22 June 2012 - Direct current (DC) or alternating current (AC)? That was the question behind the war of the currents at the end of the 19th century. Industrialist George Westinghouse and inventor Nikola Tesla wanted to use alternating current to transport electric energy, while Thomas Edison was a vigorous proponent of direct current.

But Edison’s efforts were in vain. His opponents prevailed, and alternating current grids have dominated the transmission of electric energy worldwide ever since. There were good reasons for that decision: Using transformers, alternating current can be easily adjusted to almost any desired value; in addition, alternating current makes it possible to develop meshed power grids.

However, alternating current has one major disadvantage: With high-voltage power lines for transmission over long distances, over 10% of the energy is lost for every 1,000 kilometres of distance. In this regard, direct current offers a much better balance, because high-voltage direct-current (HVDC) transmission reduces line losses by up to one third. Thanks to state-of-the-art electronic power semiconductors, the alternating current from power plants and wind farms can be converted to direct current at a reasonable expense and transmitted with very little loss.

Although the investment costs for the power converter stations are higher than with AC switchgear, thanks to the lower line costs and losses, the HVDC technology pays for itself at line distances of 600 kilometres and above and power transmission levels of 1,000 MW. It comes as no surprise that demand is soaring. The global HVDC market is expected to double to approximately €6 billion annually by 2016.

Numerous HVDC systems are already in use, particularly in China, and in Africa. One example of this is the Xiangjiaba-Shanghai HVDC power highway, which has been handling the low-loss transmission of 6,400 MW from the Xiangjiaba power plant to Shanghai, over 2,000 kilometres away, since the middle of 2010. Siemens supplied the thyristors and transformers for what is currently the world’s largest HVDC transmitting station.

The Cahora 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 Cahora Bassa dam in the northern part of Mozambique with the urban centre of Johannesburg in South Africa 1,450 kilometres distant would not have been economically feasible using a conventional AC system. The Songo converter station was built in the construction camp of the same name near the hydro power plant. At the other end of the transmission line, the Apollo converter station was built near Johannesburg, where the HVDC was converted from +/ - 533,000 volts back into alternating current and fed into the South African grid system.