A DC-link Voltage Balancing Algorithm for Three-Level Neutral Point Clamped (NPC) Traction Inverter Drive in Field Weakening Region

A DC-link voltage balancing algorithm for a three-level neutral point clamped (NPC) traction inverter drive with interior permanent magnet synchronous machine (IPMSM) is proposed. The proposed strategy is able to reduce the neutral point potential fluctuation (NPPF) considerably compared to the conventional strategy with field weakening region, when the phase current starts to lead the phase voltage. A detailed analytical study is then carried out to show the root cause of higher DC-link capacitor voltage fluctuation in the field weakening region. The proposed strategy is based on the virtual space vector, where the medium voltage vectors are used only for 1/3rd of the total duty time. In this proposed strategy the positive and negative redundant voltage vectors are used separately in a switching cycle to keep the capacitor voltage difference low, even at high load torque changes. A 6.0 kW interior PMSM is used for simulation and experimental verification. Detailed simulation and experimental studies are carried out to show the efficacy of the proposed control strategy.