A Fault-Tolerant Control Strategy for Three-level Neutral-Point Clamped (NPC) Inverter

Abstract

When the open-circuit fault occurs on neutral-point clamped (NPC) inverters, it can cause the distortion of output currents, excessively large fluctuation of output voltages, and unbalanced neutral-point potential. Currently, several existing fault diagnosis methods are able to identify the fault types, but they are unable to identify the switching devices where the open-circuit faults occur. Furthermore, even if the open-circuit faults can be identified, the implications caused by open-circuit faults need to be further addressed. This thesis is focused on diagnosing the open-circuit faults and mitigating the problems caused by these faults. An effective fault diagnosis method is proposed in his study to identify the switching devices where the open-circuit faults occur. The characteristics of different distorted currents are firstly analyzed. By normalizing the sampling currents, calculating the average currents and comparing them with thresholds, different faulty switching devices can all be identified. To address the issues caused by these faults, a fault-tolerant control strategy is proposed. By replacing the impossible space vectors caused by these faults with other vectors which can generate the same output voltages, the problems of distortion of output currents and excessively large fluctuation of output voltages can be solved. But the excessively large difference of neutral-point potential still exists. To address the above issue, a model predictive control strategy is proposed to solve the problem of excessively large difference of neutral-point potential. A cost function is built to track the reference vector and suppress the difference of neutral-point potential. The Lagrange function is used to solve the action times of different space vectors. Furthermore, it is shown that when the neutral-point potential is initially unbalanced, the developed strategy is still able to make the potential balanced and greatly reduce its fluctuation.