Why DC Supply is used for Control Circuit in Substations, not AC?

Hey, we are going to know why DC supply is used for control circuits in every substation and power station. In every substation, power stations, and battery banks are used to provide DC supply.

Actually, this question confuses us that there may be a big reason behind the uses of DC supply. You may also think that is why we cannot use the AC supply. There is no big reason behind the use of DC supply.

The main reason for using a DC supply in substations 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 be obtained from batteries. Only a battery can give a continuous power supply until it fully discharges. The battery has electrical energy storing capability that's why it can be used for backup power supply.

The control circuit for the switchgear system in a substation consists of many Circuit Breakers, isolators, and Relays which are must be in continuous ON condition.

If we use an AC supply that 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, the status of the circuit breaker, and the relay.

So during main 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, the DC supply is used for the 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 an AC Supply in the 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. Some devices are operated with AC supply also the is converted from the Battery DC supply through an inverter. So we can get a continuous AC backup power supply.

2. Static Devices or any other devices that need a DC supply can be directly connected with batteries. But when an AC supply is used, the rectifier circuit is needed.

3. Control Circuits consist of many digital devices also such as Digital Displays, Sensors, LEDs, etc. These devices always need a DC supply. So when using a DC supply in the control circuit fewer circuits are needed.

4. Most of the controlling devices are constructed with electronic components that only work with DC supply. For, example we cannot operate an electronic display with an AC supply, it will flick, so here must a steady-state DC supply is required.

5. Also, the UPS used in stations, takes power supply from the battery. When the main power supply is available, it charges the battery bank and when the main power supply is not available it takes power from the battery.


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