One dimensional feed-forward modulation of a cascaded H-bridge multi-level converter including capacitor balancing with reduced switching frequency

Abstract

Multi-level converters are well suited to medium and high power applications where the priority is for high quality a.c. currents with low harmonic content and for low switching losses. Suitable modulation schemes must find a good compromise between these two conflicting aims. This paper relates one-dimensional feed-forward modulation (1DFFM) to carrier based modulation. 1DFFM can produce high quality currents despite d.c. capacitor voltage ripple by accounting for measured capacitor voltages in the modulation process and can balance capacitor voltages by eliminating states that cause the capacitor voltages to diverge. It is shown in this paper that this method increases the device switching frequency compared to carrier-based modulation with the same sampling rate. An alternative modulation method is presented which balances the capacitor voltages by assigning commutations to bridges during each sampling period. This approach is less likely to cause two bridges to commutate at the same time so produces a lower device switching frequency than 1DFFM. It is shown experimentally that the two methods produce similar a.c. current distortion and both effectively balance the capacitor voltages .