Modular Multilevel Converters (MMC) are emerging as a promising technology for high power conversion at high voltage levels. When frequency droop regulated MMC is used as a front-end converter to integrate wind farm into HVDC grid, variation in the fundamental operating frequency results in the change of circulating current harmonic frequencies. Due to this variation in harmonic frequencies, conventional time-domain based repetitive controller with fixed sampling frequency will fail to alleviate the circulating harmonic currents. In this paper, a spatial repetitive controller (SRC) based circulating current controller is proposed to effectively attenuate the harmonics in the circulating currents even under variable frequency operation of MMC, to improve its steady-state performance. SRC has a strong periodic disturbance rejection capability, therefore, can effectively alleviate the circulating harmonic currents and is independent of the fundamental AC line frequency of the MMC. A scaled-down laboratory prototype of a single phase, 5-level MMC is developed to experimentally validate the performance of the proposed control strategy. Experimental results are presented to substantiate the improvement in steady-state performance of the MMC with the proposed spatial repetitive based circulating current controller.
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