Research On Coordinated Control Strategy Of Wind Farm Connected Grid Via VSC-HVDC

As the fastest growing renewable energy,wind power generation has been popularized and applied all over the world because of its advantages.From the trend of wind power generation,its further development,to a large extent,is being limited by its grid connection mode.Voltage source converter based high voltage DC(VSC-HVDC)is an ideal transmission mode of large-scale wind farm and AC grid connection because of its high flexibility and reliability.Therefore,it is of great significance to study the control method of wind farm connected grid via VSC-HVDC.Firstly,this thesis analyzes fundamental structure and operational principle of doubly-fed induction generator(DFIG)and establishes the mathematical m odel of DFIG,on which,the vector control strategy of DFIG rotor side converter and grid side converter are designed.The emulational result has proved that the vector control strategy of DFIG is able to achieve the decoupling control of active power and reactive power,and also can realize the tracking control of the maximum wind energy.Secondly,the thesis analyzes fundamental structure and operational principle of VSC-HVDC and establishes the mathematical model of VSC-HVDC,on which,the double closed-loop vector control strategy of VSC-HVDC is studied.According to the operation characteristics of wind farm,the control strategy of both converters station is designed to apply in wind power intergration,which lays the foundation of researching on coordinated control strategy of doubly-fed wind farm connected grid via VSC-HVDC.Finally,when doubly-fed wind farm connected AC grid via VS C-HVDC transmission,the inertia of AC grid is comparatively lower,which raise the problem of frequency deviation under the disturbances.A coordinated control strategy is proposed,which includes the simulation inertia control of grid side VCS(GSVSC),the variable frequency control of wind farm VSC(WFVSC)and the virtual inertia control of the DFIG.In the proposed coordi nated control strategy,when the frequency of the grid side AC system changes,GSVSC firstly adjusts its DC voltage reference in response to the change of the grid frequency.Meanwhile,according to the change of DC voltage,WFVSC adjusts the frequency of the WFVSC.In response to the WFVSC frequency,the virtual inertia control system is introduced into the DFIG and the active power of DFIG is changed by absorbing or releasing the kinetic energy of the rotor.Through coordinated control,the energy stored by the DC capacitor and the rotor kinetic energy of the DFIG together provide inertial support for the system,thus improving the frequency response characteristic of the system.The correctness and validity of the proposed coordinated control strategy are verified by simulation analysis.