Discussions on Switching-Ripple Voltages on the Common DC Link of the Modular Multilevel Cascade Inverter (MMCI-DSCC)

This paper describes a modular multilevel cascade inverter based on double-star chopper-cells (MMCI-DSCC) for a medium-voltage motor drive with a passive front end. Unlike the traditional two-level PWM inverter, the DSCC can operate without common dc-link capacitors because the DSCC itself possesses energy-storage components. However, a certain amount of switching-ripple voltage is generated in the common dc link when a passive front end is utilized. In this paper, the attenuation of the switching-ripple voltage by using a dc passive filter consisting of a series-connected film capacitor and resistor is attempted. A 400-V, 15-kW downscaled model is utilized to confirm the suitability of the design procedure developed in this paper. Experimental results show that the switching-ripple voltage can be attenuated satisfactorily by a small-capacity film capacitor. Moreover, the loss of the filter is negligible compared to the rated power of 15kW.