Research On Stability Analysis And Security Early Warning Of Time-varying Power System

The stability analysis of power systems under large disturbance belongs to the category of nonlinear dynamical stability essentially.The traditional method often distinguished the stability based on numerical methods and engineering practical experience and quantized stability index cannot be obtained.The energy function method judges the stability of system by constructing the scalar function directly,but has certain conservativeness.It is urgent to propose a new Lyapunov function construction method to calculate the critical clearing time and transient stability margin to realize the purpose of stability analysis and safety early warning.The main contents and achievements of this paper are:Considering that it is difficult to construct the energy function,a method to construct the Lyapunov function cluster for time-varying power systems based on La Salle invariance principle is proposed.First,the nonlinear dynamics model of multi-machine power system is constructed,and the linear and nonlinear part of the system are separated.Then,the time varying boundary of the nonlinear part is described.In addition,the construction of the Lyapunov function under the time varying power system is transformed into the feasible solution to a set of linear matrix inequalities.A class of Lyapunov functions is constructed by this mehod and is defined as Lyapunov function cluster.Compared with the traditional method,this method is simpler and more effective,and has good scalability.Aiming at the problem that the calculation of critical clearing time is time-cunsuming,a critical clearing time estimation method based on Lyapunov function cluster is proposed.First,the network structure preserving model of power system considering generator and load dynamics is established,and the boundary of the nonlinear part of the system model is depicted by time varying function;on this basis,Lyapunov function of fault power system is constructed.Then a variable upper bound of the derivative of the function during the fault is determined,and the relationship between the energy accumulation during fault and the clearing time critical is quantified.In addition,the critical clearing time calculation is transformed to convex optimization problems with linear matrix inequalities constraints.Finally,an explicit calculation method of CCT is given.The simulation results show this method does not depend on the fault system trajectory,and and improves the computational efficiency.In order to solve the problem that the transient stability margin is d ifficult to evaluate dynamically,a Lyapunov function optimal selection and transient stability margin evaluation scheme is proposed.Firstly,considering the structure preserving model of power system,the Lyapunov function clucter which is suitable for transient stability analysis is presented;on this basis,simplified Lyapunov functions in three different forms are constructed,and the comprehensive properties and stability boundary of three kinds of functions are given.Then,considering Consider the network structure and system state,the energy boundary factor of the comprehensive stability index is defined which will be used in online fault severity ranking and control strategy optimization to realize system security early warning.The simulation results show the adaptability and effectiveness of the proposed method in dynamic safety early warning and control scheme selection.