An MMC Circulating Current Suppressing Controller Based on Bridge Arm Common-Mode Voltage

Modular Multilevel Converter (MMC) bridge arm circulating current increases the MMC bridge arm current distortion and bridge arm current stress, whereas it also increases converter losses and capacitor voltage fluctuations. In this paper, based on the average value of the switching function of MMC, the mathematical relationship between the second harmonic component of the Bridge Arm Common-Mode Voltage (BACMV) and the Circulating Current Suppressing Controller (CCSC) is derived in the $d^{-2}q^{-2}$ rotating coordinate system. It proposed an MMC’s CCSC based on BACMV. The proposed controller consists of two PI controllers, namely the outer controller and the inner controller. The inner controller suppresses the common-mode voltage, and the outer controller eliminates the second harmonic component of the bridge arm common-mode voltage. The MMC-HVDC simulation model is established, and PI-CCSC, PR-CCSC, and BACMV-CCSC are used for comparative study. The results show that the overshoot of PR-CCSC is the largest and that of BACMV-CCSC is the smallest in response transient process. After entering the steady-state, the fluctuation of BACMV-CCSC is the smallest, whereas that of PI-CCSC and PR-CCSC is larger. The harmonic component of the secondary current of BACMV-CCSC is 0.08% and THD content is 0.39%, which is significantly lower than the other two suppression controllers. When the load changes abruptly, compared with the dynamic simulation, BACMV-CCSC has a smaller overshoot in the transient response process and has less disturbance after reaching the steady-state, which improves the steady-state performance of the system and the circulating current suppression controller effect, and provides a new idea about MMC circulating current suppression controller.