# Ground fault protection for fault on delta side of a wye-delta step down.

I have a distribution circuit with a primary 4-wire 21 kV and secondary 3-wire 12 kV circuit. The two winding transformer is a (grounded) Wye-(closed) Delta transformer. I know that if I have a ground fault on the secondary side of the transformer, I will not have a path for ground. I am requesting to add a grounding transformer to provide a source of ground fault current during line to ground faults.

My question is, would a ground fault on the secondary side be seen on the primary side of the circuit (after adding grnd transformer)? That is, will my breaker ground relay on the 21 kV side see a ground fault on that occurred 12 kV side?

Thank you!

• Welcome to EE.SE. There's a nifty schematic editor built in to the main editor. Post a schematic. – Transistor Aug 14 '17 at 22:10
• Ground faults on the secondary of a wye-delta or delta-wye transformer appear as phase-to-phase faults on the primary side. More details when I get time. – Li-aung Yip Sep 12 '17 at 11:20
• @Li-aungYip have you had time to explain this better perhaps in another answer? – VMMF Jun 10 '19 at 13:06
• @VMMF not as yet. Thanks for the reminder. – Li-aung Yip Jun 10 '19 at 21:02

To the primary side, a grounding transformer on the secondary side is indistinguishable from a secondary that has a ground. You can prove this to yourself by simulating a Y-Y transformer with a secondary ground fault, and then doing the same for a Y-$\Delta$ with a Zigzag transformer on the secondary side.