Medium-High Frequency Impedance Modeling of MMC and System Stability Analysis Considering Voltage Measurement Characteristics

In MMC impedance modelling, the existing research does not take into account the influence of the capacitive voltage transformer (CVT) measurement characteristics on the mid and high frequency bands of the MMC impedance, which may reduce the accuracy of the stability analysis of the flexible HVDC transmission system. By taking the south channel unit of the Yu’E flexible HVDC project as the research object, and based on an analysis of the influence of stray capacitance on the measurement characteristics of a 500kV fast saturated CVT, a MMC impedance modelling method is proposed using the multi-harmonic linearization method. The impedance analysis method is used to analyze the influence of the CVT broadband measurement characteristics on the stability of the HVDC system when the MMC station is connected to different AC systems and operates at different steady-state operating points. The results show that the CVT broadband measurement characteristics will cause the oscillation frequency of the system to change under certain working conditions. Through PSCAD/EMTTDC simulation, it is verified that the proposed MMC impedance model, which takes into account the CVT broadband measurement characteristics, can improve the accuracy of the stability analysis of the flexible HVDC system.