Concept of using DC supply in substation or power stations is to provide a continuous power supply to the control circuit. DC is a reliable source of power supply because it can obtain from batteries. ... So during power failure also, we have to provide the continuous power supply to the control circuit.
If we use AC supply which can not be obtained from the battery then we cannot provide the power supply to the control circuit during the power failure. If a power failure occurs, all devices will stop working then we cannot check the fault status, status of the circuit breaker and relay.
So during power failure also, we have to provide the continuous power supply to the control circuit. As we can provide continuous power supply only using battery and battery can provide only DC supply, so all control circuit devices are designed for DC Supply.
In a substation or power station, DC supply is used for Opening and Closing of Circuit breakers, to operate protective relays, to operate isolators, indication lamps, alarm circuits, PLCC(Power Line Carrier Communication) Panels, Emergency Lights in Control Rooms, etc.
Using of DC supply instead of AC Supply in Substation also gives some extra facilities such as,
1. When protective devices are operating with DC supply they take very less reaction time than when they are operating with AC Supply.
2. Static Devices or which need DC supply can be directly connected with batteries. But when AC supply is used, the rectifier circuit is needed.
3. Control Circuits are consist of many digital devices also such as Digital Displays, Sensors, LEDs, etc. These devices always need DC supply. So when using DC supply in the control circuit fewer circuits are needed.
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