Research On High Frequency Stability And Resonance Suppression Strategy Of MMC-HVDC System

In recent years,high frequency oscillation accidents have occurred in several MMC-HVDC projects at home and abroad.The high frequency oscillation worsens the power quality.In serious cases,it will lead to converter station blocking and seriously threaten the safe and stable operation of power system.Therefore,the research on the high frequency stability and high frequency oscillation suppression strategy of MMC-HVDC system is of important theoretical and engineering significance to ensure the safe and stable operation of power system.In this thesis,the high frequency stability and high frequency oscillation suppression strategy of MMC-HVDC system are studied.The main work is as follows:(1)This thesis introduces the main circuit model,topology and control strategy of MMC-HVDC system,deduces the mathematical models of MMC converter station,current inner loop control,phase-locked loop control,circulation suppression and AC system considering the link delay,and then constructs the detailed mathematical model of MMC-HVDC system,which lays a theoretical foundation for analyzing the high frequency stability of MMC-HVDC system.(2)Firstly,the stability analysis method of time delay system based on generalized eigenvalue is applied to the high frequency stability analysis of MMC-HVDC system.This method does not need to transform the time delay term.The time delay stability margin and critical oscillation frequency of the system are directly obtained by constructing the matrix and solving the generalized eigenvalue,avoiding the repeated iterative process of finding the time delay stability margin.The effects of current inner loop control parameters,phase-locked loop control parameters and circulating current suppression parameters on the high frequency stability of the system are analyzed quantitatively based on the time delay stability margin.Secondly,the Lyapunov delay stability criterion based on free weight matrix is applied to the high frequency stability analysis of MMC-HVDC system.The criterion does not need to transform the delay term or solve the eigenvalue.By solving linear matrix inequality,the influence of link delay on the high frequency stability of system is directly analyzed.Finally,the effectiveness of the above two methods to analyze the high frequency stability of MMC-HVDC system is verified by time domain simulation.The stability analysis method of time delay system based on generalized eigenvalue and the Lyapunov time delay stability criterion based on free weight matrix applied in this thesis provide new ideas for analyzing the high frequency stability of MMC-HVDC system.(3)Through the model equivalent transformation,the influence of link delay on the MMC-HVDC system is equivalent to disturbance.A high frequency oscillation suppression strategy based on H_∞robust control is proposed,and the optimal H_∞robust controller is designed through mixed sensitivity optimization.The effectiveness of high frequency oscillation suppression strategy based on H_∞robust control is revealed theoretically by the stability analysis method of time delay system based on generalized eigenvalue.From the perspective of equivalent reduction of MMC-HVDC system link delay,a high frequency oscillation suppression strategy based on control signal advance predictive control is proposed.By predicting the control signal,the link delay of the MMC-HVDC system is equivalent reduced and the high frequency stability of the system is improved.The effectiveness and adaptability of the two high frequency oscillation suppression strategies proposed in this thesis are verified by time domain simulation.