Material Insights · Understanding the Material World

AC transmission: How it works

Why engineers push voltage into the hundreds of thousands

What this tab covers How the energy actually moves, step by step: the science that makes it work and the engineering that makes it practical.

AC transmission carries bulk electricity from power stations to cities over overhead lines at hundreds of thousands of volts, because raising the voltage lets the same power flow with far less current and therefore far less heating loss.

Step by step

  1. GenerateGenerators produce alternating current (AC), whose voltage reverses 50 or 60 times a second, in step across the whole connected grid.
  2. Step upA transformer at the power station raises the voltage (for example to 345, 500 or 765 kV) and lowers the current by the same factor, so the power stays the same.
  3. TransmitThree conductors (one per phase), usually aluminium strands around a steel core, hang from steel towers for tens to hundreds of kilometres.
  4. Switch and protectSubstations use circuit breakers and protection relays to route power and isolate faults within a fraction of a second.
  5. Step downNear the load, transformers lower the voltage in stages and pass the power to the distribution network.

The science

Power equals voltage times current, but the heat lost in a wire equals current squared times resistance (P = I^2 R). Doubling the voltage halves the current for the same power and cuts resistive loss to about a quarter. Transformers make this possible, and they work only with AC: a changing current in one coil creates a changing magnetic field in an iron core, which induces a voltage in a second coil; the ratio of turns sets the new voltage. AC lines also absorb or generate reactive power through their inductance and capacitance, which makes voltage harder to control on long lines.

The engineering

A line's rating is set by how hot its conductors may run (the thermal limit) and, on long lines, by voltage stability. A 765 kV single circuit has a thermal limit nearly four times that of a 345 kV circuit, but its natural loading (surge impedance loading, where the line neither absorbs nor supplies reactive power) is much lower. Higher voltages need taller towers, longer insulator strings and wider clearances.

Key numbers

Global power line length (all voltages) About 80 million km (IEA, 2023)1 Thermal limit, 765 kV single circuit About 6,625 MVA5 Thermal limit, 345 kV single circuit About 1,793 MVA5 Surge impedance loading, 765 kV single circuit About 2,435 MW5 First 1,000 kV AC line (China) 640 km, commercial operation 20099

Sources

  1. Lack of ambition and attention risks making electricity grids the weak link in clean energy transitions (press release for Electricity Grids and Secure Energy Transitions) IEA, 2023
  2. Discussion of EHV and HVDC (M. Tackett, MISO, presentation to ERCOT) MISO / ERCOT, 2023
  3. China's first self-developed UHVAC demonstration project enters commercial operation T&D World, 2009