ELECTRICITY TRANSMISSION GRIDs

In many parts of the world, electricity is transmitted from power stations to consumers via an electricity transmission grid.

Until recently, the wisdom of this arrangement was never challenged. But a recent report by Greenpeace UK ("Decentralising power: an energy revolution for the 21st century") implies rather strongly that we can do without an electricity distribution grid.

Greenpeace are right to stress the advantages of combined heat and power (CHP) and microgeneration but they are quite wrong to imply that we can do without an electricity distribution grid.

Without a grid, electricity supply systems become very inefficient and this is particularly true with renewable forms of energy.

If for example, the wind is blowing strongly in Cornwall but it has stopped blowing in Watford, it is very useful if the surplus wind power from Cornwall (the excess above what the local people can use) can be transmitted to Watford (or vice versa). Without this facility, excess power in any one area is simply wasted.

Without a transmission grid, it would not be possible to take advantage of the large amounts of energy that may be obtained from wave energy devices, offshore wind farms, tidal lagoons, and so on.

HVAC and HVDC

The high-tension power lines that most people are familiar with use alternating current (AC) for reasons that were worked out by Nikola Tesla - a fascinating story in its own right. High voltages are needed to ensure that electricity can be transmitted over long distances without losing too much of the energy in the form of heat. AC has the advantage that voltages can be raised or lowered easily using transformers. Nikola Tesla invented an electric motor that would run on AC electricity to save having to convert it to direct current (DC).

But over very long distances, AC transmission lines become increasingly inefficient (because of the effects of 'capacitance' and 'impedance' in the system). So, although voltage conversions are not so easy with DC electricity, long distance transmission lines use DC electricity. With modern HVDC transmission lines, transmission losses are very low (about 3% for each 1000 km). And they are also cheaper to build than HVAC lines because only two wires are needed instead of three.

HVAC and HVDC transmission lines are often carried overland via pylons because this saves the cost of insulating the cables. But, for extra cost, they can be insulated and laid under the ground or under water.

Large-scale HVDC transmission grids

Given the advantages of transmission grids for making efficient use of available energy, to facilitate trading of electricity, and for ironing out peaks and troughs in supply and demand—and given the advantages of HVDC transmission lines for long-distance transport of electricity—there are at least three recent proposals to build large-scale HVDC transmission grids:

• A Supergrid of this type has been proposed by Airtricity as a means of reducing the effects of intermittency in wind power across Europe and to facilitate the trading of electricity (see also this press release in MS Word). In this proposal, it is envisaged that all the cables would be laid under the sea.
• The Trans-Mediterranean Renewable Energy Cooperation (TREC) propose an HVDC grid spanning the whole of Europe, the Middle East and North Africa (EUMENA) to take advantage of the enormous quantities of energy falling as sunlight on hot deserts (see also TREC-UK).
• A report in Engineer Live describes a proposal for a world-wide grid of HVDC transmission lines.

Last updated: 2006-11-06 (ISO 8601)