Corner grounded delta

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William Robertson

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Our system had 490 phase to phase. A phase 490-ground B phase 490-ground C phase 0-ground. Now it’s A phase 290-ground B phase 290-ground C phase 277-ground. I don’t know what changed. Does anyone have any ideas.
 
ptonsparky said:
Sounds like maybe you lost your corner ground. WAG.
Do you own the transformer or the POCO?
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If the corner ground was "lost", wouldn't that affect the other two phase to ground voltages?

It would be nice if we knew what the phase to phase voltages were before and after this happened.

Definitely going to have to watch this thread.
 
Looking at the new voltage readings they appear to be close enough to where the supply system is now a Wye configuration.
 
What was it to ground before? And what is it phase to phase now?
 
Hv&Lv said:
This is what I think also.
Has the POCO changed anything outside? Changed the bank out recently?

Sometimes they look and say “480” and forget there are a couple of ways to build 480.
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So if the PoCo changed from Delta to a Wye and no other changes made inside, what did they do with the neutral at the transformer bank?

Then, would the corner grounded phase have a path back to the transformer through the earth? The earth connection having high enough resistance to keep the current below any OCP. I guess it would depend on whether the PoCo grounded the neutral or not.
 
oldsparky52 said:
So if the PoCo changed from Delta to a Wye and no other changes made inside, what did they do with the neutral at the transformer bank?

Then, would the corner grounded phase have a path back to the transformer through the earth? The earth connection having high enough resistance to keep the current below any OCP. I guess it would depend on whether the PoCo grounded the neutral or not.
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No. A lot would have to be changed inside. I answered and then it hit me “that’s not right” before I could edit.
I think we need good voltage references from the same points. Apples to apples..
Edit: thinking about it now I’m not sure anything has changed. He just took voltage readings from different reference points.
 
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Hv&Lv said:
No. A lot would have to be changed inside. I answered and then it hit me “that’s not right” before I could edit.
I think we need good voltage references from the same points. Apples to apples..
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I put the exact voltages in my original post. They were taken from the same points about 1 month apart.
 
William Robertson said:
I put the exact voltages in my original post. They were taken from the same points about 1 month apart.
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Yeah. I read that now.. I saw 480 phase to phase and skimmed too fast. “Read every word...”

does everything inside work now?
 
If source got changed from delta to wye, you wouldn't have 480 nominal to ground on any of the "phases".

Somewhere something must have created an "autotransformer" either unintentionally or a fault has occurred that did this. About has to be open circuit or high resistance in the grounded conductor at the same time though, or problems with grounding and bonding system.

Any load connected in wye configuration could do this if the wye point gets grounded when it shouldn't be.
 
Ungrounded systems are MOY open circuit to ground and you should not expect that. The Earth/bonded system even if it’s just sitting on concrete is still a conductor, however poor. Bonding just keeps the impedance low. AND the insulation is a very high resistance but also it has capacitance. The definition of a capacitor is two conductors separated by an insulator, any insulator, even a vacuum. A capacitor blocks DC but passes AC.

So your system is now capacitively grounded. But since you don’t and probably can’t control the system capacitance to ensure it is perfectly balanced the current across each phase to ground is slightly different. So owing to AC Ohms Law or V = IX the three voltages are never equal or necessarily make sense. In fact due to the way it interacts with arcs, the voltage to ground can get up to 600-800% of line to line voltage during an arcing fault. Since arcing faults have much lower current than shorts breaker/fuse tripping takes a lot longer: in the mean time it does a lot of damage (arc flash) and the excessive overvoltage surges shred motors in the first couple turns.

As others said you lost your grounding. This is like a floating neutral...finding it isn’t easy.
 
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