Research On Analysis And Suppression Of Sub-synchronous Oscillation Caused By Wind Farm With VSC-HVDC System Based On Impedance Method

With the rapid development of wind power,more and more large-scale wind farms are incorporated into the power grid.However,the wind power resources and load centers in China are mainly in the form of reverse distribution.Especially for the long-distance offshore wind farm grid connection,compared with the traditional AC grid connection method,VSC-HVDC system has unique advantages for wind power integration.At the same time,there is the threat of inducing the system sub-synchronous oscillation,which affects the safe and stable operation of the system.In this paper,the problem of sub-synchronous oscillation caused by DFIG-based wind farm integration through VSC-HVDC system is studied.The equivalent effect model of DFIG-based wind farm integration through VSC-HVDC system is established,and the oscillation mechanism and influence vibration are analyzed by impedance analysis and time domain simulation.The dominant factors of the oscillation characteristics are analyzed,and the optimization method for the controller parameters is proposed to suppress the sub-synchronous oscillation of the system.The main research contents of this article are as follows:(1)The impedance model of the DFIG-based wind farm integration through VSC-HVDC system is established,and the stability criterion for the sub-synchronous oscillation of the interconnected system is derived.Based on the detailed mathematical model of the DFIG-based wind farm integration through VSC-HVDC system,the impedance model of the DFIG-based wind farm and the VSC-HVDC wind farm-side converter station(WFVSC)is established,and the equivalent impedance model of the interconnected system is obtained.Based on the frequency-impedance characteristics of the system,the stability criterion of the sub-synchronous oscillation of the interconnected system is derived,which provides a basis for the occurrence conditions of the sub-synchronous oscillation of the interconnected system.At the same time,the mechanism of sub-synchronous oscillation caused by DFIG-based wind farm integration through VSC-HVDC system is revealed from the perspective of impedance.(2)Based on the impedance model,the influencing factors of the sub-synchronous oscillation characteristics of the system are analyzed.The impedance sensitivity is introduced to analyze the dominant factors of the controller parameters.Based on the established impedance model and the corresponding impedance frequency characteristic curve,the influencing factors of the sub-synchronous oscillation characteristics are analyzed.The results show that the number of grid connected wind turbine units,the wind speed fluctuation and the setting of each controller parameter will affect the oscillation characteristics of the interconnected system.The more the number of wind turbines and the lower the wind speed,the more prone to sub-synchronous oscillations in the interconnected system.For each controller parameter,with the increase of the ratio coefficient of voltage loop and current loop,the equivalent impedance of the system increases and the oscillation frequency decreases,while the influence of the ratio coefficient of current inner loop on the rotor side of DFIG is opposite.Furthermore,the impedance sensitivity analysis is introduced to verify that the proportional coefficient of the controller parameters plays a leading role in the equivalent impedance of the system,while the integral coefficient has little effect.(3)Based on the impedance method,an optimization design method of controller parameters is proposed to suppress the sub-synchronous oscillation of the system.According to the small disturbance stability mechanism of interconnected system and the amplitude-frequency characteristic curve of system impedance model,the conditions for the stability of interconnected system are derived,and the controller parameters are optimized for the purpose of suppressing the sub-synchronous oscillation of the system.Based on the above results,the main factor affecting the stability of the system is the proportion coefficient,so the relationship between the amplitude of the system impedance ratio amplitude-frequency characteristic and the control proportion coefficient of WFVSC is drawn,and the optimization range of the proportion coefficient satisfying the system stability conditions is obtained.Further,the feasibility of the proposed controller parameter optimization design scheme is verified by the time-domain simulation.