The DC earth fault relay is designed to monitor unearthed DC systems for insulation deterioration or faults. A DC earth fault relay is a protective device used in DC power systems to detect earth faults (also known as ground faults). DC voltage is required in a substation for protection, breaker operation and monitoring purpose and therefore it is to be seen that DC voltage supply in a substation remains healthy. As Dc voltage is present everywhere in the system ,chances of earth fault in the DC system are very likely, therefore DC earth fault relay is used to detect such ground faults in the DC system.
Why do we use DC in a substation has been explained here.
DC earth fault relay working principle
The DC earth Fault Relay (EFR) like other relays also requires input from a CT to function. There is a set limit to the earth’s fault current when it exceeds the specified value for the set time delay, the output tripping contacts get activated. The relay monitors the insulation resistance between the DC circuit (positive and negative conductors) and the earth.
Kindly see in the drawing below that +Ve supply (+110v in 220v DC system) and -Ve supply (-110v in 220v DC system) is connected to the ground from the center point between the 02 nos. equal value resistances (Rx) which are connected to the DC (+Ve) and DC(-Ve) as shown in the diagram. Voltage is divided into two equal halves using this voltage divider circuit.
In 220 V DC system when there is NO earth fault in the system, Ideally
+Ve to Earth Voltage should be +110 V
-Ve to Earth Voltage should be -110 V
In case there is earth fault in DC system , we will not get above balanced voltages, if there is absolute +Ve to ground fault then we will get following voltages:
+Ve to E=0 V, -Ve to E=220 V
For partial faults in +Ve DC supply, we may get +Ve to E=40 V, -Ve to E=180 V
In above cases fault current will flow through C.T and it will actuate the DC earth fault relay.
Even if one of the +Ve or -Ve DC supply gets grounded completely ,the overall DC system will work normally. But due to some fault if both +Ve and -Ve supply gets grounded then DC will become zero and a substation will be without any protection. Therefore role of this relay is very important .
Detection and troubleshooting of DC earth fault is very time consuming and tedious , therefore there are kits available in the market called DC earth fault locator which can locate the DC fault. If no such DC earth fault locator is available ,even then the faults can be identified and rectified manually .
We have explained steps of troubleshooting DC earth fault here: How to rectify DC earth fault using a multimeter-06 Steps of troubleshooting.