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wye-delta overvoltage backfeed

wye-delta overvoltage backfeed

wye-delta overvoltage backfeed

(OP)
In another thread, Waross spoke having experience with these wye-deltas. I'm hoping he will respond along with anyone else with experience in the matter.

We recently had some damage when a fuse opened and backfeed on the downstream side of that phase occurred from an ungrounded wye-delta bank. When doing a bit of searching on the subject, I came across IEEE C62.22 which shows how an arrester could experience as much as 2.7 per unit voltage for this condition when load is unbalanced (situation normal). Sounds like an extreme disadvantage for so common a connection. Is this your experience? Thanks.

RE: wye-delta overvoltage backfeed

stevenal, I have absolutely no experience with this connection for distribution transformers. But I always wondered why US used and still using this connection. What is the advantage? If you search IEEE database, you will find at least a dozen AIEE Transaction papers on transformers ( during 1950's) with this connection, explaining load sharing, overvoltage issues etc,with mind blogging equations and graphs. In India, I have seen nothing else but 3 phase delta grounded star distribution transformers, probably running to millions. Probably waross or cucky may be able to answer your question. I have references of those mighty IEEE papers that I can share if interested.

RE: wye-delta overvoltage backfeed

Quote (stevenal)

for so common a connection.
This is not a common connection. While it was common in Central America, I have only seen this connection once in North America. That was about 20 years ago. I have since searched for that transformer bank on Goggle Maps, Steer view, and by driving around the area when I was in the vicinity but I can no longer find it.
I once found an REA publication which recommended floating the wye point if the wye/delta connection was used.
That avoids the circulating currents but introduces the possibility of transient overvoltages during switching.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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