High-Frequency Modeling for Determination of Motor Voltage Stresses in a 6.6 kV Variable Speed Drive

Variable speed drives (VSDs) cause motor overvoltage stresses to result from fast-front voltage waveforms emerging from the converter, in particular when filters are not installed at the converter output terminals. This work investigates modeling procedures for predicting the overvoltages on a 970 kW 6.6 kV induction motor that is fed from 41-m single-core cables. It is shown that frequency-dependent models for the cables and motor, developed from frequency sweep measurements, can properly simulate the motor overvoltage waveforms that result from converter side voltages with sub-microsecond voltage rise times. The models can also reproduce the damping observed in a 275 kHz oscillation in the motor voltage when the VSD system is operated at full voltage with a series inductor filter included. Usage of simplified models leads to substantial errors in the waveforms.