Transmission congestion is not unique to wind. Few electricity generating technologies can be sited precisely where power system engineers would like them to be -- close to demand. Coal fired power stations are predominantly sited in the vicinity of coal mines, with transmission built to get the electricity to where it is needed. That is why the power flows in both England and Germany are substantially north to south.
Coincidentally, the best wind resources in both Britain and Germany are also in the north of both countries, increasing the north-south power flows. Once a transmission line is "full" some generation must be "constrained off" or curtailed. Constraining coal or gas plant leads to fuel savings, which mitigate the negative economic consequences of reducing the load factor of the plant. If nuclear, hydro or wind plant are constrained off, however, profitability takes a hit. Because the technologies are relatively capital intensive compared to those that burn fossil fuel they need to operate as often as possible to repay their loans.
Connect and manage
A particular problem with wind energy compared with conventional generation is that its average output is significantly less than its peak output. This means that a 500 MW connection, say, built primarily for wind plant will not be utilised as fully as it would be if a 500 MW gas plant were built, but the cost will be the same. It may not necessarily be worthwhile building connections with capacity sufficient to carry all the renewable and conventional generation in a particular area. A concept referred to as "connect and manage" is being investigated in the UK to determine the trade-offs between the cost of transmission connections -- not necessarily rated at the full output of all the generation -- and the penalties associated with occasional constraints.
The windiest regions are often the most remote and the need for more transmission has surfaced in most countries with thriving markets for wind power, Spain and North America being typical examples. The American Wind Energy Association, lobbying for upgrades and new connections to transmit power from the "wind-rich Great Plains to energy-hungry urban areas," emphasises that transmission reinforcements will not only facilitate the development of wind energy, but will also improve system security by providing alternative paths for electricity flows when there is a fault on a particular transmission line.
The additional costs of transmission reinforcements are site-specific. A UK study from 2004, Total Cost Estimates for Large-Scale Wind Scenarios in the UK (main article), estimates that the additional transmission costs for 20% wind energy would add about EUR 4.5/MWh to the price of wind energy. In view of the likely concentration of UK wind energy in Scotland, the electricity market regulator, Ofgem, has recently authorised upgrading a key link in Scotland at a cost of EUR 840 million. Similar action seems likely in Spain and Pennsylvania.