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Phase to earth voltages are heavily unbalanced
When we run the DG with neutral ungrounded, the phase to earth voltages are heavily unbalanced. And it is changing during the course of running.

We have a 415V DG Set for emergency shutdown of a thermal power plant.

The DG neutral was kept unearthed as per consultant specification for maximum availability. The load is mainly motors and battery charger and UPS. When we run the DG with neutral ungrounded, the phase to earth voltages are heavily unbalanced. And it is changing during the course of running also.

What could be happening?

By W.L. Mostia on 10 August, 2018 - 3:25 pm

Might you supply the actual voltage values you are talking about. In an ungrounded system, if you read from phase to ground, you are likely reading the distributed capacitance voltage. I do not know what a 415 V system will read, but a 120 AC system can read from 30-90 VAC to ground in a properly working system. Are your phase-to-phase and phase-to-neutral voltages correct?

In an ungrounded system, it is typical for lights are installed across the phases to indicate a phase-to-ground fault or some equivalent system is used. You are not indicting a phase-to-ground fault, are you? This can cause some voltage issues in an ungrounded system, e.g. the grounded phase will read ~1.73X the other phase to ground voltages.

Personally, I would have recommended a grounded system for a standby generator with monthly testing, periodic inspection, and good PM.

William (Bill) L. Mostia, Jr. PE
ISA Fellow
Sr. Functional Safety Consultant
SIS-SILverstone, LLC.
http://www.sissilverstone.com
281-728-3722

By W.L. Mostia on 10 August, 2018 - 4:48 pm

In my recent post, I made a misstatement as indicated below in { }, which needs to be deleted. I also corrected a typo.

"Might you supply the actual voltage values you are talking about. In an ungrounded system, if you read from phase to ground, you are likely reading the distributed capacitance voltage. I do not know what a 415 V system will read, but a 120 AC system can read from 30-90 VAC to ground in a properly working system. Are your phase to phase and phase to neutral voltages correct? In an ungrounded system, it is typical for lights are installed across the phases to indicate a phase to ground fault or some equivalent system is used. You are not indicating a phase to ground fault, are you? This can cause some voltage issues in an ungrounded system, { Delete:: e.g. the grounded phase will read ~1.73X the other phase to ground voltages}.

Personally, I would have recommended a grounded system for a standby generator with monthly testing, periodic inspection, and good PM."

William (Bill) L. Mostia, Jr. PE
ISA Fellow
Sr. Functional Safety Consultant
SIS-SILverstone, LLC.
http://www.sissilverstone.com
281-728-3722

The actual voltages are as follows.

L1-L2-415V, L2-L3-416V, L3-L1-416V
L1-N-124V, L2-N-346V, L3-N 335V

After running for some time it becomes like

L1-L2-415V, L2-L3-419V, L3-L1-416V
L1-N-008V, L2-N-414V, L3-N 409V

The measurement is done using PT's (415/rt3/110/Rt3) and the PT neutral is earthed.

We understand that there is no ground fault, because when we earth the neutral both ph-ph and ph-E(N) voltages are balanced.

By Bob Peterson on 19 August, 2018 - 4:51 am

If it is not connected to Earth, it is unsurprising that the phase to Earth voltage is not the same from phase to phase.

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Hold a charged battery in your hand without touching the terminals. Take a voltmeter and measure the voltage to ground (earth) from either terminal. It will usually start at a "high" voltage then decay to something NOT ZERO--could be very close to zero, but it will not be zero (0.000, or however many decimal places the meter being used has).

That is an ungrounded electrical "system"--the battery has a charge, but it is not grounded, so when the voltage to ground from either terminal is measured with respect to ground it should not be surprising that the voltage(s) are not zero, and could be anything in between battery voltage and not zero.

When a synchronous generator (three-phase) is not grounded, the same will happen. When you measure terminal to terminal, you will see rated generator terminal voltage (presuming the generator is running at rated (synchronous) speed, and the excitation system is working properly with the proper generator terminal voltage setpoint). Now, try measuring each phase to ground, and voila! Connect the generator to grounded loads, and the voltages with respect to ground are going to be varying.

In such a condition, how to sense an earth fault??