I have 6.6kV resistance grounded system ( Earth fault current limited to 100A) in that for all outgoing feeders including motor feeders i need to calculate earth fault relay setting with a 1250A/1A CT. I have seen other projects - as all relay setting ( for 51 & 51N ) are
10% - 20% is taken as a thumb rule. I really want to know the philosophy behind the earth fault relay setting for a resistive grounded system
I am a bit rusty on this nowadays, so will start by stating the obvious. 10% of 1250A is 125A which is greater than your maximum earth fault current. In addition if the relays have an IDMT characteristic they will not operate for a current less than 130% of the setting ie about 165 A for the 10% setting.
There is a further problem in that with a lot of relays as the percentage setting decreases the impedance of the relay increases. This can force the CTs into saturation, and further compromise the setting.
Then you will presumably want the earth fault settings on the feeders to grade with the incoming supply.
John
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