Adaptive Current Limiting Control in Half-Bridge MMC for DC Fault

In a modular multilevel converter (MMC), the dc side fault current is characterized by a fast-rising rate and high peak values. To address this problem, an adaptive current-limiting control (ACLC) method, which is based on the bridge arm current, has been proposed. A dynamic ACLC model, aiming to suppress the fault current, has been constructed by introducing of a bypass ratio that reduces the dc voltage. This model incorporates the bridge arm current as a critical variable, giving a linearized current limit suppression curve for adaptive bypass ratio output. Additionally, the ac/dc coupling mechanism within the converter has been thoroughly analyzed. The influence of the ACLC on the ac side current has been examined, providing a theoretical basis for the upper limit of the bypass ratio constraint in both strong and weak ac systems. A half-bridge high-voltage direct current transmission technology based on the modular multilevel converter (HB-MMC-HVDC) network model was built in the real-time digital simulator to test the proposed ACLC method. The results show that the proposed ACLC method reduces the peak current values by 12.33% and 34.52% under strong ac and weak ac systems, respectively, compared with other fault current-limiting (FCL) methods, and has a better current-limiting effect.