Recent advances in circulating current suppression strategies for modular multilevel converters

Abstract

Due to the fluctuation in power generation from renewable energy sources, multiple power electronic devices are required to ensure the stable operation of the electrical grid. Among these, the inverter, as a crucial power electronic device, is extensively employed in emerging technologies like flexible DC transmission. The modular multilevel converter (MMC) is currently widely used in various fields such as flexible DC transmission and wind or solar distributed power regulation systems. However, during the start-up and shutdown processes, the inter-phase circulating currents generated in the submodules can reduce the MMC's dynamic performance, safety, and stability. This paper provides an overview of the current research status of circulating current suppression in modular multilevel converters. Firstly, it introduces the principles of commonly used Proportional-Integral (PI) control and Quasi-Proportional Resonance (QPR) control. Secondly, it discusses the drawbacks of these two control methods and elaborates on several improvement approaches. Building upon this, it analyzes a novel control method that combines PI control with QPR control. Finally, it summarizes different circulating current suppression methods and offers prospects for future research directions.