The electric grid in the United States is made up almost exclusively of alternating current (AC) transmission and distribution lines. This is largely due to the fact that it is relatively easy to change the voltage of AC power, and transporting electricity at higher voltages results in few line losses and greater efficiency. The electric power transmitted over AC transmission lines is exactly the same as the power we use every day from AC outlets, but at a much higher voltage.
On AC transmission lines, the voltage and the current move in a wave-like pattern along the lines and are continually changing direction. In North America, this change in direction occurs 60 times per second (defined at 60 hertz [Hz]). Conversely, the voltage and current on a direct current (DC) transmission line are not time varying, meaning they do not change direction as energy is transmitted.
AC had many early advantages over DC, but technological innovations, such as the ability to efficiently convert between AC and DC, have since allowed for the construction of high voltage DC (HVDC) transmission lines. In fact, HVDC is the preferred solution for moving large amounts of renewable power over long distances, and Clean Line’s other projects will deliver power using HVDC transmission lines. However, the Western Spirit Clean Line, which will be approximately 140 miles in length, is considerably shorter in distance than Clean Line’s other transmission projects, which are several hundred miles long each. AC is the most ideal, cost efficient technology type for a project of the Western Spirit’s size.