Earth faults

[HADYTEX]

Hello

Consider a 3 phase system with a residual ea rth fault relay connected to 3 current transformers in star formation.

I know that the earth fault relay will trip if phase current on any one phase exceeds the other two phases by the pickup value (let’s say 1000amps). This causes zero sequence current to flow due to imbalance of the phasss. So if phase A is 2000amps phase b is 1000 amps and phase c is 1000 amps the relay trips on a 1000amp earth fault. Or if phase a is 1000amps and the other two phases are 0 amps the same thing happens.

What I don’t understand is if the network is operating with say steady state balanced load currents of 100amps per phase, then phase a experiences a 1000amp earth fault, won’t the relay see and imbalance of 900amps? But in reality the earth fault level is 1000amps? The network will become a two phase network with phase a down to earth?

 

[protoslash]

If fault on your system is 1000amp, you shouldn't set the relay pick up right at 1000amp.

Your ground fault relay pick up can be set to as low as 120% of the max system unbalance.

 

[waross]

You can't add and subtract three phase currents the same as single phase currents. You must consider the phase angles of the currents.
If you have no line to neutral loads then unequal phase to phase directed currents will sum to zero.
If you have line to neutral loads you must add a neutral CT in such a way that it sees normal line to neutral load currents but does not see ground fault currents.

Bill
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"Why not the best?"
Jimmy Carter