Wye-Wye vs. Delta-Wye Xfmr 1

[cabletray]

An earlier thread stated that protective relaying can be difficult using wye-wye xfmrs. But I don't see any protection problems using a 480 V primary (outdoor xfmr) fed from a standby generator having a molded-case breaker and zero-sequence sensor. The sensor would sense ground faults originating in either the primary or secondary side. On the other hand, using a delta-wye xfmr, the zero-sequence sensor would not detect ground faults occuring on the secondary side of the xfmr. When you also consider that a standby step-up xfmr, if seldom used, might be purchased as a refurbished unit rather than new, and wye-wye units tend to be cheaper than delta-wye, then wye-wye might be a better choice in this situation. Am I missing something?

 

[dpc]

With a wye-wye, your ground fault sensor will see all the ground faults, but you might not want it to. With the wye-wye, your generator ground fault sensor may trip on faults far removed from the generator that could be cleared by something downstream. With the delta-wye, ground faults on the wye side are not seen by the ground fault protection on the delta side, so the ground fault protection can generally be more sensitive without reducing coordination.

Also, the delta-wye will be a better grounding transformer than the wye-wye, making more fault current available for line-to-ground faults.

But it all depends on the application and desired level of protection versus coordination. Either one will probably work. There are a lot of wye-wye transformers out there.