How do high-voltage power lines work?
The electricity in your fan was made hundreds of kilometres away, seconds ago. Follow it up to 400,000 volts and back down to 230, and work out live why the towers are so tall, the wires sag on hot days and the whole grid hums at exactly 50 Hz.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Up for the journey, down for your home. A power station makes three-phase AC at about 21 kV. A transformer lifts it to 400 or 765 kV for the long trip across the country, and substations step it down to 220, 33 and 11 kV. The transformer at the end of your street makes 415 V, and one phase of that, 230 V, reaches your sockets.
- Why the voltage is so high. A wire turns I²R into heat, so losses grow with the square of the current. The same power at ten times the voltage needs a tenth of the current and loses a hundredth as much. 500 MW over 300 km loses about 3 % at 400 kV, and nearly 30 % at 132 kV on the same wires. India loses about 17 % of its electricity between power stations and meters, mostly in the low-voltage local network and unbilled power.
- Steel, glass and aluminium. A lattice tower holds three bare phases apart. Strings of glass discs insulate them from the steel: about 9 discs at 132 kV, 23 at 400 kV, 40 at 765 kV. The wires are ACSR, aluminium strands round a steel core, often in bundles of two or four. On a hot day with a heavy load they warm up, stretch and sag, so CEA rules fix a minimum height: 8.8 m for 400 kV.
- Three wires, and when DC wins. Three phases 120° apart add up to zero at every instant, so no return wire is needed and the power flows steadily. For very long lines, above roughly 600 to 800 km, direct current wins: India's Raigarh–Pugalur link carries 6,000 MW at ±800 kV over about 1,800 km.
- One giant machine at 50 Hz. Every big generator on India's grid spins in step. When supply falls short, they slow and the frequency drops. Governors respond in seconds, load despatch centres in minutes, and relays shed load in steps from 49.4 Hz. In July 2012 the defences failed and over 600 million people lost power. Since 31 December 2013 India has run as one synchronous national grid.
- Fields and safety. Under a 400 kV line the fields are a few kV/m and around 10 to 20 µT, near or within the ICNIRP public reference levels of 5 kV/m and 200 µT, and they fall off quickly to the side. A bird on one wire is safe; a kite string, ladder or pole that bridges a wire and the earth can kill. Stay well away from fallen wires and call the helpline 1912.
| Term | Meaning |
|---|---|
| Transmission line | A high-voltage line (765, 400, 220 or 132 kV in India) that carries bulk power over long distances. |
| Substation | A yard of transformers and switches where voltage is stepped up or down. |
| I²R loss | Heat made in a wire: current squared times resistance. Higher voltage means less current and far less loss. |
| Insulator string | A chain of glass or porcelain discs that hangs a live wire from a steel tower. |
| ACSR | Aluminium conductor, steel reinforced: aluminium strands for current around a steel core for strength. |
| Sag | How far a wire dips between towers. It grows as the wire warms and stretches. |
| Three-phase | Three AC waves 120° apart on three wires; they add to zero, so no return wire is needed. |
| HVDC | High-voltage direct current, used for very long lines and undersea cables. |
| Grid frequency | 50 Hz in India. It falls when demand exceeds supply and rises when there is too much generation. |
From one New York block lit by DC in 1882 to a single 50 Hz grid tying all of India together.
- 1882 · Edison lights a city block with DC (Thomas Edison, New York City, USA)
- 1891 · Three-phase AC travels 175 km (Oskar von Miller, Mikhail Dolivo-Dobrovolsky and Charles Brown, Lauffen am Neckar to Frankfurt, Germany)
- 1896 · Niagara’s power reaches Buffalo (Westinghouse and the Niagara Falls Power Company, Niagara Falls to Buffalo, USA)
- 1897 · India’s first hydroelectric station (Darjeeling Municipality, Sidrapong, near Darjeeling)
- 1902 · 147 km to the Kolar Gold Fields (Mysore State; planned under Dewan K. Seshadri Iyer, Shivanasamudra to Kolar Gold Fields, Karnataka)
- 1965 · 735,000 volts (Jean-Jacques Archambault and Hydro-Québec, Manicouagan to Montréal, Canada)
- 2012 · The largest blackout in history (Northern, eastern and north-eastern grids, Northern and eastern India)
- 2013 · One Nation, One Grid, One Frequency (POWERGRID, Raichur, Karnataka to Solapur, Maharashtra)
The full story, with 27 moments, charts, people and 49 sources: glassbox.how/e/powerlineclear/history. The data lives in history.json.
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|---|---|---|
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