Edison was right: high-voltage direct current is making a comeback

High-voltage direct current (HVDC) loses around 3% of energy per 1,000 km, compared with 6%-10% for alternating current, and becomes cost-effective from around 600 km on overhead lines and 50 km in submarine cables. This is not a return to the past: it is a different technology, designed to carry large amounts of power over long distances.

The war of the currents, settled at the end of the 19th century, was not won by alternating current because it was better at everything. It won because it was easier to transform and distribute among millions of customers. That problem remains solved today with AC, which is why the domestic grid is not going to change. What has changed is something else: the distance over which we want to move electricity.

## Why direct current loses less energy

In an alternating current line, the current does not travel only through the center of the cable. At 50 or 60 hertz it tends to concentrate in the outer layer of the conductor, a phenomenon known as the skin effect, which reduces the usable cross-section and increases the effective resistance. In direct current that effect disappears and the current is distributed evenly across the entire cross-section of the cable.

There is a second, less intuitive reason. Alternating current does not only carry useful power: it also manages reactive power, which does no actual work but forces more current to move and causes voltage drops and extra consumption. Direct current only moves active power. Add to that the fact that in underground and submarine cables AC adds losses from eddy currents and from the insulation itself, dissipated as heat.

The result is losses of around **3%** per **1,000 km** in HVDC, compared with the estimated **6%-10%** for alternating current over the same distance. Put another way: **30%** to **40%** fewer losses than an equivalent AC line.

## The distance at which it starts to pay off

Direct current has a cost that alternating current does not: the current has to be converted at each end. The input and output converter stations are expensive, and that weighs on any short stretch. The economic question is not which one loses less, but at what distance the savings in losses and cable pay for those stations.

The threshold used is around **600 km** for overhead lines and **50 km** for submarine and underground cables. Below those distances, alternating current remains the sensible option. Above them, direct current starts to win, and the longer the route, the clearer the margin.

## Where electricity comes from changes the calculation

For decades, alternating current prevailed because generation was close to consumption and because transforming voltage was trivial. Now offshore wind farms are being built far from the coast, and large solar and wind resources are located in regions that do not match demand centers. The farther away generation happens, the more each percentage point of loss weighs.

That explains why HVDC has stopped being a rarity. It does not replace the AC grid that reaches homes: it complements it on the stretches where moving electrons over hundreds or thousands of kilometers starts to get expensive. Direct current is not coming back to take over everything. It is coming back to take over the long haul.

## Key points

- HVDC loses around **3%** per **1,000 km**; alternating current, between **6%** and **10%**.
- Direct current cuts losses by **30%** to **40%** compared with an equivalent AC line.
- The break-even point is around **600 km** for overhead lines and **50 km** for submarine cables.
- With alternating current, the skin effect at **50/60 Hz** concentrates the current on the conductor's surface and raises the effective resistance.
- The input and output converter stations make HVDC more expensive and only pay off over long distances.