Resonance Overvoltages in Medium Voltage Multilevel Drive System

Simulations and high voltage field tests indicate that unacceptable and unexpected motor terminal overvoltages and waveform shapes can occur in the normal operating range with multilevel drive systems. These waveforms (if unattended) would have resulted in premature motor insulation failures. A case study of an 11 kV multilevel system is discussed. 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.