Direct Power Control Of Doubly Fed Induction Generation System

With the growing of global energy crisis and environmental pollution,wind energy,which is a highly efficient renewable energy sources,has received sustained attention from the society and has been extensively exploited and utilized in various countries.With the continuous development of wind power technology,various types of wind turbines have been developed.The wind power generation system based on double-fed induction motor(DFIG)has become the mainstream variable speed constant frequency wind turbine due to the advantages of small capacity,high efficiency and reliability.The control strategy of wind turbine converter is the key part of energy conversion control.The common control methods of DFIG are vector control(VC)and direct power control(DPC),and VC is the main control strategy of wind power generation system.However,in recent years,DPC has attracted more and more attention due to its simple control structure and good dynamic performance.Therefore,this paper analyzes the direct power control strategy of DFIG from the theoretical analysis to the model simulation,and compares it with vector control in many aspects.The key contents and main work of this paper include:(1)Based on the detailed mathematical model of DFIG wind power generation system,the PWM converter is divided into grid-side converter and rotor-side converter separately,and the theoretical analysis and the specific implementation method of the grid-side converter and the rotor-side converter under ideal grid voltage condition are given.(2)The theoretical analysis and implementation method of LUT-DPC based on switch table are given for the grid-side converter and the rotor-side converter under ideal grid voltage conditions,respectively.Aiming at the shortcoming of LUT-DPC’s switching frequency is not fixed,the current harmonics are large and the power ripple is large,the theory of direct power control(SVM-DPC)based on space vector modulation in DFIG system is analyzed and implemented,which can obtain a fixed switching frequency,reduce the current harmonic content,improve the power pulsation,and has a good dynamic and steady-state performance.(3)A detailed design method is given for the parameters of PI regulator used in VC and SVM-DPC for DFIG according to the parameter design method of typical type I and typical type II systems.Based on the completion of the system parameter tuning,the control performance of the two control modes is compared.The precision of VC steady state is the highest,but the design parameter of VC is twice that of SVM-DPC,so the design is more difficult and the system is more complicated.DPC system is simple in structure,good in parameter robustness and fast in dynamic response.(4)The comparison of VC and DPC(mainly SVM-DPC)in DFIG system under different working conditions is carried out.The power fluctuation of SVM-DPC is larger than that of VC under the wind fluctuation condition.Under the condition of grid voltage dropping,SVM-DPC is more effective in suppressing system oscillation during electromagnetic transient process.But SVM-DPC easily causes the problem of rotor current oscillation.The control strategy of SVM-DPC is improved to restrain the rotor current oscillation and improve the LVRT characteristic of DFIG,which makes the system stable fast after Crowbar cutting out and does not cause Crowbar’s re-action.Compared with VC,grid-side converter using the LUT-DPC control strategy and the rotor-side converter using the SVM-DPC control strategy can improve the stability of power when it is incorporated into the weak grid.(5)A hardware-in-the-loop simulation platform for DFIG based on RTDS is established,and detailed hardware and software design methods are given.The real-time performance and accuracy are much higher than offline simulation.Finally,conclusions are verified on this simulation platform.