Research On Close Oscillation Modes Of Electric Power System

There are complex oscillation phenomena in power system.When the system damping is weak or negative,the oscillation is not easy to calm down or even diverge,which threatens the stable operation of power system.The renewable energy sources such as wind and photovoltaic connected to the power system and the electronization of power system make the oscillation problem more serious.Therefore,it is of great significance to study the oscillation characteristics of power system.In this paper,the close oscillation modes of power system are studied.The main research work is as follows.The small signal model of multi-machine power system for electromechanical oscillation modes analysis is established.The distribution range of power system oscillation frequency is analyzed by using Gersghorin Circle Thorem.It is shown that the oscillation frequency of power system is bounded.When the power system scale is large,the phenomenon of close oscillation modes will appear.Furthermore,the mechanism and conditions of close oscillation modes in power system are analyzed.Based on the matrix perturbation method,the analytical expression of the first order perturbation of eigenvectors is derived.According to the expression,the modal instability of close oscillation modes is analyzed.A numerical example is used to verify the modal instability of the close oscillation modes in power system.A sensitivity method for identifying close oscillation modes in power systems is proposed.Compared with the Modal Assurance Criterion(MAC)method,this method is independent of parameter perturbation,simple in calculation,can accurately identify close oscillation modes in power systems,and can also judge the degree and direction of variation of each component of oscillation modes.The10-machines and24-machines numerical examples are given to verify the method.The small signal models of the grid-connected systems of doubly-fed wind farm and direct-drive wind farm are established respectively.The eigenvalue calculation shows that the grid-connected system of doubly-fed wind farm has close oscillation modes in low frequency and subsynchronous band,and the grid-connected system of direct-drive wind farm has close oscillation modes in sub-synchronous band.By calculating the participation factors of the close oscillation modes,it is found that the participation factors are sensitive to the parameter changes,which indicates that modal instability also exists in the wind farm grid-connected system.