Abstract：In recent years, the problem of wide-band oscillations in MMC-HVDC system has attracted more and more attention. The interaction between the modular multilevel converter (MMC) and the system network is the main cause of oscillations. In the literature, the existing MMC single-side impedance/conductance modeling methods often do not take into account the dynamic processes of the opposite-end converter. Therefore, this paper focuses on the stability analysis of the whole two-terminal MMC-HVDC system by establishing a three-port hybrid parameter model of the converter, which can take into account the AC-DC coupling effect. Firstly, this paper derives the AC and DC port power conservation equation of the MMC converter considering the dynamic interaction process of harmonics between the bridge arms based on the harmonic state space method. On the basis of it, a three-port hybrid parameter model of the MMC converter is established. Then, the stability of MMC interconnected systems is analyzed based on the established three-port hybrid parameter model, which avoids the calculation of the number of zeros and poles in the right half plane of the open-loop transfer function matrix. Compared with the traditional single-side impedance stability analysis method, an accurate evaluation of the system stability can be guaranteed. Further, the sensitivity analysis formulae of the eigenvalues according to the elements of the three-port hybrid parameter model is derived, and the cause of the oscillation is disclosed. Finally, a DC current feed-forward additional damping control strategy for improving the stability of the DC side of the interconnected system is proposed and validated by the simulation. The results of this paper are as follows: firstly, the three-port hybrid parameter model established in this paper is entirely consistent with the actual values obtained from the frequency sweeping. Secondly, the analysis based on the examples shows that the traditional stability determination method will face the problem of misjudgment due to neglecting the number of right half-plane poles of the equivalent open-loop transfer function matrix. The open-loop transfer function matrix of the hybrid parameter model developed in this paper does not contain right half-plane poles. Thirdly, the stability analysis and sensitivity analysis show that the DC side of the double-end MMC-HVDC system may oscillate, which is induced by the interaction between the receiving end converter station and the sending end system. Based on the results of the sensitivity analysis, the design of additional damping control can be carried out. Based on the above results, the conclusions of this paper are as follows: (1) The three-port hybrid parameter model established in this paper can accurately reflect the small-signal characteristics of the AC and DC ports of the MMC. (2) The stability analysis method based on the three-port hybrid parameter model of the MMC and the generalized Nyquist criterion can accurately analyze the stability of the double-end MMC-HVDC system. (3) The negative resistive-inductive characteristics presented at the DC port of the receiving-end MMC converter and the capacitive characteristics presented at the sending-end MMC converter are the main causes of the DC-side oscillation. Besides, the additional damping control based on DC current feedforward can effectively suppress the DC side oscillation.
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