Wye To Delta Step Up Transformer Issues

[mblackway@bradleyelectro.]

I have a 1000KW generator feeding 277/480 wye to a step-up transformer, to 13.8kv primary delta. Once we leave the transformer and go to some switchgear, we have voltage fluctuating mostly on A phase. B phase and C phase are a little high on our readings at the switchgear through our processor. Everything on the switchgear was changed out. This was originally caused by too small of arrestors that blew on startup and a ground wire that was not tightened from the transformer. We have tried to lift the neutral and bond on the XO terminal, but we still get the same readings. Could the 1/0 ground wire be too small, and could there be a difference in potential between the grounding counterpoise of the transformer pad and the counterpoise of the switchgear? It seems as though we are hunting for a reference, but I cannot pinpoint the problem.

 

[electrofelon]

I have a 1000KW generator feeding 277/480 wye to a step-up transformer, to 13.8kv primary delta. Once we leave the transformer and go to some switchgear, we have voltage fluctuating mostly on A phase. B phase and C phase are a little high on our readings at the switchgear through our processor. Everything on the switchgear was changed out. This was originally caused by too small of arrestors that blew on startup and a ground wire that was not tightened from the transformer. We have tried to lift the neutral and bond on the XO terminal, but we still get the same readings. Could the 1/0 ground wire be too small, and could there be a difference in potential between the grounding counterpoise of the transformer pad and the counterpoise of the switchgear? It seems as though we are hunting for a reference, but I cannot pinpoint the problem.

Can you clarify what exactly you did when you 'lifted the bond' ? Did you remove the neutral connection entirely or just remove the XO to ground connection? I think what you want is leave the XO unbonded, unconnected, and do not connect the 480/277 neutral to it.

 

[electrofelon]

Also will add: Not sure exactly where you are taking your voltage readings, but sounds like the MV is ungrounded, and if so remember phase to ground voltage will be somewhat variable and meaningless.

 

[mblackway@bradleyelectro.]

Yes, the bond strap and neutral wiring were removed and taped up. However, it was not at that time removed from the generator since it is a complex job to remove it. Switchgear primary readings are as follows, but this is too much fluctuation at a microprocessor level. Here was the readings we got.

Transformer: Primary Side • H1A to H2A – 13.4KV • H1A to H3A – 14.4KV • H2A to H3A – 13.4KV • H1A to Ground – 7.5KV • H2A to Ground – 8.9KV • H3A to Ground – 8.1KV
Transformer: Secondary Side • X1 to Ground – 277V • X2 to Ground – 277V • X3 to Ground – 277V • X1 to X2 – 480V • X1 to X3 – 481V • X2 to X3 – 481V
Switchgear: • A Phase to B Phase – 14.3KV • A Phase to C Phase – 13.9KV • B Phase to C Phase – 13.9 KV • A to Ground – 8.0KV • B to Ground – 9.1KV • C to Ground – 7.3KV

 

[jim dungar]

The primary side Wye being grounded is often a problem. If the primary side is not allowed to float, the secondary voltages will often have poor voltage regulation.

If you are 13.8kV delta on your secondary, how have you grounded it?
Without a reliable ground reference your L-G voltages will not be consistent. They are impacted by their coupling capacitance and charging currents.

 

[mblackway@bradleyelectro.]

Yes, I have now figured out that the wye secondary to delta primary has to many constraints and the switchgear should have been configured for that set up. With no reference ground we should have had a 2 Potential Transformers phase to phase in an open delta to read the voltages at a micro level. Seems the site engineer did not realize this or know this at the time. Thank you for you help.

 

[electrofelon]

But even ignoring the phase -G voltages, it seems something is amuck. So just to be sure, the voltage you post and post number for were after the XO had been allowed to float?

 

[jim dungar]

Was your switchgear ordered for a wye? I would be worried about the surge arrestors and maybe the bussing insulation to ground.

 

[synchro]

Yes, the bond strap and neutral wiring were removed and taped up. However, it was not at that time removed from the generator since it is a complex job to remove it. ...

Are you sure that a bond between Xo and G was removed, and not just a bond strap between the transformer core and the enclosure which was removed? (assuming that this is a dry transformer)