Analysis Of Nonlinear Forced Oscillation Of Power System

Forced oscillation is an important factor threatening the stability of power system.With the development of high proportion of renewables,the power system presents more noticeable nonlinear characteristics.At present,most of analytical methods for forced oscillation are part of linearization technique,but there exist some errors in linearization technique with strong non-linear characteristics.As for nonlinear analytical technique such as time-domain simulation and normal forms method,time-domain simulation fails to offer key information including detailed equation to forced oscillations and internal parameters.Besides,singular coefficient exists in normal form in terms of modal resonance.Therefore,resonance is generally avoided.Additionally,normal form has not succeeded in fully explaining non-linear characteristics of frequency deviation.Multiple-scales method is a kind of quantitative analysis of non-linear dynamics,which not only provide explicit expression to equation of forced oscillations,but analyses non-linear characteristic neglected in the linearization technique.So,the use of multiple-scales method can focus on the studies of nonlinear term forced oscillation.Based what is said above,major work is accomplished as follows:(1)Single machine infinite bus system is the research object.Shortcomings of the linearization method should be analyzed in terms of simulation conditions of periodic small and large disturbance as well as heavy load working conditions.Results have demonstrated that linearized forced oscillation re sponse is less accurate under periodic larger disturbance and heavy load working conditions.And then,nonlinear parts in the system model should be reserved in an effort to improve the analysis precision.(2)Multiple-scales method is used to study nonlinear forced oscillations.The nonlinear forced oscillation mechanism under the single-machine infinity system is analyzed.The oscillation model containing nonlinearity should be established and then expressions such as oscillation equation and amplitude-frequency characteristic curve should be deduced.The nonlinear analytical solution contains the influence of high frequency multiplier components and DC components,and the accuracy of the nonlinear analytical solution is verified by numerical simulation.Secondly,after analyzing the impacts of in key parameters on nonlinear forced oscillation,it is possible to know greater impacts of damping and periodic disturbance on nonlinear forced oscillation.An increase in disturbance amplitude can lead to greater oscillating amplitude while an increase in damping can suppress the oscillation.And then both amplitude-frequency characteristic curve and phase plane method have explained frequency deviation and jumping phenomenon in the nonlinear system.Moreover,the research is also conducted on oscillation under special conditions of non-damping and free oscillation When disturbance frequency is equal to natural frequency,nonlinear forced oscillation without damping is a type of beat frequency oscillation.The nonlinear phenomena mentioned above are verified by time domain simulation.(3)Nonlinear forced oscillation under saturation conditions of inverter is discussed.The single-inverter infinite-bus system is the research object.Second-order dynamic model under the strategy of control of virtually synchro nous machines is considered and then forced oscillation response curve and amplitude-frequency characteristic after inverter saturation should also be analyzed.At the time of inverter saturation,changes will take place in internal parameters in the system and there will be potential effects of forced oscillation source.When the power system increases the virtual inertia of the equipment,it also reduces the damping ratio of the system,and the negative impact caused by the forced oscillation should be properly considered.