Analysis and Suppression of a Common Mode Resonance in the Cascaded H-Bridge Multilevel Inverter

The common mode (CM) voltage and current issues due to the parasitic ground capacitance have been well reported for the PWM converters. However, most of the previous work focused on the CM voltage generated at the motor side, or the input/output filter design for the standard compliance purpose. This paper identified a unique ground loop resonance phenomenon for the cascaded H-bridge multilevel inverter, which is generated internally due to the stray inductance of the bridge interconnection cable and the bus-to-ground parasitic capacitance. This resonance will increase the voltage and current stress in the power stage and therefore reduce the reliability of the system. The mechanism of the resonance is analyzed in detail and the corresponding suppression approach is discussed, proposing the preferred CM choke placement scheme. Simulation and experimental results provided verification of the resonance analysis and the effectiveness of the proposed suppression approach.