Why does VFD cause instability in generators.

 Standby Engine–generator sets have a relatively high impedance and poor frequency regulation in comparison to the utility power. The VFD can have extremely fast response times to step changes in motor torque. The fast response of the VFD with the slow response of the generator could create the generator to became unstable and create a series of over voltage and under voltage surges. When the load on the VFD slows down, the rectifier become reverse biased and the input current drops off to zero immediately. Upon return of the load, the input current increases back to normal. These regular step changes in current cause the generator regulator to cause significant oscillations in the generated voltage.  Extending the acceleration and deceleration time may improve the VFD and generator issues.

The second issue is that the higher generator impedance causes the harmonic current distortion at the solid state VFD terminals to significantly increase causing the voltage waveform to become flat topped. The increased harmonic distortion causes all the other loads on the generator to see a distorted voltage waveform, thus increasing the current distortion on all the generator loads.