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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 transmission 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. Here are some of the benefits of transmission grids, especially large-scale HVDC grids described below:

  • Reducing wastage. Without a grid, electricity supply systems waste energy and this is particularly true with renewable forms of energy. If for example, the wind is blowing strongly in Scotland, producing more electricity than the local people can use, that surplus energy is simply wasted unless it can be moved to places where it is needed. If there were affordable systems for bulk storage of electricity, that would make a difference but it would not remove the need to move electricity from areas of surplus to areas of need.
  • Accessing sources of renewable energy. Without a transmission grid, it would not be possible to take advantage of the large amounts of energy that may be obtained from large-scale but remote sources of renewable electricity such as wave farms, offshore wind farms, tidal lagoons, and tidal stream generators—and concentrating solar power!
  • Opening up new sources of energy. A related point is that a large-scale transmission grid can open up entirely new sources of energy that might not otherwise be considered. For example, there is potential to import geothermal energy into the UK from Iceland via a submarine HVDC transmission line.
  • Smoothing out variations in supply and demand. Another advantage of transmission grids is that, if they cover a large area like Europe or EUMENA, they reduce the variability of energy sources such as wind. The wind may stop blowing in any one spot but it is very rare for it to stop blowing everywhere across an area the size of a continent. Without a large-scale grid, it may be necessary to maintain conventional power stations on 'spinning reserve' to supply electricity at short notice if the wind drops, and this spinning reserve is wasteful. In a similar way, large-scale grids can help to smooth out variations in demand.
  • Reducing the need for ‘plant margin’. A transmission grid helps to reduce the amount of 'plant margin'—the difference between actual generating capacity in any area and the theoretical minimum generating capacity—that is required. This is because a large-scale grid smooths out much of the variability in electricity supply and demand and because spare generating capacity that is needed to meet contingencies can be shared across a relatively wide area, thus reducing the amount that is allocated to any one area.
  • Security of supply. A related point is that large-scale transmission grids help to ensure the security of electricity supplies in any one area. This is because any local shortage of electricity or local peak in demand can almost always be met from one or more other areas where there is spare capacity.
  • A large-scale supergrid is needed for the proper working of a single market for electricity across a wide area. The UK government and the European Commission wish to create a single European market for electricity (as we have in the UK), unbundling power generation from power transmission and promoting competition between different suppliers and sources of electricity. A large-scale HVDC supergrid is needed for the proper working of that single market.
  • Getting the best available price for electricity. Transmission grids that cross time zones may increase the value of electricity by moving it, at any one time, from areas where it is cheap to areas where it will fetch a good price. More generally, large-scale grids allow customers to obtain electricity from wherever it is cheapest at any one time, and that may vary throughout each day.
  • Stabilisation of frequencies and voltages. An HVDC supergrid (see below) can help to stabilize frequencies and voltages in the HVAC grids to which it connects.
  • Export potential. The UK (and Scotland in particular) has great potential for wind power, wave power, and power from tidal lagoons and tidal streams. If these are developed as they should be, stronger grid connections to the continent will be needed to facilitate exports of renewable electricity from these sources (see Study backs undersea cable to export Scotland's wind and wave power). Likewise for Ireland, bearing in mind that it has a much smaller population than the UK (see Ireland launches ocean energy initiative).

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. Also, DC is normally the preferred option for submarine cables.

Although long-distance transmission of electricity over HVAC transmission lines is not efficient, it appears to be possible, up to certain limits, to make efficient long-distance 'transfers' of electricity via HVAC transmission grids, provided that there are electricity generators and electricity consumers at points along the route. This is explained on a page about the cascading principle.

With modern HVDC transmission lines, transmission losses are very low.note And they are also cheaper to build than HVAC lines because only two cables 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 as envisaged in proposals from the wind energy company Aitricity for a pan-European HVDC supergrid constructed using submarine power cables. With "HVDC Light" technology from ABB, and for distances greater than about 500 km, submarine or underground cables are only about 10% to 20% more expensive than overhead lines.

note Transmission losses in HVDC transmission lines are normally about 3% per 1000 km but losses can be reduced by adding capacity (at more cost) or vice versa. In addition to the actual transmission losses there are losses at each end in converting from AC to DC and converting from DC to AC. Taking both ends together, conversion losses for an HVDC transmission line are normally about 1.5% to 2%. So, over a distance of 2000 km (which is about the distance between North Africa and the UK), total losses would be about 6% (for transmission) + 2% (for conversions) = 8% overall.

Large-scale HVDC transmission grids

Given the advantages of transmission grids detailed above, and given the advantages of HVDC transmission lines for long-distance transport of electricity, there are several proposals to build large-scale HVDC transmission grids:

These grids would not replace existing HVAC grids: they would be designed to integrate with them and complement them.

In line with the European supergrid concept, the report "Energy, Politics, and Poverty" from the University of Oxford (PDF, 763 KB, June 2007) calls for "completion of the physical European grid for ... electricity" and a unified market for energy in Europe.

EU energy commissioner Andris Piebalgs has endorsed this concept (see EU's Piebalgs says grid infrastructure needed quickly for offshore wind energy, 2008-03-31).

Incremental development

Quite apart from the strategic proposals mentioned in the previous section, the process of creating a large scale HVDC grid in EUMENA is beginning to happen in an incremental manner, in much the same way that the Internet developed piece-by-piece. Here are some examples of current proposals and developments, most of which involve submarine power cables:

Costs

There are some figures for the cost of proposed HVDC transmission lines, and comparisons with other costs, on the page about CSP costs.

Cost-benefit analyses of large-scale transmission grids show that they are very good value for money (see Interstate transmission superhighways: paving the way to a low-carbon future, RenewableEnergyWorld.com, 2008-07-30).

Links

Last updated: 2009-01-05 (ISO 8601)