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, power station. In every substation, power stations, 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 why we cannot use the AC supply. There is no big reason behind the uses of DC supply.

The main reason for 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. Only a battery can give 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, Relays which are must be continuous ON condition.

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 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, 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. Some devices are operated with AC supply also the is converted from the Battery DC supply through an inverter. So we can get a continues AC backup power supply.

2. Static Devices or any other devices 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.

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 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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