A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage DC (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in DFIG-based wind farm with series-compensation line or wind farm integration through two-level VSC-HVDC transmission system, because the internal dynamics of the MMC may have significant

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... mpact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space (HSS) modeling method is applied to model the multi-frequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method. Index Terms-Modular multilevel converter (MMC), HVDC, wind farm, stability, controller parameter design. Statement: This paper is original and has never been submitted to any other journals or conferences. Jing Lyu is the corresponding author for this work.