Wear-out failure analysis of modular multilevel converter-based STATCOM: The role of the modulation strategy and IGBT blocking voltage

The modular multilevel converter (MMC) is an attractive option for medium- and high-voltage static synchronous compensator (STATCOM) application. For this application, three aspects are relevant. Firstly, reliability and efficiency are directly related to the operation costs of this converter. Secondly, either high IGBT blocking voltage capability, featuring few number of submodules (SMs), or low-voltage IGBT modules, with a larger number of SMs, can be employed in an MMC-STATCOM. Lastly, either modulation strategies with fixed or variable switching frequency can be employed. Nevertheless, the literature does not present works evaluating these three aspects of an MMC-STATCOM at the same time. Therefore, this paper analyzes the impact of modulation strategies and number of SMs on wear-out failure and efficiency of an MMC-STATCOM. The effect of all SM components is considered. Two distinct modulation strategies are selected: phase-shifted pulse-width modulation (PS-PWM), with fixed switching frequency, and nearest-level control with cell tolerance band algorithm (NLC-CTB), with variable switching frequency. Four commercially available semiconductor blocking voltages are considered to evaluate the converter with different number of SMs. The results demonstrate that the number of SMs and the modulation strategy affect the MMC-STATCOM performance. Indeed, the converter presents different reliability at system-level and energy losses per year, depending on the IGBT blocking voltage and modulation strategy. The design with higher IGBT blocking voltage is the most affected.