Ground faults on the secondary of transformer as seen by the primary 3

[gdeep]

Hi,

I have a 12.47kV/4.16kV 5MVA delta-wye(10A,10s resistive grounded system) feeding a 5kV switchgear. The NGR is rated for 10A, 10s and I am using SEL-751A protection relays.

1) A ground fault on the secondary of the transformer would be seen by the primary as L-L fault?
2) How would this 10A ground fault current be reflected on the primary? Would be there be a considerable increase in currents on two phases of the XFMR primary and would I be able to see negative sequence currents( I am using 800:5 CT)
3) Are there any good references I could use? I have tried simulating a model in PSCAD and I could see currents go up in two phases of the primary but I cannot verify the magnitude of the currents just the relative increase to the pre fault currents.

Any inputs would really be appreciated
Thanks

 

[Mila15]

Thanks everyone! I guess, the mistake I made was using the formulas developed for normal balanced conditions during fault conditions. I can't simply multiply winding current by sqrt(3) to obtain line currents during fault conditions. Vector summation is needed here, which is what sibeen and LiteYear explained.