Fault recovery strategy for hybrid cascaded H-bridge multi-level inverters

In the configuration of the multilevel inverter which is used in this study, two cascaded H-bridge cells are connected in series with each phase of a three-phase three-level neutral-point clamped (NPC) inverter. The NPC inverter is fed by a single DC source; whereas, all of the cascaded H-bridge cells are supplied by capacitors. In this paper, in order to regulate the voltage across the H-bridge capacitors, a small fundamental harmonic is added to or subtracted from the original PWM reference of each H-bridge cell. In addition, the operation of the inverter during a fault in the cascaded H-bridge cells is studied and a method for recovery from fault and compensation of fault using the remaining H-bridge in the corresponding phase is proposed. This method allows acceptable operation of the inverter even when one of H-bridge cells is not functioning. Verification of the method using simulation shows the proper operation of the voltage capacitor regulation and strategy of fault recovery.