An Impedance Model of a VSC-HVDC System Considering DC-side Dynamics for AC grid Stability Analysis

AC side impedance modelling of an HVDC converter station usually ignores the DC-side dynamics, instead, the dynamics characteristic of cable and another converter station is represented as a constant power source or a constant voltage source. The influence of DC-side power network dynamics for AC-side small-signal stability is not yet well analysed. This paper presents a three-port transfer matrix model of the three-phase voltage source converter (VSC) which intuitively characterizes both AC- and DC-side dynamics by relating AC- and DC-side current and voltages. This model not only derive the equivalent impedance of the DC-side power network but also easily obtain a modified AC impedance model of VSC considering the impedance characteristic of the DC-side network. With comparing the impedance characteristics of the modified impedance model and the original impedance model, this paper shows that if the DC-side dynamics are ignored in an HVDC system, it may lead to an incorrect result of stability assessment. The proposed three-port transfer matrix model simplifies the impedance modelling process of the HVDC transmission system and the modified AC impedance model provides accurate stability analysis of the HVDC transmission system. Simulation results are given to validate the correctness of the proposed impedance model and the effectiveness of the modified AC impedance model for stability analysis.