Study On Chaos Theory Mechanism Of Low Frequency Oscillation And Chaos Control In Power System

With large interconnection power grid and long distance electric transmission becoming the developing trend of modern power system,the power system becomes more complex,in which the low frequency oscillation phenomena happen occasionally.For the security and stability of power system,the low frequency oscillation is a key problem to solve.While the mechanisms of low frequency oscillation based on linear models are studied in depth,it is still hard to explain some phenomena.As a typical nonlinear dynamic system,power system will emerge chaos under certain specific conditions because of its own nonlinear characteristic,which shows as an irregular low frequency oscillation.This kind of low frequency oscillation,caused by nonlinear characteristic,is hard to be eliminated with traditional linear control methods.Therefore,analyzing the low frequency oscillation characteristic of chaos system based on chaos theory and proposing a nonlinear control method is significant to the security and stability of power system.In this thesis,conditions of chaos in power system and low frequency oscillation characteristics of chaos system are analyzed.Based on this,a control method is designed to suppress the low frequency oscillation.The main contents are as following:Firstly,mechanisms of low frequency oscillation in power system are introduced;the chaos theory and bifurcation theory are mainly expounded;characteristics and judgment methods of chaos are summarized.The Lorenz system is used as an example to illustrate the characteristics of the chaotic system in MATLAB.Secondly,a nonlinear power system is analyzed as a research object.The condition that chaos occurs in power system is obtained by Melnikov method and inferred qualitatively by maximum Lyapunov index and Poincare map.Through timing diagrams,bifurcation maps and attractor diagrams,low frequency oscillation characteristics of chaos system are analyzed to depict the phenomena from stability to instability with the change of external disturbance amplitude and internal parameters.The simulation results indicate that high disturbance amplitude,low damping and high mechanical power all will lead to chaos in power system.Finally,to suppress low frequency oscillation by chaos,a sliding mode controller is presented.The main disadvantage of sliding mode variable structure control is high frequency chattering.The thesis improved the sliding mode algorithm,using fuzzy control to adjustapproaching parameter.The controller keeps the robustness property and accelerates the speed of approach while eliminates the high frequency chattering.The theoretical analysis and numerical simulation results show that this controller can suppress chaos effectively,suppress low frequency oscillation and ensure the stable operation of power system.