 Electric power transmission

Electric power transmission transfers bulk electrical power between generating areas, major network junctions and receiving systems. Transmission lines carry electric current; electrical transformers change the operating electric potential difference in alternating current networks. Raising that potential can reduce the electric current required for a specified transferred power, and therefore reduce resistive heating. It also changes insulation and equipment requirements.

For an original fixed-output direct current comparison, deliver $P_L=10\mathrm{MW}$ through a line with total conductor electrical resistance $R=2\Omega$. At receiving-end electric potential difference $V_L$, useAt $V_L=10\mathrm{kV}$, the electric current is $1000\mathrm{A}$, conductor heating is $2\mathrm{MW}$, and the source must supply $12\mathrm{MW}$ at $12\mathrm{kV}$. At $100\mathrm{kV}$, the corresponding values are $100\mathrm{A}$, $20\mathrm{kW}$ and $10.02\mathrm{MW}$ at $100.2\mathrm{kV}$. This compares the same receiving load and line electrical resistance; it does not assume equal source power in the two cases.

The resulting loss reduction by a factor of one hundred follows from squaring the tenfold electric current reduction. It does not imply lossless transfer: electrical transformers, converters and conductors have their own losses. The DOE treatment explicitly separates winding copper loss from useful transformer output.^[ref]

Long alternating current lines also have distributed inductance and capacitance, so a resistive balance alone cannot determine their phase relationships or operating limits. An electric power transmission calculation must specify whether its electric potential differences are sending-end or receiving-end values, and whether stated power is active power or apparent power. Those conventions keep conductor-current calculations consistent with power factor.