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DC side dynamics are ignored in most of the research on grid-connected converter, which is dealt as a constant voltage source. In this case, the analysis result of small-signal stability is affected to some extent. In this paper, the impedance model of grid-following converter is reconstructed, then followed by stability analysis of the system considering DC side dynamics. Firstly, the applicable conditions are analyzed, that is to say, DC side is equivalent whether to voltage source or controlled current source. It is demonstrated that the DC side dynamics must be considered in case of small-signal stability analysis. Then, the impedance model of the grid-following converter is established based on harmonic state-space (HSS). Secondly, the stability of the system with different DC side structures is analyzed by Bode criterion under different grid strengths. Subsequently, it is revealed that the influence mechanism of grid strength on the stability of grid-following converter. In addition, the influence of different links on impedance characteristics of the system is analyzed, including phase-locked loop, current loop and filter. Finally, the theoretical analysis and electromagnetic transient simulation results show that under the condition of weak grid, the interaction between phase-locked loop and grid is strengthened, which reduces the small-signal stability of the system. Moreover, the critical short-circuit ratio of the system considering the DC side dynamics is larger.

The DC-side response characteristics of the VSC-HVDC transmission system under AC-side sub-synchronous disturbances may lead to the propagation of AC/DC oscillations, and then there may be non-fundamental frequency oscillation on the DC-side. To clarify the interactive influence mechanism and propagation evolution law of the AC and DC side sub-synchronous oscillations in the VSC-HVDC transmission system, the frequency characteristics of the DC-side oscillation power are investigated by considering the effect of converter control links. First, the AC-side voltage and current frequency responses under single/multiple SSO components are analyzed. Secondly, the frequency characteristics of the DC-side power after the injection of single/multiple SSO components are investigated. Meanwhile, the effect of system control parameters on the sub-synchronous responses of the DC-side power is investigated. Finally, the theoretical analysis is verified with PSCAD/EMTDC simulations. The results show that under the influence of the AC-side SSO components, the DC-side of VSC-HVDC faces the risk of multi-frequency oscillations; when there are multiple SSO components on the AC-side, there is a coupling between the components, and the system control parameters affect the amplitude of the DC-side oscillation power. [ABSTRACT FROM AUTHOR]