Comparison of Losses and Thermal Performance of a Three-Level Three-Phase Neutral-Point-Clamped dc-ac Converter under a Conventional NTV and the NTV2 Modulation Strategies

Several pulsewidth modulation strategies have been proposed for the three-level three-phase neutral-point-clamped DC-AC converter. Among them, the nearest-three virtual-space-vector (NTV² ) pulsewidth modulation (PWM) guarantees the DC-link capacitor voltage balance under any operating condition, provided that the addition of the three phase currents equals zero. However, this is achieved at the expense of a higher number of switching transitions per switching cycle, as compared to conventional nearest-three space vector (NTV) PWM. This paper presents a detailed loss and thermo-dynamical model of the converter. The model is used to quantitatively compare the converter losses and device thermal stress incurred by the NTV² PWM and a conventional NTV PWM, operating at the same switching frequency, and for different AC load angles. The results show that under low load angles the NTV² PWM presents a similar amount of losses and device junction temperature stress compared to the conventional NTV PWM, thanks to a better switch utilization