Research On The Low Voltage Ride-through Technology Of DFIG-based Wind Turbines Based On Rotor-side Port Impedance Characteristics

With the increasing penetration of wind power into the grid,in order to ensure the power system stability,the transmission system operators require the wind turbines(WTs)to have low-voltage ride through(LVRT)capability.Doubly-fed induction generator(DFIG)-based WTs have occupied the highest share of the global wind energy market,nevertheless,DFIG is difficult to fulfill the LVRT requirement due to its special structure.Currently,the common LVRT solution is to adopt the crowbar circuit,due to the advantages of the simple control and low cost.However,there are some defects such as the loss of controllability and the absorption of reactive current.As a result,some improved LVRT control strategies are proposed,which have the advantages of no hardware device and flexible control.However,owing to the limited converter rating,these are still many challenges.As a consequence,this dissertation is dedicated to the relevant research about the improved control strategy based LVRT technology of DFIG-based WTs,and it is organized as follows.In order to reveal the essence and performance differences of the typical improved LVRT control strategies,an analysis method based on the rotor-side port impedance characteristic is proposed.Firstly,the dynamic model and transient behavior of DFIG are presented,which lays the foundation of the latter research.Then,from the perspective of the rotor-side port impedance characteristics,the essences of these LVRT control strategies are revealed.Moreover,in terms of the transient rotor voltage characteristics,the transient rotor current characteristics,the decay rate of the stator flux,the electromagnetic torque characteristics,the output power characteristics,and the implementation complexity,the performance differences of these LVRT control strategies are obtained.Finally,the correctness of the analysis results is demonstrated by the simulation.In order to improve the controllable LVRT capability of the DFIG-based WTs,a constant-inductance-emulating control(CIEC)method is proposed.Firstly,learning from the previous research idea,the optimal rotor-side port impedance characteristics that satisfy the constraints of the rotor voltage and rotor current is revealed.Then,a CIEC method is proposed,by controlling the rotor-side port impedance to emulate a suitable inductance.Under the proposed method,the constraints of the rotor voltage and rotor current can be satisfied,and the controllable LVRT capability of DFIG can be enhanced.Moreover,the electromagnetic torque ripple can be eliminated effectively.Furthermore,the controllable operating range of the proposed method is obtained,as well as its key influencing factors.Finally,the proposed method is verified by simulation and experiment.In order to accelerate the attenuation of the stator flux to shorten the transient process,a dynamic-inductance-emulating control(DIEC)method is proposed.Firstly,the key optimization control objectives during the different LVRT stages are discussed.Then,the decay time-scale of the transient stator flux under the five control modes is analyzed and evaluated.Furthermore,on the basis of the aforementioned CIEC method,a DIEC method is proposed with fully considering the stator flux dynamic.Under the proposed method,the rotor-side port impedance is controlled as a dynamic inductance which gradually becomes smaller.As a result,the attenuation rate of stator flux can be maximized in the premise of satisfying the constraints of both rotor voltage and rotor current.Finally,the proposed control method is verified by simulation.In order to improve the control precision of the inductance-emulating control method,a feedforward current reference control(FCRC)method is proposed.Firstly,the required rotor current reference to improve the controllable LVRT capability is revealed.Then,a FCRC method is proposed,by introducing the additional feedforward of current references into the current loop.This method can precisely track the transient current reference,and it has the advantages of the simple design and fast dynamic response.Furthermore,based on the presented equivalent circuit of current loop,the physical essence of three current control methods including FCRC method is analyzed and explained.Finally,the proposed control method is verified by simulation and experiment.