Research On The Analysis And Optimization Control Of The Power Response Characteristics For DFIG-Based Wind Turbine Under Frequency Disturbances

Driven by the target of carbon peaking and carbon neutrality,the generation side structure of the power system is undergoing a significant revolution,characterized by large-scale wind power continuously integrated into the power system.There is no doubt that wind power exploitation brings substantial economic and environmental benefits.However,with the high penetration of wind power,the frequency dynamic behaviour of the power system has changed profoundly,showing the unfavourable feature of low inertia.As a result,the frequency stability of the power system is seriously threatened.To solve the deteriorating frequency problem,frequency control has become a mandatory requirement for wind power integration.Inertial control,as the initial stage of frequency control,shows critical importance for the frequency dynamic of the power system.The performance of the inertial response directly determines the initial rate of change of frequency(RoCoF).Compared with other stages of frequency control,the power dynamic of the wind turbine(WT)during the inertial response is more significant.Correspondingly,the problems are also more prominent.Therefore,this paper focuses on the power dynamic during the inertial response for the doubly-fed induction generator(DFIG)-based WT,centring on the inertial control realization,inertial characterization method and inertial control method.The main content of this paper is as follows:(1)Typical structure of DFIG and influences of inertial control on system frequency dynamic: The inertia of the power system greatly reduces after the WT replaces the synchronous generator(SG)integrated into the grid.To illustrate the impacts of the inertial control by the WT,the operating characteristics and typical structure of the DFIG are first introduced.Then,the frequency regulation stages of the power system and the corresponding evaluation indexes are described.Finally,the influences of the control parameters on system frequency dynamic and power characteristics of DFIG before and after adopting the virtual inertial control are compared,which highlights the importance and necessity of inertial control by WT(2)Power transfer capability and its enhancement strategy for DFIG: During the inertial response,the WT not only requires energy sources but also needs to transmit the power to the grid according to the power reference by inertial control to realize the frequency support.To quantify the power transfer capability of DFIG,a static power model under stiff and weak grids is established,and the dominant factors limiting power transfer are analyzed in detail.On the basis,the influence of common reactive power control(RPC)modes on active power is illustrated.Subsequently,the specific RPC mode that can ensure maximum active power output is obtained.Finally,combined with the dq-axis current priority of the rotor side and grid side,a grid-side converter(GSC)dominated RPC method is proposed.The proposed method further improves the power transfer capability of DFIG,which ensures the effective implementation of inertial control.(3)Phase motion equation and its equivalent inertia characteristics for DFIG: The inertia of SG can be considered constant,while the inertia of DFIG is determined by the mechanical structure and control system.To characterize the inertia of DFIG,the internal voltage of DFIG is obtained by analogy to the definition of SG.Further,the similarity and differences in the power transfer mechanism between the WT and the SG are compared.And the dynamic response of the phase-locked loop(PLL)during frequency disturbance is described in detail.On this basis,by considering the control coupling among the active power control loop,voltage control loop and PLL,the phase motion model is derived.Moreover,the inertia coefficient and damping coefficient of the WT are obtained.Finally,the impacts of controller parameters and operating points on inertia are analyzed.The analysis results explain the fundamental issues of why the WT show no inertia under typical vector control and what are the equivalent inertia characteristics of DFIG.(4)Inertia PLL control strategy and its operating characteristics for DFIG: Under typical vector control,most inertial control methods optimize the power loops for the inertial response.However,some researches indicate that optimizing other control loops can also obtain the inertial response.To understand the nature of inertial control,this paper investigates the basic principles of different inertial control methods from the perspective of power dynamics.The mechanism of optimizing the PLL dynamic to obtain the inertial response is specifically discussed.On this basis,the phase motion equation of the SG and the basic structure of the PLL are directly compared,and the characteristics of the PLL are described from the angle of inertia.Moreover,an inertia PLL(i PLL)control method is proposed.Finally,the inertia characteristics and the dynamic characteristics of the proposed i PLL are analyzed in detail.The equivalent inertia of i PLL presents constant characteristics as the inertia of SG to some degree.