Analysis On Subsynchronous Oscillation Damping Characteristics And Suppression Strategy Of Virtual Synchronous Wind Power Grid-connected Syste

Virtual synchronous generator(VSG)mimics the external characteristics of synchronous generator and have the active ability to provide inertial support and frequency response capability for the grid,proposing a novel solution to the prominent problems of low inertia and weak damping in the wind power systems.On this basis,it is increasingly gaining attention in the field of wind power control technology.However,the wind power system represented by double-fed induction genetator(DFIG)is subjected to the subsynchronous control interactions when connected to the grid by series-complemented capacitors.Furthermore,the access to virtual synchronous control also significantly affects the mechanism and dynamic characteristics of subsynchronous control interactions and there is an urgent need to research and analyze the damping characteristics of subsynchronous control interactions with the VSG involoved.Therefore,this paper focuses on the subsynchronous control interaction problem of virtual synchronous DFIG grid-connected system under series-complemented network,and can be contained that carrying out the full-order small-signal modeling,the analysis of the subsynchronous control interaction(SSCI)damping characteristics and the oscillation suppression strategy research of the virtual synchronous DFIG grid-connected system,which provides the theoretical basis and technical support for the study of the grid-connected stable operation of virtual synchronous DFIG grid-connected system under series-complemented network.The main research contents of the paper include as follows:(1)In order to dissect out the influence of control parameters on the SSCI of wind power system,based on the grid-connected system structure of virtual synchronous DFIG grid-connected system,the full-order small signal state space model of virtual synchronous DFIG grid-connected system is constructed.Firstly,this paper focus on the small signal model of VSG and phase-locked loop(PLL),and takes into account the physical connection relationship of each module in the system to modularize the full-order small signal state space model of the system with the VSG involved or not.Secondly,the characteristics of the SSCI phenomenon of the system before and after the virtual synchronous access are compared and analyzed.Finally,the validity of the proposed small-signal model is verified based on the simulation model of an actual wind farm in North China.(2)In view of the unclear occurrence mechanism and damping characteritiscs of SSCI in VSG connected DFIG wind power system,the damping characteristics of VSG in the SSCI mode are quantitatively evaluated by adopting the oscillation frequency and damping ratio as quantitative indicators,and the sensitivity of VSG control parameters to SSCI is obtained by caculating the participation factor.Finally,the validity of the modeling method and the accuracy of mechanism analysis are verified by time-domain simulation.The results demonstrate that although VSG weakens the influence of the inner-loop proportionality coefficient of the rotor-side converter on the SSCI,the inner-loop proportionality coefficient of the rotor-side converter has the greatest influence on the SSCI,and the integral coefficient of VSG excitation control has a higher degree of influence on the SSCI than the damping coefficient.(3)In order to figure out the problem that the grid-connnected system of DFIG with virtual synchronous control may induce synchronous oscillation,a SSCI suppression strategy based on improved active disturb rejecting control(ADRC)is proposed.Firstly,the mechanism of ADRC control to suppress SSCI in the virtual synchronous DFIG grid-connected system is derived,and an extended state observer(ESO)with filtering function is constructed to estimate and compensate the SSCI in real time,so as to achieve the purpose of suppressing sub-synchronous oscillations.Then,the stability of ESO,the estimation error and the closed-loop control system of the grid-side converter are discussed by using the Rouse criterion.Finally,the validitiy and adaptability of the proposed control strategy to SSCI suppression is verified based on the time simulation model.