Utility capacitor switching and adjustable-speed drives

The authors consider the effect of capacitor switching on adjustable-speed drives. Surges of less than 200% magnitude caused by utility capacitor bank switching can disrupt operation of adjustable-speed drives. Pulse width modulated (PWM) drives are more sensitive to surges because the drive transistors have lower overvoltage tolerance than silicon controlled rectifier (SCR) drives. The authors use the results of a case study to analyze the circumstances of drives tripped by utility capacitor switching. It is shown that low loss utility capacitors increase the surge magnitude by decreasing system damping. Surge magnification is possible at buses remote from the capacitor, and the three conditions for magnification are described. It is also shown that drive sensitivity to the surge depends on whether the drive filter inductor is saturated by the surge