Does anyone have any resource info on why- Wye- Wye connected transformers are used in certain situations and how they correlate to helping or hindering Ferro Resonance? After reading what I can find, and listening to the POCO, I am confused.
Situation we have- 34.5kv Ungrounded Delta service from the POCO to customer. Customer has 4160v generators and Wye- Delta (step up) transformers to generate the 34.5kv in the event of a utility outage.
POCO states that whenever they have a fault on their line, the fault current is seeking the H0 to ground bond on customer’s transformer, instead of going back to substation… and is burning up their static lines.
They state this is a case of Ferro Resonance and customer must replace both of their Wye Delta Transformers with Wye-Wye xfmrs.
Having never heard that term before, I started doing some hw and am now more confused than I was before! Any help understanding these topics would be greatly appreciated…
Ps- Eng. firms are being brought in to analyze the situation and recommend a solution, but I would really like to understand what’s going on…
Ty!
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Super interesting situation. Tbh I’m no expert in this area but I’ll take a stab at it.
The ferroresonance thing is completely not a concern. Here is a great video explaining ferroresonance.
I’m also not buying that changing a customers transformer wiring is going to prevent the “POCO”?? ( I assume this means utility co.??) from having issues. I’d say if the utility is burning up “static straps”?? (Again, sorry don’t know what you mean by this) they the utility needs to upsize those conductors b/c they are likely undersized.
It’s been a while since this post do you have any updates???
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The customer hired a 3rd party engineering firm and yes, they said they are the wrong types of transformers for the application, though I never heard why.
The original protection schemes were apparently never commissioned properly and didn’t provide the protection intended or advertised.
My employer has been hired to replace both transformers, associated cabling and upgrade the protection schemes to engineered requirements, though we have not seen them yet. The transformers are on order but still many, many months away.
Seems like there were multple issues all compounding together. Many hearsay tidbits of info have come out but none verifiable, such as “someone” received a kickback to buy the transformers they did, the commissioning company had new and inexperienced commissioning technicians…who, when they couldn’t get things to work as engineered, changed things to get their tests to pass… etc. idk
In short, sounds like they are going to be rebuilding almost the whole system.
My apologies that I don’t have a better answer or resolution to this matter.
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Back when i was working for a utility company I saw it in action… although most efficient, WYE WYE has its downsides. More so on 3 phase WYE-WYE pad mounted transformers. the design of the core itself. The effects are amplified if the windings are 3-core and/or little to no 3 phase load is present. 5-core windings & 3 phase load help reduce the effects of ferro resonance… also we are talking underground, so the cable alone amplifies the effect.
I took a trouble call one night because everything was on fire for 2 miles down a distribution line (25kv)… 2 of 3 fuses were blown feeding this line. only 1 phase energized. it was a long line but fortunately no more than 8-10 customers on the line. 7 out of the 10 where WYE-WYE pad mounts.
1 off the phases that had a blown fuse just 1!! had everything on fire. single phase transformers, lighting arrestors, & the padmounts were spraying oil like ive never seen before… all of the transformers were cooking and getting worse every where i turned. fires on both sides of the highway… total chaos lol.
long story short… construction hit a pole knocked the phases together (blew 2 fuses) but one of the phases wrapped around the phase that stayed energized. A-B-C phases… B&C blew and were isolated from the system. B wrapped around A and now we had 2 A phases entering all 3phase equipment… the isolated phase (C) was the one lighting everything on fire. all of the equipment was burned up on that line including customer property…
the now 2 parallel A phases going in the wye wye cores had and incredible flux effect on the idle C phase. 7 transformers were basically working together to end the world. the only transformer to explode like a bomb was an apartment complex with zero 3 phase load at the time. 3 phase was only there for the fire pumps (least loaded had the most catastrophic failure). tons of things factored in creating this perfect storm, but yes ferro resonance was included, it didn’t help that about a mile of that line was 1000kcmil AL cable… the root cause analysis took me about week of looking at meter reports, relays, PI feeder data… verifying phasing from the substation all the way down to the pt wires feeding each meter. I used a satellite phase ID so that help a ton there… i flipped up every stone possible.
super rare, but it has the possibility to be highly destructive.
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