Nonlinear Sliding Mode And Crowbar Circuit Control For Doubly Fed Wind Power Generation System

Wind energy is another form of solar energy,and has the advantages of inexhaustible,no pollution,low cost and so on.Wind power generation has the best cost effectiveness compared with solar energy,nuclear energy and other new energy sources.Therefore,many countries in the world are actively developing wind power.In order to normalize the grid-connection of new energy sources,the countries launched their new grid codes to the new energy sources,including the low voltage ride through capability(LVRT),a clear strict requirement.Doubly fed induction generator(DFIG)is the main part of the wind power market because of its many advantages such as small convertor capacity,light weight,low loss and variable speed constant frequency operation.Therefore,it is of great significance to study the low voltage ride through of DFIG.First of all,in this paper,the model of DFIG is established based on ABC three-phase static coordinate system and two-phase rotating coordinate system.Then the working principle of DFIG is analyzed and the vector control strategy is introduced on account of the model of power decoupling and synchronous rotating coordinate system.Secondly,due to the weak low voltage ride through capability of the DFIG system under the traditional vector control strategy,the departed control algorithms need to be improved,so the theory of state feedback exact linearization and variable structure control is introduced.Nonlinear and tight coupling mathematical model of DFIG is linearized by state feedback exact linearization,and the low voltage ride through controller is designed by using the nonlinear variable structure control theory due to its fast dynamic response speed.The 6*1.5MW system is built in MATLAB,and the simulation results show that the state feedback linearization and variable structure control has better control performance than the traditional vector controlFinally,aiming to the power grid voltage drop,adding the Crowbar circuit in the rotor side and unloading circuit in the DC side,the influence of the selection of Crowbar circuit resistance and the switching time on the low voltage ride through effect is analyzed.When the Crowbar circuit is accessed,because doubly fed generator in asynchronous motor running absorbs reactive power from the grid,which is not adverse to the recovery of the grid voltage,a given value of inactive current can be attained through the stator reference voltage subtracting stator terminal voltage,after a PI regulator,then subtracting real-time reactive power and passing a PI regulator,which can optimize convertor reactive power control strategy in grid side.After the Crowbar circuit operation,the grid side converter is controlled to output reactive power to support the grid for recovery voltage.