Ungrounded Delta System Abnormal Voltage Readings

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nuckythompson

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Nova Scotia
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Electrical
Hi all,

I have a 4160V/600 delta/delta ungrounded transformer that feeds a switchgear of 600V breakers for various use.

We have a built in voltmeter that measures phase to phase voltages. It is reading:

Phase A-B: 600V
Phase B-C: 300V
Phase C-A: 200V

Any ideas what is going on here?
 
If your system is truly ungrounded, then Line to Ground voltages are meaningless and unpredictable.

You are measuring the coupling capacitance of your system, among other things your readings will depend on loading, meter impedance and system short circuit capacity.
 
nuckytaylor said:
I have a 4160V/600 delta/delta ungrounded transformer that feeds a switchgear of 600V breakers for various use.

We have a built in voltmeter that measures phase to phase voltages. It is reading:

Phase A-B: 600V
Phase B-C: 300V
Phase C-A: 200V

Any ideas what is going on here?
Click to expand...
Open primary fuse?
 
wwhitney said:
But the OP was about phase-phase voltages.

The reported voltages are physically impossible, as they violate the triangle inequality: 600 > 300 + 200.

Cheers, Wayne
Click to expand...
Yes, phase to phase thank you. Also, I was just guessing. It is an old analog meter fed from bus PT's. I double checked and the voltages are closer to

Phase A-B: 600V
Phase B-C: 350V
Phase C-A: 250V
 
LarryFine said:
Open primary fuse?
Click to expand...
nuckytaylor said:
Yes, phase to phase thank you. Also, I was just guessing. It is an old analog meter fed from bus PT's. I double checked and the voltages are closer to

Phase A-B: 600V
Phase B-C: 350V
Phase C-A: 250V
Click to expand...

I agree with Larry. With a primary fuse open, it would be "single phasing" and the 600V A-B voltage is then distributed across the 350V on B-C and 250V on C-A.
 
Loss of one primary phase would place two of the secondary side voltage in series combination which would be in parallel operation with the one transformer still receiving two phase voltages. If the bank was unloaded, you would read 600 volts, 300 volts, 300 volts. The most likely reason you read a 350 volt and 250 volt reading is due to the connected load on the secondary side.
 
LarryFine said:
No, on the 4160v primary. You didn't mention PTs at first.

Why? Does the 600v equipment still operate correctly?

Is the problem with the 600v feeders, or only the meters?
Click to expand...

The 600V equipment is still operating correctly. There are PT's on the 600V bus feeding a voltmeter that shows phase to phase voltages. This is where I am getting my readings.
 
nuckytaylor said:
The 600V equipment is still operating correctly. There are PT's on the 600V bus feeding a voltmeter that shows phase to phase voltages. This is where I am getting my readings.
Click to expand...
Then I would check the meter circuit wiring, and see whether the meter(s) agree with your voltmeter.
 
Are there two PT's that are connected in open delta? If so, maybe the common connection between the PT primaries is supposed to connect to the phase C bus but is disconnected. Then there would be just A and B connected to the PT primaries and providing 600V single phase across them. And so the voltage at the PT primary terminal for C would be somewhere between the 600V that is across A and B. In your case 350V of the 600V is across B-C, and 250V across C-A.
 
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