Analysis Of Sub-synchronous Oscillation In DFIG Plant And The Suppression Strategy Based On SVC

The wind energy,as a renewable and clean energy,has become one of the most potential energy because of its advantages of low cost,large reserves of resources and can be utilized on a large-scale.Owing to the reverse distribution pattern of China's wind energy resource rich region and the power load center,the wind power development in China exhibit the characteristics of "large-scale centralized development,long-distance transmission of high voltage".In order to improve the transmission capacity of the line,the series capacitor compensation technique is widely used,but at the same time it is easy to induce the sub-synchronous oscillation(SSO)of the system.In this paper,the SSO problem in the process of wind power transmission is studied:First of all,the generation mechanism,main analysis methods and control measures of SSO in wind power grid connected system are analyzed.With the doubly fed induction generator(DFIG)as the main research object,the mathematical model of the wind turbine,pitch angle control,generator,stator side converter and its control and the rotor side converter and its control are established in detail.And the vector control of DFIG is also studied.Secondly,the single machine equivalent model of DFIG plant is established in the PSCAD/EMTDC simulation environment.The generation mechanism of induction generator effect(IGE)and sub-synchronous control interaction(SSCI)in wind power grid connected system is theoretically deduced,and the conditions for their generation are obtained.Then a kind of equivalent replacement method is put forward to replace the DFIG converter system in the form of controllable equivalent power supply to analyze the influence of the converter control link.Combined with the time domain simulation method and characteristic root analysis method,the conclusions are obtained:when the DFIG runs below the synchronous speed,the SSO is generated by the interaction of IGE and SSCI,and when it runs above synchronous speed,there is no IGE,and the system generates SSO mainly attribute to SSCI;the converter section of the DFIG takes part in the SSO process,and it's negative damping effect is the main reason for the rapid divergence of the system power oscillation.On this basis,a SSO suppression strategy for DFIG plant based on static Var compensator(SVC)is proposed.The mechanism of the SVC inhibitory system SSO is analyzed theoretically,and the condition that SVC can provide positive damping for the system SSO is obtained.The SVC controller is designed,with the output voltage and active power signal of wind farm used as the control signal.For SVC parameters setting,a method of SVC parameters self tuning based on look-up table and interpolation algorithm is proposed to improve the adaptability of different operating conditions.Finally,the SVC parallel model is established in PSCAD/EMTDC simulation platform.The SVC suppression system SSO is simulated,and the results show that the SVC suppression strategy proposed in this paper can effectively suppress the system SSO.