Cahora-Bassa (previously spelled Cabora Bassa) is a separate bipolar HVDC power transmission line between the Cahora Bassa Hydroelectric Generation Station at the Cahora Bassa Dam in Mozambique, and Johannesburg, South Africa.

History

The system was built between 1974 and 1979 and can transmit 1920 megawatts at a voltage level of 533 kilovolts DC and 1800 Amperes. Thyristor valves are used, which unlike most other HVDC schemes are mounted outdoors and not in a valve hall. The valves are grouped into eight, 133 kV six-pulse bridges in series at each end. The long powerline runs through inaccessible terrain, so it is mostly built as monopolar lines apart. In case of a single line failure, transmission with reduced power is possible via the surviving pole and return through the earth.

thumb|The Cahora Bassa HVDC power station under construction in 1974

thumb|The Apollo converter station under construction in late 1973

thumb|The Apollo HVDC converter station and distribution lines. The Cahora Bassa HVDC line can be seen as the tall pylons on the left. The line in the centre of the picture with a single conductor is the electrode line of HVDC Cahora Bassa.

Cahora-Bassa was out of service from 1985 to 1997 because of the Mozambican Civil War in the region. The project was beset with technological challenges, most notable of these being the adoption of solid-state rectification devices in a large-scale commercial installation. Mercury-arc valves had been the de facto standard for HVDC up to this time. Cahora Bassa was the first HVDC scheme ordered with thyristor valves, though its operation was delayed. It was also the first HVDC scheme operational in Africa, and the first anywhere in the world to operate above 500 kV. Significant commercial hurdles, culminating in hearings at an International Arbitration Tribunal seated in Lisbon, in 1988, also had to be overcome.

Following a refurbishment exercise, the scheme was put back into commercial operation in October 1997. Between 2006 and 2009 the thyristor valves at the Apollo converter station were replaced by more modern water-cooled thyristor valves.

Description

Construction and ownership

The Cahora-Bassa transmission project was a joint venture of the two electrical utilities, Electricity Supply Commission (ESCOM, as it was known prior to 1987), latterly Eskom, Johannesburg, South Africa and Hidroelectrica de Cahora Bassa (HCB), a firm owned 15% by the government of Portugal and 85% by Mozambique. Equipment was constructed and supplied by ZAMCO, which was a consortium of AEG-Telefunken JV, Brown Boveri Company, and Siemens of Germany. Brown Boveri subsequently became part of ABB and AEG subsequently became part of Alstom.

The commercial arrangements also included Electricidade de Moçambique (EDM) which took supply from Cahora Bassa through a wheeling arrangement with Eskom. Effectively, Eskom supplied southern Mozambique (Maputo) from the then Eastern Transvaal at 132 kV with the sales deducted from the HCB supply to Eskom. The tripartite agreement was suspended due to force majeure when the line from Cahora Bassa was unavailable in the 1980s.

The system was commissioned in three stages starting in March 1977 with four six-pulse bridges, and in full operation of eight bridges on 15 March 1979.

Transmission line

The power line runs from the Songo converter station, which is near the hydroelectric station and normally operates as a rectifier, to the Apollo converter station near Johannesburg, which normally operates as an inverter. Each of the self-supporting steel towers along the route carries two bundles of four 565 square millimetre (1120 kcmil) cables, and a single 117 square millimetre (231 kcmil) grounding conductor. There are approximately 7,000 towers with an average span of .

The maximum span is using reinforced towers. Earth return for unipolar operation is provided by buried graphite electrodes at each station. The DC line has smoothing reactors and surge arrester capacitors at each station.

Northeast of Apollo Converter Station the poles of HVDC Cahora Bassa cross under several 400 kV AC lines at 25°54'58"S 28°16'46"E respectively 25°54'57"S 28°16'51"E in such a low altitude that the area under the line may not be walked on and is fenced in [https://www.google.de/maps/@-25.9157096,28.2794915,3a,90y,92.64h,82.19t/data=!3m6!1e1!3m4!1sshsRGl2ZtzIIiEjYg4pNEA!2e0!7i13312!8i6656].

The two lines are named as Zeus and Apollo respectively.

Thyristor valves

Cahora Bassa was one of the first HVDC schemes built with thyristor valves from its inception. Unusually, the thyristor valves are outdoor mounted. In the original installation they were oil filled for both cooling and electrical insulation. The only other HVDC scheme in the world equipped in this way from the outset was the first phase – now decommissioned – of the Shin Shinano frequency converter in Japan. Each valve tank contains two valves, forming a double-valve connecting the two DC terminals to one single-phase, two-winding converter transformer. Each six-pulse bridge contains three such tanks and hence each station contains 24 double-valves.

The development work for the thyristor valves began in the late 1960s when the only thyristors available at the time were, by today's standards, small, and were rated only 1.6 kV each.