Neutral-Point-Shift-Based Active Thermal Control for a Modular Multilevel Converter Under a Single-Phase-to-Ground Fault.

The thermal design of a highly reliable modular multilevel converter (MMC) is significant for the voltage-source-converter-based high-voltage direct current system, especially under the unbalanced ac grid fault. In this paper, the analytical thermal model of the MMC considering the fault ride-through strategy is established to explore the MMC dynamic thermal behavior under the single-phase-to-ground fault. With the established thermal model, the proportional relationship between the MMC dc current and submodule thermal imbalance factor is derived. By exploring the features of dc current, it is discovered that the asymmetrical ac voltage makes the dc current different among three phases, leading to the thermal imbalance among three phases. To solve this issue, the neutral-point-shift-based active thermal control method, which can reshape the MMC ac voltage and balance the three-phase dc current, is proposed. With the proposed control method, the thermal distribution among three phases becomes balanced and the junction temperature of the most stressed power device is significantly reduced. Furthermore, the performance analysis shows that the proposed method brings little effects on the circulating current suppression. Finally, the major theoretical conclusions are verified by a laboratory MMC test bench. [ABSTRACT FROM AUTHOR]