Suppression of third-order harmonic current in transformerless MMC

Abstract

The modular multilevel converter (MMC) based high voltage DC (HVDC) system can be effectively used for bulk power transmission. The MMC topology for the voltage source converter (VSC) has several advantages. In this work, the transformerless operation of MMC is explored. The internal dynamic of MMC can induce a third-order zero-sequence harmonic current. Its effect on the system is analysed, including the adverse impacts on energy requirement per arm and power transfer capability. The internal dynamic equations of the transformerless MMC configuration are derived, and two different controllers: the proportional-resonant (PR) controller and proportional-integral (PI) controller, were applied to suppress the unwanted third-order current. The performance analysis of these controllers is presented and the results indicate that the controllers efficiently suppress the third-order harmonic current. Moreover, the electromagnetic transient (EMT) models of MMC under different configurations have been developed on the real-time digital simulator (RTDS) platform. An analysis of internal variables of the MMC is also included to ensure that the controller does not adversely affect the system. Lastly, the state-space model is developed, and the stability is analysed.