Impact Mechanisms Of DFIG Based Wind Farms' Active Power Recovery Control On Rotor Angle Stability And Control Strategies

Nowadays,the global wind power industry,led by China,has been developing rapidly.A large number of wind generators have been integrated into the power grid.In China,as the leading model in the wind power market,double-fed induction generators(DFIGs)are widely used in the form of large-scale wind farms(WFs)in practical engineering.Comparied with synchronous generators(SGs),DFIGs have different dynamic characteristics when the fault occurs.As for their active power control in transient process,China,s "Technical rule for connecting wind farm to power system" has made relevant requirements:since the fault is cleared,the un-triped wind farm's active power should recover to the pre-fault level at speed of 10%rated power per second or faster.When large-scale DFIG-based WFs are connected to the power grid,their active power may continue to change the electromagnetic power of SGs after the first swing process,which seriously affects the rotor angle trajectory.It results some more complex rotor angle instability phenomenon,which poses new challenges to the safe and stable operation of modern power systems.Therefore,it is of great theoretical and practical significance to carry out research on the impact mechanisms of DFIG-based WFs on rotor angle instability and improvement measures.From perspective of the "Technical rule for connecting wind farm to power system" and the measured data of WFs,this paper introduces the controls of DFIGs and the transient model of DFIG-based WFs,and then deeply explores impacts of DFIG-based WFs' post-fault active power control on the rotor angle instability and its modes.Based on the research results,two control strategies for rotor angle stability improvement are also proposed.The main research contents are summarized as follows:The controls and dynamic characteristics of DFIGs are introduced.Based on the relationship between rotor angle stability and DFIGs' dynamic characteristics,two main modeling methods(mechanical modeling and non-mechanical modeling)and their applicability in the rotor angle stability analysis are described.The transient models of the DFIG and DFIG-based WF are given,which are suitable for the rotor angle instability analysis in this paper.It also provides a theoretical basis for the use of DFIG-based WFs in subsequent research.The impact mechanism of DFIG-based WFs' post-fault active power control on rotor angle instability of the first and the second swings is proposed.Based on the extended equal area criterion(EEAC)and the DFIG-based WF's transient model,an equivalent one machine infinite bus(OMIB)system model that is suitable for the multi-machine system integrated with DFIGs is built.By using the model,analytical calculations of two-machine system connected with a DFIG-based WF are analyzed.It reveals that the low post-fault active power of the DFIG-based WF increases the possibility of the second swing stability.The interrelation between the first swing instability and the second swing instability of wind power integrated system is investigated.It reveals that:as the post-fault active power recovery rate of the DFIG based WF gradually gets slower,the system which only had possibility of the first swing instability will have possibility of the second swing instability and it eventually decreases the entire system's rotor angle stability.Simulation results show that the mechanism analysis is correct.The research extends the analysis method of rotor angle stability of wind power integrated systems that only focuses on the first swing instability.For the further research on the instability problem,the impact mechanism of DFIG-based WFs' active power recovery control on the instability in the whole transient process is proposed.As the post-fault active power generally restores from during-fault level to the pre-fault level or nearby for maintaining system's post-fault steady state,impacts of the active power recovery control on the equivalent OMIB system model' parameters and rotor angle instability are presented.It indicates that the recovery control increases the possibility of the first swing instability and odd multi swing instability and decreases the possibility of even multi swing instability.Then,impacts of DFIG-based WFs' active power recovery controls with different recovery rates on rotor angle instability are put forward,which considers the impact degree on each swing instability and relation of all swings.As the recovery rate(assumed to be consent in each control strategy)of the DFIG-based WF is set from fast to slow due to different control strategies,it reveals that there will form three instability modes(in which the critical instability is the first swing instability,odd multi-swing instability,and the second swing instability,respectively).Simulation results validate the effectiveness of the proposed mechanism.The research further expands the range of analysis method for rotor angle stability of wind power integrated systems,and provides theoretical support for the wind farm's transient control that considers the instability in the whole transient process.To solved the new rotor angle instability problems of wind power integrated systems,the control strategy of DFIG-based WFs' post-fault active power is proposed.Based on the proposed impact mechanism,the control principle is firstly proposed,and the recommended values of control time and control amount are given.By utilizing an"online pre-decision and real-time matching" scheme,an engineering application method for the control principle is then proposed,where the control strategy procedures and detailed parameters calculation are demonstrated.Finally,the effectiveness and comprehensiveness of the proposed control strategy are verified by numerical simulations.The proposed principle can provide a more direct and clear guidance for the design of rotor angle stability improvement through DFIG-based WFs'controls.Moreover,the proposed strategy has a good potential in the practical engineering application.For the further optimizing of the improvement measure,a coordinated control of DFIG-based WFs' post-fault active power and SGs' tripping with the aim of improving rotor angle stability and control economics is proposed.To achieve it,the effect of each control object on the stability margin of each swing is firstly presented.Based on this,a principle of the coordinated control is put forward.WFs' control period is designed as six stages and the value of post-fault active power in each stage is suggested to improve the stability of the first five swings and maintain the post-fault steady state.To decrease the tripping amount of SGs,SGs are tripped only when WFs1 control effect is not sufficient to avoid the instability.Then,by utilizing "online pre-decision and real-time matching" scheme,an engineering application method for the control principle is proposed,where the control strategy procedures and detailed parameters calculation are demonstrated.The numerical simulations show that the coordinated control has a better damping effect and a less control cost(less amount of tripped SGs)than the traditional DFIG-based WFs' control or SGs' tripping.The proposed principle can provide a more direct and clear guidance for the design of transient controls of wind power integrated systems.Moreover,the proposed strategy has a good potential in the practical engineering application.