Abstract:

In high-voltage direct-current (HVDC) systems based on MMCs, SPG fault on the MMC valve side may lead to serious consequences and affect the reliability of the system. Therefore, this paper analyzes the characteristics of SPG faults on the valve side of traditional hybrid MMC based HVDC system, and proposes a hybrid MMC topology based on thyristor branch loop to protect system under SPG faults on the valve side. In the hybrid thyristor branch loop based MMC, each arm is composed of full bridge submodule and half bridge submodule, and the thyristor branch connects the three-phase lower arm inductors to form a loop. In the case of SPG fault, the hybrid MMC based on thyristor branch loop can suppress the lower arm current, clamp the AC voltage on the MMC valve side, and shut off the MMC fault current by touching the thyristor branch loop. The proposed hybrid MMC based on thyristor loop has the advantages of low overvoltage, short current self-shutdown time, small number of full bridge submodule and low power consumption. In this paper, the effectiveness of the proposed topology and control is verified by PSCAD/EMTDC simulation and experiment.

Key words: high-voltage direct-current system, modular multilevel converter, single phase to ground fault on the valve side, thyristor-branch-loop