The Analysis And Nonlinear Suppression Strategy For Sub Synchronous Oscillation Of Doubly-fed Generator

With the rapid development of wind power and the large number of applications of series compensation and direct current transmission technologies,the occurrence of subsynchronous oscillations in wind power generation systems has become increasingly frequent.Subsynchronous oscillations will cause shafting oscillations and power oscillations of the unit,causing shaft-line fracture and system instability in severe cases.This paper takes the doubly-fed wind turbines as an example to study the subsynchronous oscillation in the system.The content is as follows:(1)For the modeling of the shafting system,this paper proposes a three-mass block shafting model based on the two-masses shafting model,that is,taking into account the flexibility of the blade,the blade is equivalent to a mass at its mass point.With the wind turbine mass block and the generator block,a three-mass block shafting model is constructed,which can not only reflect the dynamic process of the shafting but also reflect the dynamic process of the blade.In addition,other components of the doubly-fed wind turbine were also modeled,including the wind turbine model,variable pitch structure model,generator model,and converter model.(2)In this paper,the eigenstructure analysis method was used to analyze the axial model of two-mass and three-mass blocks respectively:1)For the two-mass shafting system,the natural frequency,amplitude and elasticity of the shafting oscillation were studied.The results show that the natural frequency of the shafting increases with the increase of the elastic coefficient of the shafting system,and the amplitude of the oscillation decreases with the increase of the elastic coefficient;2)For the model of the three-mass block shaft system,the blade and the shaft system are studied.The relationship between the natural frequency and amplitude of the oscillation and the elastic coefficients of the blade and the shaft system has been studied.The results show that the elastic coefficient of the shafting only affects the oscillation frequency and amplitude of the shafting system.The effect is similar to that of the two masses,and the oscillation of the blade is affected.The frequency and amplitude have little effect;but the leaf elasticity coefficient has an effect on both,but it only has a greater influence on the oscillation frequency and amplitude of the blade,and has less influence on the shafting system.(3)The nature of subsynchronous oscillation of doubly-fed wind turbines is still due to the lack of system damping that causes the continuous oscillation of shafting and machine-electric power.Therefore,the rotor excitation control can be used to adjust the system electromagnetic power and increase the system damping.In addition,considering the adjustment of the blade pitch angle on the mechanical power,the mechanical power and electromagnetic power can be controlled by the control of the pitch angle.Reach balance more quickly and calm oscillations.In this paper,the nonlinear control theory of target holographic feedback is used to design the rotor excitation control law and rotor rotor excitation and pitch angle coordination control laws for the two-mass and three-mass block shaft system respectively.The MATLAB simulation results verify the design of this paper.The control law can quickly suppress the oscillation of the shaft system and the blade,protect the safety of the shaft system and the blade,and improve the stability of the system.