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Alternating current (AC) vs direct current (DC)

Whether to factories or household appliances, whether via the transmission system or local distribution grids, electrical current is transported as alternating current or AC throughout all of Germany. But high voltage direct current or HVDC can also be used to transport electricity across large distances.

Direct current (DC) technology is not new. Thomas Edision, the inventor of the light bulb, already used direct current to usher the USA into the electric age. In the end, however, alternating current became the worldwide standard for electricity transmission.

Advantages only for large distances

Direct current technology was given a new boost thanks to the development of converter technology with semiconductors. This made it possible to transform alternating current to direct current with little loss. This requires so-called converter stations at both ends of the line that transform alternating current from the transmission system to direct current and back.

Experience with both systems

The high voltage direct current transmission offers advantages for the transfer over very large distances, as the losses are lower compared to alternating current. The downside of HVDC is that the construction costs are much higher than for alternating current due to the need for converter stations. For this reason, HVDC is currently only suitable for point-to-point connections over large distances.

Example of a converter station

For over ten years, 50Hertz has been operating the Kontek cable together with the Danish system operator Energinet.dk. The Kontek cable is an HVDC link that connects Germany and Denmark via the Baltic Sea. The line is about 170 kilometres in length. With its voltage level of 400 kV, it has a transfer capacity of 600 MW. The onshore connection point is the converter station in the Bentwisch substation near Rostock.

Efficiency is decisive

When choosing the transmission technology for the connection of wind farms in the Baltic Sea, 50Hertz decided in favour of the most efficient solution, namely alternating current.


Christian Brehm

Public participation

Christian Brehm
Tel: +49 30 5150-3556
Fax: +49 30 5150-3112
E-Mail: christian.brehm@50hertz.com


The German electricity grid

The German electricity grid is organised in four levels. At the highest level, the supra-regional transmission grids, electricity is transmitted across long distances at a high voltage of 380 or 220 kilovolts from the large power plants to the consumption centres - and also to the European neighbours. The second level includes the distribution grids of the regional electricity companies. They distribute the electricity at a 110 kV voltage (high voltage) in a large area, and supply the large-scale industry. The third level includes the local grids (medium voltage at less than 110-kV) supplying the industry and commerce. The lowest voltage level (low voltage at less than 1 kV) is used to supply households and smaller enterprises. The different voltage levels are connected to each other by means of substations, where voltage is transformed to a higher or lower level.