AC = Alternating Current
DC = Direct Current
HVDC = High Voltage DC
In order to transmit electric power in bulk, you need high voltage, high current, or both to make high power. Watts (power) = volts (“voltage”) * amps (current).
Voltage, Current, Wire Size, Wire Height
The thickness of wire required to carry electricity depends only on the current, so it is very advantageous to keep the current low so you can use thinner, lighter, cheaper wire. To do that and still transmit a lot of power, you have to use very high voltage. High voltage wires have to be placed high up away from other things, but it is better than using heavy, expensive wire the size of a tunnel.
AC vs. DC
So we use high voltage to keep the current down, but what does using high voltage have to do with using AC or DC? A transformer (they are all over the place, you may have seen our heard one blow up) is the device used to convert electric power to high voltage for transmission and then back to low voltage for use. Transformers only work with AC electricity, not DC. This is the reason that Westinghouse’s AC system beat Edison’s DC system.
The AC vs. DC battle is not over! Modern technology enables stepping up the voltage of DC electricity to high voltage for power transmission. There are HVDC lines around the world now. China especially is investing in HVDC power transmission as well as the Baltic states to reduce their reliance on Russian energy.