Overmodulation strategy of NPC type 3-level inverter for traction drives

This paper proposes an overmodulation method for improving the voltage utilization of NPC type 3-level inverter for traction drives. The conventional railway vehicle has used a vector control to MI=0.907 and a slip-frequency control from MI=0.907 to the six-step mode. The slip-frequency control has less output torque dynamics than the vector control. In this paper, the linear region of NPC type 3-level inverter output voltage is extended to the six-step mode by using overmodulation strategy. As a result, vector control can be adapted in the whole regions and it leads to better output torque dynamics comparing with the conventional slip frequency control in overmodulation region. The overmodulation range is divided into two modes depending on the modulation index (MI), In the overmodulation region I, the reference angles are derived from the Fourier series expansion of the reference voltage corresponds to the Ml. In the overmodulation region II, the holding angles are also derived in the same way. Therefore, it is possible to obtain the linear control and the maximized utilization of PWM inverter output voltage.