Effect of variable speed drives

[Besoeker]

Or similar.
The PWM waveform can cause partial discharge (PD) even when the applied voltage is lower than the insulation breakdown voltage.
It is a cumulative effect. When the insulation is subjected to continual partial discharge pulses, it progressively degrades. How long it takes depends on a number of factors not least among them being the PWM inverter switching frequency.

This from a report I did a few years when users were less aware of such potential problems than now:

Voltage Overshoot
At the points it was measured the voltage overshoot is within the limits of Technical Specification IEC TS60034-17 third edition. The limit given for the switching times measured is about 1200V. Using figures from the Gambica Technical Report No1, (second edition) the peak voltages recorded give a very low probability for partial discharge and even if it did cause PD, the number of pulses to breakdown would exceed 1013. Bearing in mind that the switching frequency is below 1 kHz the motor will never see that number of pulses.

Correction to the above.
" the number of pulses to breakdown would exceed 1013".
should read:

"the number of pulses to breakdown would exceed 10¹³. "

The cut and paste didn't format the same way here.

 

[don_resqcapt19]

I'm also guessing most or even all of them have line reactors on the output and it will help smooth the voltage peaks.

Only a couple of the very largest that have conductor runs over 500 feet have reactors. One is a deep well pump with a run of about 1,300 feet to get to the well head and the pump in about 400 feet down the well.