We tripped a substation!

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Industrial Electrician
Catchy title huh? Lol

A few days ago we lost power at our plant, and after some digging and talking to our utility, I ended up finding out that our substation was showing 6 amps on b phase and ground, thus, shutting the substation down.

I'll spare you all of the troubleshooting steps, but here are the "end" facts.

At times, A Phase, B phase and ground are showing 6 amps to ground at 21kv. I say "at times", because during initial troubleshooting, we did not see the ground fault initially. This ground fault can be sustained for a period of time (let's call it 1 minute).

We have isolated this issue to a 21kv to 480v delta wye transformer. This transformer also has a breaker on the 21kv side, that when opened, the ground fault disappears. So initially, one would think that something on the secondary side is causing the issue. But after thinking a little deeper about this, I'm of the mindset that I don't believe our utility would be seeing a ground fault on the secondary side of this transformer. I believe the issue is much more likely between the bottom of the transformer breaker and the 480 side coils, i.e, the buss bars or the 21kv coils. A quick background on this particular transformer... it blew up a year ago... enough said.

So the reason I came here is to hopefully ask a simple question, and hopefully get a simple answer (although I know this never happens lol).

If the secondary side of a delta wye transformer has a ground fault, will this be detectable on the primary side?
 
Location
California
Occupation
Industrial Electrician
Awesome, so then it's gotta be either the buss bars or the 21kv coils. My assumption (haven't actually looked, but it makes sense to me) is that either the buss bars got knocked loose and over time/temperature are now touching something they shouldn't be, or that the oil has slowly been leaking out and we are now seeing affects of it being very low on oil? Obviously could be other things, but at least this narrows down the issue to the transformer.

I was kinda surprised at how tough it was to find this answer online. There was only one site that stated "ground fault current cannot pass through a delta wye transformer". Other than that, pretty difficult to find. I would maybe chaulk it up to being "electrical common sense", but its surprising how many people disagreed with me.
 
Location
California
Occupation
Industrial Electrician
Ok, so pretty much, the primary side will just see the current on the phase/phases and not the ground (in the event of a secondary ground fault), if I'm understanding it correctly.
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
Ok, so pretty much, the primary side will just see the current on the phase/phases and not the ground (in the event of a secondary ground fault), if I'm understanding it correctly.
Yes. Some of the current in the delta will correspond to the ground fault current on the secondary side, and so be "abnormal", but it will not show up as a ground fault.
So, a ground fault on the secondary side which, for whatever reason, does not result in an actual overload will not cause a primary trip.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Have you confirmed that this is a 21kV delta : 480/277V wye transformer?

In particular this ransformer doesn't have a grounded wye primary?

A grounded wye primary is possible on MV transformers, and this grounding is an intentional ground. Protective relaying on such systems confuse me :)

Jon
 
Location
California
Occupation
Industrial Electrician
This isnt the exact transformer in question, but the only difference is that the KVA on the problematic one is 1500/1725.

So yes, this is a 21kv delta 480/277 wye.

IMG_20230304_143256.jpg
 
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