Small-Signal Impedance Modeling and Stability Analysis of the MMC-FTF Converter

In the dc collection and transmission of offshore wind power system, the single-phase modular-multilevel-converter-based front-to-front (MMC-FTF) converter is the key equipment to realize the voltage level conversion from medium-voltage convergence to high-voltage transmission. However, the small-signal stability of high-voltage direct-current (HVDC) system based on MMC-FTF converter is still unclear. In this paper, the dc impedance models of the primary and secondary sides of the MMC-FTF converter are established for the first time based on the multi-harmonic linearization method. Then, impedance criterion is used to reveal the oscillation risk existing in the interconnection between MMC-FTF and dc submarine cable, and the influence of controller parameters on the system stability is quantitatively evaluated for the first time according to the phase angle sensitivity. Finally, based on MATLAB/Simulink simulation and hardware-in-the-loop (HIL) real-time simulation, the correctness of the results is verified.