A modified PWM scheme for three-level inverters with neutral point voltage balancing for EV applications

Abstract

In neutral-point-clamped (NPC) inverter, effective mitigation of neutral point (NP) voltage fluctuations still remains a challenging task. The popular pulse width modulation (PWM) schemes usually increase the number of switching transitions and, therefore, increase the switching loss while compensating NP voltage oscillations. In this paper, a modified modulation strategy by combining conventional sinusoidal PWM (SPWM) and double signal PWM (DSPWM) is proposed. The main attribute of the proposed strategy is the tradeoff between switching loss minimization while allowing some low-frequency oscillations in NP. By incorporating a variable bias signal, the modulation mode shifts seamlessly from SPWM to DSPWM and vice versa. Further, to mitigate existing unbalance in dc-link capacitor voltages that might have been created because of several non-idealities and dynamic operating conditions of the traction inverter for electric vehicles (EVs), an unbalance compensation signal of appropriate polarity is added with the modulating signals that satisfactorily generates compensating neutral current in the right direction. Simulation and experimental results are provided to verify the effectiveness of the proposed scheme.