Zero Sequence Current Source

[raggibd]

I am currently designing an inverter based distributed energy resource facility in which, upon review, the utility is requiring an additional zero sequence current for effective grounding on the high-voltage side of my transformer. I have a 1500kVA oil-filled padmount step-up transformer (G-WYEelta) to interconnect with the customer-owned medium voltage system. The examples the utility has provided all use G-WYE:G-WYE transformers. Does the G-WYEelta transformer need the additional zero sequence source on the MV side? I thought this configuration of transformer provided that source on the high voltage side of the transformer.

 

[HamburgerHelper]

The grounded wye on the high side should provide them with their fault current. For industrial loads, we usually have delta-G-wye transformers so the customers system can't trip our ground protection. The g-wye on your transformers secondary will draw zero sequence current from the primary for a secondary fault. It will be less though if you also have a delta tertiary.

 

[raggibd]

Forgive me for being unclear. The low-voltage side being supplied by the DER is 480 volt and is a grounded Wye. The high voltage side is 13.8kV and is in a Delta configuration and is connected to customer-owned MV gear (also being supplied by utility MV distribution lines). So "primary" and "secondary" may get confusing. Does your response take this configuration into account?

 

[cranky108]

A grounded wye to grounded wye transformer typically does not supply zero sequence current. If the third winding were to be a Delta, then it would supply zero sequence current.

So it sounds like they are asking you to either purchase the transformer with a delta winding, or purchase a zig-zag transformer. An alternite would be to purchase a transformer with a phantom delta.

You can do one of these, or if you just want to poke them in the eye, propose a grounded wye capacitor bank, which also provides zero sequence, but not the way they want it.

 

[HamburgerHelper]

Cranky,

Why doesn't a grounded-wye - grounded-wye not usually supply zero sequence? Are they usually grounded through some impedance?

 

[davidbeach]

If both sides are grounded, the wye-wye transformer provides a zero-sequence connection between the sides, but does not act as a source. If there isn't a zero-sequence source on the low side, it won't provide zero-sequence current to the high side.

 

[HamburgerHelper]

Yeah, you're right. It seemed counter intuitive to me until I look at the sequence networks.

 

[prc]

The way raggibd explained, LV delta of 1.5 MVA transformer will be connected to delta HV of the DER transformer. Then MV will be completely unearthed system. Is it allowed? Will a grounded star/grounded star transformer with a 3 phase 3 limbed core construction(to get phantom tertiary) or a HV delta/LV star transformer meet the requirement?

 

[cranky108]

The primary is the high voltage, the secondary is the lower voltage.

prc, your question seems a little confusing. What voltage is what? A delta winding is also not a zero sequence source, but on a delta/grounded-wye transformer, it does provide a zero sequence source to the wye side.

A system with no ground reference can be a good and bad thing. In a hospital, a ground fault will not require the system be removed from service. In a distribution system, you can't tell where the fault is, until there is either a fire in the dead pine needles, or a second phase goes to ground resulting in a phase to phase fault.