An improved control method for suppressing capacitor voltage fluctuation of medium-voltage motor drives based on cascaded H-bridge

The cascaded H-bridge (CHB) has been an effective alternative to medium-voltage motor drive applications. Submodules with pulse width modulated (PWM) rectifiers have the ability of regenerating power and controlling reactive power, but capacitors of submodules suffer the problem of voltage fluctuation at twice the inverter frequency. Power decoupling control is effective to decrease pulsating voltage, but the current to generate decoupling power is complicated to calculate, which relies on real-time monitoring converter output, stator voltage and current of motor. And the use of phase-locked loops (PLL) to obtain the power angle of motor threatens the transient process stability and increases complexity. This paper proposes a simplified method to calculate decoupling current without using PLL and monitoring parameters mentioned above, and transient performance of capacitor voltage is improved. Simulation results based on 6kV, 710kW motor verify the validity of the proposed method.