A Novel Circulating Current Controller for MMC Capacitor Voltage Fluctuation Suppression

This paper proposed a second harmonic capacitor voltage eliminating controller (SHCVEC) for capacitor voltage fluctuation suppression of modular multilevel converters (MMCs). The proposed controller consists of two PI controllers, namely the outer controller and the inner controller. The outer controller determines the reference value of the circulating current, and the inner controller generates the circulating current in each arm. The second harmonic capacitor voltage is controlled to zero by the outer PI controller, so there will be no static error between the generated second harmonic capacitor voltage and its reference value, which is the main improvement compared to other conventional methods. The PSCAD/EMTDC simulation verification of SHCVEC is carried out based on a 2-terminal MMC-HVDC system, and the effectiveness and feasibility of the proposed controller are proved. Furthermore, a comparison in terms of capacitor voltage fluctuation and rms arm current is made between the SHCVEC and the conventional circulating current suppressing controller, and the results demonstrate that the capacitor voltage fluctuation could be significantly reduced with only a small increase of rms arm current.