Mechanism And Characteristics Analysis Of Subsynchronous Oscillation Caused By DFIG-based Wind Farm Integrated Into Grid Through VSC-HVDC System

Due to the inverse distribution of wind energy and electricity load centers in China,a suitable method for large-scale wind power transportation is in desperate need.Currently the most wind turbines in our country are belong to double fed induction generator(DFIG).And the voltage source converter based HVDC(VSC-HVDC)technology based on IGBT has an advantage in the situation of high-power and long distance electricity transmission.Therefore,VSC-HVDC will be applied to the large-scale transportation of wind power in our country.However,subsynchronous oscillation(SSO)may occur due to the interaction between DFIG-based wind farm and VSC-HVDC system,which will increase the operation risk of this method.In this context,the mechanism and characteristics analysis of SSO results from the interaction between DFIG-based wind farm and VSC-HVDC system is investigated in this paper.The time-domain simulation method,eigenvalue analytical method and impedance analytical method are all taken into account to analyse the SSO phenomenon.The impedance analytical method is adopted to explore the mechanism of SSO problem;moreover,the impact of the operating conditon of DFIG and PI controllers' parameters of system on the SSO characteristics are analysed.The main progresses in this paper are as follows:(1)The simulation model of multiple DFIGs interfaced with VSC-HVDC system is established,and the SSO phenomenon is investigated by the time-domain simulation.The eigenvalue method is also adopted to analyse the participation factors of the SSO mode.(2)The impedance model of DFIG-based wind farm and wind farm VSC(WFVSC)station are established.And on this basis,an impedance-based simplified equivalent circuit model of multiple DFIGs interfaced with WFVSC station is established.This model can be further represented as a RLC series resonance circuit,which can be used for judging the start-oscillating condition intuitively.(3)The impedance analytical method is adopted to explore the mechanism of SSO problem.Moreover,the impact of factors including online numbers of DFIGs,wind speed,loading level and PI controllers' parameters of system on the SSO characteristics are analysed based on the simplified model.Sensitivity indicators are introduced to assist the analysis of the impact of PI controllers' parameters on the system impedance.(4)The impedance models of DFIG which considers the influence of phase locked loop(PLL)is derived to study the effects of the PLL parameters on the system stability.