A simplified charge balancing algorithm for modular multilevel converter

Abstract

Modular multilevel converter (MMC) is used in several applications of medium and high voltage transmission system as it has several advantages over other power electronic converters. For conventional converters, capacitor charge balancing is not a huge problem. Only the load ripple current flows through the DC link capacitor. Main load current doesn't affect the capacitor and it flows through the DC bus. For a MMC, charge balancing of all the module capacitors is a complicated task as the full load current flows through all the modules when MMC is operational. The module capacitors get charged and discharged very rapidly. Few charge balancing algorithms are already proposed in the literature. They have some disadvantages due to high execution time and complexity, as number of modules need to be inserted or bypassed is calculated in every step of operation. In this paper a simplified charge balancing algorithm is proposed depending upon the phase disposition modulation technique. In the proposed algorithm there is no need to calculate the number of modules that need to be inserted or bypassed during every step of operation. In addition, gate signals are given to the module switches directly. The new technique is fast and less complex. A nine level modular multilevel converter is simulated using the proposed algorithm in Matlab Simulink environment. All simulation results are presented in the paper.