current, and voltage imbalance

[sparkyroger]

I have a 12470 pri wye - 480v sec delta transformer bank consisting of (3) 333kva pole mount transformers. With lite load we have a small amperage imbalance. While the load increases, the amperage imbalance gets worse. By looking at the recordings taken, it looks like a capacitance issue. I am trying to narrow the problem down without any success. One of three transformers have been changed, but that did not fix the problem. A service ahead of this bank does not have these problems, so i think it is safe to say it is a transformer or secondary cabeling issue. Another thing noticed was that phase A - ground = 280v, phase b - ground = 480v, and phase c - ground = 290v. I know that it is an ungrounded delta, but usually to groung per phase will = around 300v. Any ideas out there?

 

[alehman]

Sounds like you have an unbalanced load. What is the load?

The neutral shift could well be due to unbalanced capacitance on the secondary distribution. Do you have any long single-phase circuits?

 

[sparkyroger]

The load is three phase motors on conveyors, screens, and crushers. There is really no single phase load to speak of, only lights.

 

[LionelHutz]

Well, current imbalance is cause by either voltage imbalance or is just a natural characteristic of the machine. So, you either have a phase-phase voltage imbalance problem or you have a load that runs at a current imbalance.

The phase-ground voltages may point to some type of capacitance to ground or resistance to ground problem but they could also just be a red herring. If one voltage was much closer to 0 then I'd investigate it further but not with those voltages.

 

[sparkyroger]

I know that this problem has just risen in the past few weeks. We have been having soft start motor starters fault out with this phase imbalance. I just can't put my finger on what is causing this imbalance. Over the weekend with nothing running, the phase imbalance is still showing up on recordings. If I turn the capacitor bank off, the imbalance gets worse.

 

[wareagle]

Sparky
Have you checked the transformer impedances. Are they the
same?

 

[sparkyroger]

Transformer impedances are the same.

 

[LionelHutz]

OK, what are the phase to phase voltages when this current imbalance exists????

 

[Borti]

An open delta winding would cause similar problems. I would test transformers ratio and winding resistance.

 

[sparkyroger]

Phase to phase voltages are :484v,485v,484v.
We also had sampled the transformer oil and had it analyzed. The results came back with no internal transformer problems.

 

[apowerengr]

If I understand your description of the service transformers correctly, for most utilities this bank would be operated ungrounded wye-delta thus avoiding the issue of the bank becoming a ground-fault source. As Borti suggests, if this bank is connected grounded wye-delta and one can is burned out or the utility has lost a phase to the bank, you could have voltage imbalances or the bank could be balancing the primary system's voltage imbalance. The only way to tell is to ground the high-side neutral (if it's not already), isolate the secondaries and take voltage checks phase-to-phase on all three transformers.

 

[LionelHutz]

Those are the 3 voltages as measured coming into your plant?

Are you measuring these currents and voltages after the transformers connections where you just have the 3 phases?

Have you looked at the phasing of the 3 phases to see if they are 120 degrees apart?

If you are measuring in your plant and the phase-phase voltages are balanced yet the phase currents are not I do not see how this could be a problem with the source unless there's a phase shift.

 

[Borti]

If you can rule out the transformer problem, try sectionalizing your load while monitoring the currents.

 

[wareagle]

Sparky posted "Another thing noticed was that phase A - ground = 280v, phase b - ground = 480v, and phase c - ground = 290v. I know that it is an ungrounded delta, but usually to ground per phase will = around 300v."
If you have a delta secondary Phase to gnd should be zero.
If you have one phase grounded the voltage should be
480, 480 and 0.
I agree with Borti. Put you voltmeter at the main and start the motor. If the voltage drop is large, the problem is between the main and the transformer. If the drop is small the problem is between the main and the motor. Then start isolating the circuit.

 

[waross]

It could be the "Discontinous Ground Fault Syndrome" or arcing ground fault.
If you have a piece of equipment such as a transformer, motor or ballast that is failing to ground, the system capacity may interact with the induction of the windings to form a crude spark gap transmitter. The winding may also transform the RF to a higher voltage.
Reading voltages to ground of 280V, 290V, and 480V is not reasonable if all the voltages are at 60Hz.
What is reasonable is 280V 60Hz, 290V 60 Hz, and a combination of about 270V 60Hz with a higher voltage at a higher frequency superimposed or "riding" on the phase. Your voltmeter interprets the combination of voltages as 480 volts. Depending on the type of meter you are using, the reading may or may not be dependable.
Try looking at the voltages with a scope and look for a high frequency riding on one phase.
Your current readings indicate the possibility of other problems but they may be related.
respectfully

 

[PhilDevine]

Unbalanced line to ground capacitance would be my first thought and could result in common mode noise issue on the front end of your drives.

 

[7anoter4]

I think one of the transformers may be damaged.
[two of the Delta connections are diconnected and one
phase winding is grounded].See the attachment.

 

[7anoter4]

Sorry,
B phase seems to be damaged [not C as shown in the drawing].
See pls. another attachment.