Line-to-Neutral Phase Imbalance

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mkmorris

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Location
Spartanburg, SC
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Reliability Engineer
We were having issues on a circuit last night, so we measured the line voltage upstream of the distribution panel and found the following. Apparently this has been an ongoing issue for 5 months and I am walking into it now. Line voltage measured this morning was as follows:

L1-L2: 452 VAC
L1-L3: 452 VAC
L2-L3: 456 VAC

The upstream transformer is Delta 13,200 down to Wye 480. We have a voltage drop because the 13.2kV we receive from utility is closer to 12.5kV coming in based on the taps. We hope to change the taps in time to get closer to 13.2kV when production allows us the downtime. Line to Ground was very imbalanced and I could not understand why. We have a resistive grounding system (That is in alarm), but I am not sure if that system would cause that imbalance, or if more likely there is an issue with the upstream transformer. I have not dealt with resistive grounding systems yet in life which is why I ask.

L1-G: 198.0 VAC
L2-G: 256.0 VAC
L3-G: 368.2 VAC

I cannot get the downtime to test the transformer (DLRO, TTR, DOBLE, etc). Any ideas on what may cause that line-to-neutral imbalance is greatly appreciated. Currents all seem balanced on the 480 side and upstream from the 13.2 breaker.
 
Something has to pull the neutral point over and L1-G voltage down. What are the line and the grounding resistor current readings? If the lines aren't fairly balanced, I might be looking for a L1-G fault. (It may be easier to chase this without downtime than the transformer's bond.)

The best troubleshooting tool right now is probably going to be your ammeter.
 
zbang said:
Something has to pull the neutral point over and L1-G voltage down. What are the line and the grounding resistor current readings? If the lines aren't fairly balanced, I might be looking for a L1-G fault. (It may be easier to chase this without downtime than the transformer's bond.)

The best troubleshooting tool right now is probably going to be your ammeter.
Click to expand...

It might also be an open-circuited grounding resistor that's allowing the L-G voltages to float. A current measurement on the grounding resistor (as zbang has mentioned), as well as a voltage measurement across it, could determine if this is the case.
 
A resistive ground system is just what the name describes. Instead of a solid bond between the system and ground you have a resistor.

In the event of a ground fault, the resistor limits current flow but causes extremely unbalanced L-G voltages.

Think about the situation where you have a bolted fault; the faulted line will be at 0V to ground, neutral will be at 277V to ground (with 277V applied to the grounding resistance), and the other two lines will be at 480V to ground.

So the numbers you are seeing are not a solid line-ground fault, but might be caused by a fault from a motor winding or in a heating element.

Or, as has been noted, if the grounding resistance has failed, then the system could now be ungrounded with L-G floating haphazardly.

-Jon
 
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