A Control Strategy Research On The LVRT Capability Of DFIG Wind Farm Based On STATCOM

People's eyes have turned to the development and utilization of renewable energy when the environment of China deteriorates gradually along with the repaid development of national economy.Wind power generations as a potential and enormous renewable energy,which is a significant part of the world energy strategy.With the development of wind power capacity,the proportion of the wind power in power grid is also growing,the impact of wind power on the grid stability region cannot be ignored.The doubly-fed induction generator(DFIG)has become a mainstream model of the wind power generation due to its various advantages.The drops of grid voltage was caused by the asymmetrical fault,wind turbines off-grid operation will exert serious impact on the stability of the grid.Therefore,this dissertation focuses on research work on the asymmetrical fault characteristics and how to improve the low voltage ride through(LVRT)ability of wind turbines.Static synchronous compensator(STATCOM)can dynamically add the reactive power to the grid which is suitable to improve the LVRT ability of DFIG.However,the problem about the decoupled technology and the control strategy of STATCOM in wind power system need further study.The premise of the study of LVRT is to analyze the transient process of DFIG when the power grid voltage drops.Firstly,the basic structure and operation principle of DFIG are analyzed,and then the mathematical model of DFIG under three-phase static coordinate system and two-phase synchronous rotating coordinate system are established.And in this condition the electromagnetic transient of DFIG and the faults characteristic under the gird asymmetrical faults situations are analyzed in detail.And then the maximum peak voltage of rotor side is regarded as the constraint condition to evaluate the LVRT ability of DFIG,and the maximum peak voltage based on three kind of asymmetric fault condition is derived respectively.At the same time the faults characteristics of DFIG is simulated in MATLAB environment.Secondly,this dissertation purposed a method to improve the LVRT ability of DFIG by installed a STATCOM in the grid side.The basic structure and operation principle of STATCOM are analyzed,and then the mathematical model of STATCOM is established.Because the generation of reference reactive current detection and control for STATCOM are critical to obtain perfect performance.In terms of the traditional detection method of reactive current of STATCOM that will generates detecting error under grid-side asymmetrical faults,in this dissertation,an improved detection method is proposed.The linear active disturbance rejection control(LADRC)technology is applied to the current decoupling of the strong coupling mathematical model of STATCOM.And then the control method based on LADRC technology on dq axis of STATCOM is designed,so the parameter method is used todetermine the parameters for the controller and the control strategy of STATCOM is completed.Finally,this dissertation builds up a simulation model for the entire wind power control system included a STATCOM,and then the LADRC control method and the PI control method are simulated respectively in this condition.The simulation result shows that the LADRC control method has better rapidly and robustness than the PI control method,and the LADRC control method is more effectively to improve the LVRT ability of DFIG.