Ground fault relay protection

[Power2all]

I am coordinating a relay associated with a MV breaker with a LV breaker downstream for ground fault. There is an existing relay upstream of the new relay which has an instantaneous setting for ground fault but no definite time setting. My thought was to use instantaneous with definite time to not allow tripping below a 0.5 sec which is above the LV breaker curve. Any ideas on what best practise is?

 

[dpc]

If the transformer is delta-wye there is no need to be concerned about coordinating the MV ground fault with the LV ground fault, since the MV ground fault device will not see LV ground fault current.

 

[Power2all]

OK, Thanks. How about the two relays, is it better to have definite time or is current sensing in the instantaneous range sufficient?

 

[dpc]

Not really enough information, but I'd probably use a short time delay on the primary instantaneous (definite-time) to avoid nuisance tripping on inrush. You can also use an inverse time.

For the LV side, you need to coordinate with the any other downstream ground fault relays.