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Neutral current on 13.2kv generator 2

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andiant

Electrical
Nov 16, 2005
1
Have a new wye 13.2kv genset with neutral lead bonded to ground on generator chassis along with frame ground. System is used for emergency backup and for peak shave during peak hours. All is well with building loads, but when peak shaving with utility there is 25 amps on the neutral and the 489 relay trips unit offline with ground current. Service coming in is 13.2kv delta (no neutral)stepping down to 4160 or 480 volt wye. Secondary of transformers do have neutral bond to ground. Do I see neutral grounding resistors or bonding neutral ground elsewhere coming in as a possible fix?
 
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Impedance grounding the generator would help, and would also limit the damage to the generator should it ever have an internal ground fault. Another option would be a 13.2 kV delta - 13.2 kV grounded-wye transformer on the output of the generator, thereby separating the generator zero-sequence network from the system zero-sequence network.
 
Medium-voltage generators should not be solidly-grounded unless you have to serve line-neutral loads.

You might want to put a scope on the current and see if it is fundamental or has third harmonics in it. But if the generator handles isolated operation OK, this probably is not the problem, and I think the SR489 filters out the harmonics.

If the generator connects directly to the utility distribution system at 13.2 kV, that's a bad idea. Their load imbalance could result in neutral current in your generator.

An isolation transformer, as David mentioned, is a good, if costly, solution.
 
Install a single phase grounding transformer between the generator neutral and earth with a resistor on the secondary of the transformer. Size the resistor for 10-40 amps primary (13.2 kV) current during a solid ground fault. Adjust your downstream relaying accordingly. The 489 has a grounding transformer voltage input that does ignore 3rd harmonics. (I don’t think the neutral overcurrent has a 3rd harmonic filter, better check.)

The added impedance will prevent the circulating currents you are seeing. If there is another grounded wye transformer on the 13.2 kV system somewhere, a path for unbalance current flow exists from the generator to the remote transformer and back through earth. IMO, any difference or unbalance in voltage between the utility voltage and the generator voltages will show up as neutral current flow.

Your generator may be putting out perfectly balanced voltages, but the utility system may have some imbalance at your service. The difference voltage is the driving voltage for the neutral/earth current.

Adding the grounding transformer increases the impedance of the earth-neutral circuit knocking down the ground currents.
 
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