Transient Modeling Of Doubly Fed Induction Generator-based Wind Turbine And Transient Stability Analysis Of Its Grid-connected Power Systems In Rotor Speed Control Timescale

With rapid development of wind power generation in China,wind turbine(WT)is gradually replacing synchronous generator(SG)and plays a main role in local system.It changes the transient behavior of power system.In recent years,there are several transient accidents with unknown mechanism happens in domestic and overseas,which seriously threaten the safe and stable operation of WT-connected power systems.The transient characteristics of power generation equipment variation is the important factor that leads transient stability mechanism of the WT-connected power systems to change.Different from the SG characterized by the rotor motion equation,the transient characteristic of the WT is determined by its diversified control structure.Typical WT's control structure is composed of multi-timescale(rotor speed / DC voltage / AC current control timescales)normal control and multi-timescale transient switching control,which leads the WT's transient characteristics are multi-scale and diversified in different fault.Affected by WT's transient characteristics,the transient problems in WT-connected power system present to be multiscale and intermittent.Therefore,the traditional transient stability theory which based on the transient characteristics of SG,may lose efficiency.Thus,it is essential to reconsider the WT's transient characteristic and multi-timescale transient stability problem of WT-connected power system.Present researches focus on modularization and mathematical modeling for numerical simulation.They mainly consider the effect of WT on transient stability of SG,while the understanding of WT's characteristics and the transient stability mechainsem are not taken into account.Moreover,there is also a lack of systematic research on the transient stability of WTdominated power system.Therefore,this paper proposes a unified modeling method for transient process analysis of WT-dominated power system at first.The transient models of DFIG-based WT in rotor speed control timescale are proposed,in which different normal and transient controls are considered.Then,this paper proposes a systematic analytical thinking that analyzes the transient problems of WT-connected power system.The transient characteristics of DFIG-based WT and their impact on the transient stability of typically simple DFIG-based WT dominated power system are analyzed.The main contents are as follows:(1)The multi-timescale transient characteristics of DFIG-based WT and transient stability of rotor speed control timescale in grid-connected system are described.The multi-timescale normal control and multi-timescale transient switching control of DFIG-based WT are introduced.The analytical framework of the multi-timescale transient characteristics of DFIG-based WT and the multi-timescale transient stability of DFIG-based WT-connected system are established,which proposes to understand the multi-timescale transient characteristics from fault degree(viz.,non-severe & severe fault)and fault time(viz.,short/middle/long time),and to divide the multi-timescale transient stability instantaneous timescale(about 1ms),AC current timescale(about 10ms),DC voltage timescale(about 100ms)and rotor speed control timescale(about 1s).The meaning and challenge of transient stability of DFIG-based WT-connected system in electromechanical are carefully interpreted.(2)To describe the transient characteristics,the transient models of DFIG-based WT in rotor speed control timescale under diversified control are established.Based on the “power excitation-internal voltage response” modeling concept,the transient models of DFIG based WT in rotor speed control timescale are uniformly established with consideration of normal control and transient control respectively.The relationship between various transient models and the transient responses of power systems with different fault degrees and fault stages is illustrated.The proposed models directly reflect the physical processes of DFIG-based WT,and provide helps for understanding the operation mechanism of DFIG-based WT in power system transient responses.(3)The transient characteristics of the DFIG-based WT under normal control and the transient stability in rotor speed control timescale of SMIB system under non-severe fault are analyzed.Based on the proposed model with normal control,the transient characteristics of DFIG-based WT under different active power control strategies are summarized.In the SMIB system,the impacts of the transient characteristics of DFIG-based WT on the transient stability mechanism of system are expounded,based on which the transient stable/unstable responses of system under non-severe fault are analyzed.An additional instabilizing mechanism,which is caused by the nonlinearity of phase-locked loop(PLL)branch,is found.Accordingly,a new phase instable phenomenon is also found.Finally,the impact rules of different factors on the transient stability of the SMIB system under non-severe fault are analyzed.(4)The transient characteristics of DFIG-based WT under transient control and the transient stability in rotor speed control timescale of SMIB system under severe fault are analyzed.Based on the proposed model with transient control,the transient characteristics of DFIG-based WT under transient control strategies are summarized.In the SMIB system,the transient responses and the key factors during severe fault stage and climbing stage are analyzed respectively.It also pointed out that,the responses during severe fault stage and climbing stage will influence the initial states of fault recovery stage,and thus decide the transient stability.Finally,the influence rules of key parameters on the transient stability of the SMIB system are obtained,based on which the rules are also explained physically from the point of energy.(5)The transient stability in rotor speed control timescale of DM system under different fault degree is analyzed.In DM system,the impact of transient characteristics of DFIG-based WT on the interaction path between DFIG-based WT and SG is analyzed.Further,the impact of interaction on the transient stability of system in rotor speed control timescale is analyzed.The transient stabilizing mechanism and the stable/instable responses in non-severe and severe faults are revealed.Finally,the influence rules of different control parameters and the preparation of wind power on the transient stability of DM system are summarized.The proposed unified modeling method and the systematic analytical thinking could provide a new research thought and reference for WT-connected power system transient problems analysis.