Research On Sub-synchonous Oscillation Suppressing Strategies For DFIG-based Wind Farms Into Series-compensated Transmission Systems

Due to the continuously increasing demand for renewable energy within the entire world,wind power generation has experienced fast growth during the past few decades.Wind power plants located far from load centers frequently require the update of the transmission infrastructure to accommodate the additional power.Series capacitor compensation is considered to be a more economical solution to increase the power transfer capability of an existing transmission line compared to construction of new transmission lines.Several wind farms in the United States and China already are connected to series-compensated lines to evacuate bulk power from wind resources.However,series capacitor compensation may induce sub-synchronous oscillation(SSO).SSO events have been reported since 2009 in series-compensated doubly fed induction generator(DFIG)-based wind farms in southern Texas,southwestern Minnesota,and northern China(Hebei Province).Therefore,it is necessary and urgent to study countermeasures for SSO damping over a wide range of operating conditions of wind farms.In this paper,sub-synchronous oscillation suppressing strategies for DFIG-based wind farms into series-compensated transmission systems are studied:(1)In order to study the SSO characteristics,the small-signal models of DFIG-based wind farms into series-compensated transmission systems are established.First,the impact of active power control of DFIG on SSO is analyzed considering wind turbine operating characteristic.Furthermore,the general rules of reactive power influencing SSO are studied in the entire operation region of DFIG.Then,eigenvalues and modal analysis tools are used to analyze and understand the interaction effect between control parameters in the rotor-side converter(RSC).(2)The impedance model of RSC control loop is derived to explain the impact of RSC on SSO.On the basis of this impedance model,a VIC is proposed to damp SSO by reshaping the output impedance of RSC,and its principle for SSO suppression is elaborate based on the basis of the impedance model of the rotor control loop for DFIG.To improve the performance and robustness of the VIC,an optimal VIC design method is proposed,which aims to ensure the maximum operating region considering wind speed variation.(3)A robust design method for the SSDC based on the practical small-signal stability region(PSSSR)is proposed.First,the concept of PSSSR is proposed,which ensures that every operating point in the stability region meets the damping requirements of power system operators.and the mathematical expression of its boundary is deduced.In addition,the boundary characteristics of PSSSR is studied.Then the rotor speed,active power and reactive power output of the DFIG are selected as the key uncertain parameters in the DFIG operating region due to their great impact on SSO.Furthermore,an optimal SSDC design method based on PSSSR,which aims to ensure the maximum operation range of the DFIG,is proposed and realized using a genetic algorithm.A detailed analysis is performed using eigenvalues and nonlinear time-domain simulations to evaluate the performance and robustness of the SSDC designed using the proposed method.