Dynamic Stability Analysis And Control Strategy Research Of DFIG-based Wind Power Generation System During Weak Grid Faults

Affected by the reverse distribution of wind energy resources and power load,the wind power has the characteristics of large-scale development and long-distance transmission in China,which leads to the weakening of the power grid strength and frequent short circuit faults.Moreover,with the increase of wind farm capacity,the dynamic coupling between the wind farm and the power grid is also increasing,which makes the wind power system have the risk of oscillation instability during the shortcircuit fault of weak power grid.Different from the transient instability of the traditional power system,the oscillation instability of wind power system during the short circuit fault of weak power grid involves the interaction of multi-time scale control,and its behavior law is very complex.The oscillation frequency presents the characteristics of wide frequency and belongs to the category of dynamic stability problem.This new oscillation problem seriously threatens the safe and stable operation of wind power system,and also restricts the utilization of wind power.However,the research on the oscillation problem of wind power system during the short circuit fault of weak power grid is still at the initial stage,which leads to the unclear oscillation mechanism,unknown oscillation pattern and lack of effective optimal control strategy.In addition,during the short-circuit fault,especially during the asymmetric short-circuit fault,the frequency coupling problem of wind power generation system is also very acute.At present,there is no effective research method to study of the frequency coupling problem during the faults,which leads to the lack of comprehensive understanding to the frequency coupling mechanism.Therefore,in order to improve the dynamic stable operation performance of the wind power system during the weak power grid fault,this paper takes the doubly fed induction generator(DFIG)-based wind power generation system(WPGS)as the research object,and studies the oscillation instability mechanism,oscillation influence factors and optimal control strategy of DFIG-based WPGS during the symmetric and asymmetric shortcircuit fault of the weak power grid.Based on the complex frequency domain impedance model,the frequency coupling mechanism of the DFIG-based WPGS during the symmetric and asymmetric short-circuit faults of power grid is explored.Considering the factors such as transmission line impedance and voltage unbalance degree,the coordinated control strategy of DFIG-based WPGS during the small value unbalanced fault of weak power grid is optimized.The contents of this dissertation are as follows:(1)The small-signal model for the DFIG-based WPGS considering phase-locked loop(PLL)and line impedance during the symmetrical fault of weak grid is established.The dynamic coupling between the control loops of DFIG-based wind turbines(WT)and the impedance of power grid during LVRT is described.The mechanism of the oscillation instability during symmetric short-circuit fault of weak grid is revealed.Based on the small signal model,a comprehensive modal analysis of wind power system during the short-circuit fault of weak grid is carried out,and the oscillation mode of the weakest damping of the system is determined.It is pointed out that the oscillation of the system caused by the interaction of the controller has the wide-range-frequency oscillation characteristics.In addition,the influence law of different parameters on the dynamic stability of the system is explored through the characteristic root locus.Combined with the voltage vector relationship during the short-circuit fault of weak grid,this paper proposes the improved LVRT control schemes from two aspects of either optimizing the dynamic active/reactive current distribution or decreasing PLL bandwidth to enhance the system small signal stability.The time domain simulation and experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed control schemes.(2)In order to explore the frequency coupling issue of the DFIG-based WPGS during short-circuit fault,this paper takes the symmetric short-circuit fault as a special case of the asymmetric short-circuit fault,and establishes a complex frequency domain impedance model of DFIG-based WPGS under short-circuit fault.Then,combining the DFIG-based WT and power grid interaction relationship,the frequency coupling mechanism of DFIG-based WPGS during the LVRT was clarified,and the frequency transfer evolution law of the system during short circuit fault is described in detail.Formed a general frequency coupling analysis method,which lays a theoretical foundation for clearly understanding the frequency coupling phenomenon of DFIG-based WPGS during symmetric and asymmetric short-circuit fault.The correctness of the theoretical analysis is verified by simulation and experiment results.(3)In order to deeply analyze the dynamic stability of DFIG-based WPGS during the asymmetric short-circuit fault of weak grid,the equivalent complex impedance model of DFIG-based WPGS is derived.According to the input and output characteristics of the complex impedance model,the dynamic interaction between the positive-and negativesequence impedance of the DFIG-based WT and the power grid during asymmetric fault of weak grid is described in detail,and the mechanism of the oscillation instability is revealed.Based on the principle of the characteristic root invariance,the complex impedance model is reduced and the equivalent complex transfer matrix is obtained.Furthermore,the effected of the voltage imbalance degree,rotor speed,PLL and the positive-and negative-sequence current loop control parameters on the dynamic stability of the DFIG-based WPGS during the weak grid asymmetric fault are comprehensively analyzed by the generalized Nyquist criterion,and the optimal selection method of controller bandwidth in the fault process is proposed.The correctness of the theoretical analysis is verified by the time domain simulation and the experimental results.(4)The negative sequence current output capacity of DFIG-based WT under the small value unbalance fault of power grid is studied.On this basis,combined with the negative sequence current demand of each control target,the controllable operation region of the DFIG-based WT with voltage unbalance degree and active power output as the limiting conditions is investigated.Furthermore,considering the influence of transmission line impedance and voltage imbalance degree,an optimized coordinated control scheme of the rotor-grid side converters during the small value unbalance fault of weak grid is proposed.It can effectively reduce the influence of the DFIG-based WT on the voltage imbalance degree of the point of common connection,and significantly enhances the stable operation performance of the DFIG-based WT under the weak grid small value unbalance fault.The effectiveness of the proposed control scheme is verified by time domain simulation.