Wye-Delta, Delta Wye Transformers for step-up /step-down
Wye-Delta, Delta Wye Transformers for step-up /step-down
(OP)
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?
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?






RE: Wye-Delta, Delta Wye Transformers for step-up /step-down
But you ought to reconsider so that you have a grounded 12kV system.
RE: Wye-Delta, Delta Wye Transformers for step-up /step-down
RE: Wye-Delta, Delta Wye Transformers for step-up /step-down
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?
RE: Wye-Delta, Delta Wye Transformers for step-up /step-down