Research On The Space-time Distribution Characteristics Of Frequency Dynamics Based On The WAMS

Frequency is an important index of power quality and one of the most important variables reflecting the balance of active power supply and demand. Frequency would fluctuate which is caused by sudden trip of huge generators in power system, other studies have shown that the change of frequency has the time-space distribution characteristics. With more and more high-capacity power plants(fossil fired power plant installing 1000 megawatts alone or nuclear power plant) dived into network operation, the risk of large active power vacancy event would increase. The time-space distribution characteristics of dynamic frequency is a very complex topic, theoretical analysis method could only study the characteristics of several simple systems. The shortcomings of detection methods are main restriction. The WAMS which is based on PMU can realize synchronous measurement of the dynamic information in the whole system. The WAMS can provide observation techniques for the study of dynamic frequency and its time-space distribution characteristics in power system. The further study of dynamic frequency and its time-space distribution characteristics after a failure has great significance for avoiding instability of frequency and improving the UFLS control.In this paper, the four incidents of active power vacancy which are all caused by abnormal trip of nuclear power plant in Northeast power grid are studied. The criteria for describing the dynamic process of frequency is established, which include the initial moment of response, the initial frequency, the minimum of frequency and the average change rate of frequency, according to the frequency data of 45 monitoring sites recorded by the WAMS. The relations between the key features of frequency fluctuation and the geographical distance from monitoring site to the disturbance source are analyzed. The time-space distribution characteristics of dynamic frequency are revealed from a different perspective.At the theoretical analysis, the temporal property of dynamic frequency and its relations with the parameters of system are studied through the model of dynamic frequency in single-machine power system; the time-space distribution characteristics of dynamic frequency are also researched by the electromechanical transient simulation model in multi-machine power system. The influence of active power vacancy, the inertia constant and dead band of speed governor to the time-space distribution characteristics is studied, the results of which are compared with the measurement data. The simulation results show that the speed of frequency change spread would increase when the system has larger active power vacancy and smaller inertia constant. And the minimum of frequency is strongly correlated to the active power vacancy and inertia constant, so as to the average change rate of frequency. The decrease of active power vacancy as well as the increase of inertia constant and dead band of speed governor will led to the delay of the time when frequency reaches its lowest point.The data of dynamic frequency based on the WAMS show a profound impact on study of time-space distribution characteristics. The combine of measurement data and simulation study could reveal the mechanism of dynamic frequency and its main influences better. The study of dynamic frequency process is much meaningful to avoid instability of frequency and improve the UFLS control.