Active Thermal Control of Hybrid MMC Under Over-Modulation Operation

Abstract

Hybrid modular multilevel converter (Hybrid-MMC) has been considered as a more promising topology in VSC-HVDC by virtue of its DC fault ride-through ability. Additionally, the voltage modulation ratio m can be promoted by utilizing the negative arm voltage output so that the power transmission and efficiency can be enhanced. However, under over-modulation operation $(m \gt 1)$, the current and thermal distribution in HBSM and FBSM become different from that of traditional modulation $(m \lt 1)$ due to the more complex modulation mode and operation principle. In this paper, the thermal distribution under different modulation index m and different power factor operation is emphasized in research. Under a high modulation ratio and a high power factor, the thermal unbalance among the devices of SM becomes more critical. In term of this issue, symmetrical modulation based active thermal control, which utilizes FBSM's redundant switch status, is proposed. With the proposed thermal control method, the thermal distribution among FBSM' devices becomes more balanced and the thermal stress of the power device with the highest temperature is reduced significantly. In the end, the theoretical analyses are verified based on a laboratory MMC prototype.