Open-Circuit Switch Fault-Tolerant Operation of Five-Level Nested Neutral Point Clamped Converter

Abstract

Five-level nested neutral point clamped (5L-NNPC) converter is a compelling topology to use in renewable energy conversion, grid-connected facilities, and motor drives. However, an open-circuit fault (OCF) in any of the switches in the NNPC results in the inverter shutdown. Therefore, to tackle this issue, this article presents a comprehensive fault-tolerant strategy including OCF detection, diagnosis, and postfault operation in 5L-NNPC. As an IGBT switch fails, the failed switch is diagnosed through a model-based two-layer strategy. Furthermore, taking advantage of model predictive control (MPC) and reconfiguring the switching states, a postfault operation strategy with the ability to provide partial power is developed. Due to the OCF, some switching states are removed and the phase voltages are unbalanced in postfault conditions. Therefore, the weighting factors of the multiobjective cost function are tuned automatically to improve the functionality of MPC, especially, in terms of balancing the voltages of flying capacitors (FCs). Accordingly, the introduced method exploits the remaining capacity of the converter, while the voltages of the capacitors are maintained. Simulation and experimental results verify the accuracy of the detection and diagnosis method and also, the capability of the converter to supply partial power to the load after an OCF occurrence.