd. Make) TIE TRANSFORMER:

1 2 2

Four nos. Of Tie Transformers are provided for feeding power to station auxiliaries like Cooling water & Raw water pumps, Coal Handling & water treatment Plants, Ash & Fuel Handling pumps, Cooling towers and lighting requirements of station & colony. SHUNT REACTORS Long lines when lightly loaded, the receiving end voltage raises, due toferranti effect. Shunt Reactors produce lagging MVAR there by control the receiving end voltages during lightly loaded conditions. Shunt reactors also limit the short circuit fault levels. Therefore, Shunt reactors are provided on both the ends of Nagarjuna Sagar lines 1 & 2, the length of these lines being about 267 km. 8.3. SWITCH YARD OPERATION ACTIVITIES: As mentioned elsewhere, RSTPS switchyard is handling bulk power and its operation and Maintenance has become critical. Any ambiguity in the operation of the switchyard may lead to such disasters like grid failure, station outages crippling not only the normal life of people but also the very economy of the country. Even in less serious situations such as cascade tripping of Auto Transformers due to unplanned over loading has caused under utilization of our generating capacity many times. The operation of switchyard calls for a very alert staff that shall have to sense the abnormalities in time and prompt to concern timely to enable normalcy of the system. The following are some of the identified activities of 400 KV switchyard operations. 1.Identifying of faulty equipment, safe isolation of equipment without disturbing other system as much as possible, raising job cards, arranging shutdowns, trial charging and normalization of 400 KV SWYD. And 132KVSwyd, associated equipment like CBs, Isolators, Ats, TTS, Shunt Reactors, ACDBs, DCDBs, Battery Plant, Charges PLCC equipment, Swyd. Compressors and lighting. 2. Daily inspection of indoor/outdoor swyd equipment, checking of oil leakages, temperatures and any other abnormalities like sparks etc. SF6 gas pressures, compressed air pressures, running period of compressors, availability status of emulsifier system, swyd. And station P.A. system and PLCC communication system etc. monitoring of physical conditions of swyd equipment. 3. Analyzing and locating of fault leading to feeder/Transformer trip, reporting emergencies to the higher authorities, coordinating with other agencies like AP Transco/ Genco, PGCIL in clearing faults and normalization of system. 4. Close monitoring of grid parameters, coordinating with IOCC, SRLDC, OS (SR), OS (ED), LDC (APSEB), and Shift charge Engineer & Desk Engineers for smooth operation of grid system, timely action to ensure continuity of power supply. 5. Quick arrangement of startup power supply in case of grid failures, station outages.

6. Continuous monitoring of system parameters like voltage, frequency, line and Transformer, loading unit generations, MVAR and MW net exported. recording and corrective action where the abnormality found. 7. Preparing of daily power generation / export/import energy reports, exchanging data with IOCC, OS (ED), OS (SR), collection of generation details from other power projects and storing. 8. Assisting the shift in-charge in transmitting the flash report, availability report, unit trip/synchronization messages, shutdown messages, generation back down messages, modification of availability declarations, feed back to shift in charge, the deviation if any in total generation with respect to the declaration. 8.4. SWITCHYARD EQUIPMENT To perform switchyard operation activities perfectly, operation staff should have good knowledge about the equipment provided in switchyard as well as in control room. They should be familiar with the control system adopted here and a good understanding about the procedures to be followed during the emergencies, outage requirements and charging. Brief description about switchyard equipment is given below. CIRCUIT BREAKER It is an automatic device capable of making and breaking Electrical Circuitunder normal and abnormal conditions such as short circuits. SF6 is the arc quenching media for all the 400 KV and 220 KV breakers installed in the switchyard. Pneumatic operating system is provided in AEG, ABB and NGEF make breakers and Hydraulic operating system is provided in BHEL make breakers. 132KV breakers provided in 132 KV lines are of Minimum oil type operating on spring charge mechanism. ISOLATORS Isolator is an off load device provided in conjunction with circuit breaker to disconnect the equipment or the section, which is to be isolated from all other live parts. The isolators provided in the switchyard are of central break type. The operation of Isolators can be done from control room (remote) or local. Motorized operation for opening & closing of Isolator is provided, however Isolators can also be opened & closed manually in the event of nonavailability of motorized operation. EARTH SWITCH Earth switch is mounted on the isolator base on the line side or breaker side depending upon the position of the isolator. The earth switch usually comprises of a vertical break switch arm with the contact, which engages with the isolator contact on the line side. Earth switch is required to discharge the trapped charges on the line or equipment (under shut down) to earth for maintaining safety. Earth switch can be operated only from local either by electrical operation or manually.

BUSBAR Bus bar is an Aluminium tube of 4” IPS having wall thickness of 0.4”, where all incoming and outgoing feeders are connected in a schematic way to enable smooth operation and Maintenance of equipment without any interruption to the system. At RSTPS one and half breaker scheme is provided for 200 MW generator feeders and 400 KV outgoing lines, Twobreaker scheme is provided for 500 MW generator feeders SURGE / LIGHTING ARRESTERS Surge Arresters are provided to ground the over voltage surges caused by switching and lighting surges. Surge Arresters provide leakage path to the ground whenever the system voltage rises above the specified value. They are equipped with surge monitors, which measure the leakage currents and a counter to record the number of surges taken place.

CURRENT TRANSFORMER (CT) Current Transformers are provided to step down the current to low values suitable for measuring protection and control instruments. Current Transformers also isolate measuring and protective devices from high system Voltage. CTs in the switchyard consist of five secondary cores. Core 1&2 are used for bus bar protection, 4 & 5 are for main 1&2 protection and core 3 is for measuring instruments. CAPACITIVE VOLTAGE TRANSFORMER (CVT) CVTs step-down the system voltage to sufficiently low value (110 V) for measuring, protection and synchronizing circuits. CVT has a H.F. terminal point for receiving & transmitting the high frequency signals for carrier protection and communication. WAVE TRAP Wave Trap is a parallel resonant circuit tuned to the carrier frequency connected in series with the line conductor at each end of the protected transmission line section. Wave trap offers high impedance path for high frequency signals and low impedance path for power frequency current. This keeps carrier signal confined to the protected line section and does not allow the carrier signals to flow into the neighboring sections. 8.5. SWITCH YARD CONTROL ROOM EQUIPMENT The control room is the place where the conditions of the system are monitored, controls initiated and operations are integrated. Control room consists of the following equipment. CONTROL PANELS Corridor type flat control panels are provided in U shape with doors at both the end panels. Between the front and rear panels, there is adequate space for inspection of interior wiring. The controlling knobs are provided on front panel for opening & closing of breakers and isolators. The close/open position of the breakers / isolators / earth switches is indicated through lamps or semaphore indicators. The relative position of each equipment is shown in

the mimic single line diagram that is painted on front side of the control panels. The indicating instruments (MW, MVAR, voltage, current etc.) and annunciation windows are provided on the top of front panel for monitoring of the equipment. Breaker monitoring and protective relays such as LBB, Auto enclosure, check synchronization, Trip circuit monitoring, Annunciation relays and energy meters are mounted on the rear side of the panel. RELAY PANELS Relay panels are of cubicle type, flat independent boxes with a door at backside. All the protective relay units related to one bay are divided into two groups viz. Main 1 protection, stub protection, O/V protection and their auxiliary & trip relays as group 1 and Main 2 protection, U/v protection and their auxiliary & trip relays as group 2 relays. Group 1 & group 2 relays are mounted on front side of two separate panels side by side. Fault locator and disturbance recorder of the corresponding bay mounted on front side of the third panel. A separate glass door is provided front side of all the panels to cover the relays from dust. EVENT LOGGER Even Logger recognizes the changes in signal-input states, plus time data allocation for sequential recording of events. It displays the events in a time sequential of 1/ sec, such as opening/closing of breaker poles, Isolator poles, E/S etc. pressure high/low of air, SF6, N2 Oil etc. Alarm Appeared/reset of all protection / trip relays, it also displays the status of equipment, in service/ out of service in a regular period say 8 hrs. This is one of the important diagnostic equipment available to operation staff to understand the type of emergency in a flick of a second. MASTER CLOCK One maser clock (make Keltron) is provide in switchyard control room to synchronize the timings of all the Event loggers, DAS (Data Acquisition system of units), Disturbance Recorders, clocks provided in control rooms, etc., to maintain a uniform time, so that the sequence of events can be recorded and analyzed to know the cause of disturbance. GENERATION DATA ACQUISITION AND MONITORING SYSTEM (GDAMS) At large switchyard control rooms like RSTPS it is essential to record and continuously monitor the parameters of the Generation & transmission system. NTPC is the largest power utility of the country generating power from 20 thermal/ gas power stations at various places of the country. Many more power stations are yet to come. To manage all these power stations efficiently and effectively NTPC has established an operational services control room at corporate office in New Delhi, where generation data from all the stations is to be monitored continuously. To facilitate the above function, Generation Data acquisition and Monitoring system is provide at all NTPC switchyard control rooms. CMC Ltd. has supplied the necessary software on Micro Soft Windows NT environment and installed the PC based GDAMS network in Server Client configuration. GDAMS scans automatically the real time measurements like load on units, load flows on feeders. Bus voltage, grid frequency, MVR loads, etc. for every second through RTUs and record it. The Acquired data

is linked up to OS control room though satellite communication channel. The types of data displays available in GDAMS are given below. TYPE OF DISPLAYS Alpha Numeric Display Mimic Diagram Display

Graphical display Threshold display Alarm display Trend display

Displays direct of measured parameter along with name of parameter in tabular form In this Display the single line diagram of the circuit with position of the breakers along with real time power flow is indicated. This displays the graph of quantities In Threshold blackout display the threshold values of quantity are displayed Alarms are displayed to draw the attention of operator In this display the trend of the quantity real values in a specified time blocks are shown.

The data Acquisition by GDAMS is more vital in analyzing the faults, forecasting the local trends, impact of the line and unit outages, estimation of variations in frequency and voltages in different seasons, generating reports. DISTURBANCE RECORDER All 400 KV lines connected to this switchyard are provide by the Disturbance Recorders (D/R), D/R is a PC based or Microprocessor based on line monitoring equipment D/R is the most vital diagnostic equipment in analysis of post fault trappings. FAULT LOCATOR When a line tripped on fault, the Fault Locator provided in the Relay panel indicates the approximate distance of the fault location so that Maintenance group easily tract the fault and clear it. When F.L. indicates zero or very less distance, operation staff should assume that the fault is in the switchyard equipment, and check for all equipment connected to the concerned bay, which was tripped on fault. INDICATING & RECORDING INSTRUMENT The following measuring instruments were providing on control panels of all bays. a) At the top of the control panel. 1. Ammeters in three phases. 2. Volt (KV) meters in three phase 3. Reactive power (MVAR) meter 4. Watt (MW) meter 5. Winding Temperature indicating meter (for only Transformer bays) 6. Tap position indicating meter (do) b) Rear side of the each bay control panel 1. Main energy meter (export) 2. Check Energy meter (do)

3. Main energy meter (import) 4. Check Energy meter (do) PLCC (power line carrier communication) In order to achieve fast clearance and correct discrimination for faults in 400 KV transmission network, it is necessary to signal between the points at which protection relays are connected. PLCC is high frequency signal transmission along with actual overhead power line. IT is robust and therefore reliable, constituting a low loss transmission path that is fully controlled by the power authority. PLCC is required for the following cases. Inter tripping In inter trip (direct or indirect trip) applications; if the command is unmonitored by a protective relay at the receiving end, reception of the command causes circuit breaker operation. Permissive tripping Permissive trip commands are always monitored by a protection relay. The circuit breaker can be operated only when reception of the command coincides with operation of protective relay responding to a system fault. Blocking Blocking commands are always monitored by a protection relay. The circuit breaker can be operated only if the command is absent when the protection relay is operated by a fault. Telemetry Telemetry refers to science of measurement from remote location. The various measurements obtained from transducers converts into signals and these signals transmit to remote control rooms through PLCC ex. All lines and generators of RSTPS parameters like MW, MVAR, etc linked up to IOCC through PLCC. Telephone PLCC can be used as a speech channel. All substations connected to RSTPS are providing by one direct telephone (hot line) for speedy communication. Communication is also available for all PGCIL S/S through PLCC telephone exchanges. 220 V DCDB & BATTERY PLANT 220 VDC supply is required mainly for the following applications. a) Control supply for 400, 220, 132, 33 KV breakers, and associated equipment. b) Control supply for relaying and protection circuits. c) Annunciation & indication circuits. d) Emergency lighting.