Study On Mechanism And Control Strategy Of Sub-synchronous Oscillation Caused By DFIG-based Wind Turbines Connected To Grid

The environmental and social problems which caused by traditional energy sources are becoming more and more serious.The development of new energy sources has received much attention.Among them,wind energy is an environmentally friendly renewable energy source,and the wind power capacity is increasing.Using Power electronic control technique,wind power generating unit can achieve maximum utilization of wind energy and flexible output,but at the same time,its complex dynamic characteristics will inevitably bring new challenges to the stable operation of the power grid.Recent years,the sub-synchronous oscillation phenomenon associated with wind farms has seriously affected the stable operation of wind power grid-connected and even power systems itself.Therefore,it is urgent to study the influence of grid-connected wind turbines on power system.The main content and results of this article are as follows:Aiming at the problem of the sub-synchronous oscillation mechanism research model of the DFIG(Doubly fed Induction Generator)grid-connected system,a dynamic energy model of the DFIG reflecting the sub-synchronous oscillation characteristics is established.On this basis,the analytical method is used to study the dynamic changes of the DFIG energy and its damping characteristics during the sub-synchronous oscillation process.That is,the dynamic energy of the DFIG under sub-synchronous oscillation is composed of periodic components and non-periodic components,wherein the non-periodic component energy determines the damping characteristics of the generator.When the change rate of non-periodic component energy is positive,the generator exhibits a negative damping characteristic,and the oscillation diverges Conversely,when the change rate of the non-periodic component of the port energy is negative,the oscillation gradually converges as the DFIG exhibits positive damping.The simulation results verify the accuracy of the dynamic energy model of DFIGAiming at the research on the interaction characteristics of sub-synchronous oscillation energy between wind turbines or wind farms,a method for identifying the dominant path of sub-synchronous oscillations of DFIG is proposed,which analyze the oscillation dominant path and select the vibration source generator from the perspective of the dynamic energy frequency characteristics of the DFIG.Firstly,according to the propagation path of the disturbance component in the generator,the response characteristics of the converter control to the disturbance frequency of the input generator port is derived.On this basis,considering the frequency characteristics of the generator dynamic energy in the same oscillation mode,an energy spectrum similarity function model to realize the identification of the dominant path of oscillation is proposed.Finally,acof cording to the relationship between energy and the damping characteristics the DFIG,the vibration source generator is determine accurately.The simulation results verify the correctness of the sub-synchronous oscillation dominant path identification method.Aiming at the research problem of the sub-synchronous oscillation suppression measures of the DFIG,a sub-synchronous oscillation control strategy based on the parameter adjustment of the converter is proposed.Firstly,considering the main influencing factors affecting sub-synchronous oscillation in the process of sub-synchronous oscillation,the influence mechanism of wind speed,grid-connected distance and inverter control on sub-synchronous oscillation are derived.Then analyze and construct the stable boundary conditions of sub-synchronous oscillation.On this basis,the problem of sub-synchronous oscillation suppression is transformed into control parameter optimization problem.Finally a multi constraint parameter adjustment scheme under multi-restrictions conditions is proposed.Simulation results verify the effectiveness of the sub-synchronous oscillation suppression strategy.