Energy Balancing between HBSMs and FBSMs of Hybrid MMC during Fault Blocking

Abstract

Hybrid modular multilevel converter composed of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) has the ability to block the dc fault current. For hybrid MMC, both FBSMs and HBSMs will discharge to the dc side when the dc fault occurs. However, after the converter is blocked, only FBSMs absorbs the fault energy from the dc side and the arm inductors. Thus, the FBSM capacitor voltage may get too high, and the voltage difference between the FBSMs and the HBSMs will be large, making restart of the converter difficult. Balancing the energy between the HBSMs and the FBSMs can solve the two problems. The circulating current caused by line voltage could be used to balance the fault energy by adjusting the specific FBSMs state dynamically. In this paper, a control method is proposed with a dual-loop control strategy, which can reduce the voltage difference between HBSMs and FBSMs in the same arm to around 1.52% of their normal voltage. In the meantime, the overvoltage of the FBSMs is reduced to 35.8% from 58.2%. Thus, the overvoltage can be relieved and the restart can be accelerated.