Earth Fault Protection with Neutral Earthing Resistor

[benscott172]

Hi all I have a question that's been tying my head in all sorts of knots recently. I'm working on an 11kV distribution system with an NER at the 33-11kV transformer which limits the SLG fault current by 250 amps. When I run the simulator (SKM) the DLG fault current is lower than this value approximately 161 amps, however, the total fault current, seen at the point of the fault it nearly 4000 amps. If the EF protection curve is set to protect for SLG and DLG then the lower neutral current for the DLG fault will cause a slower tripping time then that of the SLG fault even though the total fault current is far more severe!!! Clearly this is not right, my only though is that the DLG fault will be picked up by the overcurrent, which has a higher pickup but will trip the breaker much faster. If that is the case though, there maybe possible conflicts between the OC and EF grading, causing undesired tripping. Has anyone come across this problem before? If so please help, thanks...

 

[davidbeach]

Problem? Basic obedience to Ohm's Law is a problem?

 

[dpc]

With that neutral impedance, the DLG fault is going to look a lot like a L-L fault so relying on your ground fault element for protection may not the right approach - you must also have phase overcurrent protection as well. Your ground relaying has to be set to protect the resistor, which probably has only a 10 second rating. It might be instructive to look at the sequence currents for the DLG fault. Maybe you can use a negative sequence overcurrent element in addition to the ground fault sensing element.