Departmental of Electrical Engineering

Transmission and Distribution of Electrical Power (TDE)

lecture series on

Chapter No. 8

SUB-STATIONS

Prepared by- J R Goyanka.

Contents

IntroductionSub-Station and points to be consider while

laying out SubstationClassification of Sub-StationsSelection of site for sub-stationSymbols used for equipments in sub-stationEquipments used in sub-stationLayout of sub-station

Introduction to Sub-Station

Sub-Station is a place where some of the characteristics of Electric Power is changed, such as Electric supply voltage is changed Conversion from DC to AC and DC to AC Change in frequency takes place

Points to be consider while laying out Sub-Station

It should located at the center of gravity of load.

It should provide safe and reliable arrangement. (Safety- facilities and clearance for repairs and maintenance for abnormal occurrences’ such as possibilities of explosion or fire.)(Reliability- for good design and provision of suitable protective gear, etc.

It should be easily operated and maintainedIt should involve minimum capital cost.

Classification of Sub-Stations

According to service requirementi. Transformer sub-

stationsii. Switching sub-

stationsiii.Power factor

correction sub-stations

iv. Frequency changer sub-stations

v. Converting sub-stations

vi.Industrial sub-stations

According to constructional featuresi. Indoor sub-stationsii. Outdoor sub-

stationsiii.Underground sub-

stationsiv. Pole mounted sub-

stations

Classification of Sub-StationAccording to service requirement

Transformer sub-stations Those sub-stations which change the voltage level of

electric supply.Switching sub-stations

They simply perform the switching operations of power lines.

Power factor correction sub-stations Those sub-stations which improves the power factor of

the system. Such sub-stations are located at receiving end of TL. Synchronous condensers are used as power factor

improvement device.

Classification of Sub-Station According to service requirement contd…

Frequency changer sub-stations Those sub-stations which change the supply

frequency. Frequency change may be required for industrial

utilization or export of electric power.Converting sub-stations

Which change ac power into dc power with ignitron to supply for special purpose such as traction, electroplating, electric welding, etc.

Industrial sub-stations Which supply power to individual industrial concerns.

Classification of Sub-Station According to constructional features

Indoor sub-stations For voltage up to 11kV (and in some critical case up to

66kV), the equipment of the sub-station is installed indoor.

Outdoor sub-stations For voltage beyond 66kV, equipments required

clearance, such as between conductor and space required for switches, circuit breakers and other equipments

Classification of Sub-Station According to constructional features contd…

Underground sub-stations In thickly populated area; space available is limited

and the cost of land is highPole mounted sub-stations

This is an outdoor sub-station with equipment installed overhead on H- pole or 4-pole structure.

It is the cheapest form of sub-station for voltage not exceeding 11kV (or 33kV in some cases).

Electric power is almost distributed in localities through such sub-station.

Underground Sub-station

Pole-Mounted Sub-station

PARTICULAR INDOOR SUB-STATION

OUTDOOR SUB-STATION

SPACE REQUIRED FOR SUB-STATION

Less More

TIME REQUIRED FOR ERECTION

More because additional building constructional is required

Less

FUTURE EXTENSION Difficult due to less space available

Easy, space availability is large

FAULT LOCATION Difficult because all the equipments are enclosed

Easy because all the equipments are in full view

CAPITAL COST More due to building construction

Less, as no specially designed required

MAINTENANCE Difficult as space between equipments is less

Easy

PROTECTION AGAINST NATURAL CALAMITIES

More Less

Selection of site for sub-station

1. Availability of land2. Near to load centre3. Atmospheric pollution

Symbol used for Equipments in Sub-station

Symbol used for Equipments in Sub-station

Equipments used in Sub-station

1) Bus-bars2) Insulators3) Isolating Switches4) Circuit Breakers5) Power Transformer6) Instrument Transformers (CT & PT)7) Metering and Indicating Instruments8) Earthing Switch9) Lightening Arrestor10) Miscellaneous Items

1. Bus bar

When number of generators or feeders operating at the same voltage have to be directly connected electrically, bus-bars are used as a common electrical component.

Bus-bars (are common conductor) are copper rods or thin walled tubes and operate at constant voltage

Hence any fault taking place on the bus bar will result into complete black out.

The most commonly used bus-bar arrangement Single bus-bar arrangement Single bus-bar system with sectionalisation Double bus-bar arrangement

1.a Single bus-bar arrangement

1.b Single bus-bar system with sectionalisation

1.c Double bus-bar arrangement

1.d Ring bus bar system

Types of Bus Bar

Two types Strain type

similar to overhead transmission line ACSR conductors or aluminium conductors are supported by

strain type insulators The tension in conductor is limited up to 900kg.

Rigid type This bus bars are hollow aluminium pipes general supported

by pedestal insulators The main advantage of rigid type of bus bar is the clearance

between the conductors remain same, also the maintenance is easy as the connections are not very high from the ground.

Due to its large diameter, the corona effect is less Limitations – high cost and require large space

Voltage

Size for Strain type

Size for Rigid type

66kV 37/2.79 mm 40 mm

132kV 37/4.27 mm 65 mm

220kV 61/ 3.99 mm

80 mm

400kV 61/4.27 mm 100 mm

2. Insulator

Purpose of insulator They support the conductors(or bus-bars) Confine the current to the conductors

Material used for insulator Porcelain, glass

Types of insulators Pin, suspension, strain, shackle, post etc.

Post insulator is used for bus-bars. A post insulator consists of porcelain body, cast

iron cap and flanged cast iron base.The hole in the cap is threaded so that the bus-bars

are directly bolted to the cap.

3. Isolating switches (Isolator)

It is used to disconnect a part of the system for general maintenance and repairs.

Its an knife switch , designed to open a circuit under no load (no current) (only).

The entire sub-station had been divided into V-section

4. Circuit Breaker

Is used to open or close the circuit under normal as well as fault conditions.

It is so designed to operate manually (or by remote control ) under normal conditions and automatically under fault condition, for which relay circuit is used.

Voltage Up to 66kV – bulk oil circuit breaker Above 66kV – low oil circuit breaker For still higher voltage level, SF6, vacuum or air blast

circuit breaker is used

5. Power Transformer

Used to step-up or step-down the voltageNow a days 3 phase transformer is used, although

3 single phase bank of transformer could also be used to

Advantages of using 3 phase transformer Only one 3-phase load-tap changing mechanism can be used Installation is much simpler than the three single phase

transformerFor ratings up to 10 MVA, naturally cooled, oil

immersed transformer is usedFor higher ratings, air blast cooled transformer is

used

6. Instrument Transformer (IT)

The lines in sub-station operates at high voltage and carry current thousands of amperes. While measuring instruments and protective devices are designed for low voltages(generally 110V) and current (about 5A).

Therefore they will not work satisfactorily if mounted directly on the power lines. Thus instrument transformer is used.

The function of these instrument transformer is to transfer voltage and currents in the power lines to the value which are convenient for the operation of measuring instruments and relays.

Types of IT’s 6.a Current Transformer (CT)6.b Potential Transformer (PT)

6.a Current Transformer (CT)

CT is essentially a step-up transformer which steps down the current to a known ratio.

Primary of this transformer consists of one or more turns of thick wire connected in series with the line.

The secondary consist of large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of the current in the line.

Example- a CT rated at 100/5A is connected in the line to measure current. If the current in the line is 100 A, then current in the secondary will be 5A.

6.b Potential/ Voltage Transformer (PT)

It is essentially a step-down transformer and steps down the voltage to a known ratio.

The primary of transformer consists of large number of turns of fine wire connected across the line.

The secondary winding consists of a few turns and provides for measuring instruments and relays a voltage which is known fraction of the line voltage.

Example- suppose PT rated at 66kV/110V is connected to a power line. If line voltage is 66kV, then voltage across the secondary will be 110V.

Instrument Transformer (IT) contd…

7. Metering and Indicating Instruments

There are several metering and indicating instruments (ex. Ammeter, voltmeter, energy meters etc.) installed in sub-station to maintain watch over the circuit quantities.

The instrument transformers are invariably used with them for satisfactory operation.

8. Earthing Switch

One terminal of earthing switch is connected to the line and other is connected to the ground.

The main purpose of the switch is to discharge the static charges of the line when line is open circuited.

9. Lightening Arrestors

Lightening arrestors provides protection to sub-station equipment against the lightening stroke.

Sometimes it is also called as a surge diverter.

There are different types of lightening arrestors such as rod gap, horn gap, valve type and expulsion type lightening arrestor.

10. Miscellaneous Items

In addition to above there are certain miscellaneous items such as fuses, power line carrier communication equipment, battery system, shunt reactors, capacitor bank etc.

Layout of Sub-station

Generally there are two bus-bars, main bus bar and auxiliary bus bar.

Each bus bar act separately and capable of handling entire load.

The load is connected to the main bus bar, and when there is fault or maintenance carried on main bus bar then the load is transferred to auxiliary bus bar.

The incoming and outgoing lines can be connected to entire bus bar with the help of bus bar coupler which consists of circuit breaker and isolators.

Layout of Sub-station

Key diagram of 66/11kV sub-station

Key diagram of 11kV/400V Indoor Substation

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