Abstract: A system for regulating the voltage by switching capacitor banks and for preventing damage due to harmonic resonance in an AC power supply line after a capacitor bank is switched. The system includes a detector for detecting the harmonic distortion in the supply line, and multiple capacitor banks connected in parallel to the supply line. At least one switch is in series with each capacitor bank to connect or disconnect the capacitor bank to the supply line. A controller operates the switches and receives input from the detector. After a capacitor bank is switched to improve conditions in the supply line, the controller compares the total harmonic distortion to an action threshold and switches capacitor banks as programmed when the total harmonic distortion exceeds the action threshold for a predetermined period of time in order to avoid a resonant condition that could cause damage to equipment.