Modulation Strategy Impact on the Energy Storage Requirements of Modular Multilevel Converter-Based STATCOM

The modular multilevel converter (MMC) has proved to be a suitable converter topology for high-voltage direct current systems (HVDC). Some efforts can be identified in the literature to apply the MMC in static synchronous compensators (STATCOMs). However, the MMC has some challenges in the modulation schemes and energy storage requirements. Indeed, the literature has not presented a clear analysis of the modulation strategy impact on energy storage requirements. Thus, this paper introduces the spreading factor index to compare different modulation strategies in terms of capacitor voltage balancing capability. In addition, the impact of energy storage requirement on different modulation strategies is presented. In order to demonstrate the methodology, two strategies were selected: phase-shifted pulse-width modulation (PS-PWM) and nearest-level control with cell tolerance band algorithm (NLC-CTB). Subsequently, the MMC performance for each modulation strategy was compared in terms of energy storage requirements, capacitor voltage balancing capability, losses and current distortion. Besides, different operation conditions were analyzed in a case study based on a 15 MVA STATCOM. The results indicate that PS-PWM reduces the energy storage requirements, while the NLC-CTB modulation reduces the converter power losses.