Power Transmission: Moving Energy Through Space Instead of Time
Transmission is the least visible part of the energy system and one of the most consequential. It is also the part where the binding constraint is almost never technical. The engineering has been understood for a century; what limits it is permitting, land and the fact that nobody wants a pylon.
Why Voltage Is So High
Power lost as heat in a conductor equals the current squared times the resistance. Since power delivered is voltage times current, the same power can be sent as high voltage with low current or low voltage with high current - and because the loss term goes with the square of current, the high-voltage option wins by an enormous margin. Raising voltage tenfold cuts current tenfold and losses a hundredfold.
That single relationship built the grid as it is. Generators produce at perhaps 20 kilovolts, transformers step that up to 400 or 765 kilovolts for long-distance transmission, and a cascade of substations steps it back down to 230 volts at the socket. The transformer, which only works with alternating current, is the reason alternating current won the argument in the 1890s.
Modern high-voltage lines lose around 2 to 3 percent per 1,000 kilometres. The whole of a national grid, transmission and distribution together, typically loses 5 to 10 percent between generator and consumer - a figure that is remarkably low for moving energy across a country and is often assumed to be much higher.
There is a limit to how far alternating current goes usefully. A long cable behaves as a capacitor, charging and discharging 50 or 60 times a second, and the current doing that consumes conductor capacity without delivering anything. Overhead lines manage several hundred kilometres. Submarine cables, where the surrounding seawater makes the capacitance far larger, run into the wall at roughly 80 kilometres.
What HVDC Changes
Direct current has no cycles, so there is no repeated charging of the cable's capacitance and no distance limit from that source. Losses fall to about 3 percent per 1,000 kilometres against 6 to 8 for comparable alternating current, and the cable itself carries more power because there is no reactive component.
The cost is at the ends. Converter stations, which turn alternating current into direct and back, are expensive - hundreds of millions for a large link - so HVDC only pays above a break-even distance of roughly 600 to 800 kilometres overhead, or 50 to 80 kilometres submarine. Below that, ordinary alternating current wins.
Above it, HVDC is transformative. China has built links exceeding 3,000 kilometres carrying 12 gigawatts, moving hydro and wind from the west to the coastal cities. The North Sea is being progressively wired with submarine HVDC connecting offshore wind farms to several countries at once, so a single wind farm can sell into whichever market needs it.
A further advantage is control. An alternating-current interconnection ties two grids into synchronism, so a disturbance in one propagates to the other. An HVDC link transfers power without that coupling and its flow is set electronically rather than by physics, which means an operator can decide exactly how much power goes where. That is why HVDC is used to join grids that run at different frequencies, and increasingly to add controllability inside a single grid.
Transmission as an Alternative to Storage
The comparison that matters for a renewable grid is between a wire and a battery. Both solve the same underlying mismatch, and they do it along different axes: storage moves energy from one hour to another, transmission moves it from one place to another.
The reason transmission is often cheaper is statistical. Wind at two sites fifty kilometres apart is highly correlated; at a thousand kilometres it is barely correlated at all. Aggregate output across a continent is therefore far smoother than at any individual farm, with fewer deep troughs and fewer curtailed peaks. Every megawatt-hour that a line carries from a windy region to a calm one is a megawatt-hour that did not need to be stored.
Solar adds a second dimension. A grid spanning several time zones sees its solar peak spread across more hours, and an east-west link lets morning generation in one region serve breakfast demand in another. Europe and China both exploit this; the United States, whose grid is divided into three weakly connected interconnections, largely does not.
None of this removes the need for storage, because a continent-wide still, dark week is a real event in winter. It reduces how much storage is needed, which matters because storage is the expensive part. Studies of high-renewable systems consistently find that restricting transmission raises total system cost substantially, and that the cheapest configurations build both.
Why It Does Not Get Built
The technical problems of transmission were solved decades ago. The obstacle is that a line crosses land belonging to people who receive no benefit from it, and the legal systems of most democracies give those people standing to object at length.
Major transmission projects routinely take 10 to 15 years from proposal to energisation, and the great majority of that is consultation, environmental assessment and litigation rather than construction. Germany's north-south corridors, intended to carry Baltic wind to Bavarian industry, were substantially delayed and then partly rerouted underground at several times the cost, specifically to reduce opposition.
The cost allocation problem compounds it. A line benefits the region that exports, the region that imports and the overall system, in proportions that are genuinely difficult to calculate and that regulators must settle before construction. Cross-border lines add the further difficulty that the benefit and the pylon may fall in different countries.
The consequence is visible in the statistics. Curtailment - renewable generation switched off because the network cannot carry it - has risen sharply in regions where generation was built faster than wires. That is energy already paid for and then discarded, and it is the clearest measurable cost of under-building transmission. It is also why grid investment has become the binding constraint on renewable deployment in several countries, ahead of the cost of the generation itself.
Frequently asked questions
Why is electricity transmitted at such high voltage?
Because losses go with the square of current. For a given power, raising voltage lowers current proportionally, so tenfold higher voltage means a hundredfold lower loss. Transformers step generator output up to 400 or 765 kilovolts for long distances and back down for consumers, which is why alternating current became the standard.
What is HVDC and when is it used?
High-voltage direct current. It loses about 3 percent per 1,000 kilometres against 6 to 8 for alternating current and has no distance limit from cable capacitance. Converter stations at each end are expensive, so it pays above roughly 600 to 800 kilometres overhead or 50 to 80 kilometres submarine.
How much electricity is lost in the grid?
Typically 5 to 10 percent between generator and consumer across transmission and distribution combined. High-voltage lines themselves lose only 2 to 3 percent per 1,000 kilometres; most of the remainder occurs in the lower-voltage distribution network closer to the customer.
Can transmission replace storage?
Partly. Wind and solar output at sites a thousand kilometres apart are barely correlated, so a well-connected grid sees far smoother aggregate output and needs less storage. It cannot cover a continent-wide still, dark week, so the cheapest systems build both - but studies consistently find that restricting transmission raises total cost.
Why do transmission lines take so long to build?
Permitting, not engineering. Major lines take 10 to 15 years, mostly in consultation, environmental assessment and litigation, because a line crosses land whose owners gain nothing from it. Cost allocation between exporting region, importing region and system adds a further dispute that regulators must settle first.
