DC System in Substation

         In Substation Wheather it is MV or HV, DC supply is used for Protection, Metering, Monitoring and Measurement purpose. We all know that our whole distribution system is AC so why DC supply used in substation. 

WHY DC?

1. We can store DC in battery.

2. DC has less complex as it has only magnitude.

         If we use AC supply from substation for protection then in fault condition whole system has no power resulted in turn off of your all relays so you cannot extract fault data or Disturbance recorder from numerical relay to analyse fault and didn't conclude anything about fault. So Something is necessary which is available in blackout condition to restore your system.

DC Voltage level can be 24V, 48V, 110V and 220V DC.

Block diagram of DC System

1. Battery Charger:- Every Substation has Float cum Boost Charger installed to charge battery. When battery is discharge then it's voltage will be very low so Battery charger come into boost mode,  in this mode constant current charging done and battery voltage rise. Around 80% capacity of battery returned in the constant current region.

         When Battery is charged then battery charger come into float mode which is constant voltage mode of charging. This mode will maintain the battery indefinately without boiling out electrolyte or overcharging the battery.

2. Battery Bank:- Every substation has seperate battery room in which series connection of cell is done. Lead acid battery used in substation. 

3. DC Distribution Board:- It is junction of DC Supply. From here DC supply distribute in different circuit. For protection, DC Fuse is provided.

Connection of Battery:- To make 110V DC two connection is possible.

• Positive wire hold 110V and Negative wire hold 0V as you earth negative terminal.
• Mid point earthing in which positive wire hold +55V w.r.t ground and negative wire hold -55V w.r.t. ground. Mid point earthing method shown in figure.

     

Connection of Batteries

           As you can see To obtain +55V and -55V, Mid Point must be earthed. Now if you check voltage with multimer at Positive to ground then it will be +55V and at negative to ground it is -55V.  

  

Why this type of connection?  

• In this type of connection if positive terminal earthed or someone touched it then only 55V will came across it. If you use negative earthed system then whole 110V will be came across it which is fatal than 55V.
• Troubleshooting of wiring problem become easy.