Analysis Of Wind Turbine's Electromechanical Transient Characteristics Through Modeling As Well As Its Optimization For Power System Transient Stability Improvements

In recent years,the wind power cumulative installed capacity in China and other countries in the world has been increasing rapidly.The large-scale integration of wind power,whose transient characteristics are drastically different from the traditional synchronous generators,has posed severe challenges to many aspects of the power system stability,among which is the power system transient stability.The challenge is especially serious in areas such as Gansu Province China,where the inverse distribution of wind resources and electric loads results in a regular heavy power flow through the long transmission lines connecting the wind farms and the load center.Different from the traditional synchronous generator whose electromechanical transient characteristics,especially the active power characteristics,are mainly determined by its electromechanical components rather than by its electromechanical controls,the electromechanical transient characteristics of the wind turbine,including the active power characteristics,are mainly determined by its electromechanical controls rather than by its electromechanical components.Therefore,the optimizing space of the traditional synchronous generator's electromechanical transient characteristics,especially the active power characteristics,is quite limited,while the electromechanical transient characteristics of the wind turbine,including the active power characteristics,can be optimized in a greater range and in a more flexible way to improve the power system transient stability.However,on the one hand,the current studies on the optimization of the wind turbine's electromechanical transient characteristics are mainly focused on its reactive power characteristics,proposing various optimizing controls of reactive-current-boosting kind,based on the relevant theoretical analyses and operating experiences of synchronous generator's reactive power characteristic optimizations such as the forced excitation,and have not paid much attention on the optimization of the wind turbine's active power characteristics.On the other hand,the few studies focusing on the active power characteristic optimization are lack of in-depth theoretical analyses,and the optimizing controls they proposed are also lack of sound theoretical support.The thesis focuses on the analysis of wind turbine's electromechanical transient characteristics through modeling as well as its optimization for power system transient stability improvements,to address the severe challenges of power system transient stability brought about by the large-scale integration of wind turbines that possess remarkably different characteristics from traditional synchronous generators.First,the thesis systematically analyzes the related theoretical issues of the three participants of the transient stability of power systems with wind power integration,i.e.,the wind turbine,the synchronous generator and the electric network.Then,the thesis optimizes the relevant transient controls of the wind turbine's electromechanical transient characteristics,paying a special attention on its active power characteristic optimization,to improve the transient stability of such power systems as those whose wind power has to be transported to the remote load center through long transmission lines and therefore face severe transient stability challenges.The contents of the thesis can be summarized as follows:(1)The thesis analyzed the influence mechanism of the phase-locked-loop PLL on the wind turbine's electromechanical transient characteristics,and established a timedomain model of the wind turbine's electromechanical transient characteristics considering the PLL's influence.The analyses stated that the PLL influences the wind turbine's electromechanical transient characteristics by affecting the relationship between the phase of the wind turbine's dq rotating coordinates and that of the orientating grid voltage vector,and the absence of the PLL in a wind turbine model could result in a distinct electromechanical-timescale deviation of the model's active and reactive current responses.The thesis then established a time-domain model of the wind turbine's electromechanical transient characteristics considering the PLL's influence.(2)The thesis established a qualitative physical model of the wind turbine's electromechanical transient characteristics based on the magnitude/phase motion equations,and analyzed the qualitative physical characteristics of the existing wind turbine as well as its shortcomings and possible optimizing directions.The thesis used the power system magnitude/phase-motion-equations modeling method to establish a qualitative physical model of the existing wind turbine,and analyzed the qualitative physical characteristics of the existing wind turbine through the model.The analyses stated that the existing wind turbine only responds to the system voltage magnitude dynamics,and does not respond to the angle dynamics in the electromechanical timescale.The thesis then analyzed the adverse impacts of the deficient angle-dynamics response on the power system transient stability,and proposed an optimizing direction,i.e.,the existing wind turbine should strengthen its response to the system voltage angle dynamics.(3)The thesis proposed a new method to optimize the swing dynamics of the synchronous generator in single-machine infinite-bus systems.The thesis analyzed the swing dynamics both in a quantitative way and in a qualitative way after dividing each swing period into four stages,and proposed a new method to optimize the swing dynamics,i.e.,the stability problems relevant to the swing dynamics of the synchronous generator in singlemachine infinite-bus systems,including the aperiodic stability problems and the oscillatory stability problems,can all be optimized by mildly decreasing the magnitudes of the synchronous generator's rotor acceleration in stage 1 and stage 3,and by wildly increasing the magnitudes of the rotor acceleration in stage 2 and stage 4.(4)The thesis analyzed the influence of the network characteristics by studying a simple power system with wind power integration.The simple power system was comprised of two synchronous machines and one wind turbine connected in a chain by transmission lines.On the one hand,the thesis quantitatively analyzed the influence of the network characteristics on the unidirectional interaction-strength/influence-capability of the wind turbine's active and reactive currents on the synchronous generator's swing dynamics,focusing on four network characteristics,i.e.,the network strength,the network structure,the fault location and the network power flow,and reached many preliminary valuable conclusions.On the other hand,the thesis also quantitatively analyzed the transient characteristics of the electrical frequency at different locations of the network.The analyses stated that different from the electrical frequency during the steady state when the electrical frequency at any location of the network can all reflect the rotating speed of the synchronous generators in the network,during the transient state,only the electrical frequency near a synchronous generator can reflect the rotating speed of that specific synchronous generator.(5)The thesis designed an optimizing control of the wind turbine's active current to improve the transient stability of such power systems as those whose wind power has to be transported to the remote load center through long transmission lines.The optimizing control was based on the thesis' s theoretical analyses of the three participants of the transient stability of power systems with wind power integration,i.e.,the wind turbine,the synchronous generator and the electric network,and was for the situations where the wind power has to be transported to the remote load center through long transmission lines.The optimizing control was also well verified through multiple time-domain simulations.