Generator Voltage Imbalance

[bwgibb]

Remote facility with four 4160V 2600kW generators in parallel. All gen's each have a NGR on the neutral. There is voltage imbalance of 6%, regardless of which or how many generators are running, with zero load. The bus line-to-line voltage is equal (4.15kV) but the line-to-neutral varies (2.3kV for phase A, 2.6kV for phase B, and 2.5kV for phase C, roughly).

Each generator has Y connected PT's on the 4160V side, right at the stator connections. This MAY cause some circulating currents if the Y connected PT neutral is grounded, but it shouldn't cause a floating neutral.

The common bus has a DELTA-WYE PT. Both PT's (bus and each generator PT) show the same imbalance.

All NGR's have been inspected to ensure continuity to ground.

A couple points... The voltage imbalance is apparent regardless if gen 1, 2, 3 or 4 is on, and is apparent whether there is no load (all feeder breakers open on the common bus) or any value of load on (1200kW load bank and/or plant load).

Having a tough time finding where the floating neutral is coming from. Anybody have some ideas?

 

[davidbeach]

What current does NGR allow for an SLG fault? What is the charging current of the system? If the NGR doesn't allow more than charging current, you're still running a capacitively grounded system (aka an ungrounded system).

But that aside, what does it matter? You're on a system where only phase-phase voltages matter; the phase-ground voltages are an interesting curiosity unless you're trying to troubleshoot some abnormality (this isn't). What happens if you measure phase-neutral rather than phase-ground?

What's the primary voltage on your wye connected PTs? If you have a 2400 volt rating you might want to replace them with PTs having a 4160V primary rating, but leave the ratio the same. If, for instance, you have 2400:120 (20:1) PTs you might consider replacing them with 4200:210 PTs, still connected in wye. That 2400V primary would only be appropriate on a solidly grounded system. I've seen part cycle PT saturation cause all sorts of trouble on high resistance grounded system when the PT was sized for the supposed phase-neutral voltage; replacement with properly rated PTs solved the problems.

 

[waross]

DavidBeach;
A few years ago you shared an issue with strange neutral voltages. I have always wondered about that. Did that turn out to be a saturation related issue or metering errors?
Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Saturation. Anyway it want away when phase-phase rated PTs replaced the original phase-neutral rated PTs.

 

[waross]

Thanks for the closure David.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter