Sub-station

We know that the electrical energy is generated in different types of generating stations located at distant places from the consumer’s premises. For example,nuclear and coal fired based thermal power generating stations are commissioned far away from the populated area from safety point of view. So, to transfer such a huge quantum of electrical energy from generating stations to the ultimate consumer,number of transformers and other switching equipments are required to step up or step down the voltage. Any station comprising of transformers and switching equipment is known as sub-station. In general the sub-station are of following three types: -

i. Pole Mounted Type Sub-station

As the name indicates, this type of sub-stations is mounted on Reinforced Cement Concrete (R.C.C.) pole structure consisting of H-type (Double Pole) or square- type (Four Pole). A platform is prepared at suitable height by rolled steel joist, for placing the 11/0.440 kilovolts distribution transformer

A gang-operated switch (G.O. switch) is used to energise the transformer. These types of sub-station are commissioned on roadsides and hence do not require any building structure and sub-station attendant for its operation. So, these are cheapest and most economical as compared to other types of sub-station. These sub-stations are most widely used for rural electrification and small colonies or medium consumers in urban areas. The input voltage to this type of sub-station is 11 kilovolts and output is 440 volts between any two-phase wires and 230 volts between any phase and neutral wires.

ii. Outdoor Type Sub-station

These types of substations are commissioned away from the populated areas from safety point of view and is used to control the 11kv to 132 kv supply. Transformers,circuit breakers and other accessories are commissioned out side the building but are controlled from a building through wires by using remote instrumentation

We have two 132 kv incoming feeders from two different sub- stations or generating stations. Both the incoming feeders terminate on separate 11 kv bus bars through isolators, main oil circuit breakers (MOCB) current transformer(CT), potential transformer (PT) and 132/11 kv step down transformers, etc. All these equipment are commissioned in a closed boundary. The current transformer and potential transformers are used to monitor the line potential and current and hence the units supplied to the sub-station. We can shift the entire load on either feeder by selecting the proper bus bar and bus-coupler. Both the source can also be used independently. This bus coupler is used to couple both the bus bars to use single incoming feeder. Lightning arrester (LA) is used to discharge the excessively induced voltage in the line conductors to ground, induced due to the lightning effect.

Earth switch is used to connect all the conductors to ground, to avoid accidents when the line is under permit to work (PTW). The output of this sub-station is connected to 11/0.440 kv distribution transformer. Sub-station attendants are required for the operation of the outdoor type sub-stations.

iii. Indoor Type Sub-station

In this type of sub-station, the 11/0.440 kv distribution transformer, main oil circuit breaker (MOCB), L.T. air circuit breaker (ACB) and distribution panels are commissioned in a building whereas the isolators and gang operated switches are commissioned outside the building. Transformer may be located outside the building in shed duly covered boundaries. The circuit diagram of the indoor type sub-station

We have two incoming feeders from different outdoor sub- stations. Gang operated. switches are used to select the incoming source of supply.Isolators are used to physically isolate the transformer from incoming or outgoing feeders/circuits. Main oil circuit breaker is used to switch on or off the 11/0.440 kv distribution transformer whereas the air circuit breaker is used to operate the low-tension distribution panel. We can operate both the circuit breakers under fully loaded conditions whereas the isolators should be operated when the load is switched off. Oil circuit breaker 1 and 2 are used to energise the two different distribution transformers. Lightning arrester (LA) is used to arrest and discharge excessively induced voltage in the line due to lightning effect.