Power Systems Transient Stability Assessment And Control Based On WAMS

With the increase of system-size, the commission of all kinds of new transmission technology and the deregulation of power industry, the dynamic behavior of power systems becomes more complex, and easier to lose transient stability. Moreover the large blackout might occur if the disturbance cannot be handled properly. The lack of quantitative power system stability analysis approach is one of the main reasons for losing good chance to implement the emergency control and even finally resulting in the large outage. The rapidly developing wide area measurement system (WAMS) gives a new way for quick on-line quantitative transient stability analysis and control. Therefore, this dissertation aims at studying the methods of transient stability assement and the scheme of emergency control based on trajectories from WAMS. They are mainly including:The first step is to propose the three methods of perturbed trajectory prediction based on the synchronous trajectories information from WAMS. A method of perturbed trajectory prediction based on trigonometric functions is presented firstly. The algorithm is simple and the prediction speed is very fast. But without combining power system models, it is only suitable for short time extrapolating; then, by combining the power system models with the principle of atmosphere self-recall equations, a method of perturbed trajectory prediction based on atmosphere self-recall equations is also presented. This method not only needs self-historical data, but also introduces other related data (i.e. electricmagnetic power, mechanical power, etc). Therefore, it is more accurate than ordinary curve extrapolated methods. Because the impact of load characteristic can not be ignored for transient stability analysis and control of power systems, a new method for perturbed trajectory prediction is proposed with the consideration of load characteristic. It is unnecessary to pre-know the network topology, fault mode, position and load type in advance. Because of introducing the static ZIP load model, the result of this method fits practical power systems well.The mode of multi-swing instability occurs frequently in modern power system, so, the second step is to propose the two methods of multi-swing stability assessments. Beginning with the individual machine energy function in COI coordinate and proving the relationship between kinetic energy of individual machine and stability of individual machine, a method of multi-swing stability assessment based on curve curvature of kinetic energy-rotor angle is proposed. In this method, only the speed deviation of generators is used and the algorithm is simple, so, real-time stability assessment can be realized by this method. In order to avoid calculating the sensitivity of stability margin to control variation for formulating controlstrategies, the relationship between transient energy variation rate of the individual machine and its stability is proved, then a new criterion of multi-swing stability based on transient energy variation rate and the unstability index of individual machine are presented . For introducing the information of generator input mechanic power, output magnetic power and rotor speed, the accuracy of the method is high and the calculation speed is rapid. When the unstabiligy index of individual machine is used in formulating a control strategy, it is not necessary to computing the complex sensitivity of stability margin to control variation.Therefore, this method can be well fit for study on control strategy of transient stability. Finally, a control strategy of generator and load shedding based on transient energy variation rate of generator is presented. In this method, it is unnecessary to spend long time to search the optimal solution. Since it chooses the most unstable generator to do the control, the control action is most effective, and it can be suitable for rapidly emergency control. Then a scheme of closed loop correcting control is proposed. It is unnecessary to make contingency set and calculation speed is fast for this method, which can avoid the problem of stratagy-mismathch. Because the stability of power system will be assessed after each control, it is quite useful for avoiding the problem of excessive control.