Substation

Sub-Station

Introduction The present day electrical power system is a.c. i.e. electric power is generated, transmitted and distributed in the form of Alternating current.

The

electric power is produce at the power station, which are located at favorable places, generally quite away from the consumers. It is delivered to the consumer through a large network of transmission and distribution. At many place in the line of power system, it may be desirable and necessary to change some characteristic ( e.g. Voltage, ac to de, frequency p.f. etc.) of electric supply. This is accomplished by suitable apparatus called sub-station for example, generation voltage (11kv or 6.6kv) at the power station is stepped up to high voltage (Say 220kv to 132kv) for transmission of electric power. Similarly near the consumer’s localities, the voltage may have to be stepped down to utilization level. This job is again accomplished by suitable apparatus called sub-station.

Definition of sub-station :“The assembly of apparatus used to change some characteristics (e.g. Voltage al to de freq. p.f. etc) of electric supply is called sub-station”

Classification of sub-station Govt. Polytechnic, Washim

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Sub-Station There are several ways of classifying sub-station. However the two most important way of classifying them are.

I) According to service requirement :According to service requirement sub-station may be classified into. 1) Transformer sub-station :Those sub-station which change the voltage level of electrical supply are called TIF s/s. 2) Switching sub-station :These sub-station simply perform the switching operation of power line. 3) Power factor correction S/S :These sub-station which improve the p.f. of the system are called p.f. correction s/s. these are generally located at receiving end s/s. 4) Frequency changer S/S :Those sub-stations, which change the supply frequency, are known as frequency changer s/s. Such s/s may be required for industrial utilization. 5) Converting sub-station :Those sub-station which change a.c. power into d.c. power are called converting s/s ignition is used to convert AC to dc power for traction, electroplating, electrical welding etc.

6) Industrial sub-station :-

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Sub-Station Those sub-stations, which supply power to individual industrial concerns, are known as industrial sub-station.

II) According to constructional features :According to constructional features, the sub-station are classified as : 1) Outdoor Sub-Station :For voltage beyond 66KV, equipment is invariably installed outdoor. It is because for such Voltage the clearances between conductor and the space required for switches, C.B. and other equipment becomes so great that it is not economical to installed the equipment indoor. 2) Indoor Sub-station :For voltage upto 11KV, the equipment of the s/s is installed indoor because of economic consideration.

However, when the atmosphere is

contaminated with impurities, these sub-stations can be erected for voltage upto 66KV. 3) Under ground sub-station :In thickly populated areas, the space available for equipment and building is limited and the cost of the land is high. Under such situations, the substation is created underground. Fig. Shows a typical underground sub-station in which transformer, switch gear & other equipments are installed.

The design of underground s/s requires more careful consideration.

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Sub-Station 1) The size of the s/s should be as minimum as possible. 2) There should be resonable access for both equipment & personal. 3) There should be provision for emergency lighting and protection against fire. 4) There should be good ventilation.

4) Pole-mounted sub-station :This is an outdoor sub-station with equipment installed overhead on H.pole or 4-pole structure. It is the cheapest from of s/s for voltage not exceeding 11KV(or 33KV in some cases). Electric power is almost distributed in localities through such sub-station. For complete is given bellow. Fig. Shows the typical 4-pole sub-station, it is a distribution substation placed overhead on a pole. Fig No.2 shows the schematic connections, the transformer & other equipment are mounted on H-type pole. The 11KV line is connected to the T/F (11KV/440V) through gang isolator and fuses. The lighting arresters are installed on the H.T. Side to protect the sub-station from lighting strokes.

The T/F step down voltage to 400 V, 3

phase, 4 wire supply. The voltage between any two lines is 400 V & between line & neutral is 230V.

The oil ckt breaker installed on the L.T. side automatically

isolates the mounted sub-station. T/F are generally in the event of fault generally 200KVA T/F is used.

Comparison between outdoor & indoor s/s Govt. Polytechnic, Washim

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Sub-Station

Sr.No.

Particulars

Out door Sub-station

1. 2. 3. 4.

Space required Time required for erection. Future extension Foult location

More Less Easy Easier

because

equipments 5. 6. 7.

Capital cost. Operation Possibility

of

escalation

view. Low Difficult fault Less clearance

is

in

Indoor Sub-station

Less More Difficult the Difficult full the

because

equipment

is

enclosed. High Easier because More. can

be

provided

Equipment in a Transformer Sub-Station The equipment required for a transformer Sub-Station depends upon the type of Sub-Station, Service requirement and the degree of protection desired. TIF Sub-Station has the following major equipments.

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Sub-Station 1) Bus - bar :When a no. of lines operating at the same voltage have to be directly connected electrically, bus-bar are used, it is made up of copper or aluminum bars (generally of rectangular X-Section) and operate at constant voltage. Fig. Shows the arrangement of Duplicate bus-bar, generally it consist of two bus-bars a “main” bus-bar and spare bus-bar. The incoming and outgoing lines can be connected to either b/b. With the help of a bus-bar coupler, which consist of a ckt breaker and isolators. However, in case of repair of main bus-bar or fault accusing on it, the continuity of supply to the circuit can be maintain by transforming it to the spare bus-bar for voltage exceeding 33KV, Duplicate bus-bar is frequently used.

2) Insulators :The insulator serves two purpose. They support the conductor ( or bus bar ) and confine the current to the conductor. The most commonly used material for the manufactures of insulators is porcelain. There are several type of insulator (i.e. pine type, suspension type etc.) and there used in Sub-Station will depend upon the service requirement.

3) Isolating Switches :In Sub-Station, it is often desired to disconnect a part of the system for general maintenance and repairs. This is accomplished by an isolating switch

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Sub-Station or isolator. An isolator is essentially a kniff Switch and is design to often open a ckt under no load, in other words, isolator Switches are operate only when the line is which they are connected carry no load. For example, consider that the isolator are connected on both side of a cut breaker, if the isolators are to be opened, the C.B. must be opened first.

4) Circuit breaker :A circuit breaker is an equipment, which can open or close a ckt under normal as well as fault condition. It is so designed that it can be operated manually ( or by remote control) under normal conditions and automatically under fault condition. For the latter operation a relay wt. is used with a C.B. generally bulk oil C.B. are used for voltage upto 66 KV while for high voltage low oil & SF6 C.B. are used. For still higher voltage, air blast vaccum or SF6 cut breaker are used.

5) Protective relay :“A protective relay is a device that detects the fault and initiates the operation of the C.B. to isolate the defective element from the rest of the system”. The relay detects the abnormal condition in the electrical ckt by constantly measuring the electrical quantities, which are different under normal and fault condition. The electrical quantities which may change under fault condition are

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Sub-Station voltage, current, frequency and phase angle. Having detect the fault, the relay operate to close the trip ckt of C.B.

6) Instrument Transformer :The line in Sub-Station operate at high voltage and carry current of thousands of amperes.

The measuring instrument and protective devices are

designed for low voltage (generally 110V) and current (about 5A). Therefore, they will not work satisfactory if mounted directly on the power lines. This difficulty is overcome by installing Instrument transformer, on the power lines. There are two types of instrument transformer. i)

Current Transformer :A current transformer is essentially a step-down transformer which

steps-down the current in a known ratio, the primary of this transformer consist of one or more turn of thick wire connected in series with the line, the secondary consist of thick wire connected in series with line having large number of turn of fine wire and provides for measuring instrument, and relay a current which is a constant faction of the current in the line. ii)

Voltage Transformer :It is essentially a step - down transformer and step down the voltage

in known ratio. The primary of these transformer consist of a large number of turn of fine wire connected across the line. The secondary way consist of a few turns and provides for measuring instruments and relay a voltage which is known fraction of the line voltage.

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Sub-Station 7) Metering and Indicating Instrument :There

are

several

metering

and

indicating

Instrument

(e.g.

Ammeters, Volt-meters, energy meter etc.) installed in a Sub-Station to maintain which over the ckt quantities. The instrument transformer are invariably used with them for satisfactory operation.

8) Miscellaneous equipment :In addition to above, there may be following equipment in a SubStation. i)

Fuses.

ii)

Carrier-current equipment.

iii)

Sub-Station auxiliary supplies.

Conclusion In this way I concluded that my seminar report on Sub-Station also we understand what is mean by Sub-Station, classification of Sub-Station, comparison between outdoor & indoor Sub-Station & various components & instrument used in Sub-Station.

Reference :Govt. Polytechnic, Washim

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Sub-Station V. K. MEHATA

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Sub-Station

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Sub-Station

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Sub-Station

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