Cahora Bassa (HVDC)

Cabora-Bassa (also spelled Cahora Bassa) is the name for an HVDC power transmission system between the Cahora Bassa Hydroelectric Generation Station in Mozambique, and Johannesberg, South Africa. The bipolar HVDC power line can transmit 1920 megawatts at a voltage level of +/-533 kilovolts and 1800 amperes. Thyristor valves are used, which unlike other HVDC schemes are mounted outdoors and not in a hall. The convert|1420|km|mi long powerline runs through inaccessible terrain, so it is mostly built as monopolar lines convert|1|km|mi apart. In case of a single line failure, transmission with reduced power is possible via the surviving pole and return through the earth.

In the 1980s Cabora-Bassa was not in service because of civil warfare 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, whereas mercury arc valves had been the "de facto" standard for HVDC up to this time. Significant commercial hurdles, culminating in hearings at an International Arbitration Tribunal seated in Lisbon, in 1988, also had to be overcome.

Description

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 82% by the government of Portugal and 18% by Mozambique. Equipment was constructed and supplied by ZAMCO, which was a consortium of AEG-Telefunken JV, Brown Boveri Company, and Siemens AG of Germany.

The commercial arrangements also included Electricidade de Mocambique (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 bridges, and in full operation of eight bridges in June 1979.

The power line runs from the rectifier/converter station Songo, which is near the hydroelectric station, to the inverter/converter station Apollo near Johannesburg. Each of the self-supporting steel towers along the route carries two bundles of four 565 square millimetre (1120 kcmil) cables, one per pole, and a single 117 square millimetre (231 kcmil) grounding conductor. There are approximately 7,000 towers with an average span of convert|426|m|yd. The maximum span is convert|700|m|yd 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.

The thyristor valves are outdoor mounted, and oil filled. Each valve is a double assembly (two sets of thyristors in parallel). Each station has eight valves, for a total of 22,656 thyristors installed at each end of the line. AC filters tuned to the 5th, 7th, 11th and 13th harmonics of the 50 Hz power supply are installed at each station, approximately 195 MVAr at Apollo and 210 MVAr at Songo.

=Repairing the war da

After the civil war ended in 1992, one of the many effects of the decade of strife was the damage to the HVDC transmission lines. Nearly all of the 4200 transmission line towers located on the convert|893|km|mi of line in Mozambique needed to be replaced or refurbished. This work was started in 1995 and took until late 1997 to complete. The system was restored to full power transmission capacity by 1998.

Subsequently, Eskom has commenced electricity supply to Mozambique at 400 kV, under terms similar to the original wheeling agreement, from the Arnot Power Station in Mpumalanga, via Swaziland. The principal purpose of this infrastructure is to provide bulk electricity supplies to the Mozal Aluminium Smelter operated by BHP Billiton.

The Memorandum of Understanding, signed on 2 November 2007, means that Mozambique will by the end of 2007 be in charge of a project located on its soil but on which it had no control for the past 30 years due to contractual obligations with Portugal.

The new arrangement gives Mozambique 85 percent of the Cahora Bassa Hydroelectric (HCB) project while Portugal will retain only 15 percent. The project has a capacity to produce 2,000 megawatts of electricity and is one of the main suppliers of power to the Southern African Power Pool.

Mozambique will, however, need to pay US$950 million to the Portuguese government as compensation for the post civil-war reconstruction and maintenance of the dam.

The civil war resulted in serious damage to the transmission infrastructure, forcing the Portuguese government to fork out about US$2.5 billion to repair it.

External links

* [http://www.taprojects.co.za/media_articles/media_article_2.asp Restoration of the transmission line]
* [http://groups.colgate.edu/cews/archives/2002_2003/resources/04-22-03.htm Bibiliography of references on the social and economic impact of the project]
* [http://www.eskom.co.za/eia/Enviro%20assessments/Apollo-Phoebus-Vulcan/Map10COMBINFO.pdf Location of Apollo]

References

* "Eustace F. Raynham", Apollo - Cahora Bassa: enigma and diversions, EE Publishers, ISBN 0-620-32261-6, [http://www.eepublishers.co.za/view.php?sid=1121|Abstract]