Alternating current (AC) refers to the alternating current whose current direction changes periodically with time, and the average value of operation in a cycle is zero. Unlike DC, its direction will change with time, and DC does not change periodically. Usually the waveform is sinusoidal. Alternating current can effectively transmit electricity. In fact, there are other applications, such as square wave and triangular wave. The mains power used in life is alternating current with sinusoidal waveform. The frequency of alternating current refers to the number of periodic changes in its unit time. The unit is Hertz, which is inversely related to the cycle. The frequency of alternating current in daily life is generally 50 Hz or 60 Hz, while the frequency of alternating current involved in radio technology is generally large, reaching the measurement of kilohertz (kHz) or even megahertz (MHz). The AC frequency of power systems in different countries is different, usually 50 Hz or 60 Hz. UHV AC line The main advantages of UHV AC transmission are: (1) Improve transmission capacity and transmission distance. With the expansion of power grid area, the transmission capacity and transmission distance of electric energy are also increasing. The higher the required grid voltage level, the better the effect of compact transmission. (2) Improve the economy of power transmission. The higher the transmission voltage, the lower the price per unit capacity. (3) Save line corridors. Generally speaking, one 1150kv transmission line can replace six 500kV lines. The use of UHV transmission improves the utilization rate of the corridor.