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Moving energy: wires, pipes and ships: Why electricity travels at high voltage

Understanding Energy / Module 9: Moving energy: wires, pipes and ships Part 1 of 4

Why electricity travels at high voltage

Electricity is the most flexible energy carrier, but it cannot be put in a box and shipped. It flows through metal, and metal resists. Much of the design of a power system, from the height of the towers to the size of the transformers, follows from one simple relation between current and heat.

1.1 Heat in a wire grows with the square of the current

When current flows through a conductor, some of its energy turns into heat. The heating loss equals the current squared multiplied by the resistance of the wire, written I^2R. The power delivered, on the other hand, equals voltage multiplied by current. Put these two facts together and the logic of the grid follows. To send a given amount of power, raise the voltage and the current falls in proportion; the heating loss then falls with the square of that.

A worked example shows how strong the effect is. Suppose a town needs 100 MW and the line feeding it has a resistance of 5 ohms, roughly what 100 km of a single heavy conductor might have. Treat it as a simple circuit. At 400 kV the current is 250 amps and the loss is 250 x 250 x 5 watts, about 0.3 MW, or 0.3% of what is sent. At 10 kV the current would be 10,000 amps and the calculated loss 500 MW, five times the power being delivered. Such a line could never work: the wire would overheat long before. Raising the voltage forty times cuts the loss sixteen hundred times.

That is why transmission lines run at 220 kV, 400 kV and 765 kV, and in China at up to 1,000 kV AC and 1,100 kV DC. The price of high voltage is insulation and clearance. Air breaks down if conductors come too close to each other, to the tower or to the ground, so higher voltages need taller towers, longer strings of insulators and wider corridors.

Thicker conductors also cut resistance, but metal is heavy and costly, so engineers balance metal against voltage. Most overhead lines use strands of aluminium around a steel core: aluminium for conductivity per kilogram, steel for strength over long spans. The same arithmetic explains why wiring at low voltage must be short and thick, and why the last few kilometres of the system, where voltages are lowest, lose a surprising share of the energy.

1.2 Alternating current won because its voltage is easy to change

The first central power stations of the 1880s supplied direct current at low voltage, which could serve only customers within a kilometre or two. Alternating current won the contest that followed for one practical reason: its voltage can be changed easily. A transformer is two coils of wire wound on an iron core. Alternating current in one coil creates a changing magnetic field, which induces a current in the other, and the ratio of turns sets the ratio of voltages. There are no moving parts, and large units pass on more than 99% of the energy they handle.

So a modern system steps voltage up and down several times. A generator produces perhaps 15 to 25 kV. A step-up transformer at the power station raises this to transmission voltage. Substations near cities step it down to sub-transmission and then to distribution levels of around 11 to 33 kV. Transformers on poles or in street kiosks make the final drop to 230 or 240 volts in India and Europe, or 120 volts in North America.

AC has its own drawbacks. The current reverses 50 or 60 times a second, and every generator on a connected network must turn in step with every other. Long AC lines also behave partly like capacitors and coils, storing and releasing energy in each cycle. This reactive power does no useful work, but it takes up line capacity and must be managed to keep voltages within limits. The longer the line, the bigger the problem, and beyond several hundred kilometres engineers add compensating equipment or switch to direct current.

China has pushed AC further than anyone, opening a 1,000 kV line 640 km long in 2009. Yet even there the longest links, which carry hydro, coal and wind power thousands of kilometres east, are direct current. The Energy table tiles "AC transmission" and "Transformers and power electronics" show the equipment in more detail.

Process flowchart From power station to socket
Inputs Electricity at the generator100 units, about 20 kV Process steps 1 Step-up transformer

raises voltage to 220 to 765 kV; well under 1% lost

2 Transmission lines

hundreds of km at high voltage; about 1 to 2% lost (indicative)

3 Substations

step voltage down to 11 to 33 kV

4 Distribution lines and transformers

local wires down to 230 V; the largest share of losses

Outputs Electricity at the meterabout 95 units in a well-run grid

The 5% total is the US average for 2018 to 2022; the split between stages is indicative. Where theft and unbilled supply are common, far less is paid for.

Source: US EIA, How much electricity is lost in electricity transmission and distribution in the United States? (2023), https://www.eia.gov/tools/faqs/faq.php?id=105

1.3 Direct current returns for long distances and under the sea

Modern power electronics have brought direct current back for bulk transmission. A high-voltage direct current (HVDC) link has a converter station at each end. The first turns AC into DC using banks of semiconductor valves; the second turns it back into AC in step with the receiving grid. In between, the line carries a steady current on two conductors, one positive and one negative, which is why voltages are written as plus or minus 800 kV.

DC has three advantages. With no alternating fields there is no reactive power to manage, so the whole conductor carries useful power and losses per kilometre are lower. Cables under the sea or underground behave like long capacitors: an AC cable spends much of its capacity charging and discharging itself, so beyond a few tens of kilometres DC is the only sensible choice. And DC links can join grids that are not synchronised or that run at different frequencies, with operators setting exactly how much power flows and in which direction.

The price is the converter stations. They are large and expensive, and each loses about 0.7 to 1% of the power passing through it. So HVDC pays only over distance. EPRI puts the break-even against AC at roughly 480 km for overhead lines and about 50 km for cables. A plus or minus 800 kV line in China, from Xiangjiaba to Shanghai, loses about 7% over roughly 2,000 km including both converters, or about 3.5% per 1,000 km.

The record holder is China's Changji to Guquan link: plus or minus 1,100 kV, 3,293 km and 12 GW, carrying power from Xinjiang in the far west to Anhui in the east. In Europe, HVDC links stitch together national grids across seas. Viking Link between Britain and Denmark, 765 km of land and subsea cable carrying 1.4 GW, was the longest link of its kind when it opened in 2023. Such links let windy countries export surpluses and import in calm spells, so they also work as a kind of shared storage. The "HVDC links" and "Underground, submarine and superconducting cables" tiles go further.

See it in the table

H₂O WaterAl AluminiumSt SteelHVAC AC transmissionHVDC HVDC linksCab Underground, submarine and superconducting cablesPE Transformers and power electronics

Check your understanding

A line carries the same power but its voltage is doubled. What happens to its resistive losses?

Doubling voltage halves the current. Loss goes with the square of current, so it falls to a quarter.

Why are subsea power cables longer than about 50 km almost always DC?

Cables act like long capacitors. With AC, charging current takes up much of the conductor's capacity; DC avoids this.

What is the main extra cost of an HVDC link compared with an AC line?

Converters are expensive and lose about 0.7 to 1% each, so HVDC pays off only over long distances or under water.
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