Modeling And Stability Study Of AC/DC Power System With VSC-HVDC

Because of its advantages in large-capacity long-distance transmission and asynchronous interconnection,high-voltage direct current transmission has been widely used.Due to the rapid development of self-shutdown power electronic devices,voltage source converters have been produced,and the penetration rate of flexible DC transmission based on it in the transmission system has also continued to increase.With the development of flexible DC transmission and the introduction of a large number of power electronic devices,the system has gradually exposed the problems of low inertia and weak damping.The low-frequency oscillation caused by the weak damping mode has become an important hidden danger for the stability and safety of the power grid.The introduction of a large number of power electronic devices also makes different time scales exist in the power system,and the system model becomes more complicated.If the original model of each module is still used for stability analysis and simulation calculation,it is easy to bring about the problems of large memory occupation,long simulation time and dimensional disaster.Therefore,it is necessary to study a simple and practical simplified model of AC-DC power system with VSC-HVDC to facilitate calculation and simulation,and to carry out in-depth discussion and research on system stability based on the simplified model.Firstly,this paper comprehensively considers the main circuit and control part of the AC/DC power system with VSC-HVDC,establishes the mathematical model of each module,and linearizes the nonlinear model of the system at the equilibrium point to obtain a small interference analysis model for the AC/DC power system with VSC-HVDC.The Prony identification is used to verify the established small interference stability model of the VSC-HVDC power system.The results show the accuracy of the small interference calculation results of the established model.Secondly,the small disturbance stability of the AC/DC power system with VSC-HVDC is analyzed by the eigenvalue method and the sensitivity method.The analysis results show that the system has 8 oscillation modes,and the control parameters have a great influence on the small disturbance stability.The singular perturbation theory is used to divide the fast and slow variables in the AC/DC power system with VSC-HVDC,and the fast variables are ignored in order to quickly reduce the dynamic order,and 3 models of reduced order for the system are obtained.The small interference stability of the original model and the 3 reduced-order models is analyzed and compared.The results show that the reduced-order model can retain the original oscillation mode of the system to varying degrees,and provides suggestions for studying the influence of control parameters on the small-interference stability.Finally,an appropriate reduced-order model is selected to analyze the influence of the control parameters on the stability of small disturbances,and the controller parameters that have a greater effect on the oscillation mode are determined in combination with the results of the sensitivity analysis.The genetic algorithm is used for optimization,and the small disturbance stability and dynamic response curve of the photovoltaic power generation system before and after parameter optimization are simulated and compared.The research results show that the optimization results of the original model and the reduced-order model are basically the same,and they can quickly return to a stable operating state under small disturbances,with good damping characteristics and improved stability.Moreover,the optimization method based on reduced-order model has faster calculation speed and higher efficiency,which verifies the effectiveness of the proposed method.