Study On The Analysis Method Of SSO For DFIG Wind-farms Based On Frequency Domain Impedance Model

With the rapid development of China's economy,energy demand is increasing day by day.The limited nature of fossil fuels determines that it should increase the proportion of new energy sources in energy consumption such as wind energy.Even though the Series capacitor compensation technology has alleviated the problem of load center and energy reverse distribution in some extent,but also bring the risk of sub-synchronous oscillation(SSO)to the wind power system.At present,there is no mature analytical method for SSO of wind power system.In this paper,the DFIG frequency domain impedance model is established by combining the IEEE first synchronous oscillation model and the actual grid model in China.The frequency domain impedance analysis method for SSO of DFIG connected to grid with series compensation is deduecd,and the influence factors on SSO of DFIG connected to grid with series compensation are analyzed.Firstly,the mathematical model of each part of the system is established by combining the IEEE first synchronous oscillation model and the actual grid model in China;and the existing DFIG transient modeling method is improved,a point-to-point table of wind speed and rotor speed is established by least squares method based on maximum power control method in wind speed model.The DFIG equivalent frequency domain impedance model considering the grid side converter and the rotor converter is established based on induction motor impedance model and control loop characteristics,then a single DFIG equivalent impedance expression is derived.Secondly,the frequency domain impedance analysis method for SSO of DFIGs connected to grid with series compensation is gived based on DFIG equivalent frequency domain impedance.The qualitative analysis of SSO of wind power system is realized by analyzing the frequency characteristic curve of the real part and imaginary part of the system.And the rationality of the analytical method is further verified by simulations on PSCAD/EMTDC and numerical method with Matlab.The results show that the proposed method can accurately and qualitatively analyze whether the wind power system has SSO risk,as well as the SSO is convergent or divergent.Lastly,the influencing factors of SSO characteristics of DFIGs connected to grid with series compensation is studied.It turns that the number of windings,wind speed,DFIG numbers and control loop parameters will affect the SSO characteristics of the wind power system.