Three Phase Transformer Polarity 2

[emoltzan]

I have a question that came up the other day when we were putting tie breakers between 2 480V switchboards. We have a 2 5kV breakers each feeds a different 4160-480V Delta-Wye transformer. These transformers each feed a separate switchboard with breakers. We tied these switchboards together with a tie breaker on each to backfeed if one of the transformers were lost. My question comes here; we measured voltage between the 2 switchboards and measured A-A 280V B-B 280V C-C 280V. This must be coming from the transformer having differing polarity as the voltage between A-A B-B C-C should be 0 to avoid closing into a bolted fault. Can someone explain how 280V is developed?

-Thanks

 

[jghrist]

You have two phases swapped on one of the primaries. This makes a 60 deg phase shift on the secondary. The voltage between like phases will be the 277 V line-ground voltage.

 

[emoltzan]

I understood that much, I was more looking for a diagram/explanation of how the 277V was obtained.

 

[stevenal]

Perhaps this faq can help: faq238-1154

 

[emoltzan]

Based on that, and from what we thought earlier it looks like a 60 deg phase shift when comparing both secondaries. I was attempting to do some phasor comparison with 60deg angle between two phasors with equivalent magnitude (480V)and still not coming up with the 280V we saw.

 

[stevenal]

The wye side phasors are not 480, they are 480/sqrt(3) = 277.

 

[Palletjack]

It’s not polarity wrong, displacement is wrong.
Check the voltages across A&B, and A&C, see what you come up with.

60 degree shift tells me the delta primary is landed different on the two transformers

A DABY can be a DACY (DY1, DY11) simply by landing an ABC rotation differently.
Assume ABC rotation. DY1 A phase is landed on H1, B phase landed on H2, C phase on H3..
DY11 is A phase landed on H3, B phase landed on H2, C phase landed on H1.
SO YOUR 30 degree lead becomes 30 degree lag, therefore 60 degrees between transformers.

With an ACB rotation its the opposite.