Feedforward Modulation Technique for More Accurate Operation of Modular Multilevel Converters

Abstract

Modular multilevel converters have become the prominent topology for medium- and high-voltage applications. The performance of these converters highly depends on the accuracy of the used modulation approach, for which the capacitor voltage of submodules (SM) are usually assumed to be equal. This article exhibits that ignoring the capacitor voltage differences among SMs adversely affects the system performance. This becomes more obvious the larger the capacitor voltage differences are. Hence, this article proposes a more accurate feedforward modulation approach that takes into account either the instantaneous capacitor voltage value and the real output voltage in the modulation stage. As a result, in applications where larger SM voltage differences are expected, the current distortion and control performance are improved. Particularly, switching–saving approaches benefit from this method as it enables their operation with reduced switching losses without the downsides of increased distortion due to capacitor voltage differences. The proposed approach is analyzed and compared with the nearest-level modulation and with the level-shift PWM. Simulations and experimental validation are presented to confirm the effectiveness of the proposed technique.