A Fault-Tolerant Strategy for Safe Operation of Cascaded H-Bridge Multilevel Inverter Under Faulty Condition

In the postfault operation of cascaded H-bridge (CHB) inverters, a CMV is injected into the phase voltages of the converter to achieve the maximum balanced line-to-line voltages. However, when the nonregenerative CHB inverter feeds a low power factor (PF) load, the FCMV may cause real power to return from the load to the dc side. On the other hand, when the inverter feeds a high PF load, the real power of some H-bridge cells may exceed the rated value. This article proposes a strategy consisting of two methods to tackle the mentioned problems. Also, three regions are determined, i.e., the nonoverload area, the nonreturnable power area, and the attainable zone for the FCMV. Then, a SOA is obtained by overlapping these regions. The first method (IMFCMV) determines the CMV waveform with the least possible FCMV. This avoids the power flow back; however, it does not guarantee the nonoverload operation of cells. So, it should be checked whether this FCMV is causing to overload operation of cells or not. If not, the proposed strategy uses the IMFCMV method. If yes, the second method (SFCMV) is used to shift FCMV into the SOA. In the SFCMV method, the determined CMV by the first method is modified so that its FCMV is located in the SOA. The simulation analysis and experimental results on the hardware prototype are provided to support the proposed strategy.