Economically transporting energy over distances of hundreds of kilometers was and still is one of the central challenges for electrical engineering. For exceptionally long distances, high-voltage, direct-current (HVDC) power transmission has been an especially efficient solution since the 1970s. Forty years ago, Siemens built the first major HVDC line in Southern Africa to bring electricity from Mozambique to South Africa over a distance of 1,420 kilometers.
Problems recognized – but initially no solution in sight
In transporting electricity over greater distances, two problems arise first. Energy losses occur due to the ohmic voltage drop along the line. In order to keep these as low as possible, high alternating voltages are used for transmission in the early days of electrical engineering, which means that the proportionate voltage loss is reduced due to the low current intensity and the ohmic resistance of the line. However, with high voltage alternating currents, the so-called skin effect occurs, which pushes the current to the edge of the cable cross-section – the second problem. 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). This technology was developed to such an extent that in the early 1970s it was possible to start planning such high-voltage direct current (HVDC) transmissions over distances of over 1,000 kilometers.
1,420 km cable length – converter stations in Africa
The Cahora Bassa (called Cabora Bassa during the colonial period of Mozambique) 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 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 Apolle converter station was built near Johannesburg, where the high-voltage direct current was converted from +/ - 533,000 volts back into alternating current and fed into the South African grid system. The Cahora 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.
Robust and durable – the technology works and is modernized today
Operation of the new line began in May 1975, yet was repeatedly interrupted due to regional hostilities and damages inflicted to sections of the line. Fortunately, the engineers had designed the line with a bipolar direct current of + / - 533 kV and possible overhead return line. As a result, after destruction of one half of the line, the system was still functional. But it got worse.
After a long interruption due to the civil war in Mozambique between 1981 and 1992 and its aftermath, electricity production could not be resumed until 1998. Initially operated under Portuguese management, the government of Mozambique purchased most of the shares in the power plant in 2006. Six years later, the remaining 15 percent of the shares were split between a Portuguese and a Mozambican company. With over 2,000 megawatts, the power plant is still the most powerful in southern Africa today. A large part of the electricity is exported to South Africa. Siemens will ensure that Mozambique remains one of the main exporters of electricity in the region. Since 2017, the company has been carrying out extensive maintenance and modernization work at the HVDC station at the dam. By 2020, the order worth 26 million euros will be completed and power transmission will continue to be secured.
Volker Leiste | Dr. Ewald Blocher