third harmonic neutral current

[e5fornow]

I have two 1800 kVA emergency standby generators that can parallel the utility. When either engine, alone or in parallel, is in parallel with the utility, I have excessive amounts of third harmonic neutral current. This will cause the utility and generator breakers to trip on ground faults. I have verified that I have same pitch generators(11/16). I am concerned that my utility transformer is to blame. I currently have a 1500kVA wye-wye transformer with both the primary and secondary neutrals grounded. It has been a concern that the high side neutral should not be grounded. I am in the process of trying to have the Utility transformer changed to a 2500kVA delta-wye. Do you think this will help?

 

[Alex68]

Hi e5fornow
have you discovered what the 3rd harmonic source is?
Perhaps it is the generator itself, since a particular internal construction. Then the question is: does it work with the neutral connected to ground?
Usually we run the emergency generator with the neutral isolated from ground because it is an "emergency" device and it must run also in case of phase to ground fault.

Surely a delta-wye transformer will help because the 3rd harmonic is stopped by the delta winding. You can also try to disconnect one of the neutral of the existing transformer from ground, the one of the generator side should be better.

Anyway I think the first thing to do is to find the harmonic source and the path of this harmonic current. And then you can decide what to do.

 

[paddyB]

The 3rd harmonic is generated by the pitch of the machine. 2/3 pitch eliminates it. However you have the correct pitch for grid connection. Neutral earthing is the cure. You need a fault limiting reactor or resistor on the neutal to earth to limit the third harmonic.

 

[jghrist]

With a wye-wye transformer, you might also find your generator trying to feed unbalanced load on the utility system. This can also cause tripping of the ground protection.

 

[dpc]

How is the generator grounded? A delta-wye utility transformer would be preferred over a wye-wye, but this may not solve your third-harmonic problem.

You may also need to review how your ground relaying is connected. Current in the NEUTRAL of a four-wire system should not cause operation of ground-fault relays.

What voltage is the secondary? If it is 480V and you have a 4-wire system, the ground relaying may not be properly connected.

All generators produce some third harmonic voltage, some more than others.

 

[jwerthman]

The third harmonic current is caused by the third harmonic voltage from the generator circulating through the utility transformer. With a GrdY-GrdY connection, the harmonic current will circulate out into the utility system.

Changing the utility transformer to a delta (primary) - GrdY (secondary) will only make your problem worse because the zero sequence path for the third harmonic current will be much lower impedance than it is now. It would eliminate the third harmonic from getting into the utility primary system, but the secondary third harmonic current will most likely increase.

Some practical solutions are:
1. Reactance ground the generator neutrals with sufficient impedance to limit the circulating third harmonic current. If the generators are supplying a 4-wire system, the required impedance may cause too much line-to-neutral voltage regulation when the system is isolated from the utility.
2. Unground the generator neutrals when operating in parallel with the utility transformer. The neutral(s) can be grounded when operating isolated from the utility or they can be left permanently ungrounded if a grounding transformer is installed to provide a system ground when isolated from the utility. If a grounding transformer is added, make sure that the total ground fault current from the utility and the grounding bank is not excessive.
This is a common problem that comes up over and over again.

 

[jbartos]

Suggestion: Normally, the utility side has the delta connection and secondary side has the wye connection. The proper generator operation would require:
1. To be grounded and the utility transformer ungrounded when the generator supply is the only one.
2. To be ungrounded when it is paralleled with the utility, and the utility transformer is grounded on the lower side, assuming the secondary or load side has the low system voltage level.

The grounding transformer could be used for the 3phase 3wire based system voltage. It would not be appropriate for 3phase 4wire based system voltage.

 

[dpc]

If this is a 4-wire 480/277v system, you cannot separately ground the generator neutral, if you want to run in parallel with utility. You need to bring the generator neutral back to the service as a neutral and connect to the neutral of your service.

This should eliminate the nuisance tripping of the ground fault protection, since the third harmonic current will be contained in the neutral wiring, not the ground.

You don't mention the actual amount of third-harmonic current, but normally, as system such as this can tolerate a fairly high third-harmonic current without problems. The effective RMS current is the square root of the sum of the squares of each harmonic. If your third-harmonic current is 30% of the fundamental, this corresponds to a heating effect of 4.4% additional. So for a 30% third harmonic current, you need to de-rate the machine by about 4%.