Cabora-Bassa

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Cabora-Bassa (also spelled Cahora Bassa) is the name for an HVDC-power system between the static inverter station Songo, Mozambique, which is situated near the Cahora Bassa Hydroelectric Generation Station based on the Cahora Bassa dam near Zambezi falls in Mozambique, and the converter station Apollo near Pretoria, South Africa. The HVDC-power line from Cabora-Bassa is bipolar HVDC, which 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 1420 kilometre long powerline runs through inaccessible terrain, so it is mostly built as monopolar lines 1 km 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.

[edit] Description

The Cahora-Bassa transmission project was a joint venture of the two electrical utilities Electricity Supply Commission, ESKOM, Johannesburg South Africa and Hidroelectrica de Cahora Bassa (HCB), a firm owned 82% by the government of Portugal and 18% by Mozambique[4]. Equipment was constructed and supplied by ZAMCO, which was a consortium of AEG Telefunken, BBC, 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 Eastern Transvaal at 132 kV but the sales were 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.

Each of the self-supporting steel towers along the route carries two bundles of four 565 square millimetre cables (one per pole) and a single 117 square millimetre grounding conductor. There are approximately 7,000 towers with an average span of 426 m. The maximum span is 700 m 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 arrestor 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. [1]

[edit] Repairing the war damage

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. [2] Nearly all of the 4200 transmission line towers located on the 893 km 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.


[edit] External links

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