Resonance Overvoltages in Medium-Voltage Multilevel Drive Systems

Simulations and field tests indicate that unacceptable motor-terminal overvoltages and waveform shapes can occur in the normal operating range with high-output-voltage multilevel drive systems. These waveforms (if unattended) can result in premature motor insulation failures. A case study of an 11-kV multilevel system is presented. Simulations and calculations confirm the theory of resonance overvoltages. Different solution possibilities are analyzed. Further simulations and investigations are performed to determine the optimal carrier frequency. Test results confirm that the proposed modification has the desired effect, with waveforms well within limits stipulated by international standards. Simplified equations and recommendations are provided to determine suitable application solutions. Further simulated resonance case studies are presented, considering the effect of the system configurations, motor size, and cable length. Suitable carrier-frequency selection methods are presented to solve the problem.