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

Abstract

In the post-fault operation of cascaded H-Bridge (CHB) inverters, a common-mode voltage (CMV) is injected into the phase voltages of the converter to achieve the maximum balanced line-to-line voltages. However, when the non-regenerative CHB inverter feeds a low power factor load (PF), the fundamental component of CMV (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 paper proposes a strategy consisting of two methods to tackle the mentioned problems. Also, three regions are determined, i.e., the non-overload area, the non-returnable power area, and the attainable zone for the FCMV. Then, A safe operating area (SOA) is obtained by overlapping these regions. The first method (Injecting Minimum FCMV (IMFCMV)) determines the CMV waveform with the least possible FCMV. This avoids the power flow back; however, it does not guarantee the non-overload 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 (Shifting FCMV (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.