Modeling Of Wind Turbines Based On Amplitude-phase Motion Equation Method In Dc-link Voltage Control Timescale And Its Applications In Power System Stability Analysis

To meet the needs of national energy security,environmental protection,and international climate change,China's wind power generation is increasingly developed.Especially in China's “Three North” area,the penetration of wind power generation has reached a high level and the wind power generation has gradually become one of the major power source in this district's power grid.As a typical power electronics device,wind turbine(WT)has many different features compared to traditional synchronous generator(SG),which brings some new dynamical stability problems to the power systems with large-scale wind power.During these dynamical stability problems,the ones closely related with the energy exchange and accumulation between devices' DC-link capactors are called the dynamical stability problems in DC-link voltage control(DVC)timescale in this thesis.With regard to the dynamical stability problems in DVC timescale in wind power integrated systems(WPIS),several studies have tried to analyze its small-signal stability,but mainly focused on the single-machine situation.While for the transient stability analysis of WPIS in DVC timescale,only a few studies are related with it,and just considering a specific control loop in WT.To extend these researches to the multi-machines scenario and to the consideration of complete controls in WT,this thesis presents a small-signal model and a transient model respectively to understand WT's small-signal and transient external characteristics in DVC timescale,based on the amplitude-phase motion equation method.Meanwhile,the proposed models are well applied to the small-signal and transient stability analyses of WPIS in DVC timescale,respectively.The details are as follows:(1)The sequential control feature of PMSG/DFIG-based WT is introduced.According to the energy storage elements and the response speeds of the corresponding controllers,WTs generally have dynamical characteristics at three different timescales,viz.,AC current control timescale,DVC timescale(about 0.1s)and rotor speed control timescale.As a result,the dynamical stability problems in WPIS also have a multi-time scale feature.The relationship between the multi-time scale features of WT and WPIS's stability problem is discussed,which indicates that the deep understanding of WTs(viz.the modeling of WTs)is the basis of analyzing the stability of WPIS.Focusing on the WPIS's dynamical stability problem in DVC timescale,the meanings and challenges are carefully interpreted.(2)To consider the dynamical interactions between multi-WTs in DVC timescale,especially in the weak grid condition,small-signal models of PMSG/DFIG-based WTs based on amplitude-phase motion equation method are respectively established to understand WTs' external characteristics in DVC timescale.Firstly,the basic concept and idea of amplitude-phase motion equation method are explained.Then,by synthetically considering the complex control effects of WT in DVC timescale,the small-signal model in motion equation form,of which the inputs are the active/reactive power imbalances and the outputs are the phase/amplitude dynamics of WT's internal voltage,is developed.With the model,characterisctics of equivalent inertia and damping coefficients of WT in DVC timescale can be clearly understood.And,the model depends only on the parameters of WT itself,without including the information of power network,which makes the model can be easily extended to the modeling of multiple WTs.Finally,the physical meaning of the proposed model is interpreted,and comparisions of eigenvalues show that the proposed model can hold the main behaviors of concern.(3)The general thought based on self-stabilizing and en-stabilizing concepts to analyze the dynamical interactions between different objects and the system stability affected by the dynamical interactions is introduced.Utilizing this thought and the proposed WT's small-signal model,the dynamical interaction between WT internal volage's phase and amplitude motions in DVC timescale in single WT integrated system,the dynamical interaction between different WTs in DVC timescale in multiple WTs integrated system,and the dynamical interaction between WT and high-voltage direct current(HVDC)transmission system in DVC timescale in WT-HVDC system are analyzed,respectively.And,the influence rules of different factors on the small-signal stability of corrensponding system in DVC timescale are also analyzed.The procedure and method to equivalently transform the analysis of multi-machines system to the analysis of equivalent single-machine system are carefully introduced.It is worth noting that the above method is not limited to the senarios used in this thesis,it can also be applied to the multi-machines scenarios including other power electronics devices,such as photovoltaic(PV)power generation and FACTS devices.(4)Considering the WT's complete controls in DVC timescale and preserving the nonlinearities in WT,the transient model to specially describe WT's transient characteristics in DVC timescale is proposed,based on amplitude-phase motion equation method.The relation between the active/reactive power and phase/amplitude dynamics of WT's internal voltage is developed in this model,which can help the power engineerings understand the transient characteristics of WT in DVC timescale just like the familiar SG.Comparisons of time-domain simulation responses verify the correctness of the proposed transient model.By comparing with the classical model of SG,WT's different features,viz.,high-order,asymmetry and coupling features,are summarized.Based on these different features,the open-loop transient response characteristics of WT's internal voltage to different power disturbances are analyzed.Meanwhile,the significance of WT's open-loop transient characteristics in WPIS's closed-loop transient analysis is discussed.(5)The monotonic instability phenomenon of WT internal voltage's phase in DVC timescale,which is similar to the loss of synchronism phenomenon of SG's transient power angle,is discovered in different wind power integrated scenarios.Based on the proposed WT's transient model and by introducing the concept of “dynamic gear box” to abstractly understand WT's phase motion,the mechanism of the above instability phenomenon is revealed,which indicates that it dues to the power relationship of WT's input and output powers turning from output power greater than input power to output power less than input power again when the internal voltage's phase is still increasing.Learning from the EEAC theory,the method to evaluate the degree of transient stability of WPIS is given based on the system's disturbed trajectories.The method can quantificationally evaluate the influence of different factors on the system's transient stability to some degree.Hereby,the influence rules of the strength of power grid and WT's control parameters on the transient stability of WPIS in DVC timescale are analyzed.And,the physical explanation of the analysis result is also given based on the mechanism understanding.