Thyristor based modular multilevel converter with active full-bridge chain-link for forced commutation

Voltage source converter (VSC) such as modular multilevel converter (MMC) has been intensively studied for high voltage direct current (HVDC) applications due to its flexible terminal voltage adjustment, compatibility with weak ac grid and independent active/reactive power control over conventional line-commutated-converter (LCC). However, LCC using thyristors still offers advantages such as high current capability and low semiconductor losses, making it dominant in the construction of large scale converter station. This paper investigates the potential of using thyristor to build the MMC sub-module circuitry, where an auxiliary full-bridge chain-link (FB-CL) is adopted for the controlled transition and forced commutation of main thyristor-bridge, forming the proposed active forced commutated (AFC) thyristor cell. The AFC-MMC combines the advantages of LCC and MMC by the hybrid use of thyristor and self-commutated device, including the high power capacity, optimized efficiency, flexible control and enhanced fault-tolerant ability.