unbalanced phase voltage in 3 phase 480V system

[vinod]

We have energized a 480V distribution panel yesterday. When we measured the voltage between Phase to ground (neutral), we got an unbalanced readings.

reading as follows;

Phase A to Ground >>>>326V
Phase B to Ground >>>>241V
Phase C to Ground >>>>287V,

however, the line voltage readings are balanced. ie 480-490v.

The nominal voltage between Phase to ground is 277V. we found this information on transformer name plate.

Please explain, What could be the cause of this unbalanced phase voltages?

 

[ActionDave]

It sounds like the XO on the transformer is not bonded or not bonded correctly.

 

[Smart $]

Faulty neutral between point of measurement and transformer is the usual culprit.

 

[vinod]

Thanks a lot for the clarification....
I will investigate the neutral and ground connections...

 

[Besoeker]

The nominal voltage between Phase to ground is 277V. we found this information on transformer name plate.

Wouldn't that be phase to neutral?
In service, the neutral may get connected to ground. It's probable. But the transformer nameplate can't give you phase to ground voltage.

 

[kwired]

Wouldn't that be phase to neutral?
In service, the neutral may get connected to ground. It's probable. But the transformer nameplate can't give you phase to ground voltage.

Correct, transformer nameplate will tell you voltage to the X0 terminal, should there be any kind of impedance inserted beyond that point (intentional or not) you may get other readings - and with readings like the OP has it is a good possibility that this is what has happened.

 

[Jraef]

If it is even a 4wire system! Having wildly variable ground reference voltage readings is common on an ungrounded Delta system, still quite common in industrial settings.

 

[kwired]

If it is even a 4wire system! Having wildly variable ground reference voltage readings is common on an ungrounded Delta system, still quite common in industrial settings.

I agree if it were an ungrounded system, but the fact that OP mentioned this:

The nominal voltage between Phase to ground is 277V. we found this information on transformer name plate.

We kind of must assume it is a 480/277 volt wye system and should be required to be grounded at the neutral - though it could have high impedance ground system installed, but if that is not malfunctioning should still give us balanced voltages to ground.

Unless a 480/277 system is permitted to be ungrounded. If it is permitted you can not have any 277 volt loads or it must have the neutral grounded AFAIK. Only situation I have heard of using a 480/277 spource ungrounded or more likely with a grounded phase would be when using a standby generator that is 480/277 to back up the ungrounded or grounded phase system, either case the neutral is not utilized though there is a point in the system where it exists. One could have a center tap in one phase (or even all three phases) of a delta system but not use it and effectively have about the same situation.

 

[kingpb]

If it is even a 4wire system! Having wildly variable ground reference voltage readings is common on an ungrounded Delta system, still quite common in industrial settings.

Your statement is correct. However, based on the fact that they are supposedly reading 277V off of the transformer nameplate, I would have to accept it is most likely a 480Y/277V transformer.

 

[Jraef]

I agree if it were an ungrounded system, but the fact that OP mentioned this: ...

Your statement is correct. However, based on the fact that they are supposedly reading 277V off of the transformer nameplate, I would have to accept it is most likely a 480Y/277V transformer.

Damn, I missed that detail... <slaps forehead>

I guess I saw the screwy phase-to-ground readings, applied my experience in recognizing that floating ground issue and failed to see the evidence not supporting it.

Still, the nameplate stating that it is a Wye transformer on the 480V side does not mean that the X0 is connected. (I'm not defending my stupidity here, I'm moving on...)

Given that the OP is in Saudi Arabia where 480V would not be commonly used, I would venture to guess that he has a piece of machinery from North America and someone has used a transformer to boost their voltage up to 480V. If so, and there were no Neutral loads, you would not want to connect the X0 terminal. The problem in that case then would just be that the OP is looking for something he is never going to find, a consistent voltage reference to ground on a floating ungrounded Wye system.

 

[Phil Corso]

Vinrod,

Phase-to-ground readings are varied because the phase-to-ground capacitance of connected apparatus (cable, xfmrs, motors, generators, surge-arrestors, etc) vary! Indicative of an ungrounded 3-phase system!

Regards, Phil Corso