Stability Analysis And Suppression Strategy On Sso Of Direct-derive PMSG Wind Farms

Most of the renewable energy equipment,such as wind power generation,are placed in remote areas.There is long-distance energy transmission as they connect to the grid.Therefore,the line impedance is large,and the system shows the characteristics of weak grid.In recent years,the sub-synchronous oscillation(SSO)caused by the dynamic interaction between direct drive permanent magnet synchronous generator(D-PMSG)wind turbine and power grid seriously threatens the safe and reliable operation of power grid system.Under the condition of weak grid,the interaction between the converter controller of DPMSG and the power grid is the main reason for SSO.In this paper,the grid side converter of direct drive wind turbine is taken as the research object,and the impedance analysis method is used to study the influence of parameters on the sub-synchronous and super-synchronous oscillation characteristics as well as the oscillation mechanism of the system under weak grid.A method to suppress the sub-synchronous oscillation is gonging to be proposed to improve the system stability of D-PMSG.Firstly,the machine side system is equivalent to a controllable current source,and then the main circuit impedance model of grid side converter will be established in the synchronous rotating coordinate system.In the practical D-PMSG system,there is disturbance on the DC link,which should be taken into account during the modeling.Then,based on the impedance model of the main circuit,the output impedance model of the grid side converter considering the effects of current loop,direct voltage control loop and phase-locked loop(PLL)is derived.Finally,the accuracy of the model will be verified by the method of frequency scanning.Based on the output impedance model of the grid side converter,the effects of the short circuit ratio(SCR)of the system and the change of controller parameters and main circuit parameters on SSO of the system will be analyzed.The simulation model of D-PMSG with rated power of 2MW is gonging to be built in MATLAB/Simulink,and the theoretical analysis will be preliminarily verified by the simulation.The simulation and theoretical analysis results show that the influence between the controller and the power grid aggravates when the SCR decreases,so that the system is more unstable.In addition,in a certain range,the increase of the bandwidth of PLL and the decrease of the proportional coefficient of the current loop and voltage loop will increase the negative damping frequency range of the output impedance,resulting in the resonance of the point of common coupling(PCC)voltage and current.Thus the influence of changing the steady state point on the stability characteristics of D-PMSG will be analyzed.The additional damping method is going to be proposed to reduce the frequency range of negative damping by depressing the influence of disturbance caused by each controller,so as to improve the characteristics of the system and enhance the stability of the grid-connected system.Finally,the theoretical analysis will be verified by the experimental results,and the effectiveness of the proposed method will be proved.