Installation considerations for multi-motor AC drives and filters used in metal industry applications

This paper investigates the zero sequence current that develops in the ground circuit of a multi-motor drive application. The motivation was to reduce the magnitude of common mode electrical noise in the system ground grid and to reduce occurrence of ground fault sensor trips as a result of this noise. The phenomenon of line to ground cable charging current (I/sub l/g) during drive switching is a main component of zero sequence current and is discussed with basic equations describing its magnitude. Results of a high frequency characterization study of all components in the zero sequence path was required to obtain estimates of surge impedance for these equations. Measured surge impedance was also used in a simulation program designed to predict I/sub lg/ magnitude for various system conditions. A PWM output filter and an input isolation transformer with the neutral high resistance grounded were two solutions investigated to reduce the zero sequence current that were both simulated and measured on site.