Cahora Bassa

The Cahora Bassa HVDC transmission system is an important source of imported power for the South African grid.

The link provides 1,920 MW of power transmission capacity from a hydropower plant on the Zambezi River in northern Mozambique, and was put into service in three stages, from 1977 to 1979.

The HVDC system includes the Songo converter station in Mozambique near the Cahora Bassa hydropower plant, and the Apollo converter station in South Africa near Johannesburg, which were built by the ZAMCO consortium (AEG-Telefunken, Brown Boveri Company (BBC) and Siemens). The power link is owned by the power utilities, Hidroeléctrica De Cahora Bassa (HCB) in Mozambique and Eskom in South Africa. The system comprises two parallel monopolar lines across a 1,400-km long route; 900 km is in Mozambican territory.

The Apollo station upgrade

In 2006, Eskom awarded ABB a contract to upgrade the Apollo converter station in South Africa to boost its transmission capacity from 1,920 MW to 2,500 MW now, and prepare it for a future upgrade to 3,960 MW. The upgrade significantly increases the station's power availability and reliability, and reduces required maintenance.

ABB’s unique air-insulated outdoor thyristor valves replaced the old oil-insulated valves and were mounted on the existing support insulators. This produced significant savings, since it required only minimal changes to the station's physical arrangement. The upgrade was completed quickly with minimal transmission disruption.

The Songo station refurbishment

In 2012, HCB awarded ABB a contract to replace key high-voltage equipment, such as transformers, DC smoothing reactors, arresters and measuring equipment at the northern converter station, Songo in Mozambique. The refurbishment enhances power availability and improves system reliability. Commissioning will take place in two parts - DC equipment in 2013, transformers in 2014.

Main data

Commissioning year: 1977 - 79
Apollo refurbishment: 2008
Songo refurbishment: 2013-2014
Power rating: 1 920 MW
No. of poles: 2
AC voltage: 220 kV (Songo), 275 kV (Apollo)
DC voltage: ±533 kV
Length of overhead DC line: 1 420 km
Main reason for choosing HVDC: Long distance
Application: Connecting remote generation
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