A modified circulating current suppression control based on MMC grid connected using NLM

Abstract

High total harmonic distortion (THD) occurs when conventional circulating current suppression and modulation strategies fail to control arm circulating current in a modular multilevel converter (MMC) controlled by a virtual synchronous generator. This paper proposed a joint circulating current suppression technique that improves the proportional resonant controller's control mode and introduces the nearest level approach modulation (NLM) strategy for arm circulating current rectification. The MATLAB/Simulink program is used to conduct the simulations. The results show that: when the grid frequency does not fluctuate, the joint suppression strategy's arm current THD is 2.79%, 0.92% lower than the quasi-proportional resonant controller's (QPRC) THD; under primary frequency modulation, the suggested strategy's THD of the a-phase current is 1.75%, 1.03% lower than the QPRC's THD. Under typical operating conditions and with primary frequency management of the grid-type MMC, the results show that the proposed combined circulating current suppression technique may successfully lower total harmonic distortion (THD), eliminate bridge arm circulating current, and improve power quality.