bus overvoltages on a high leg delta service

[T Gibbs]

I hope that 763v is really just a "ghost" voltage, i.e. if you put a load on it, it will go away, because anything over 600v is illegal, plus the wire insulation is only rated for 600v.

 

[DickDV]

I'm with jraef on his statement about something being terribly wrong and it needs to be fixed fast before someone gets killed.

And, no, 480V delta is not normally corner grounded. I'd go so far as to say it shouldn't be corner grounded even tho there are a couple of rogue contractors here in Michigan that routinely do that.

You need to get your power distribution system straightened out. Forget all this other idle speculation and get it done right away.

And, it would be really nice to let us know what was really wrong. We could all learn something from your experience.

 

[jraef]

Actually, if you have a legacy Delta system, you are REQUIRED to have it grounded somewhere now, or have a Ground Fault Monitoring system. The most common grounding method, if done, is to corner ground it. My point was that if that is a two transformer delta service with a corner ground, and NONE of the three phases is reading at zero volts referenced to ground, then it is floating. And if one of them is floating at 763V referenced to ground, that is (likely) a "ghost" reading in that it is just the capacitive coupling to the ground reference, but if you see that, then that means something ELSE is seriously wrong.

But as to it being "High Leg" delta, no way with the data provided. If it was 480 Line to Line (as stated) and one phase was center tapped to provide 240V Line to Neutral (weird), the "High" leg referenced to ground then would be the sq. rt. of (480^2 - 240^2), so 416V and the other two legs would read 240V referenced to ground, not 400V. There is no math formula that gets the 400V and 763V values. Something is broken somewhere.

 

[T Gibbs]

I noticed that the OP hasn't posted since the first one. I wonder if he's still here.

 

[jraef]

I noticed that the OP hasn't posted since the first one. I wonder if he's still here.

We may have been too slow in warning him...

 

[ganutenator]

On a side note, I had bus over voltage faults on a VFD because the drive was set to output a higher voltage than was coming in. i.e. the drive was set to motor FLA (460) but only had 420 on the line leads feeding the VFD. The drives that are faulting are probably sensing a problem in the plant.

 

[T Gibbs]

On a side note, I had bus over voltage faults on a VFD because the drive was set to output a higher voltage than was coming in. i.e. the drive was set to motor FLA (460) but only had 420 on the line leads feeding the VFD. The drives that are faulting are probably sensing a problem in the plant.

Sounds like you had some serious issues if you only had 420v.

 

[ganutenator]

I took a stab at the voltage. I don't remember how much lower is was than motor FLA.

 

[Gil47]

I can't give a mathematical reason for the voltages you are seeing,
but a gut feeling and thats all it is, that maybe you have a VFD
that is not supplying current on one of the three phases, possibly because of a blown fuse,
and maybe this phase is now attached to earth, introducing some sort of
AC or even DC waveform on top of your three phase supply.

A VFD will still give a a three phase output to a motor and will still run OK
as long as not heavily loaded even with one supply phase not supplying power.

Maybe you could test each of the three phases to your VFD's to see if power is being supplied
to the drives three phases they should be within 10% of the others.

Then again it may just be a ghost voltage as other have already suggested.

 

[duckman]

Going since May 18, people have been offering advice with no response from the person who was so concerned that he needed help and 4 days later people are still giving advice on a "hypothetical"
Was the problem 2 VVVF dives or his Supply, will we ever find out?
I wonder some times whether there is a forum member that comes up with these possible "hypothetical" to keep all the forum members brains ticking over.

 

[Mickey]

I wonder some times whether there is a forum member that comes up with these possible "hypothetical" to keep all the forum members brains ticking over.

"Brain Ticking" posts were more common in the earlier days of the forum. Search "Terry Woods" for one member, there were others of course (Peter is another).

 

[Gil47]

"Brain Ticking" posts were more common in the earlier days of the forum. .

Sums it up for my interest, have not got enough to do or just avoiding the obvious jobs.


To give direction to my previous post
480 V divided by (square root of 3) 1.732 = 277 V
480 +277 = 757 V close to the stated peak voltage

Now 277 V x ( by square root of 2 ) 1.414 = 392 V
also close to the other stated voltages

Brain tickle then provided possible explaination.