Research On The Comprehensive Strategy Of Fault-Ride-Through Of Double-Fed-Induction-Generator Power System

Double-Fed-Induction-Generator(DFIG)has become one of the main types of wind farms because of its excellent power generation performance.However,due to the structure that the stator of DFIG is directly connected with the grid,the grid faults will easily lead to a series of adverse transient reactions of DFIG,including overcurrent of rotor,overvoltage of DC bus and oscillation of electromagnetic torque,etc.,which may cause DFIG to disconnect from grid in serious cases.With the increase of capacity of grid connected DFIG,it is very important to improve the faults ride through(FRT)capability of DFIG.The main research contents of this paper are as follows:Firstly,the mathematical model of each part is established according to the physical model of DFIG.Then the transient characteristic formula of the DFIG under the different fault conditions including symmetrical voltage drop,asymmetric drop and voltage surge is deduced.So,the mechanism of the adverse transient response of DFIG is obtained.In view of the poor control performance of traditional PI controller for DFIG under power grid faults,according to the operation control principle of DFIG and the idea of sliding mode control(SMC),based on the active disturbance rejection control(ADRC),the control strategy of converter of DFIG based on SM-ADRC control is proposed.In order to reduce the number of setting parameters,the nonlinear disturbance observer(NDOB)is selected to replace the extended state observer(ESO).Through the fast separation sequence of positive and negative voltage,the ability of PLL in asymmetric fault of power grid is enhanced.In consideration of the serious fault of the power grid,the hardware protection circuit is needed to put into operation to ensure that the DFIG can ride through the fault successfully.The Crowbar circuit and the rotor series dynamic resistance(SDR)are combined to complement advantages and disadvantages of each other,and the appropriate protection circuit is input or removed according to the fault degree.Considering the most serious situation of power grid fault,the expressions of rotor transient current are derived with Crowbar and SDR put into use,and the protection resistance value is obtained by drawing the relationship of value between protection resistance and rotor transient current.Aiming at the problem of overvoltage and fluctuation of DC bus under grid faults,Super capacitor energy storage system(SC)is connected in parallel at DC bus capacitor,and the voltage of DC bus is stabilized by controlling the energy storage and discharge of SC.According to the relationship between the reactive power demand of the grid and the reactive power limit of the grid side converter(GSC),the reference value of the reactive power is selected reasonably to guarantee the grid side converter output the appropriate reactive power.Considering that in the process of Crowbar circuit put into operation,the DFIG in the operation state of asynchronous motor needs to absorb the reactive power to excite field from the grid.The reactive power characteristics of DFIG in the asynchronous operation state are deduced,and the pitch angle control is optimized according to the relationship between reactive power and slip of reactive power characteristics.Finally,based on the integrated protection circuit(Crowbar,SDR and SC)and the control mode of reactive power(control of GSC reactive power and control of pitch angle),the adaptive control strategy is proposed,which the corresponding protection circuit can be put into and cut off and the corresponding control mode of reactive power can be switched in different situations and times,so as to improve the FRT ability of DFIG in all aspects.