Carrier-Based Fault Tolerant Control Strategies for Three-Level T-Type Inverter With Unbalanced Neutral-Point Voltage

For three-level T-type inverter (3LT2I), the unbalanced neutral-point (NP) voltage is required in some photovoltaic (PV) generation systems. Meanwhile, the unbalanced NP voltage easily leads to the fault of power switches. However, the existing fault tolerant control (FTC) strategies are only capable of the balanced NP voltage conditions. In order to simultaneously realize the FTC and unbalanced NP voltage control (UNPVC), a half cycle FTC (HC-FTC) strategy and a whole cycle FTC (WC-FTC) strategy are proposed in this paper. The carrier is modified to avoid using the impossible switching states and realize FTC. Meanwhile, the mathematical models of NP current under both normal and fault conditions are established based on volt-second balance principle, and thereby the zero-sequence component (ZSC) derived by the model can be used to realize the UNPVC whether the switch is fault or not. In addition, the ZSC is selected in detail to improve the NP voltage control ability in HC-FTC. Furthermore, the application range of HC-FTC is obtained and the WC-FTC with wider application range is proposed. The two proposed strategies can be selected according to the requirements. Finally, experimental results validate the effectiveness of the proposed strategies.