Research On Voltage Stability Of Power Systems Including Wind Farm And Bifurcation Phenomena In The Systems

With the enlargement of grid-connected wind farms both in its capacity and scale ,it makes a increasing impact on conventional power systems. It has become an important field to study the mutual impact between the wind farms and conventional power systems in stabiliy analysis of power systems. Based on the established static and dynamic mathematic model, this dissertation using voltage stability theory and bifurcation theory studied the voltage stability of power systems including wind farm in order to reveal the mechanism of steady state voltage instability of power system containing wind farm and the variation process of power systems'voltage stability and structural change of the solution caused by injecting wind power.The main research results and conclusions are shown below:1),On the base ofπtype circuit of wind turbine generator(WTG), the weighted sum method was improved to establish the equivalent model; The continuation power flow method(CPFL) and direct method involving wind power system were deduced considering WTG's power versus voltage characteristic and power increasing pattern in combination with general continuation method and direct method. CPFL can trace the equilibrium solution manifolds in large scope and offer detailed imformation of systems; Direct method can calculate the saddle-node bifurcation point effectively and accurately.2),Using CPFL combined with direct method, the dissertation focused on steady state voltage stability analysis of multi-machine power systems which connected with the equivalent model of wind farm.The simulation shows that the increase of wind power injection leads to the increasing demand for reactive power, which is badly in need of reactive power support coming from PV node and slack node,meanwhile, this support should be strong enough; Reactive power compensation generated by capacitor banks of WTG play an important role in effectively raising voltage level on low voltage bus of wind farm and improving the injection limit; Also reactive power compensation can raise voltage level at saddle-node bifurcation point and postpone saddle-node bifurcation.3),Basing on modal analysis , the voltage stability of power system connected by wind farm was analyzed to reveal voltage instability mechanism in the power system,which provides the theoretical basis for the reactive power reserve and selecting the Point of Common Coupling.4),Established the capacitor's dynamic mathematic model which is appropriate for the ananlysis of power systems including wind farms.With WTG and capacitor's dynamic mathematic model, bifurcation analysis in simple power system connected by wind farm shows that Hopf bifurcations and saddle-node bifurcation are found in the feasible region of extended power flow equations and the calculation results is precision enough.