Cahora Bassa (HVDC)

Cahora Bassa HVDC
Cahora Bassa HVDC
	The southern line crossing through the Kruger National Park
	Route of the Cahora Bassa HVDC scheme
Location
Country	Mozambique, South Africa
Coordinates	15°36′41″S32°44′59″E / 15.61139°S 32.74972°E ; 25°55′11″S28°16′34″E / 25.91972°S 28.27611°E
From	Cahora Bassa Dam, Mozambique
To	Johannesburg, South Africa
Ownership information
Owner	Eskom, Hidroelectrica de Cahora Bassa (HCB)[ citation needed ]
Construction information
Manufacturer of substations	AEG-Telefunken, Brown Boveri Company, Siemens (original equipment); ABB Group (upgrade)
Commissioned	1977–1979
Technical information
Type	overhead line
Type of current	HVDC
Total length	1,420 km (880 mi)
Power rating	1920 MW
DC voltage	±533 kV
No. of poles	2

Last updated April 03, 2024

Cahora-Bassa (previously spelled Cabora Bassa) is a separate bipolar HVDC power transmission line between the Cahora Bassa Hydroelectric Generation Station at the Cahora Bassa Dam in Mozambique, and Johannesburg, South Africa.

History

The system was built between 1974 and 1979 and can transmit 1920 megawatts at a voltage level of 533 kilovolts DC and 1800 Amperes.^[1] Thyristor valves are used, which unlike most other HVDC schemes are mounted outdoors and not in a valve hall. The valves are grouped into eight, 133 kV six-pulse bridges in series at each end. The 1,420 kilometres (880 mi) long powerline runs through inaccessible terrain, so it is mostly built as monopolar lines 1 kilometre (0.62 mi) 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 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 valves 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, in 1988, also had to be overcome.

Following a refurbishment exercise,^[2] the scheme was put back into commercial operation in October 1997.^[3] Between 2006 and 2009 the thyristor valves at the Apollo converter station were replaced by more modern water-cooled thyristor valves.^[4]

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, Johannesburg, South Africa and Hidroelectrica de Cahora Bassa (HCB), a firm owned 15% by the government of Portugal and 85% 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. Brown Boveri subsequently became part of ABB and AEG subsequently became part of Alstom.

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) 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 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, 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) cables, and a single 117 square millimetre (231 kcmil) grounding conductor. There are approximately 7,000 towers with an average span of 426 metres (466 yd).

The maximum span is 700 metres (770 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.

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 in .

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. Each valve tank contains two valves, forming a double-valve connecting the two DC terminals to one single-phase, two-winding converter transformer. 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.^[2] In the first phase of the project (4 bridges at each end) each valve contained 280 such thyristors in series with two in parallel^[1] – 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.^[2]

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 stations: one at Gamaboi in South Africa and one at Catope in Mozambique.

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. Nearly all of the 4200 transmission line towers located on the 893 kilometres (555 mi) of line in Mozambique needed to be replaced or refurbished. This work was started in 1995 and took until late 1997 to complete.^[3] 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.

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 was awarded a contract to replace the thyristor valves at the Apollo station.^[4] 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

Site	Coordinates
Apollo Inverter Station	25°55′11″S28°16′34″E / 25.91972°S 28.27611°E / -25.91972; 28.27611 (Apollo converter station)
South African Electrode Line Terminal	25°50′04″S28°24′02″E / 25.83444°S 28.40056°E / -25.83444; 28.40056
Gamaboi PLC Repeater Station	23°55′36″S29°38′32″E / 23.92667°S 29.64222°E / -23.92667; 29.64222 (Gamaboi PLC Repeater Station)
Pole 1 crosses border between South Africa and Mozambique	22°32′06″S31°20′39″E / 22.53500°S 31.34417°E / -22.53500; 31.34417
Pole 2 crosses border between South Africa and Mozambique	22°31′15″S31°20′22″E / 22.52083°S 31.33944°E / -22.52083; 31.33944
Catope PLC Repeater Station	18°01′00″S33°12′18″E / 18.01667°S 33.20500°E / -18.01667; 33.20500 (Catope PLC Repeater Station)
Mozambican Electrode Line Terminal	15°43′20″S32°49′04″E / 15.72222°S 32.81778°E / -15.72222; 32.81778 (Mozambiquian Electrode)
Songo Converter Station	15°36′41″S32°44′59″E / 15.61139°S 32.74972°E / -15.61139; 32.74972 (Songo Converter Station)

Gallery

Media related to Cahora Bassa (HVDC) at Wikimedia Commons

Related Research Articles

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.

The Cahora Bassa lake—in the Portuguese colonial era 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 Africa, only Lake Volta in Ghana, Lake Kariba, on the Zambezi upstream of Cahora Bassa, and Egypt's Lake Nasser are bigger in terms of surface water.

The HVDC Cross-Channel is the 73-kilometre-long (45 mi) high-voltage direct current (HVDC) interconnector that has operated since 1986 under the English Channel between the continental European grid at Bonningues-lès-Calais and the British electricity grid at Sellindge. The cable is also known as IFA, and should not be confused with the new IFA-2, another interconnect with France that is three times as long but only half as powerful.

The Pacific DC Intertie is an electric power transmission line that transmits electricity from the Pacific Northwest to the Los Angeles area using high voltage direct current (HVDC). The line capacity is 3.1 gigawatts, which is enough to serve two to three million Los Angeles households and represents almost half of the Los Angeles Department of Water and Power (LADWP) electrical system's peak capacity.

The Nelson River DC Transmission System, also known as the Manitoba Bipole, is an electric power transmission system of three high voltage, direct current lines in Manitoba, Canada, operated by Manitoba Hydro as part of the Nelson River Hydroelectric Project. It is now recorded on the list of IEEE Milestones in electrical engineering. Several records have been broken by successive phases of the project, including the largest mercury-arc valves, the highest DC transmission voltage and the first use of water-cooled thyristor valves in HVDC.

HVDC Kingsnorth was a high-voltage direct-current (HVDC) transmission system connecting Kingsnorth in Kent to two sites in London. It was at one time the only application of the technology of high voltage direct current transmission for the supply of transformer stations in a city, and the first HVDC link to be embedded within an AC system, rather than interconnecting two asynchronous systems. It was also the first HVDC scheme to be equipped with self-tuning harmonic filters and to be controlled with a "Phase Locked Oscillator", a principle which subsequently became standard on all HVDC systems.

The Inga–Shaba EHVDC Intertie is a 1,700 kilometres (1,100 mi)-long high-voltage direct current overhead electric power transmission line in the Democratic Republic of Congo, linking the Inga hydroelectric complex at the mouth of the Congo River to mineral fields in Shaba (Katanga). It was primarily constructed by Morrison-Knudsen International, an American engineering company, with the converter equipment supplied by ASEA. Construction was completed in 1982 and it cost US$900 million. The scheme was, for many years, the longest HVDC line in the world.

The HVDC Itaipu is a High-voltage direct current overhead line transmission system in Brazil from the Itaipu hydroelectric power plant to the region of São Paulo. The project consists of two ±600 kV bipoles, each with a rated power of 3150 MW, which transmit power generated at 50 Hz from the Paraguay side of the Itaipu Dam to the Ibiúna converter station near São Roque, São Paulo. The system was put in service in several steps between 1984 and 1987, and remains among the most important HVDC installations in the world.

<span class="mw-page-title-main">Path 27</span> Electrical transmission line in Southwestern United States

Path 27, also called the Intermountain or the Southern Transmission System (STS), is a high-voltage direct current (HVDC) electrical transmission line running from the coal-fired Intermountain Power Plant near Delta, Utah, to the Adelanto Converter Station at Adelanto, California, in the Southwestern United States. It was installed by Asea, a company based in Sweden, and commercialized in July 1986. The system is designed to carry power generated at the power plant in Utah to areas throughout Southern California. It is owned and operated by the Intermountain Power Agency, a cooperative consisting of six Los Angeles-area cities, the largest member being the Los Angeles Department of Water and Power (LADWP), and 29 smaller Utah municipalities.

Shin-Shinano Frequency Converter is the designation of a back-to-back high-voltage direct current (HVDC) facility in Japan which forms one of four frequency converter stations that link Japan's western and eastern power grids. The other three stations are at Higashi-Shimizu, Minami-Fukumitsu, and Sakuma Dam.

The HVDC Rihand–Delhi is a HVDC connection between Rihand and Dadri in India, put into service in 1990. It connects the 3,000 MW coal-based Rihand Thermal Power Station in Uttar Pradesh to the northern region of India. The project has an 814 kilometres (506 mi) long bipolar overhead line. The transmission voltage is 500 kV and the maximum transmission power is 1,500 megawatts. The project was built by ABB.

The Vyborg HVDC scheme is a system of electricity transmission from the Russian power system to Finland, using high-voltage direct current. It consists of four 355 MVA (250 MW) back-to-back converter blocks, the first three of which were completed in the early 1980s and the last in January 2001. Much of the original converter equipment has been refurbished or modernised.

McNeill HVDC Back-to-back station is an HVDC back-to-back station at 50°35'56"N 110°1'25"W, which interconnects the power grids of the Canadian provinces Alberta and Saskatchewan and went in service in 1989. McNeill HVDC back-to-back station is the most northerly of a series of HVDC interconnectors between the unsynchronised eastern and western AC systems of the United States and Canada. The station, which was built by GEC-Alstom, can transfer a maximum power of 150 MW at a DC voltage of 42 kV. The station is unusual in many respects and contained several firsts for HVDC.

HVDC BorWin1 is the first HVDC facility in the world to be built for importing power from an offshore wind park to shore, and the first to use voltage source converters (VSC) in Germany. It connects the offshore wind park BARD Offshore 1 and other offshore wind farms in Germany near Borkum to the European power grid. The facility was built by ABB and has a capacity of 400 MW at a bipolar voltage of ±150 kV. HVDC BorWin1, which leads from BorWin Alpha Offshore Platform to Diele substation, consists of a 75 kilometres (47 mi) of underground and 125 kilometres (78 mi) of submarine cable.

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 dam was finished in December 1974 after much political debate. This dam is used to convert the Zambezi River power into electricity by turning turbines. That energy is then sent to South Africa. The Cahora Bassa dam forms Cahora Bassa Lake. The dam is jointly owned by Mozambique and Portugal. From independence until 2007, eighteen percent of the dam and lake was owned by Mozambique and eighty-two percent by Portugal. Portugal sold down its share to 15 percent in 2007. The Cahora Bassa dam is the largest hydroelectric power plant in southern Africa and the most efficient power generating station in Mozambique.

The Chandrapur back-to-back HVDC station is a back-to-back HVDC connection between the western and southern regions in India, located close to the city of Chandrapur. Its main purpose is to export power from the Chandrapur Super Thermal Power Station to the southern region of the Indian national power grid. It is owned by Power Grid Corporation of India.

The Chandrapur–Padghe HVDC transmission system is an HVDC connection between Chandrapur and Padghe in the state of Maharashtra in India, which was put into service in 1999.

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 power systems of different frequencies. HVDC converters capable of converting up to two gigawatts (GW) and with voltage ratings of up to 900 kilovolts (kV) have been built, and even higher ratings are technically feasible. A complete converter station may contain several such converters in series and/or parallel to achieve total system DC voltage ratings of up to 1,100 kV.

The Rio Madeira HVDC system is a high-voltage direct current transmission system in Brazil, built to export power from new hydro power plants on the Madeira River in the Amazon Basin to the major load centres of southeastern Brazil. The system consists of two converter stations at Porto Velho in the state of Rondônia and Araraquara in São Paulo state, interconnected by two bipolar ±600 kV DC transmission lines with a capacity of 3,150 megawatts (4,220,000 hp) each. In addition to the converters for the two bipoles, the Porto Velho converter station also includes two 400 MW back-to-back converters to supply power to the local 230 kV AC system. Hence the total export capacity of the Porto Velho station is 7100 MW: 6300 MW from the two bipoles and 800 MW from the two back-to-back converters. When Bipole 1 commenced commercial operation in 2014, Rio Madeira became the world’s longest HVDC line, surpassing the Xiangjiaba–Shanghai system in China. According to the energy research organisation Empresa de Pesquisa Energética (EPE), the length of the line is 2,375 kilometres (1,476 mi).

The Xiangjiaba–Shanghai HVDC system is a ±800 kV, 6400 MW high-voltage direct current transmission system in China. The system was built to export hydro power from Xiangjiaba Dam in Sichuan province, to the major city of Shanghai. Built and owned by State Grid Corporation of China (SGCC), the system became the world’s largest-capacity HVDC system when it was completed in July 2010, although it has already been overtaken by the 7200 MW Jinping–Sunan HVDC scheme which was put into operation in December 2012. It also narrowly missed becoming the world’s first 800 kV HVDC line, with the first pole of the Yunnan–Guangdong project having been put into service 6 months earlier. It was also the world’s longest HVDC line when completed, although that record is also expected to be overtaken early in 2013 with the completion of the first bipole of the Rio Madeira project in Brazil.

References

1 2 Compendium of HVDC schemes, CIGRÉ Technical Brochure No. 003, 1987, pp89–94.
1 2 3 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 ^{[ permanent dead link ]}.
1 2 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.
1 2 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, 2009, paper reference P107.

External links

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[compendium-1] 1 2 Compendium of HVDC schemes, CIGRÉ Technical Brochure No. 003, 1987, pp89–94.

[Venter-2] 1 2 3 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 ^{[ permanent dead link ]}.

[Oliveira-3] 1 2 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.

[Goosen-4] 1 2 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, 2009, paper reference P107.

[1]

[2]

[3]

[4]