HVDC > HVDC References > Africa > Cahora Bassa
Background
The Cahora Bassa HVDC transmission provides an important power import facility to the South African grid. It transmits 1920 MW of power from the Cahora Bassa generating station on the Zambezi River in northern Mozambique. The original transmission was put into service in three stages from 1977 to 1979. The system includes two converter stations, one at Songo in Mozambique and the other at Apollo in South Africa. The transmission is owned by Hidroelectrica De Cahora Bassa in Mozambique and
Eskom in South Africa.The converter stations were built by the ZAMCO consortium (AEG, BBC and Siemens).
There are two parallel monopolar lines between these two stations, covering 1,400 km, of which 900 km is in Mozambican territory. The civil war in Mozambique soon led to that the transmission line was destroyed and the transmission remained unused for many years.
After the civil war in Mozambique ended in 1992, repair works were planned for the HVDC transmission lines. Nearly all of the 4200 transmission line towers located on the 900 km of line in Mozambique
The old oil-insulated and oil-cooled thyristor outdoor HVDC Thyristor valves in the Apollo station. (Click for larger image.) | 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.
The converter stations were planned in the 1960’s for mercury arc valves, which means that there are four 6-pulse converters in each pole. However the stations were equipped with oil-insulated and oil-cooled thyristor outdoor valves. |
The Apollo converter station refurbishment
The South African power company, Eskom, awarded a contract to ABB for the refurbishment
of the Apollo converter station in August 2006. The upgraded Apollo substation, boosts transmission capacity of the substation from 1,920 MW to 2,500 MW. It also prepares for a future upgrade to 3,960 MW. The project encompasses:
- Replacement of Thyristor Valves and Valve Cooling Systems
- Replacement/refurbishment of AC filters
- Replacement of Control and Protection System
- Replacement and integration of Converter Station HMI systems
- Replacement of system transient fault recording equipment
- Replacement of the DC line fault recording equipment
- Provision and optimization of associated auxiliaries/ancillaries
This upgrading significantly increases the availability and reliability and reduce the maintenance of the Apollo station.
| ABB’s unique air-insulated outdoor thyristor valves replace the old oil-insulated valves and are mounted on the existing support insulators for the old valves. Thereby significant savings are made, since minimum changes need to be made to the physical arrangement and no new valve halls need to be built. | 
The new outdoor valve assembly at the Apollo station. (Click for larger image.) |
MACH 2TM HVDC Control and Protection System are installed to replace the old analogue control systems, originally installed in the 1970’s.
The upgrade was perfomed in shortest possible time, and with minimum disruption of the power transmission capacity of the link. The transmission capacity was reduced by 50 % during a period of three months, and by 25 % during two months.
| Main data |  |
| Commissioning year: | (1977 - 79 ) Apollo upgrade 2008 |
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| Power rating: | 1 920 MW |
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| No. of poles: | 2 |
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| AC voltage: | 220 kV (Songo), 275 kV (Apollo) |
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| DC voltage: | ±533 kV |
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| Length of overhead DC line: | 1 420 km |
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| Main reason for choosing HVDC: | Long distance |
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