Analysis Of Subsynchronous Oscillation Mechanism And Suppression Measures Of Doubly-fed Wind Farm Delivery System Based On Sequence Impedance Modeling

With the rapid development of wind power and photovoltaic energy,the power system configuration and its characteristics are more and more complex,and the task of ensuring the security of power system becomes much more arduous.Doubly-fed induction generator(DFIG)is the most widely used wind turbines,it can improve the capability of long-distance transmission by series capacitor compensation.But at the same time,the sub-synchronous oscillation(SSO)caused by DFIG is getting prominent.Therefore,it is necessary to study the system structure of the doubly-fed wind farm delivery system,to reveal the sub-synchronous oscillation mechanism and put forward feasible suppression measures combining theoretical analysis with simulation modeling.Firstly,the impedance of the doubly-fed wind farm delivery system is derived analytically.Using the method of harmonic linearization,this paper analyzes the linear modeling of phase-locked loop(PLL),the transfer of harmonic component in the wind turbines and the frequency conversion between the stator and rotor.The positive and negative sequence impedance analytical formulas of grid-side converter(GSC)and rotor-side converter(RSC)are respectively derived in three-phase static coordinate system.The influence of the external loop control of RSC on the impedance characteristics is analyzed from the view of bandwidth.Finally,by connecting GSC and RSC in parallel,the positive and negative sequence impedance analytical formulas of DFIG are established considering GSC,RSC and PLL.Then,the simulation model of the doubly-fed wind farm delivery system is built on PSCAD/EMTDC platform and the system impedance is validated.This paper introduces the model structure of the system according to the basic ideas and typical problems in model building and debugging.And the steady state and dynamic characteristics of DFIG are studied by simulation.The paper also analyzes the influence of GSC on the equivalent impedance of DFIG.The results of theoretical deduction and simulation are verified by each other and the validity of the theoretical analysis and the simulation model of DFIG is demonstrated.Finally,the sub-synchronous suppression measures of series-compensated DFIG-based wind farms are presented.Based on Nyquist stability criterion,the influence factors on system stability such as wind turbine output,control parameters of current loop controller,series compensation and numbers of fans are analyzed quantitatively.It also makes a comparison and conclusion on the influence degrees of different factors.Besides,the impedance model of DFIG with the control strategy of voltage virtual synchronous-generator is established and its impedance characteristics are studied.The effect on the sub-synchronous suppression is verified based on PSCAD/EMTDC platform and field test.According to above research,this paper puts forward effective sub-synchronous suppression measures such as adjusting wind turbine output and control parameters of current loop controller,reducing series compensation and the numbers of fans,optimizing the control strategy of DFIG.The essence of the measures is to change the impedance characteristics of the system in the low frequency band.It has guiding significance for the sub-synchronous suppression and the system stability in practical engineering.