Control scheme of transformer magnetic bias suppression in cascaded h-bridge multilevel inverters

When a transformer is connected to a cascaded H-bridge (CHB) multilevel converter, transformer magnetic bias usually appears. The mechanism of the transformer magnetic bias, is discussed, and it is concluded that the DC components of the transformer primary-side voltage and secondary-side current result in magnetic bias. Then several control strategies are proposed and compared to eliminate the DC-component voltage of the primary-side by suppressing the DC-component inductor current in the filter of the CHB inverter. The voltage feedback associated with hysteresis controller and magnetic bias suppression controller has faster dynamic response and more complete magnetic bias suppression. This control strategy is able to eliminate magnetic bias in CHB inverter completely in the resistance load switch on and off experiment, and the voltage restoring time gets less than 20 ms.