Sensitive Ground Overcurrent on Primary of Transformer

[bceng87SS1517]

I've got an application where I have a dedicated substation breaker supplying a single XFMR(D-Y) with a switch on the primary, a secondary main, and a 400A NGR. I am looking to set the primary breaker ground overcurrent as sensitive as possible(thinking instantaneous 50G) without concern of tripping on a ground fault on the secondary of the XFMR. I know the ground fault current wont reflect as such across the (D-Y) XFMR but there will be an increase in magnitude on two of the primary phases as a result which will show up as an unbalance or ground current as seen by the relay. Any thoughts on limitations of the sensitivity that can be set without sacrificing selectivity? (Context: Voltage collapse once fault escalates to three phase is resulting in loss of drives. One thought is to use sensitive 50G setting with voltage level as torque control.)

 

[RRaghunath]

1-phase to Earth fault on Y-side will reflect as current in two phases on upstream Delta side. But, these currents will not show up as residual current when currents in three phases are summed-up. So, there is no way the 1-phase to earth fault on secondary can cause Earth fault relay on primary to operate.
Thus, the earth fault relay on primary can and shall be set sensitive and fast acting for effective protection against faults on primary.

 

[stevenal]

I agree there is no residual current under either load or secondary fault conditions. The relay, however, operates on on CT secondary current. How sensitive you can go will depend on CT accuracy.