Electric energy transmission, transform, power distribution, power assumption, together constitute the whole function of the electric system. Through the transmission, the power station and the long-distance ( several thousand kilometers) power load center is connected, power creation and application is not limited by the regions any more. Compare with the other energy transmission (coal, oil), the power transmission is low lost, high efficiency, flexible, easy to control, low environment pollution, power transmission can make the power station in the different areas be connected and balance the power of peak and valley. In the modern society, power transmission is the artery of the energy.

       The route of the power transmission have two types, overhead lines and under ground lines. The former is consist of pole, wire, insulation parts ect. The later is placing the electric wire under the ground or under the water. The power transmission also can be divided into DC and AC. In the 1880s, the DC transmission came to reality successfully, later because of the limit of the voltage raise in the transmission (the transmission cover area is related with the voltage of the power in the transmission) the end of 1890s, the DC was replaced by AC. The AC transmission successfully creation brought the electric era of the 20th century. Since the 1960s, due to the development of electric and electronic, the DC had the new development too. Cooperated with the AC, the mixture of DC and AC system became the electric system. 

       The height of the power voltage in transmission is the main parameter of the power transmission development. In the 1990s, the usual power voltage in the transmission of the worldwide are 220kv and the higher voltage of 330-765 kv, even 1000kv  and higher.