There are two 34.5kV/4160 (delta/wye) transformers. Each transformer secondary has a 6 ohm neutral grounding resistor. The transformers feed switchgear which allows them to be paralleled. There are also 10 generators each with a 6 ohm NG resistor which also feeds switchgear that allows them to be paralled.
If only one transformer is feeding the load the ground fault current is roughly 400Amps (2400V/6ohm). If all the sources are in parallel and feeding the load the ground fault current is roughly 4800A (2400V/(6ohm x 15 parallel).
I am having difficulty grasping what sort of relay protection is used to protect MV cables in this circumstance. For instance at 400A the cable shield will not be damaged if the fault persist for 60 cycles but at 4800A the cable shield is damaged pretty quickly (4 cycles). Is a 51N relay the right device for this problem? Does a 51N relay come with a time versus current curve?
If only one transformer is feeding the load the ground fault current is roughly 400Amps (2400V/6ohm). If all the sources are in parallel and feeding the load the ground fault current is roughly 4800A (2400V/(6ohm x 15 parallel).
I am having difficulty grasping what sort of relay protection is used to protect MV cables in this circumstance. For instance at 400A the cable shield will not be damaged if the fault persist for 60 cycles but at 4800A the cable shield is damaged pretty quickly (4 cycles). Is a 51N relay the right device for this problem? Does a 51N relay come with a time versus current curve?