Open-Circuit Fault-Tolerant Method for Three-Level T-Type Inverter With Voltage Gain Enhancing

The conventional three-level T-type inverter (3L-T2I) can solve open-circuit fault (OCF) at any switching devices by itself. However, the conventional fault-tolerant (FT) methods for upper switch of 3L-T2I have a low voltage gain which cannot ensure the inverter operation in low voltage range. In order to improve the voltage-gain of the inverter under faulty condition, this brief presents a new topology of FT-3L-T2I by adding one normally-closed relay. This extra relay connects the mid-point of DC-link voltage and the common-point of bi-directional switches, in normal operating. In post-fault operating, this relay is opened. A new FT 2-level space-vector modulation (SVM) technique is introduced to control the proposed inverter. Under this SVM method, the operation of faulty upper switch is recovered by bi-directional switches and upper switches of other healthy phases. By this way, the voltage gain of the FT-3L-T2I can increase 2/root 3 times than conventional methods. Unlike any traditional method, under this approach, the unbalanced neutral-point voltage cannot affect the output voltage. The experimental results based on 900-kVA laboratory prototype are introduced to verify the effectiveness of the proposed inverter and SVM control strategy.