Power production must adhere to the principle of "safety first", that is, meet the needs of users safely, continuously and with high quality (voltage, frequency, pressure and temperature of steam or hot water, etc.). ) and quantity. Electric energy or heat energy can't be stored in alternating current, and power production has the characteristics of high automation and synchronous production, supply and marketing. Power plants, transmission lines, substations and users form a unified power system, and the destruction of any production link will affect the energy balance of the whole network.