Review of Circulating Current Control Methods in Modular Multilevel Converter

Abstract

An equally challenging issue with modular multilevel converter (MMC) is the circulating current which usually flows through the arms of the converter and distorts the arms currents by introducing second order harmonics, add extra losses to the converter and impacts its components ratings and amplitude of the capacitor voltage ripples. It is as a result of the variation in the instantaneous voltages among the three phases of the converter that arise from the voltage variation in the DC capacitor voltages. Circulating current needs to be properly controlled in order to improve the performance of the converter. This paper reviewed the methods of controlling the AC part of the circulating current which is responsible for the double-line frequency component in the arm currents of MMC. A mathematical model of the converter was developed where it was established that the arm current consist of three components; a fundamental component which is half of the output AC current, a DC component that is responsible for active power exchange, and a second order harmonic component. The last two components constitute the circulating current. Further analysis was conducted on one phase leg due to symmetry. In particular, common and differential modes of one phase leg of the MMC circuit were considered and analyzed independently. For the case of the differential mode, it was established that the differential voltage can be used as a control tool to control the circulating current.