Fractional frequency transmission system (FFTS) is an alternative for offshore wind power. As a AC/AC direct converter for FFTS, modular multilevel matrix converter (M3C) demonstrates many competitive advantages. However, the industrial frequency components and the fractional frequency components in M3C are directly coupled, so that the harmonic problem cannot be ignored. This paper mainly focuses on the causes of harmonic generation. Through the physical laws and theoretical calculations, the composition of each harmonic component and its sequence is obtained. The magnitude of the harmonics at each frequency is clearly seen, and the harmonic law in M3C is obtained. This lays the foundation for how to suppress the harmonics and circulating components in the control. Finally, the correctness of above research is verified by the simulation results in MATLAB/ Simulink.

modular multilevel matrix converter (M3C),harmonics,Fractional frequency transmission system (FFTS)