Unbalanced MMC Harmonic Circulation Suppression Based on PIR-VI Strategy

In recent years, modular multilevel converter (MMC) has been increasingly used in high-voltage DC transmission and grid-connected applications with unique advantages. In order to improve the efficiency of MMC, reduce the circuit loss and minimize the operation risk, many studies have been conducted on the harmonic loop current suppression of MMC circuits. When the grid is operated under unbalanced operating conditions, the DC component of the bridge arm circulating current is no longer equal and the traditional methods are no longer simply applicable. In this paper, based on the analysis of the theory of three-phase unbalanced MMC circuits, the strategy of proportional integral resonant-virtual impedance (PIRV-VI) composite controller is proposed and verified by simulation.