480V 3 phase System. One Leg to ground is unusually high. (347V/270V/272V)

[adougcole@gmail.com]

I'm just looking to see what your opinion is on this.

On our plantsite we have our own Primary Utility 21,000V system. With multiple transformer banks through out the plantsite. Primary side is an 21,000V ungrounded Y, secondary side is 480V ungrounded Delta. 480V 3-phase is our most commonly used Voltage in the sawmill.

At one particular location we have an odd reading to ground, being 347V/270V/272V to ground. I understand sometimes you'll get a high leg.. by maybe 15 or 20 volts. But this just seems like something is going bad.

Possible leg going to ground via one of our induction loads..? Winding going Bad on transformer?

We have many VFD's on this system, but we have gone through and shut off most of the loads and the voltage still remains the same.
There's one motor control center we have not been able to take off line at this point. (which we will eventually)

Here's the question, what's more probable, is this Load related or coming in from our transform bank?

 

[LarryFine]

With an ungrounded secondary, line-to-ground voltages are meaningless.

 

[Jraef]

With an ungrounded secondary, line-to-ground voltages are meaningless.

Meaningless in that the line-to-ground measurements will vary by virtue of system capacitance. So you can measure them, but that tells you nothing useful unless it's zero, in which case it tells you that you have gone from an ungrounded delta to a corner grounded delta.

 

[LarryFine]

Agreed, which is where ground-detection comes into play. Many believe it's better to intentionally ground one line than discover an accidentally-grounded one. Also, it limits over-voltage insulation or equipment damage from, for example, an accidental primary-to-secondary fault.