3rd HArmonics problem 2

[mmiro227]

We have a this site with three Caterpillar Generators that parallel. Two of them are 350kw and one is 300kw. Problem started when the third unti(350kw) was added. It is different pitch. The original two units are 15/16 pitch and the new unit is 2/3 pitch. To aliviate the third harmonics currents in the neutral, a reactor was added to this unit between neutral and ground. That solved the problem of circulating currents of this unit and the others when in parallel. The problem started when the 300kw, older unit, had to be replaced. A new generator was built to the exact 15/16 pitch of the original. Now there are neutral circulating currents between this unit and the others up to 400 amps when the unit is fully loaded. We know the problem are third harmonics according to test made with a VMI. Lifting the neutrals is not a viable solution in this case. Any ideas?
Thanks

 

[electricpete]

I'm not sure I understand the source of the harmonics.

As I understand it pitch will affect space harmonics within the motor but shouldn't affect output voltage. Have you monitored the voltage of generators running individually and do you see voltage harmonics there?

It seems possible that the harmonics are related to saturation. In that case you could help the problem by lowering voltage and also by lowering reactive power supplied by the generator (lowers the internal voltage drop). Just guessing here.

 

[dpc]

The generators will produce third harmonic voltage - they can be 10% of the fundamental voltage.

But I think there must be something else going on in this case. I would agree with electricpete that you could be overexciting the machine and operating in saturation. I would be suspicious that all of the 400A is really third harmonic current.

How are you regulating the machine output voltages? Do you have some type of reactive power compensation? It sounds a little like you have two (or three) voltage regulators fighting each other. You need to make sure all output voltages are roughly the same.

dpc

 

[mmiro227]

Thanks for your imput guys. We are using Caterpillar VR3 regulators, they are all identical, we made sure, by upgrading them all, also they are connected in cross current compensation. Right now we have a 20% third harmonics at full load. The excitation voltage was verified bringing the unit to full load by itself and in parallel with the others. When the unit parallel with any of the others or all of them the circulating currents at no load is 0. With only resistive load the hamonics are well below the 10%, and with the neutral lifted even at .8 power factor we are still withing limits. We are planning to add a ground resistor to limit the current to 20 amps.

 

[dpc]

Actually, the 20% third harmonic current isn't that big a problem as a thermal effect. The effective RMS current is the square root of the sum of the squares, so if your fundamental current is at 100% the addition of 20% third harmonic current increases the equivalent RMS current to only 102% of full load.

Of course, it is a concern for your ground fault relaying. But if the generators have the same pitch, there really should be no resultant third harmonic current, since the third harmonic output voltages should be the same.

Not sure what the output voltage is, but I'm sure you're aware that if you have any line-neutral loads, you can't add the resistors.

dpc

 

[gpattu]

Dear mmiro227,
I face exactly same problem. Please refer my thread No. 138-16017.
Whether 3rd harmonics are because of pitch ?
In your initial post you have mentioned about very high circulating current. whereas in your subsequent post you claim to reduce it to about 20% at full load. how do you achieved it ?
In my case I observe severe circulating current even at 0% load.
Also give me details of reactor you have used. Can e-mail me to narulaps@sancharnet.in. Thanks.

 

[AAK]

Hi,
I'm not sure 100%, but is it really the 3rd harmonic current we are dealing with here OR the normal return current from single phase loads. When paralleling generators, it is recommended that the neutral of only one of the parallelled generators be conneted to ground leaving the rest floating to avoid the problem of circulating currents. And as we know, the current uses the path of less resistivity, therefore, having a reactor in the path diverts the current from one generator to another.

Hoping that this is helpful.

AAK Best regards,
AAK