A Common-Mode Voltage Suppression Strategy of Modular Multilevel Converter-Based Static Var Generator

The conventional carrier phase-shifted pulse-width modulation strategy may cause asymmetric output phase voltages in a modular multilevel converter (MMC)-based static var generator, leading to high-frequency variations in the system common-mode voltage(SCMV). The resultant common-mode currents exacerbate insulation degradation and reduce equipment lifespan. To address this issue, this paper proposes a common-mode voltage (CMV) suppression method based on dynamic correction of the number of inserted submodules. By imposing the zero-SCMV condition as an insertion-number constraint, the proposed method dynamically corrects the numbers of inserted submodules in the three-phase arms, thereby suppressing the high-frequency SCMV fluctuations caused by the modulation process. Furthermore, integrating circulating current suppression techniques reduces the double-line-frequency alternating-current components components in inter-phase circulating currents, thereby enhancing the overall operational performance of MMC. Experimental validation on an RT-LAB-based rapid prototyping platform confirms the proposed method effectively mitigates high-frequency CMV variations. The results also show that this SCMV reduction is accompanied by increased output voltage total harmonic distortion (THD), which suggests that the proposed method is a compromise suppression strategy rather than a fully optimized modulation approach.