Differnt Pitch Generators

[dalebrichards]

I read in another forum or thread that you need to make sure that generators are the same pitch in order to operate them in parallel. This is done to prevent circulating currents.

If I connect only one three phase neutral wire on the running set is parallel to handle the single phase loads how will I get circulating currents?

I used a properly sized contactor to switch between the generators in case one shut down and I made sure the single phase loads did not exceed the value of the rated generator.

I used this system to parallel 2 Cummins Engines and 1 Cat rental set for a hospital job.

I had no circulating currents.

Did I do something wrong? I mean it worked and the Cat was a different pitch from the 2 Cummins engines. The Cat was a 1 Meg set and the Cummins sets were 1 Meg each. I commissioned the system while working for a parallel controls manufacture.

Please let me know if my thinking is incorrect but don’t you need a common tied neutral to have circulating currents?

Thanks

 

[dpc]

If the generators neutrals are all grounded, you don't need a separate neutral conductor - the current can circulate via the earth.

The circulating current is generally third harmonic current caused by third harmonic voltage generated by the generators.

How do you know you didn't have circulating current?

 

[ePOWEReng]

Generators do not need to be the same pitch to operate in parallel, but you may have to de-rate the generator if there is too much circulating current.

 

[waross]

Generator output voltage is close to a sine wave but is not a perfect sine wave. The wave form depends on the instantaneous sum of the voltages of each coil group. With different winding pitches, the voltage wave forms of the generators will be slightly different.
Any time that the voltages on parallel generators are not equal, The generator with the higher voltage will generate more VARs. With unloaded generators, this will cause a circulating current. When the generators are loaded, the load current is often much more than the no-load circulating current and there is no more circulating current. You may find that the sum of the generator currents is greater than the load current.
It gets a little more fun with different winding pitches. One set will have a higher voltage at one point in the cycle and the other set will have a higher voltage at another point in the cycle. The resulting currents will probably be at 3 times base frequency or more and will be swamped by any appreciable amount of load current. Again the sum of the generator currents will be greater than the load current.

Bill
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Jimmy Carter