Research On Sub-synchronous Oscillation Analysis And Suppression Strategy For Large-scale Series-compensated DFIG-based Wind Farm

Due to the reversed distribution of the natural resource and load centers,the large-scale,long-distance transmission and off-site consumption have become the main ways to develop and utilize the wind power resource.The series-compensation capacitor technology with the advantage of high performance price ratio,mature technology and significantly enhancing the line transmission capacity and system steady-state characteristics is widely used in the large-scale and long-distance wind generation system.While,this technology has the possibility of inducing the sub-synchronous oscillation(SSO)in the power system,which will seriously threaten the safe and stable operation of the power system and reduce its anti-disturbance capacity.On the other hand,due to the variable speed constant frequency operation,high power generation efficiency and flexibility active/reactive power control,the double fed induction generator(DFIG)has rapidly developed and become the mainstream generator type at the current wind farm.Based on the eigenvalue analysis and complex torque coefficient method,this paper focusing on the series-compensated DFIG-based wind farm aim to deeply analyze and research its SSO characteristics and influence law.In addition,this paper also proposes the corresponding SSO additional damping control strategy by using the unit’s regulation potential.The main research contents in this paper include:Firstly,the detail mathematical model of the series-compensated DFIG-based wind power generation system for the analysis of the wind power SSR is established,which includes the mathematical model of the wind turbine and shafting,induction generator,back-to-back PWM converter and its control system,dc capacitor’s dynamic process,and grid transmission line etc.On this basis,based on the eigenvalue analysis method,the linearization model and state matrix are deduced,meanwhile,the corresponding eigenvalues and correlation factors are calculated.Then the system oscillation mode analysis and the influence degree of relevant parameters can be determined.Secondly,based on the complex torque coefficient method and the DFIG electromagnetic torque linearization equation,the transfer function between the electromagnetic torque increment and rotational speed increment is deduced and its electromagnetic damping frequency characteristic curve also can be obtained.Furthermore,the explanation of the SSO mechanism of the series-compensated DFIG wind power system from the damping characteristics level can be realized.On this basis,the effects of wind speed,series compensation level,grid-connection number,transmission line and transformer resistance and the inner loop control parameters of the rotor side converter(RSC)and grid side converter(GSC)on the system damping characteristics are detailed analyzed.The correctness of the corresponding theoretical analysis can be verified by the eigenvalue analysis and time-domain simulation.Finally,referencing to the SSO suppression strategy of traditional thermal power unit,from the regulation potential of the DFIG unit itself,two kinds of SSO suppression strategies are proposed by improving the unit converter control strategy.One is to add an additional notch filter at the output of the current inner loop of converter to block the sub-synchronous component.Another is an improved additional damping control strategy based on the complex torque coefficient method and the improved particle swarm optimization(PSO)algorithm.By using the above two kinds of the suppression strategies,the SSO mode damping can be effectively improved.On this basis,by using the time domain simulation,the effectiveness and adaptability of the two kind suppression strategies are verified.