13.2KV - 480V ungrounded delta

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ernric43

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I have a 13.2KV primary with 480V secondary delta delta transformer.
I measured 500v, 500v, 36v from line to ground respectively. I have a power monitor on the transformer that is reading 999v, 999v, 442v line to line.
This was all noticed because of a Powerflex 70 that will not start up it's reading over voltage before it even starts.


Can someone explain this?
 
Ungrounded

Ungrounded

Yes the secondary is ungrounded. We are checking the jumpers but although the drive is relatively new it has ran in the past. Could this be a ground fault?
 
It's not a hard fault, but at the very least there's a line with a whole lot of capacitive coupling on the 32V phase. What's on the low-voltage phase and can you isolate loads and see how your values change?

What's weird to me is that you've got two devices, the power monitor and your VFD, reporting incorrect phase-to-phase voltages. Those are fixed by your transformer windings, so ungrounded isn't gonna make a bit of difference in that regard.

What type of monitor is hooked up there? When it says you've got 1kV L-L what does it say your L-G readings are?
 
Sketched it out. As Big J said, the simultaneous readings don't make much sense.

However, you could be looking at the symptoms of an arcing ground fault. That is a failure mode common only to ungrounded systems. I've only seen it once before - the whole system voltage raised.

If it is an arcing GF, the system has likely already sustained Over Voltage damage.

Big J's suggestion was good. Start isolating and find the fault - now would be a good time to start.

ice
 
I am asking a question here as I am not that good with ungrounded systems.

The odd L to L voltage ... Couldn't that be from an open conductor / fuse and the OP is reading a back-feed through the equipment?
 
iwire said:
...The odd L to L voltage ... Couldn't that be from an open conductor / fuse and the OP is reading a back-feed through the equipment?
Click to expand...
The only reason I'd say no is because with an open circuit I wouldn't expect any voltage drop, so I would think that if he had a backfeed it'd be at full system voltage.

That said, it can imagine a case where he had a complete open and was just reading ghost voltage, but I think he'd have overcurrent problems that he isn't describing here.
 
AB Power Monitor

AB Power Monitor

I found out that the reading on the power monitor is an over voltage condition. According to AB the monitor can only read up to 480V before it goes into over voltage reading of 900V. The voltage from the transformer is 500V. Still not sure what was going on with the drive but we swapped it with a starter and all is good.

Is there a good way to test the leg that reads 36V for capacitive coupling other than shutting things off. We did shut down the big stuff and now we're left with little 20A and 30A feeders.
 
ernric43 said:
... Is there a good way to test the leg that reads 36V for capacitive coupling other than shutting things off. We did shut down the big stuff and now we're left with little 20A and 30A feeders.
Click to expand...

Nope. That's a problem with ungrounded 480D. A ground on any one leg, anywhere is the system, shows up all over the system. Just keep shutting stuff off until it goes away. Of course your management will say that is impossible - they equipment must be kept running. Great, then live with it. Eventually another phase will fail and the fault will now be phase to phase and a CB will trip. Unless the issue is an arcing ground fault - in which case the fault will keep burning out equipment from OV.

And, nothing says the issue is capacitive coupling. If one leg has a connection to earth, not a good connection to building steel, then a non-zero low reading is the symptom.

Additionally, the "arcing ground fault" issue rasing the voltage to ground maybe 2x, 2.5x is real - that could easily have been what killed the VFD.. If you are going to continue to work around ungrounded systems, recommend reading IEEE 1100.

ice
 
On the PowerFlex drives, there are jumpers that connect the input bridge surge protection devices (MOVs) and the Common Mode Choke on the outputs, to a ground reference. On an ungrounded delta system, you MUST remove the jumpers on those connections to ground, or risk having them create damage to the VFD. In the case of the MOVs, there are three of them connected in a Wye pattern, with a 4th that references the Wye point to ground. If you don't remove the jumper, that 4th MOV attempts to become the Wye point for your entire grid, very very briefly, before it vaporizes. That destruction process can leave a carbon trace that then starts messing with your voltage readings.

It's all in the manual, but hardly anyone reads it...

MOV statement.jpg
 
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