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SUBSUB -STATIONS

The assembly of apparatus used to change some characteristic(e.g. Voltage, a.c to d.c, frequency, power factor etc) of electric supply is called a sub-station 

The assembly of apparatus used to change some characteristic(e.g. Voltage, a.c to d.c, frequency, power factor etc) of electric supply is called a sub-station 

 It should be located at the centre of gravity of load •

 It should be easily operated and maintained

•

 It should provide safe and reliable arrangement •

•

It should involve minimum capital cost

Sub-stations

Service requirement

Constructional features

Substations are classified as……..  Transformer sub stations : these are used to change the voltage levels of supply switching sub stations: stations: they simply perfome switching operations of power lines 

 Power

factor sub stations: stations : they improve the power factor of the system by using synchronous condensers  Frequency

sub stations: stations: those sub stations which change supply

frequency Converting sub stations: stations: those sub stations which change a.c. power to d.c. power 

Industrial sub stations: stations: those sub stations which supply power to industries 

Transformer substations are classified into 4 types these are…….. step up substation: the generation voltage is stepped up to high voltage 

Primary grid substation: here the voltage level is step down to 66kv from 220kv for secondary transmission 

Secondary substation: the voltage level is further step down to 11kv and these substations are generally outdoor type 

Distribution substation: the electric power from 11kv lines is delivered to distribution substation. These are generally located near consumer localities… 

According to constructional features, the sub stations are classified as:  Indoor

sub station: these substations can be installed for voltages up to

11kv Outdoor sub station: for voltages beyond 66kv these type of sub stations can be installed 

Underground sub stations: in thickly populated areas, the space available is less so we install these type of substations 

 Pole

mounted substations: this is an outdoor substation with equipment installed overhead on h-pole or 4 pole structure

It is a schematic view of pole mounted substation……

While designing a underground sub station the following points are to be followed:  the

size of substation should be minimum

 There

should be reasonable access for both equipment

 There

should be provision for emergency lights and protection against fire  There  the

should be good ventilation

transformers,switches,fuses should be air cooled to avoid bringing oil into premises

.

Electric substations are the part of power systems and used for transferring power from generating points to load centres.some of important components are… BUS BARS: various incoming and out going circuits are connected to bus bars.busbars receive power from incoming circuits and deliver power to out going circuits. 



INSULATORS: insulators serve two purposes

1) they support conductors(busbars) 2) confine the current to the conductors  most commonly used material for manufacture of insulators is porcelain.

ISOLATING SWITCHES: in sub stations it is often desired to disconnect a part of system for general maintenance and repairs. this is accomplished by an isolating switch or isolators. 

Circuit breaker: circuit breaker is used for switching during normal and abnormal conditions. it is used to interrupt short circuit currents .there are many types of circuit breakers.. 1) air blast circuit breaker. 2) oil circuit breaker 3) sf6 circuit breaker 

 power

transformers: these are used to step down or step up a.c. voltages and to transfer electrical power from one voltage level to another.

 Current

transformers: current transformer in essentially a step up transformer which steps down the current to a known ratio.

 voltage

transformer: it is essentially a step down transformer and steps down the voltage to a known ratio.

metering and indicating instruments: there are several metering and indicating instruments installed in a sub station to maintain watch over the circuit quantities. 

 in

addition to these there may be following equipment in a substation. 1) fuses 2) carrier current equipment 3) sub station auxiliary supplies

Single bus bar system…

GAS INSULATED SUBSTATION

Gas Insulated substations use sulphur hexa fluoride(SF6) gas which has superior dielectric properties. Gas insulated substations are used where there is space for providing the substation is expensive in large cities and towns

TYPES OF GAS INSULATED SUBSTATIONS Gas insulated substations are classified according to the type of configurations as………..

 isolated  3-phase  hybrid

common modules

modules

Compact Highly

phase module

modules

integrated systems

The individual circuit elements such as a pole of a circuit breaker, a single pole isolator, one phase assembly of a current transformer etc..are connected together forms an isolated phase GIS module.

In this type of module a three phase bay is assembled using the desired number of three phase elements. This reduces the total number of enclosures to one third.

It is the combination of isolated phase and three phase common elements is used to achieve an optimal solution. Hybrid GIS technology has gained popularity, specially in the medium and low voltage range

Compact GIS systems are essentially three phase common systems in which the elements such as three phase circuit breaker, current transformer etc…. Are placed

These are used in outdoor substations .highly integrated substations are ready to install substations.

The high voltage conductor made up of copper or aluminum is centrally placed in a tabular metal enclosure. In isolated phase GIS ,coaxial bus bar are common

The main parts of GIS are……. gas circuit  seals and gaskets  disconnectors  earth switch Circuit breaker Current transformer Plug in joints Enclosure Expansion joints 

Gas circuit : gas circuit has to maintain the dual purpose (i.e. arc interruption and insulation). Gas circuit should maintain the pressure throughout its life period so there should be no leakages SEALS AND GASKETS : seals and gaskets are used to capture the SF6 gas lekages.these are manufactured with nitrile rubber and viton. These serves sealing purpose

Disconnectors

or isolators are used for electrical isolation of circuit parts. They are slow acting and operating at off load. Disconnectors must be carefully designed and tested to be able to break small charging current without generating too higher over voltage.

Circuit breaker is a metal clad and uses SF6 gas for the purpose of insulation and fault interruption. The circuit breaker is directly connected to either current transformers or isolators

fast

earth switch: these are used to protect the circuit connected current transformer, voltage transformer Maintenance

earth switch: these are used for grounding the high voltage conductors during maintenance

PLUG IN JOINTS: these are designed in such a way that they can carry rated full load current uninterruptedly and can carry heavy transient currents types of plug in  joints are Static joints Quasi static joints

1

Bay width (m)

1.5

2.0

2.0

2

Bay depth

3.3

3.35

3.4

3

Bay height

3.2

3.4

3.4

4

Floor area (sq.m)

4.95

6.7

6.8

5

Volume

15.84

22.78

23.12

6

Weight

3800

5000

5700

GIS

have no risk for fire and explosion due to leakage of oil. They generate no noise and have no radio interference. The maintainence of GIS is free. It offers solutions, -Industrial areas where space and pollution problems . -Mountain areas where ice and snow are major problems.

Gas

insulations tend to be much more expensive than air insulated installations with the same ratings. Care should be taken that no dust particles enter into the live compartments which results in flash over. When fault occurs internally the diagnosis of the fault and rectifying takes very long time.