Research On Sub-synchronous Oscillation Detection And Suppressing Technology For Wind Farms Connected With Transmission Systems

In recent years,with the rapid construction of wind farms,SSO accidents caused by DFIG occured many times,which seriously affects the stability of power grid.There are many uncertainties in wind power transmission system,which make the SSO presents the characteristics of strong nonlinearity,time-varying frequency.In order to detect and suppress SSO,the following research is carried out in this paper.(1)The mathematical model of wind turbine,control system,transmission system is derived.Then,considering the influence of actual wind speed variation on the output power of wind farm,an accurate hybrid wind speed model including random wind speed,wind shear and tower shadow effect is established.A typical DFIG-based wind farm transmission system model is built on the platform of Matlab/Simulink.The program of eigenvalue analysis is compiled,which lays theoretical foundation for subsequent study of SSO.(2)An on-line power oscillation detection method based on high-order cumulants and ESPRIT identification algorithm is proposed.Firstly,the measured data of wide area measurement system is need to be preprocessed,including eliminating and correcting abnormal data which seven-point second-order forward difference algorithm and polynomial interpolation are used respectively,second-order band-pass filtering and removing direct current.Further,the higher order cumulants are calculated to shield the influence of Gaussian noise on signal identification.Finally,the ESPRIT algorithm is used to identify the higher-order cumulants,and the amplitude,frequency,phase,attenuation coefficient can be obtained.(3)A virtual resistance control strategy based on adaptive filtering technology is proposed to suppress SSO.The RSC equivalent resistance formula considering integral and proportional coefficients is derived by establishing the equivalent model of RSC,and its accuracy is verified by sweeping frequency method.To adapte the change of SSO frequency,the adaptive filtering with adaptive center frequency is adopted to prevent the failure or the beat oscillation of virtual resistance controller(VRC).In addition,a design method of proportional parameters setting process of VRC based on RSC equivalent resistance is proposed.The validity of the VRC is proved by example analysis.(4)A variable step strategy to suppress SSO based on reactive power regulation is proposed.First,the influence of reactive power output on SSO under various operation conditions is studied.The result shows the inductive reactive power have positive effect on SSO suppression.Furthermore,the operating parameters that have great influence on SSO can be obtained.The value of reactive power required to restrain SSO under different operating conditions can be calculated according to eigenvalue analysis,then the BP neural network can be obtained.When SSO occurs,the reactive power required to suppress SSO can be obtained by BP neural network according to influencial operation parameters.Then the reactive power output is adjusted to suppress SSO.The control logic of this method is simple and easy to implement,and it has strong robustness.