A New Approach to Reduce DC-Bus Capacitance in Regenerative CHB Motor Drives

The cascaded H-bridge (CHB) converters with diode-front-end (DFE) structure have been widely adopted in non-regenerative high-power medium-voltage motor drives. In order to allow regenerative capability, the diode-front end (DFE) in each power cell can be replaced by an IGBT-based active-front-end (AFE) operating as a PWM rectifier. Due to the inherent instantaneous power unbalance in the AFE-based CHB power cells, the DC-bus capacitance needs to be increased to reduce DC-bus voltage ripples. To reduce the DC-bus capacitance, control degrees of freedom provided by the PWM rectifiers can be exploited to mitigate the DC-bus voltage ripples. This paper proposes a new controller for regenerative CHB converters to perform DC-bus voltage control and ripple mitigation while maintaining high dynamic performance and canceling low order harmonics at the grid side to meet the grid standards. The effectiveness of the proposed control scheme is validated experimentally on a seven-level regenerative CHB drive.