Sizing cable to NGR

[StuChristensen]

I am doing a project that invloves using an NGR and I have to size the cable to it.

The system is 1MVA, 25KV step down to 4160V. From the transformer we're connecting to 4160V switchgear (benshaw controllers on them) to feed 2 - 800HP motors.

Can anyone point me in the right direction for this? I haven't dealt with NGR's since school.

Thanks

 

[rcw retired EE]

Determine the resistor rating, both in ohms and fault current. Rating is probably given as amps over time such as 400A for 10 seconds, 200 amps for 1 minute, or 100 amps continuous. (Se IEEE Std 32.)

Size cable for the fault current. You could save some cost by sizing the cable for the continuous rating only and verify the short time rating (See IEEE Buff Book). But the cable will be so short that the cost difference is nothing. IMO Size for the short time current.

Run 5 kV cable from the transformer neutral to the resistor and bare or 600V cable from the resistor to ground.

Resistor rating is determined by the protective relaying. How long will a ground fault last before the protection clears the fault?

 

[StuChristensen]

Drawings list it going through a 200:5 CT, with a 25A CONT rating.