Transformer voltage unbalance

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NorthLine

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Electrician
have a newly installed transformer that has strange voltage readings. all the loads on this transformer are 3phase solar inverters. the transformer is 2000 KVA 480Y/277
A to ground 304V
B to ground 261V
C to ground 296V

Phase A to B 478V
Phase A to C 479V
Phase B to C 478 V

Cant figure out why the Line to ground voltage is unbalanced and high while the phase to phase voltage is correct.

Any ideas.
 
I'm sure we have a bonding jumper installed. This transformer was previously installed without any voltage issues. We removed it from service and sent it in for bushing repair. once it was returned we hooked it back up and noticed unbalanced phases to ground.
The odd thing is I see voltage unbalance at transformer secondary paddles, in switchgear but if I test at any of the 36 inverters it shows perfect L-G 278?
Any other suggestions
 
How far is the closest inverter from the transformer? If close enough then you could measure if there's any noticeable voltage difference between the ground reference at the transformer and at the inverter, using a wire over to one of the grounds as necessary.
 
My thought is harmonic distortion produced by the inverters that may be affecting your meter readings. What kind of VM are you using?
If there are disconnects between the inverters and xfmr then open them and measure voltages at the xfmr no-load.
 
I opened the main switchgear 3000amp breaker and tested the transformer with no load, still same results. Contacted the transformer company and it sounds like they forgot to install the internal bonding strap..... not to impressed but I'll continue trouble shooting for now.
 
NorthLine said:
I opened the main switchgear 3000amp breaker and tested the transformer with no load, still same results. Contacted the transformer company and it sounds like they forgot to install the internal bonding strap..... not to impressed but I'll continue trouble shooting for now.
Click to expand...
 
I have always thought that the jumper would be field installed to give me options other than a solidly grounded system. None of the industrial ones I have installed has an XO to ground connection. We made that connection at the first disconnect by installing a system bonding jumper, except for the couple of times, there was a resistance ground used.
 
So it was a missing SBJ after all?

On the very large kva transformers that we install the SBJ is usually factory installed (maybe it's ordered that way). I don't think that the SBJ is the same as a bonding strap or is it?
 
ptonsparky said:
I've never worked on a transformer anywhere near that size, so IDK who is supposed to make the connection.
Still, nice to hear the answer. Thanks
Click to expand...
On liquid filled units the jumper is usually installed at the factory under the liquid. On dry type units it is often just a field removable/installable link.
 
NorthLine said:
I opened the main switchgear 3000amp breaker and tested the transformer with no load, still same results. Contacted the transformer company and it sounds like they forgot to install the internal bonding strap..... not to impressed but I'll continue trouble shooting for now.
Click to expand...
A simple winding resistance test would catch something like this. I always recommend acceptance testing per the ANSI/NETA ATS...
 
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