Study On The Propagation Characteristics Of Large Disturbance And Transient Energy In Ac Power Grid

Although large-scale interconnection of power grid in our country has been carried great benefits,the possibility of blackouts caused by cascading failures has been increasing.It is easily for the impact of a local fault to spread from local to a wide range of area through the interconnected power grid,which triggers a series of chain reaction,and even causes serious accidents such as blackouts and power system collapse.Actually,the nature of disturbance propagation is transient energy spreading across on the power grid.Therefore,the research in this paper focus on the propagation characteristics of transient energy.Firstly,based on the pervious methods of power system analyzing from the perspective of energy,a proper tool for the study in this paper is given.By classifying current methods for energy analyzing,an in-depth comparative analysis between the branch potential energy and energy supply on port(ESP)is made,which indicates ESP is the more proper tool to describe the power grid energy in this study.Then,through the analysis for the impact factors and based on the generator kinetic energy function,the unbalanced power distribution coefficient iK and the transient kinetic energy distribution difference coefficient iK(35)are constructed to describe the relative magnitude of the unbalanced power of generators and the relative magnitude of the transient energy distribution difference of the generator at different states,respectively.It is pointed out that the propagation of transient energy is caused by the change of transient energy distribution,which is an important factor affecting the direction and magnitude of transient energy transfer in the power grid.At the same time,a new formula for ESP calculating is given.The proposed coefficients and formula are validated by simulations on the IEEE-39 bus system.Based on the conclusions of study above,a method of power system stability assessment is put forward,which does not rely on the system disturbance trajectory any more.This method inherits the idea that the stability of the system is closely related to the transient energy aggregation on the transmission network.However,the actual algorithm only needs the information of the grid topology,the inertia of the generator and the initial generation.Given a fault,it is easily to calculate the impact strength brought by the fault on each transmission line through this assessment method.The method is fast and simple,and it can get rid of the real transient energy calculation of the power system,which relies on the system disturbance trajectory in the former method.Simulations are carried on in the Northeast Power Grid to verify the proposed method.Finally,based on the frame of power system security and stability defense system,the feasibility of application of the coefficients and conclusions given above in the security control measures is analyzed,from two aspects of active safety and passive safety.The unbalanced power distribution coefficient iK quantitatively describes the distribution of the unbalanced power on the generators at the moment of fault,which can guide the power output of generators before fault.The transient kinetic energy distribution difference index iK(35)of generators quantitatively describes the direction and magnitude of the transient energy flow on the transmission network after the fault is removed,which indicates that the generators with larger absolute values of iK(35)can be tripped to maintain the power system stability.The branch ESP index lESP reflects quantitatively the severity exerted on transmission lines by the disturbance,which helps to identify the vulnerable section of the transmission network.