Sine wave distortion on paralleled Gensets

[tbwil1]

I have three gensets recently installed at a facility with mainly UPS and other electronic loads. They are identical 2000kW sets paralleled to a common emergency bus. The utility transformer feeding the facility is rated at 7500kVA. When the bldg load is powered by the utility transformer there is no noticeable distortion of the sine wave but when the bldg is transferred to gen power, the sine wave of all three phases on the bus show spikes IN the sine wave. There is no change in the amplitude or pitch, so there's not a regulator problem, just the little distorting spikes. I understand that there is considerably more copper in the transformer and an infinite grid offers more stability than the 6000kW gen bus, but with the amplitude and pitch stable doesnt this boil down to differences in the masses of copper of the two sources? Any ideas on reactors or any cost efficient solution would be appreciated.

 

[rcw retired EE]

I've had same symptoms many times on generators feeding UPS or other large rectifier equipment. As you stated ,it is the much lower internal impedance of the utility system that minimizes the voltage distortion caused by the harmonic current.

The generators have a higher internal impedance that amplifies the voltage drop of the harmonic currents flowing through the generators. The chopped-up rectifier current creates a chopped-up voltage drop that adds/subtracts from the sine wave produced by the generator. That creates tehhnon-sinusoidal output. Adding reactors will help smooth out the current and the voltage drops.

Try measuring the THD (Total Harmonic Distortion) of the generator voltages under load. If it is within limits, there is no problem other than the voltage wave form looking ugly.

 

[edison123]

Compare the wave patterns with and without the non-linear loads (UPS, computers etc.). If there is no change in wave pattern, then the generator has inherent THD. As wilson says, there is a limit on THD. I think it is about 1%. IEEE Std 100 ???

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[edison123]

BTW, the copper doesn't have anything to do with THD.

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