Quantitative Analysis of Harmonic Transfer for Modular Multilevel Converter

The harmonic transfer of the modular multilevel converter (MMC) leads to the harmonic intersection between the ac and dc sides and introduces additional harmonics, which worsen the power quality. Especially, the low-order harmonics, which are difficult to filter out, may cause harmonic resonance and the aging of equipment. In this article, the mechanism of harmonic transfer for MMC is revealed more easily and clearly based on the relationship between arm currents and voltages. The successive approximation method (SAM) is extended for the quantitative analysis of harmonic transfer. A digital-iteration scheme (DIS) is proposed to derive the general quantitative expressions of harmonic transfer of the MMC. The DIS is appropriate for both the conditions with and without considering the effect of the control system. By the iterative calculation, the harmonics can be calculated more accurately, especially the inner harmonics. The effectiveness of the quantitative analysis is verified by comparing the calculation, simulation, and experiment results.