How do high-voltage power lines work?

The electricity in your fan was made seconds ago, maybe hundreds of kilometres away. A power station's generator makes three-phase AC at about 21,000 volts (21 kV). Then it travels through a chain of transformers and lines.

First a step-up transformer lifts it to 400 kV or even 765 kV. Giant steel towers (pylons) carry it across the country. At substations transformers step it down: to 220 or 132 kV for each state, 33 kV for a town, and 11 kV for the feeders along your streets. The distribution transformer on a pole or plinth near your home makes 415 V three-phase, and one phase of that, 230 V, comes into your house.

Why all the steps? The same power can travel as high voltage and small current, or low voltage and big current. Carrying 1 MW takes just 1.4 A at 400 kV, but 52 A at 11 kV and over 4,000 A at 230 V. Small current means thin wires and little heat lost on the way (chapter 2). High voltage is dangerous, though, so it is stepped down before it reaches people. A transformer does each step (see TransformerClear and FaradayClear).

India's lines of 220 kV and above passed 5 lakh circuit kilometres in January 2026: enough wire to go round the Earth more than 12 times.

Every wire has some resistance, so current flowing through it makes heat (see OhmsLawClear). The heat lost is I²R: current times current times resistance. Double the current and you lose four times as much.

A line carries power = voltage × current (for three-phase lines, P = √3 × V × I). So to send the same power you can use a big current at low voltage, or a small current at high voltage. Raise the voltage 10 times and the current falls 10 times, so the loss falls 100 times.

That is the whole reason for pylons and 400,000 volts. Send 500 MW over 300 km at 400 kV and about 3 % turns to heat. Try it at 132 kV on the same wires and you'd lose nearly 30 %, and the wires would overheat. Engineers can also cut R: fatter aluminium conductors, or several wires per phase in a bundle.

India's losses. About 17.6 % of electricity was lost between power stations and consumers in 2023-24 (T&D losses, CEA). Only about 3 % is lost in the big inter-state lines. Most goes in the low-voltage local network, and in power that is used but never billed. The AT&C loss, which also counts unpaid bills, fell from about 22 % in 2020-21 to about 16 % in 2024-25 (Ministry of Power).

A transmission tower is a lattice of galvanised steel angles: light, strong and cheap to carry up a hill in pieces. Its job is simple: hold three live phases high above the ground and far enough apart that electricity can't jump between them.

The wires are bare. What stops 400,000 volts reaching the steel is a string of insulator discs, toughened glass or porcelain. Each disc holds off roughly 10 to 20 kV, so the higher the voltage, the longer the string: about 9 discs at 132 kV, 23 at 400 kV and 40 at 765 kV. You can guess a line's voltage by counting them.

The wires are ACSR: aluminium conductor, steel reinforced. Aluminium strands carry the current (light, and it doesn't rust); a core of steel strands takes the pull. At 400 kV and above, each phase is a bundle of two or four wires held apart by spacers. On top run two thin earth wires that catch lightning, often with bright bird diverters so birds see them.

Metal expands when hot. On a summer afternoon with a heavy load the wire can reach 75 to 85 °C and grow by tens of centimetres, so the sag in the middle of a span deepens by a metre or more. The CEA rules set a minimum height above the ground: 5.2 m plus 0.3 m for every 33 kV above 33 kV, which works out at 8.8 m for a 400 kV line. Engineers design for the hottest day.

Look at any big line and count the wires: they come in threes. A power station's generator has three sets of coils spaced a third of a turn apart. As the magnet spins, each set makes its own AC wave, each one a third of a cycle (120°) behind the last. These are the three phases, called R, Y and B (red, yellow, blue) in India.

Here's the trick: at every instant the three currents add up to zero. When one pushes, the other two pull. So the current that goes out on one wire comes back on the other two, and no return wire is needed. Three wires do the job of six. The total power is also steady, not pulsing, which keeps big motors running smoothly.

If the loads aren't equal, the sum is no longer zero and a neutral wire has to carry the difference. That's why your street has four wires: three phases and a neutral, with homes shared out between the phases.

HVDC. For very long distances, direct current can win. A DC line needs only two conductors, has no reactive current sloshing back and forth, and loses less. But the converter stations at each end, halls of thyristor valves, are very expensive. Beyond about 600 to 800 km of overhead line, or 50 km of undersea cable, DC is cheaper overall. India's Raigarh–Pugalur link carries 6,000 MW at ±800 kV over about 1,800 km, and Champa–Kurukshetra runs ±800 kV for 1,365 km.

All the big generators on India's grid are locked together, spinning in step like the wheels of one enormous machine. Their speed sets the frequency: 50 Hz, fifty cycles a second.

Electricity can't be stored in the wires, so at every moment generation must equal demand. If a big power station trips, or everyone switches on their ACs at once, demand is suddenly bigger. The missing energy is pulled from the spinning rotors, they slow down, and the frequency falls. Too much generation and it rises. Frequency is the grid's heartbeat.

Three layers of defence bring it back. In seconds, governors on turbines sense the drop and open the steam or water valves. In minutes, load despatch centres (run by Grid-India, once called POSOCO) order stations up or down. If the fall is too big, relays shed load automatically in four steps from 49.4 Hz, cutting some feeders to save the rest. Below about 47.5 Hz generators protect themselves and trip: a blackout. On 30 and 31 July 2012 that happened across northern and eastern India, and over 600 million people lost power.

Since 31 December 2013, when the 765 kV Raichur–Solapur line joined the southern grid, India has been One Nation, One Grid, One Frequency. Solar and wind farms connect through inverters with no spinning mass, so more of them means less inertia and faster falls. Green Energy Corridors, batteries and fast-acting controls help handle that.

A high-voltage line is surrounded by two invisible fields. The electric field comes from the voltage and is measured in kilovolts per metre. The magnetic field comes from the current and is measured in microtesla (µT). Both fall away quickly with distance. The ICNIRP guidelines, used in India and worldwide, set reference levels for the public at 50 Hz of 5 kV/m and 200 µT. Under a 400 kV line you might measure a few kV/m and around 10 to 20 µT; a few tens of metres to the side, far less. The strongest fields are directly under very high voltage lines, which is why lines get a cleared corridor with no homes beneath them.

That crackle and hum near lines, loudest in rain or fog, is corona: the air right next to the wire becomes so stressed that it breaks down in tiny, glowing discharges. It wastes power and makes radio noise. Bundled conductors spread the field out to reduce it.

Why birds are safe: a bird on one wire touches only that wire. Both its feet are at the same voltage, just a few thousandths of a volt apart, so almost no current flows through it. Danger comes when you bridge a live wire and the earth, or two wires. A kite string (wet, or coated with metal "manja"), a metal ladder, a long pole or a crane can do exactly that. At these voltages electricity can even jump through the air without a touch.

Stay safe. Never fly kites near lines, never climb a tower, and keep ladders and poles well away: at least 3 m from any line and much more from big ones. If you see a fallen wire, stay at least 10 m away, keep others back, and call your electricity company, on the helpline 1912 in most states. Never try to move it.