Power system is a unified system composed of power generation, transformation, transmission, distribution, electricity and other equipment and corresponding auxiliary systems according to the specified technical and economic requirements. The basic task of power system is to produce, transmit and distribute electric energy safely, reliably, high quality and economically to meet the needs of national economy and people's life. A modern power system is composed of a large number of electrical equipment interconnected in a very wide area. Such as China has East, Northeast, North China, Central China, J Sichuan and Chongqing, northwest, South Fang Province and other regional power grids and some provincial networks, planning to achieve national networking in 2015. The advantages of networking are: It can develop and utilize all kinds of primary energy resources more economically and reasonably, and solve the problem of imbalance between energy resources and load distribution. 2 2 can stagger the peak and trough of electricity, reduce the installation and standby; 3 It is conducive to the use of standardized large-scale equipment, saving investment and improving operating economy; 4 Facilitate mutual support during failures to improve operational safety; 5 Easy to centralize management to achieve economic dispatch and reasonable distribution of power.