Abstract
Fractional frequency transmission system (FFTS) is a promising integration method for long-distance offshore wind farms. The modular multilevel matrix converter serves as the frequency converter between the 50/3 Hz FFTS and the 50 Hz main grid; however, constant circulating current component for inter-arm energy balancing is observed when the M3C operates with frequency ratio near one-third. This paper investigates the origin and compensation method for this arm power imbalance. The 3rd-order harmonics in M3C is modelled utilizing the two-dimensional phasor method proposed by the authors in previous research. Subsequent analysis shows that the arm power imbalance is generated by coupling between 3rd-order harmonic neutral-point voltage and the system-side fundamental-frequency arm current component. Then, compensation method is proposed to eliminate the 3rd-order harmonic neutral-point voltage, which resultantly retains the arm power balance and suppresses the constant circulating current. At last, the proposed model and compensation strategy is verified by simulation studies carried out in MATLAB/Simulink environment.
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