The key to connecting load and production centers over large areas

Conventional electricity transmission with three-phase lines is suitable for transporting over distances of up to a few hundred kilometers. However, requirements are changing. New energy sources such as sun and wind are becoming more important.

The largest sources of renewable energy are frequently a long distance away from the consumption centers. In addition, there is increased demand for long-distance transmission on account of the growing demand for energy from developing and emerging nations.

Physical constraints in conventional transmission technology prevent large volumes of electricity from being transmitted over very long distances (1000 kilometers or more) as this would destabilize the network and cause it to break down.   

Help should be sought in the form of the modern equivalent of technology which has been in existence since 1882 but which has barely been used in recent times, high-voltage direct-current transmission, or HDT for short. As with battery power supply, HDT involves the flow of direct current through the line. Furthermore, the losses in transmission (excluding those losses which occur in the inverter stations) are only about half as much as those which occur with the classical technology.

HDT lines also require less space because practically twice the power can be transported in a structurally comparable transmission system. In addition, the HDT facilitates the precise control of the load flow, aiding the efficiency and reliability of the network.

However, HDT costs money: before starting the journey, the alternating current has to be rectified in costly inverter stations which take up a great deal of space and produce emissions; it must then be converted back into alternating current when the electricity reaches its destination. The AC network is still required for fine distribution.

Therefore, HDT makes sense for long-distance connections: it can be used for high-capacity point-to-point connections as a submarine cable for distances of approx. 50 km or more and for overhead power lines stretching several hundred kilometers. It is also suitable for connecting wind parks to the network; their fluctuating production could otherwise disrupt the stability of the network.

Direct current production also chimes with the hope that solar energy from the African desert can be transported cost-efficiently to Europe in the future. However, the development of HDT networks is not complete yet.

About 20 HDT connections have been built in Europe over the past decade, including the world's longest submarine transmission line which runs 580 km between the Norwegian and the Dutch network (NorNed) with a capacity of approx. 700 megawatts.