AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. In electric railways, sometimes single phase AC current is used as traction current for railway traction.
Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.