Power transmission technologies

From a technical standpoint, electricity can be transported above ground (overhead or overland lines) or also below ground (underground and undersea cables).

Most high-voltage lines are operated with AC (alternating current). The use of AC has the great advantage that transformers can be employed to increase the voltage level through the lines and afterwards drop it back down for local distribution.

The conversion of the generated AC into DC (direct current) and the corresponding reconversion into AC for consumption requires technically complex power-electronic circuits. This technology, which has been available and put to use for a number of years, has been continually refined over the past two decades and is reliable even at high power levels. In contrast to AC grids, DC lines cannot be intermixed but rather can be used only as a point-to-point connection. At the present time, high-voltage DC grids are not yet in use. Their development as well as their operation parallel to AC grids are at this time still in the R&D stage.

AC power

On the European continent, electricity is today transmitted almost exclusively using AC methods. However, AC overhead lines and underground cables are power transmission systems with different characteristics. Aspects which characterise overhead lines and underground cables are primarily the type of the insulating medium, the level of the operating voltage and thus the distance between conductors and earthed components. Overhead lines and underground cables thus differ fundamentally in their operational performance.

With increasing operating voltage, the demands on underground cables become more stringent than with overhead lines. This fact is reflected in the development as well as the present and past use of both power transmission technologies. The optimal variants of overhead lines and underground cables are strongly related to the level of investment, the design, service life and the load on each line.

DC power

High-voltage DC (HVDC) is becoming interesting for the transport of larger amounts of power over long distances. HVDC lines have undergone considerable technological development in the past few years. AC power, which is otherwise commonly used, is converted to DC through rectifying circuits at the start of the lines, and at the other end it is reconverted into AC.

When it comes to losses, these lines are very efficient, but for reasons associated with costs they are suited only for transport over long distances. The lower transmission losses compensate for higher investment costs and losses during rectification only for lines extending at least roughly several hundred kilometres.