A Carrier-Based Flexible Discontinuous Modulation Scheme for Three-Level Neutral-Point-Clamped Traction Inverter

Now-a-days multilevel converters are being considered for low voltage drives and electric vehicle (EV) applications. In EV applications, operating points vary dynamically causing deep variation in power factor and modulation depth. In this paper, a flexible modulation strategy is proposed for a three-level NPC inverter to fit-in such dynamic applications. Considering discontinuous modulation as the most efficient solution for such applications, the proposed modulation strategy provides the flexibility to shift its operating point seamlessly from sinusoidal PWM to generalized discontinuous modulation by allowing a small amount of additional low frequency oscillations. Moreover, a trade-off can be obtained to get the benefit of these two modulation patterns. Additionally, focusing on to the neutral point deviation problem of the NPC inverter, the proposed modulation strategy is capable of balancing two dc-link capacitor voltages effectively throughout the range. The effectiveness of the proposed scheme for a wide range of power factor and modulation index variations is studied through simulation by developing a MATLAB/SIMULINK model for a closed loop drives system. The efficacy of the proposed modulation scheme is validated through experimentation in a laboratory-scale prototype of NPC inverter with induction motor loads.