Analysis And Control Of Sub-synchronous Oscillation In DFIG-based Wind Farms

In the wind power industry,the doubly-fed induction generator(DFIG)has become the most widely used model in China due to its excellent performance and relatively low cost.In order to increase the transmission capacity and reduce the power loss,the series capacitor compensation technology is often used in the process of transmitting the power generated by the DFIG-based wind farms.However,the application of this technology may lead to the occurrence of sub-synchronous oscillation(SSO)accidents under some conditions,which will affect the stability of the power system and the safety of wind turbines.In order to solve this problem,the following aspects are studied in this paper:(1)The establishment of the mathematical model of DFIG-based wind farms connected to the grid through series compensation capacitors.The whole system consists of the following components: wind capturing module,shafting module,induction generator module,DC link module,rotor-side-converter module,grid-side-converter module,and the transmission line module.(2)The analysis of sub-synchronous characteristics in the DFIG-based wind farms connected to the gird.In order to explore the main factors that affect the sub-synchronous characteristics of the grid-connected system,the equivalent impedance model is derived.Then the Nyquist criteria is used to analyze the stability of the system under different working conditions.For the benefit of obtaining more accurate analysis results,the small signal model of the system is derived and the eigenvalue analysis method is carried out.The analysis results of the Nyquist method and the eigenvalue method show that the wind speed,the series compensation degree of the transmission line and the control parameters of the rotor-sideconverter are the main factors that affect the sub-synchronous stability of the system.(3)The exploring of the measures to suppress sub-synchronous oscillation in the DFIGbased wind farms connected to the grid.Based on the analysis results of the sub-synchronous characteristics of the system,this paper proposes two control strategies to suppress the subsynchronous oscillation.The first one is the control strategy based on the stator-side analog resistance.By improving the control scheme of the grid-side-converter,it can increase the equivalent resistance in the stator circuit under sub-synchronous frequencies.So this method can improve the system’s damping effect to the sub-synchronous component and accelerate its attenuation.The second is the control strategy based on the rotor-side analog inductor.By improving the control scheme of the rotor-side-converter,an equivalent analog inductor can be added to the rotor circuit.This method can change the sub-synchronous frequency,thus indirectly improving the damping level of the whole system.The simulation results in time domain verify that this two methods have good suppression effects on the sub-synchronous oscillation in the DFIG-based wind farms.However,if the parameters of the analog resistance and inductor are not selected properly,instability may be brought to the system.To solve this problem,the Particle Swarm Optimization(PSO)algorithm is applied to the parameter tuning of the analog impedance-based control strategy so that the best impedance value can be found according to the operating conditions.The simulation results in time domain show that the analog impedance-based control strategy combined with the PSO algorithm proposed in this paper can better suppress the sub-synchronous oscillation without bringing other adverse effects to the stability of the system.