Dynamic Characteristics Of Doubly Fed Induction Generator-based Wind Turbine In Electromechanical Timescale And Their Applications In System Voltage Dynamics Study

Wind power is in the process of transforming from the supplementary energy in traditional power system to the dominant energy in new power system to peak carbon emissions and achieve carbon neutrality.In electromechanical timescale,the voltage and frequency dynamic stability problems introduced by the grid integration of wind power are emerging.And the attachment of inertia control to wind turbines(WTs)may bring new risks of oscillations with low frequency.It is of both practical and forward-looking significance to study the system dynamic stability problems in electromechanical timescale.The core lies in the system voltage and frequency dynamics which depend strongly on the dynamic characteristics of the internal voltage amplitude and frequency response of the gridconnected devices while being subjected to power imbalance.With the increasing penetration of wind power,the dynamic characteristics of internal voltage response of WT being subjected to power imbalance are playing a more and more important role in system dynamics.The typical WT consists of multi-timescale controls including electromechanical control and electromagnetic control.The system dynamics in electromechanical timescale are closely related to the active and reactive power exchange along with the motion of rotor and the action of electromechanical control.Therefore,the modeling of the dynamic characteristics of WT by focusing on the electromechanical control,as well as the analysis of system dynamics based on the dynamic characteristics of WT,constitute an important basis for the study of the system dynamic stability problems in electromechanical timescale.The existing electromechanical models are mostly structure-oriented based on detailed connections of hardwires and controls within WT.And they fail to illustrate the dynamic characteristics of internal voltage response of WT being subjected to power imbalance.Consequently,the existing studies on the system dynamics are mostly carried out through time-domain simulation in specific scenarios.And they fail to recognize the stability mechanisms of system dynamics,especially those of system voltage dynamics.Therefore,by taking the doubly-fed induction generator(DFIG)-based WT as the study object,this paper first proposes the modeling method and physical mechanisms of the dynamic characteristics of WT in electromechanical timescale,and then performs the study on system voltage dynamics based on the dynamic characteristics of WT.The main contents are as follows:(1)The research idea of illustrating the dynamic characteristics of DFIG-based WT and the method of reflecting the mechanism in the formation of internal voltage in electromechanical timescale are elaborated.The multi-timescale feature of the control system of WT due to the cascading phenomena of the state responses and the control bandwidths is introduced.And the sequential action process of the multi-timescale controls is explained.The multi-timescale dynamic characteristics of WT are advocated to be illustrated by the relationships of dependence of internal voltage amplitude/frequency on active/reactive power imbalance.Based on that,by focusing on the dynamic characteristics of WT in electromechanical timescale,the equivalence method of substituting the fastelectromagnetic loops with the infinity gain is proposed to reflect the mechanism in the formation of internal voltage in electromechanical timescale.(2)The method of illustrating the dynamic characteristics of DFIG-based WT with the relationships of dependence of internal voltage amplitude/frequency on active/reactive power imbalance is proposed.The equivalence method of converting the controls based on the grid state detection to the controls based on the device’s power output is explained.Based on that,the sole dependence of internal voltage amplitude/frequency on active/reactive power imbalance of the devices with grid state detection-based controls is clarified.By combining the equivalence of the fast-electromagnetic loops and the grid state detection-based controls,the models in unified structures are established to illustrate the individual dynamic characteristics of DFIG-based WT in electromechanical timescale considering various controls.This constitutes the basis for further research on the mechanisms of dynamic characteristics and system dynamics.(3)The understandings of the motion mechanisms of the internal voltage amplitude/frequency response characteristics under stimulations of the active/reactive power imbalance of DFIG-based WT are developed.The motion mechanism of internal voltage frequency abstracted as an equivalent rotor is developed by explaining the active power imbalance driving the motion of frequency and the storage of the active power imbalance supporting the motion of frequency.The motion mechanism of internal voltage amplitude abstracted as a virtual spring is developed by explaining the reactive power imbalance driving the motion of amplitude and the storage of the reactive power imbalance supporting the motion of amplitude.The motion mechanisms of internal voltage amplitude and frequency are of explicit physical significance and constitute the core of further study on system dynamics.(4)A method of analyzing the system voltage dynamics is proposed centered on the dynamic characteristics of DFIG-based WT.Based on the motion mechanism of internal voltage amplitude,an intuitive method is proposed to analyze the dynamics of internal voltage amplitude evolution from three basic aspects: the reactive power input variation characteristics,the storage characteristics of reactive power imbalance,and the reactive power output response characteristics.Based on the method,the dynamics of internal voltage amplitude evolution of DFIG-based WT are analyzed in a single-machine system.The stability mechanisms of internal voltage amplitude are explained considering various reactive power controls.The phenomena that the internal voltage first tends to stabilize and then decreases rapidly after fault in scenarios with long transmission lines or high transmission power are found,and the causes of the phenomena are revealed.