Transmission of electrical energy. It, together with substation, distribution, and electricity, constitutes the overall function of the power system. Through transmission, the power plants and load centers that are far apart (up to thousands of kilometers) are connected, so that the development and utilization of electric energy exceeds the geographical limits. Compared with other energy transmission (such as coal transportation, oil transportation, etc.), transmission loss is small, high efficiency, flexible and convenient, easy to control, and less environmental pollution; transmission can also connect power plants in different locations to implement peak and valley Transmission regulation. Transmission is an important manifestation of the superiority of electric energy utilization. In modern society, it is an important energy artery.

Transmission lines can be divided into overhead transmission lines and underground transmission lines according to the structure. The former consists of line towers, wires, insulators, etc., and is erected on the ground; the latter is mainly laid on the ground (or underwater) with cables. Transmission can be divided into direct current transmission and alternating current transmission according to the nature of the current delivered. In the 1880s, the first direct transmission was realized, and the latter was limited by the voltage (the transmission capacity was roughly proportional to the square of the transmission voltage). At the end of the 19th century, it was replaced by AC transmission. The success of AC transmission has ushered in the electrification era of the 20th century. Since the 1960s, due to the development of power electronics technology, there has been a new development in direct current transmission, which is combined with AC transmission to form an AC-DC hybrid power system.

The level of transmission voltage is the main indicator of the development level of transmission technology. In the 1990s, high-voltage transmissions with high-voltage transmissions of 330 to 765 kV and ultra-high-voltage transmissions of 1,000 kV and above were commonly used in countries around the world.

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