Analysis And Suppression Of The Sub-synchronous Oscillation Of Power Sytsem With Large-scale Wind Farm Integration

In recent years,along with the rapid development of wind power in our country,series compensation and the high voltage direct current based on voltage source converter are usually used for sending out electric energy.This not only improves the flexibility of transmission system,but also brings about the problem of sub-synchronous oscillation(SSO),which threatens the safe and stable operation of the system.The main problems are as follows:On the one hand,induction generator effect(IGE)and sub-synchronous control interaction(SSCI)may occur when the onshore wind turbines are connected to the grid by series compensation and the series compensation is not suitable or the inverter parameters are changed.And on the other hand,as the shaft stiffness coefficient of the offshore wind turbines compared to the traditional onshore wind turbines has increased,when the large offshore wind farms by VSC-HVDC access to the grid,two sub-synchronous oscillations are triggered:the torsional vibration of the fan shaft and the sub-synchronous oscillation caused by the interaction between the wind turbine and the VSC-HVDC converter.Therefore,this paper studies the sub-synchronous oscillation caused by the grid connection between onshore and offshore wind farms,and puts forward the suppression measures.The main work is summarized as follows:(1)Taking the model as the research object,which the wind farms are respectively connected by series compensation and VSC-HVDC,a detailed mathematical model of each link is established,including DFIG and its converter control model,series compensation line model,VSC-HVDC and its converter station control model,which laid the theoretical foundation for the analysis of sub-synchronous oscillation.(2)An electromagnetic transient model of onshore wind farms which is considered load is constructed,and the eigenvalue method is used to analyze the generation mechanism of IGE?SSCI and the influence of the load on oscillation characteristics.The effect of the series compensation,wind speed and rotor inner-loop converter parameters k_p,k_i on system oscillation before and after the load is verified by an example system.(3)A small signal model of VSC-HVDC connected to offshore wind farms is set up.Two oscillation modes of shaft torsional vibration and the device interaction are identified by using the participation factors,and their corresponding strong state variables are identified.According to the two kinds of oscillation modes under the condition that the parameters of the strong correlated state variables are changed,the influence of the parameters of the strong correlated state variables on the damping performance of the system is further analyzed by drawing the root locus curve,which provides the basis for selecting the input signal of the damping controller.(4)Based on the signal testing method,a coordinated additional damping control is proposed.The sub-synchronous torque relationship of the shaft is changed and the damping of the system is improved by applying DFIG_SEDC and VSC_SSDC coordinated control from the DFIG excitation device and VSC rectifier station,respectively.The effectiveness of the sub-synchronous suppression measure is also verified by the results of the time-domain simulation.