Combined Current Limiting Control Strategy of Main Converter under Flexible DC Distribution Line Fault

The DC short-circuit current of flexible DC distribution network based on modular multilevel converter has the characteristics of fast rise and high peak.In order to limit the short-circuit fault current, a combined current-limiting control strategy is proposed based on the capacitor discharge of the converter submodule and the active power fed to the AC side. Based on the change rate and amplitude of the fault current, the current limiting coefficient is adaptively changed to reduce the input number of submodules and restrain the rise of the fault current on the DC side. Considering the risk of overcurrent in the converter arm due to submodule deactivation, the dual closed-loop control of modular multilevel converter(MMC) is improved and zero clearing instruction is added to the outer loop control to suppress the active power fed in the AC side and reduce the peak current of the bridge arm. The simulation results in PSCAD show that compared with the current limiting control with infinite current and the current limiting control with submodules only, the breaking current of the DC circuit breaker is reduced by 22.87% and 4.12%, and the peak current of the bridge arm is reduced by 23.46% and 33.17%, effectively reducing the breaking current of the DC circuit breaker and the peak overcurrent of the bridge arm.