Analtsis od Surge Capacitor Lead Connections for the Protection of Motors

This analytical study uses a single- phase model to determine the effect of motor cable surge impedance and surge capacitor lead inductance on the protection of the motor against steep front motor starting switching surges. It is shown that switching surge voltages approaching 2 per unit can occur at the motor terminals unless capacitor leads are very short. The analysis covers the full range of cable surge impedances, surge capacitor lead lengths and spacings, and surge-protective capacitors usually encountered in installations of motors rated 2 kV to 15 kV. Capacitor value has negligible influence on the level of surge protection against surges with fronts 500 ns or less. Capacitor lead inductance (length and spacing) and cable surge impedance are the controlling parameters.