Mitigation of the low‐frequency neutral‐point current for three‐level T‐type inverters in three‐phase four‐wire systems

Large electrolytic capacitors are normally applied to maintain a stiff DC‐bus in uninterrupted power supply systems. However, the low‐frequency currents flow through them, which can reduce their lifespan and risk the system reliability. Therefore, this study investigates the neutral‐point current and corresponding suppression scenarios for three‐phase four‐wire three‐level T‐type inverters. First, the neutral‐point current for three‐level T‐type inverters is analysed and the mathematical expression is obtained. With the mathematical model, the neutral‐point currents in cases of different load conditions are investigated. In order to reduce the neutral‐point current and extend the lifespan of DC‐bus capacitors, a neutral‐point current suppression control strategy is proposed. The basic concept of the proposed control strategy and its effectiveness in cases of different load conditions are presented. Finally, a 30 kW T‐type three‐level inverter platform is built and the experimental results are presented to verify the theoretical analysis.