An Operation Method of VSC-HVDC under Exhaustion of SM Redundancy

In order to improve the operation reliability of modular multilevel converter based high voltage direct current transmission (MMC-HVDC) system, each arm of MMC will be equipped with some redundant sub-modules (SMs). Once the redundancy is exhausted, the MMC-HVDC system will be shut down immediately. To solve this problem, this paper proposes a method for continuous operation of MMC-HVDC after the exhaustion of SM redundancy. First, the operation principles after the exhaustion of redundancy are proposed. One is to use the variable level modulation technology to control the total energy storage of the arm at its rated level. The other one is to reshape the output power and regulate the maximum positive voltage of SMs at its maximum operating conditions. Secondly, based on the proposed operation principle, a numerical solution using the successive approximation and the dichotomy method is proposed to obtain the output power limit of MMC-HVDC after the redundancy is exhausted. Finally, Taking the Luxi back-to-back MMC-HVDC project for example, its power limit after the exhaustion of SM redundancy is analyzed to provide a technical basis for improving the operation capability.