Carrier based PWM for even power distribution in cascaded H-bridge multilevel inverters within single power cycle

A cascaded H-bridge multilevel inverter offers important advantages over the conventional two-level topologies, though it necessitates isolated input dc sources. Even power distribution amongst these sources is an important control issue for an enhanced life-time of the overall system. In currently used control schemes, for `n' number of input sources and a fundamental period `T' for the output voltage, average power drawn from each source become equal after cyclic time spans of `n*T'. In this paper, a methodology is presented using which these spans are reduced to `T', irrespective of the number of input sources. In the proposed scheme, sine-triangle pulse-width-modulation (SPWM) technique has been espoused to obtain a `multilevel reference signal'. This signal is further manipulated, so as to impart flexibility for the selection of redundant states. Actual driving signals for the power switches are then derived from the manipulated multilevel reference signal, utilizing chosen states such that the respective average currents drawn from the input sources are repeatedly equalized within one power cycle of the output voltage. Such flexibility is not directly offered by the standard multilevel SPWM methods. Simulation and experimental results indicate that for two input sources and an output voltage of 50 Hz, the conventional `carrier rotation' scheme requires repeated time-spans of 0.04 sec for even power distribution whereas the proposed scheme requires only 0.02 sec. Thus, the proposed scheme leads to a better and effective utilization of sources, especially when the number of input sources is large.