In transporting electricity over greater distances, one seeks to minimize ohmic drops along the transmission line that lead to energy losses. One of the first solutions for minimizing these losses was the use of high alternating voltages for transmission, with the result that the proportional voltage losses were reduced due to the low amperage and ohmic resistance of the line. However, the so-called skin effect occurs in high-voltage alternating current; this effect is the tendency of an alternating current to distribute itself within a conductor with the current density being largest near the surface of the conductor. As a result, a large part of the overhead line’s cross section is not efficiently used.

This effect can be reduced by using very high voltage direct current. Yet this approach was not applied in larger projects for nearly a century: One lacked the possibility to convert alternating current into high-voltage direct current (and back). The pioneering experiments carried out by René Thury in Switzerland late in the 19th century produced no lasting success.

The advent of modern semiconductor technology in the 1960s first offered a practical possibility to use so-called thyristor valves (controlled semiconductor devices) to economically and reliably convert alternating current into direct current (and back). By the late 1960s, the technology was so advanced that one began early in the 70s to plan HVDC transmission lines for distances over 1,000 kilometers.

The Cabora Bassa hydro power plant (built by a consortium of Siemens, AEG, Hochtief and others) in today’s Mozambique offered an opportunity for building such an HVDC system. A transmission line linking the Cabora-Bassa Dam in the northern part of Mozambique with the urban center of Johannesburg in South Africa 1,450 kilometers distant would not have been economically feasible using a conventional AC system.

The Cabora Bassa system could process 2,000 amperes per thyristor. The design of the converter stations used converter devices mounted on porcelain insulators in outdoor oil-filled containers. The converter devices were remotely controlled with signals via fiber-optic cables – at the time also a technical milestone.

Today, HVDC lines are normally used for distances above roughly 700 kilometers, since the overall transmission losses are then lower than with a high-voltage AC line, despite the additional losses in the converters. HVDC technology is also used for the large-scale wind farms in the North Sea (BorWin, HellWin, SylWin) to transport the electricity via submarine cable to the mainland and feed it into the AC grid through converter stations.

May 2011 – Volker Leiste