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System stability issues occur when the VSC-HVDC system is connected to the weak grid. This phenomenon may be caused by the AC and DC dynamics characteristics between the converters in the system. Although it is convenient to use the DC transmission line and the large capacity DC filter to decouple the two AC systems from each other or to make the DC side of the system equivalent to a constant voltage source and so on to facilitate the model analysis and reduce the model order, the AC and DC dynamic characteristics between the converters will be neglected. The AC and DC dynamic characteristics can lead to inaccurate results when analyzing system stability, and cannot provide theoretical guidance for system control or stability prediction after short circuit ratio changes. Therefore, this paper establishes the equivalent DC impedance model with the AC side dynamic characteristics including frequency coupling and the equivalent AC side impedance model with the DC side dynamic characteristics. Through the analysis of the equivalent system impedance model, it is concluded that the system impedance model considering frequency coupling is more accurate to analyze the system stability. The conclusion is verified by Matlab / Simulink simulation. The conclusion is validated by Matlab/Simulink simulation.

For a single converter, AC and DC are coupled to each other; for a converter cascade system, the converters interact with each other in some cases, which in turn affects the stability of the system. In this paper, the HVDC system is taken as the research object, and the harmonic linearization method is adopted to consider the interaction between the DC side and the AC side to analyze the stability of the cascade system. Experimental tests are performed to verify the effectiveness of the proposed method. According to the results, relevant measures to improve the stability of the cascade system are proposed.