Key Diagram of 11 kV / 400 V Indoor Substation

Key Diagram of 11 kV / 400 V Indoor Substation

Fig. 8.8.1 shows the key diagram of 11 kV/400 V indoor substation. The explanation is as given below.

The 3 phase, 3 wire 11 kV is tapped at the substation and connected to gang operated switch which contains isolators in each of its phase.

From the gang operated switch, with the help of underground cable the line is taken to the indoor substation and connected to the High Tension (H.T.) side of the transformer through circuit breaker. The transformer steps down the voltage from 11 kV to 400 V and secondary uses 3 phase, 4 wire system for distribution.

The CTs are placed at suitable locations which are used to supply metering and indicating instruments and relay circuits.

The secondary side of the transformer is connected to the bus bars through main circuit breaker which is normally of oil type. The 3 phase, 4 wire supply is further given to various consumers through circuit breakers.

Between any two lines the voltage is 400 V while the voltage between any one phase and neutral is 230 V. The three phase supply can be used to meet three phase type of loads such as motors, furnaces etc. while single phase supply is used for single phase loads such as residential load containing tubes, fans and other equipments.

Review Question

1. Draw and explain key diagram of 11 kV/400 V substation.