Cahora-Bassa (previously spelled Cabora Bassa) is a separate bipolar
HVDC
A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...
power transmission line between the
Cahora Bassa
The Cahora Bassa lake—in the Portuguese colonial era (until 1974) known as Cabora Bassa, from Nyungwe ''Kahoura-Bassa'', meaning "finish the job"—is Africa's fourth-largest artificial lake, situated in the Tete Province in Mozambique. In Afri ...
Hydroelectric Generation Station at the
Cahora Bassa Dam
The Cahora Bassa Dam is located in Mozambique. It is one of two major dams on the Zambezi river, the other being the Kariba. The project began to come online in 1979 after much political debate. This dam uses the Zambezi River water to generate ...
in
Mozambique
Mozambique, officially the Republic of Mozambique, is a country located in Southeast Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west, and Eswatini and South Afr ...
, and
Johannesburg
Johannesburg ( , , ; Zulu language, Zulu and Xhosa language, Xhosa: eGoli ) (colloquially known as Jozi, Joburg, Jo'burg or "The City of Gold") is the most populous city in South Africa. With 5,538,596 people in the City of Johannesburg alon ...
,
South Africa
South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
.
History
The system was built between 1974 and 1979 and can transmit 1920
megawatts
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor o ...
at a voltage level of 533 kilo
volt
The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s DC and 1800
Ampere
The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
s.
[Compendium of HVDC schemes, CIGRÉbr>Technical Brochure No. 003]
1987, pp89–94. Thyristor
A thyristor (, from a combination of Greek language ''θύρα'', meaning "door" or "valve", and ''transistor'' ) is a solid-state semiconductor device which can be thought of as being a highly robust and switchable diode, allowing the passage ...
valves are used, which unlike most other HVDC schemes are mounted outdoors and not in a
valve hall
A valve hall is a building which contains the valves of the static inverters of a high-voltage direct current plant. The valves consist of thyristors, or at older plants, mercury arc rectifiers. Mercury arc rectifiers are usually supported by ...
. 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.

Cahora-Bassa was out of service from 1985 to 1997 because of the
Mozambican Civil War
The Mozambican Civil War () was a civil war fought in Mozambique from 1977 to 1992 due to a combination of local strife and the polarizing effects of Cold War politics. The fighting was between Mozambique's ruling Marxist Front for the Liberat ...
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 valve
A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high- current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-f ...
s 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
Lisbon ( ; ) is the capital and largest city of Portugal, with an estimated population of 567,131, as of 2023, within its administrative limits and 3,028,000 within the Lisbon Metropolitan Area, metropolis, as of 2025. Lisbon is mainlan ...
, in 1988, also had to be overcome.
Following a refurbishment exercise,
[Venter, F.P., Marshall, D.A., Cuedes, C., Oberholzer, G., Re-commissioning experience with the Apollo – Cahora-Bassa HVDC scheme, CIGRÉ session, Paris, 1998]
paper reference 14–111
the scheme was put back into commercial operation in October 1997.
[Oliveira, H., Sintra, L., Lokala, J., Pembele, I.E., Lubini, I.E.., Goossen, P.V., Bhana, S., Operating experiences in the HVDC systems of the Southern African power pool Cahora-Bassa: Apollo and Inga-Shaba, CIGRÉ session, Paris, 2000]
paper reference 14–111
Between 2006 and 2009 the thyristor valves at the Apollo converter station were replaced by more modern water-cooled thyristor valves.
[Goosen, P., Reddy, C., Jonsson, B., Holmgren, T., Saksvik., O., Bjorklund, H., Upgrade of the Apollo HVDC converter station, CIGRÉ 6th Southern Africa Regional Conference, ]Cape Town
Cape Town is the legislature, legislative capital city, capital of South Africa. It is the country's oldest city and the seat of the Parliament of South Africa. Cape Town is the country's List of municipalities in South Africa, second-largest ...
, 2009,
paper reference P107
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
Eskom Hld SOC Ltd or Eskom is a South African electricity public utility. Eskom was established in 1923 as the Electricity Supply Commission (ESCOM) (). Eskom represents South Africa in the Southern African Power Pool. The utility is the larg ...
, Johannesburg, South Africa and Hidroelectrica de Cahora Bassa (HCB), a firm owned 15% by the government of
Portugal
Portugal, officially the Portuguese Republic, is a country on the Iberian Peninsula in Southwestern Europe. Featuring Cabo da Roca, the westernmost point in continental Europe, Portugal borders Spain to its north and east, with which it share ...
and 85% by
Mozambique
Mozambique, officially the Republic of Mozambique, is a country located in Southeast Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west, and Eswatini and South Afr ...
. Equipment was constructed and supplied by ZAMCO, which was a consortium of
AEG The initials AEG are used for or may refer to:
Common meanings
* AEG (German company)
; AEG) was a German producer of electrical equipment. It was established in 1883 by Emil Rathenau as the ''Deutsche Edison-Gesellschaft für angewandte El ...
-
Telefunken
Telefunken was a German radio and television producer, founded in Berlin in 1903 as a joint venture between Siemens & Halske and the ''AEG (German company), Allgemeine Elektrizitäts-Gesellschaft'' (AEG) ("General electricity company").
Prior to ...
JV,
Brown Boveri
Brown, Boveri & Cie. (Brown, Boveri & Company; BBC) was a Swiss group of electrical engineering companies. It was founded in Baden bei Zürich, in 1891 by Charles Eugene Lancelot Brown and Walter Boveri who worked at the Maschinenfabrik Oerlik ...
Company, and
Siemens
Siemens AG ( ) is a German multinational technology conglomerate. It is focused on industrial automation, building automation, rail transport and health technology. Siemens is the largest engineering company in Europe, and holds the positi ...
AG of
Germany
Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
. Brown Boveri subsequently became part of
ABB
ABB Group is a Swedish-Swiss multinational electrical engineering corporation. Incorporated in Switzerland as ABB Ltd., and headquartered in Zurich, it is dual-listed on the Nasdaq Nordic exchange in Stockholm, Sweden, and the SIX Swiss Excha ...
and AEG subsequently became part of
Alstom
Alstom SA () is a French multinational rolling stock manufacturer which operates worldwide in rail transport markets. It is active in the fields of passenger transportation, signaling, and locomotives, producing high-speed, suburban, regional ...
.
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
Maputo () is the Capital city, capital and largest city of Mozambique. Located near the southern end of the country, it is within of the borders with Eswatini and South Africa. The city has a population of 1,088,449 (as of 2017) distributed ov ...
) from the then
Eastern Transvaal
Mpumalanga () is one of the nine provinces of South Africa. The name means "East", or literally "The Place Where the Sun Rises" in the Nguni languages. Mpumalanga lies in eastern South Africa, bordering Eswatini and Mozambique. It shares bor ...
at 132 kV with the sales deducted from the HCB supply to Eskom. The tripartite agreement was suspended due to
force majeure
In contract law, force majeure ( ; ) is a common clause in contracts which essentially frees both parties from liability or obligation when an extraordinary event or circumstance beyond the control of the parties, such as a war, strike, riot, ...
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
An HVDC converter station (or simply converter station) is a specialised type of substation which forms the terminal equipment for a high-voltage direct current (HVDC) transmission line.Arrillaga, Jos; High Voltage Direct Current Transmission, s ...
, 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
A circular mil is a Units of measurement, unit of area, equal to the area of a circle with a diameter of one Thou (unit of length), mil (one thousandth of an inch or ). It is equal to pi, /4 square mils or approximately . It is a unit intended for ...
) 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 undercross 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 i
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
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
. Each valve tank contains two valves, forming a ''double-valve'' connecting the two DC terminals to one single-phase, two-winding converter
transformer
In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
. 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.
[ In the first phase of the project (4 bridges at each end) each valve contained 280 such thyristors in series with two in parallel][ – the largest number ever used in a single HVDC valve.
Phases 2 and 3 used improved thyristors with a rating of 2.4 kV each and only required 192 in series per valve – still a large number by modern standards – with two in parallel. As a result, each converter station contained a total of 22,656 thyristors.
]
Other equipment
The thyristors also had poor transient overcurrent capability, so another unusual feature of the scheme was the existence of ''overcurrent diverters'' between the valves and transformers, although these were later decommissioned at the Apollo station.[
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.
There are two ]PLC repeater station
Power-line communication (PLC) is the carrying of data on a conductor (the ''power-line carrier'') that is also used simultaneously for AC electric power transmission or electric power distribution to consumers.
A wide range of power-line commu ...
s: one at Gamaboi in South Africa and one at Catope in Mozambique.
Repairing the war damage
After the civil war
A civil war is a war between organized groups within the same Sovereign state, state (or country). The aim of one side may be to take control of the country or a region, to achieve independence for a region, or to change government policies.J ...
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 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
Arnot Power Station in Mpumalanga, South Africa, is a coal-fired power plant operated by Eskom. Coal from the Arnot coal mine directly feeds the station.
History
Construction of Arnot started in 1968, this generation unit went on line in 1971 an ...
in Mpumalanga
Mpumalanga () is one of the nine provinces of South Africa. The name means "East", or literally "The Place Where the Sun Rises" in the Nguni languages. Mpumalanga lies in eastern South Africa, bordering Eswatini and Mozambique. It shares bor ...
, via Swaziland
Eswatini, formally the Kingdom of Eswatini, also known by its former official names Swaziland and the Kingdom of Swaziland, is a landlocked country in Southern Africa. It is bordered by South Africa on all sides except the northeast, where ...
. The principal purpose of this infrastructure is to provide bulk electricity supplies to the Mozal Aluminium Smelter operated by BHP
BHP Group Limited, founded as the Broken Hill Proprietary Company, is an Australian multinational mining and metals corporation. BHP was established in August 1885 and is headquartered in Melbourne, Victoria.
As of 2024, BHP was the world� ...
.
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 pay about US$2.5 billion out of pocket to repair it.
Apollo station upgrade
In 2006 ABB
ABB Group is a Swedish-Swiss multinational electrical engineering corporation. Incorporated in Switzerland as ABB Ltd., and headquartered in Zurich, it is dual-listed on the Nasdaq Nordic exchange in Stockholm, Sweden, and the SIX Swiss Excha ...
was awarded a contract to replace the thyristor valves at the Apollo station.[ The outdoor mounting concept was retained, but each of the new housings contains a complete six-pulse bridge instead of only two valves, and the replacement thyristor valves are of a more conventional air insulated, water-cooled design using 125mm, 8.5 kV thyristors. 36 such thyristors are connected in series in each valve, without parallel connection, and the new valves are capable of a subsequent upgrade to 600 kV, 3300 A. At the same time, the AC filters were replaced.
]
Sites
Gallery
*
See also
*High-voltage direct current
A high-voltage direct current (HVDC) electric power transmission system uses direct current (DC) for electric power transmission, in contrast with the more common alternating current (AC) transmission systems. Most HVDC links use voltages betwe ...
*HVDC converter
An HVDC converter converts electric power from high voltage alternating current (AC) to high-voltage direct current (HVDC), or vice versa. HVDC is used as an alternative to AC for transmitting electrical energy over long distances or between AC pow ...
*HVDC converter station
An HVDC converter station (or simply converter station) is a specialised type of substation which forms the terminal equipment for a high-voltage direct current (HVDC) transmission line.Arrillaga, Jos; High Voltage Direct Current Transmission, s ...
*Cahora Bassa
The Cahora Bassa lake—in the Portuguese colonial era (until 1974) known as Cabora Bassa, from Nyungwe ''Kahoura-Bassa'', meaning "finish the job"—is Africa's fourth-largest artificial lake, situated in the Tete Province in Mozambique. In Afri ...
*Cahora Bassa Dam
The Cahora Bassa Dam is located in Mozambique. It is one of two major dams on the Zambezi river, the other being the Kariba. The project began to come online in 1979 after much political debate. This dam uses the Zambezi River water to generate ...
*Energy in Mozambique
Mozambique has abundant energy sources available for exploitation. As of 2021, the country was ranked first in energy potential of all the countries in the Southern African Power Pool (SAPP), with an estimated energy capacity of 187,000 MW. Ava ...
* Electricity sector in South Africa
References
Further reading
* ''Eustace F. Raynham'', Apollo – Cahora Bassa: enigma and diversions, EE Publishers,
External links
Bibliography of references on the social and economic impact of the project
Location of Apollo
CIGRÉ SC B4 Compendium of HVDC schemes, 2005
Siemens HVDC Reference List
{{Webarchive, url=https://web.archive.org/web/20140222015305/http://www.energy.siemens.com/hq/pool/hq/power-transmission/HVDC/HVDC-Classic/HVDC_Transmission_EN.pdf , date=22 February 2014
ABB HVDC Reference Projects – Cahora Bassa
Cahora Bassa HVDC transmission link – Guide: Apollo station; ABB publication No. POW-0054.
Cahora Bassa HVDC transmission link – Apollo station refurbishment project; ABB publication No. POW-0055.
HVDC transmission lines
Electric power infrastructure in Mozambique
Electric power infrastructure in South Africa
Energy infrastructure completed in 1979
Mozambique–South Africa relations
1979 establishments in Mozambique
1979 establishments in South Africa