wye-delta overvoltage backfeed

[stevenal]

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.

 

[prc]

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.

 

[waross]

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

 

[stevenal]

Thanks all.
The advantage is all on the customer side. Customer has single phase 120/240 V and 230 V three phase motor load. Not the preferred arrangement from the utility side, but it's tough to convince a long time customer they need to change equipment or install dry transformers. I'm not sure Google would be much help to see the delta secondary connections. A larger "lighter" transformer might be a clue, but I've seen unbalanced banks connected wye also. Yes we float the primary wye, just like in the IEEE C62.22 example. Wye is tied down when single phase switching is performed, but nature fails to inform us prior to single phase events caused by wind.

 

[waross]

The classic solution is to use an open delta transformer bank.
In your case, when a single phase event results in one fuse blowing, don't replace the fuse.
Anecdote warning:
In Central America, where the grounded wye/delta was common, it was also common to see one fuse open on vrtually all the wye/delta banks, until the next election.
If the ruling party changed, then all the party favorites in the top engineering jobs in the Government owned utility would be replaced. The new guy at the top would typically send out an order to "Get with it" and replace all the blown fuses.
Single phase events were common, (weekly) and the fuses would typically be replaced with the next larger size.
Eventually over-fused transformers would start to fail.
Usually, within about a year after an election, inertia and appathy would take over and the system would revert to one blown fuse on every bank.
Anecdote #2.
I had left the country and was no longer working for the island utility. I was back in the country for a visit and the manager of the utility asked my to come out to the island and walk around the system to check for any possible problems.
Sure enough, after I had spent a few years getting rid of all the wye/delta banks, here was a new wye/delta bank.
"I thought that we had agreed that we would no longer allow that connection on our system?"
"Yes, I know, but that is the national phone company's telephone exchange and we have to give them what they want."
"Do you remember when we used to have all the refrigerator burn-outs blamed on our power company?"
"Yes. That was awful."
"When was the last time that you were accused of a refrigerator burn out?"
"Not for years."
"Do you want to go back to those days?"
"No way."
"Well then we have to do something about that transformer bank."
The solution:
As you know, a fused cutout has a toggle arrangement on the bottom.
The toggle is held in place by the fuse link.
When the fuse blows, the toggle spring pulls the fuse link apart to aid in clearing.
The toggle collapses and allows the fuse holder to drop free.
We had a toggle spot welded in place without a fuse link installed.
This was put in place for one of the transformers.
The transformer bank then functioned in open delta with one transformer connected on the secondary but disconnected from the primary.
The customer never knew and never saw a difference in service.
You do have to check the transformer loading.
In our case, The single phase load was only lights and office machines for a small office.
The three phas load was one 17 KVA bettery charger.
The battery bank was three 50 KVA transformers.



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