Suppression of real power back flow of non-regenerative cascaded H-bridge inverters operating under faulty conditions

Good fault-tolerant ability is one of the advantages of cascaded H-bridge (CHB) inverters. When the faulty cell is bypassed, it is necessary to inject zero-sequence (ZS) voltage into three-phase output voltages to balance the line-to-line voltages. Conventional methods injecting ZS voltage is aimed at maximizing the achievable amplitude of the output line-to-line voltages. However, the injection of ZS voltage may lead to real power back flow in one or two phase. For CHB inverters which have no regeneration ability, the DC voltage will rise to an intolerable level. This paper analyzes the requirements for the ZS voltage to both achieve maximum output voltage and avoid real power back flow. Further, a method to suppress real power back flow by reducing the fundamental component in ZS voltage is proposed. Compared with conventional methods, the proposed method can allow the inverter to drive a load with lower power factor without leading real power back flow or sacrificing the voltage output capability. Thus the reliability of the CHB inverter is enhanced. Simulation results verify the validity and the advantage of the proposed method.