| Since the control characteristics of the wind power generation are significantly different from that of synchronous generators,the transient synchronization mechanism of phase-locked synchronous renewable energy generation units,represented by wind power generation,is gradually undergoing fundamental changes.Especially,due to the high power electronic dominated control characteristics and complex transient behaviors,wind power generation faces with the challenges of unidentified synchronization characteristics,unclear instability mechanism and various synchronization evolution shapes.As a result,there is lacking of corresponding theoretical support,physical analysis methods and instability criterions.At the same time,the key factors affecting the transient stability of wind power generation has not been described quantitatively,so it is difficult to put forward suitable transient synchronous stability control schemes.In addition,it is constrained by grid codes and the transient sequential switching schemes that,the wind power generation not only face diverse large disturbance instability risk during different fault stages,but also aggravate the drastic interaction between wind power generation and grid impedance,this may also result in small disturbance instability risk during LVRT.Consequently,the transient synchronization mechanism of the wind power generation has not been systematically understood from the viewpoint of the entire fault stage,the transient stability problems have not been revealed in a unified way,and the adaptive synchronous stability control methods considering both large/small disturbance stability has not been comprehensively proposed at present.Therefore,this study takes the phase-locked synchronous wind power generation as the research object,foucuing on its transient synchronization stability issues under the grid short circuit condition.Aiming to systematically reveal the transient instability mechanism and the fault evolution process,physically describe the synchronous characteristics and its change law during different fault stages for wind power generation.Furthermore,in order to propose an intuitive description method of transient synchronous instability characteristics,estabilish quantitative transient synchronization instability criterions as well as large/small disturbance synchronous stability assessment indexes for wind power generation,and then formulated the corresponding matching principle and the transient adaptive active stability control methods.The contents of this dissertation are as follows:(1)Firstly,according to the typical LVRT control mode of the wind power generation during fault duration stage,a large-signal equivalent circuit model is established,which can reflect its output characteristics from the perspective of circuit.Secondly,based on the spatial position relationship and physical coupling characteristics of multiple voltage vectors,a voltage-vector-triangle graphic analysis method is proposed,which is intuitively revealed the migration rule of the transient operating point,the large signal's loss of synchronization(LOS)mechanism and different fault evolution patterns.Then,the large signal synchronization instability boundaries of wind power generation are deduced by combining the vector's relative spatial motion law with the characteristics of PLL's discriminator.Furthermore,the detailed classification pricinple of the large signal stability areas are also deduced.On this basis,the “margin of the transmission line's voltage angle” is defined and used to quantitatively assess the large signal synchronization instability risk.Meanwhile,the influence laws of different key factors on the large signal synchronization stability range are identified and quantitatively revealed.Finally,the corresponding large signal synchronous control strategies are formulated from the perspectives of downloading reactive current,optimization of active/reactive current ratio and impedance counteraction,respectively.Both the correctness of the analysis results and the effectiveness of the control strategies are verified by simulations and experiments.(2)The transient responses of PLL-based wind power generation are divided into five stages during entire grid fault process by considering its sequential control switching schemes.Furthermore,considering PLL's nonlinear behaviors,the transient synchronization characteristics of wind power generation for different stages are respectively represented by equivalent rotor swing equations.Then,the synchronous characteristic indexes of virtual torque,virtual inertia,virtual damping and others are described,as well,the influence laws of grid status and system operation/control parameters on synchronization characteristics are also analyzed.As a result,the transient synchronization mechanism is revealed by essentially explaining the coupling driving relationship between the equivalent power angle and terminal voltage states under unbalanced virtual torque excitation.On this basis,the influence laws of different synchronous characteristic attributes on the accumulation/release of transient energy are explored,and the internal mechanism of various LOS phenomena caused by mismatched transient energy during different fault stages is revealed.Finally,the transient synchronous stability criterions for different fault stages are systematically proposed,including equivalent equal area criterion(EAC)and critical fault detection time and so on.The correctness of the analysis is verified by simulations and experiments.(3)The equivalent rotor motion equation of the wind power generation during LVRT is taken as the study case,the corresponding small-signal state space model was established.On this basis,the influence laws of grid status,line's impedance and system operation/control parameters on the quasi-static small disturbance synchronization stability of wind power generations during LVRT are analyzed by characteristic root locus.Finally,the equivalent synchronizing torque coefficient and quasi-static damping ratio are proposed,which are respectively used as the criterion of stable equilibrium point and quasi-static small disturbance synchronization stability index for wind power generation.(4)In order to solve the stability problems including the existence of the equilibrium point,the mismatching transient energy and the poor quasi-static small disturbance synchronization stability under grid fault condition.The key factors have the consistent effect on large/small disturbance stability are studied.From the perspectives of parameter design,operation improvement and the layout of additional stability controllers,the consistent design principle of large/small disturbance synchronous stability control strategy is analyzed,respectively.Then,the adaptive transient synchronous stability control strategy is respectively proposed from the perspectives of adjusting the operation mode,compensating the grid impedance and attaching the stability controllers.Finally,from the perspectives of the synchronous characteristic indexs,the stable range of equilibrium point,the balance of transient energy,the equivalent synchronizing torque coefficient and the quasi-static damping ratio,respectively and quantitatively analyzed the improvement mechanism of different adaptive synchronous control schemes for wind power generation.The corresponding guideline of parameter setting is also formulated.The effectiveness of the adaptive control strategies is verified by simulations and experiments.The research results will lay theoretical foundation for the transient instability mechanism analysis and stable control strategies for wind power generation.In addition,these will also provide a technical support for the safe and stable operation of the power system with integration of large-scale wind farm. |
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