I didn't want to hijack a thread. And 5e likely wants to keep focused until he gets an answer. So ....
Most of the primaries I deal with are 25kv, 69kv, 138kv, all overhead - and one 38kv UG (10 years ago). And I deal with a lot of 13.8kv secondaries as well as the standard 480 secondaries.
The one 38kvUG xfmr was provided by the utility and is wye primary. A couple of the small 25kv are pole mount xfmrs and I'm not sure about those. I'll look at them nest time I'm out there. The rest are above ground services and the primaries are all Delta.
I took a class in 1985 about transmissions lines and I recall issues with stability on UG lines above 30kv with any significant length - we studied one that was 30kv and 30 miles (I'm guessing) - Goat Island Norway maybe. And that was a big deal.
Of the xfms in the plants, fed with 13.8kv, all are Delta primaries.
So, I'm understanding what you are saying about insulation resistance and resonance, but now I'm wondering why this doesn't apply to all of the systems I deal with - Why aren't they all Wye primaries? - even the 13.8kv.
I'm guessing there is a downside.
carl
coulter said:jt (and others) -
I am absolutely not familiar with Wye connected xfm primaries, advantages, when one would want to use one. Could you recommend some references?
carl
LarryFine said:Carl, most utility transformers these days are Y-connected. Most are single-bushing units, or single-ended, meaning that one end of the transformer is connected to the system neutral.
Next time you have the opportunity, look up at the transformers on a pole and especially the primary connections.
jtester said:Carl
Wye primaries have lower insulation levels than comparable delta primary transformers, they're cheaper.
Another problem is ferroresonance anytime you have a delta in an underground system. The high capacitance of underground MV cables combined with the reactance of the transformers can create a resonant condition.
Jim T
Most of the primaries I deal with are 25kv, 69kv, 138kv, all overhead - and one 38kv UG (10 years ago). And I deal with a lot of 13.8kv secondaries as well as the standard 480 secondaries.
The one 38kvUG xfmr was provided by the utility and is wye primary. A couple of the small 25kv are pole mount xfmrs and I'm not sure about those. I'll look at them nest time I'm out there. The rest are above ground services and the primaries are all Delta.
I took a class in 1985 about transmissions lines and I recall issues with stability on UG lines above 30kv with any significant length - we studied one that was 30kv and 30 miles (I'm guessing) - Goat Island Norway maybe. And that was a big deal.
Of the xfms in the plants, fed with 13.8kv, all are Delta primaries.
So, I'm understanding what you are saying about insulation resistance and resonance, but now I'm wondering why this doesn't apply to all of the systems I deal with - Why aren't they all Wye primaries? - even the 13.8kv.
I'm guessing there is a downside.
carl