Wye-Delta, Delta Wye Transformers for step-up /step-down

[petrovich]

So we had to install two transformers to stepup/stepdown the voltage due to long distance from solar field to interconnection point.

They are 480V Y, 12KV delta & 12KV delta, 480V Y transformers while interconnection is at 480V.

The question is what happens when there is a fault on the delta side. Will the elevated voltage be seen on low voltage (480V) side or because the voltage is dictated by utility side there won't be elevated voltage on either of 480V sides during the ground fault?

 

[davidbeach]

As long as there's 12kV between phases the 480 systems won't know where ground is for the 12kV system. Could be on any of the phases, anywhere inside the triangle, or conceivably even outside the triangle.

But you ought to reconsider so that you have a grounded 12kV system.

 

[Overvoltage]

Or, at least add a set of PT's and monitor the voltage referenced to ground and alarm or trip if one of them drops below a certain threshold.

 

[GroovyGuy]

I agree with David; try to find a way to ground the 12kV system. Even a solidly grounded 12kV system would be better than an ungrounded system.
If I understand your predicament you have the capability of power flow from both ends of your l2kV system, including from the solar-field end and the Utility end.
Two possible solutions come to mind;
1) install a set of 3 CTs connected in grounded-wye broken-delta configuration, along with a 59N relay, or;
2) install a zig-zag transformer, along with a NGR and a relay.

You may need to repeat options 1) and 2) at both ends of the 12kV line ??
You will need to trip CBs at both ends of the line if a GF is detected.

I suspect that in hind site, wye connections on both 12kV windings would have been a preferred solution.

To answer your question "The question is what happens when there is a fault on the delta side. Will the elevated voltage be seen on low voltage (480V) side or because the voltage is dictated by utility side there won't be elevated voltage on either of 480V sides during the ground fault?"
Answer: A single-phase-to-ground fault on the 12kV side will haven no impact on the 480V side. That in itself is a big deal, since most faults originate as a single-phase-to-ground, and you have no means to detect this condition. I'd recommend that you buy a couple of spare transformers, you'll need them sooner than you think.

BTW, what are the xfmr sizes and how long is the 12kV line? Is it overhead ACSR or buried cable?

 

[Mbrooke]

If the 12kv delta secondary is feeding into a 7.2/12kvY feeder, can't the grounding (detectors or zig-zag) be skilled?