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Generator Voltage Imbalance

Generator Voltage Imbalance

Generator Voltage Imbalance

(OP)
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?

RE: Generator Voltage Imbalance

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.

RE: Generator Voltage Imbalance

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

RE: Generator Voltage Imbalance

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

RE: Generator Voltage Imbalance

Thanks for the closure David.
Yours
Bill

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

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