Namibia needed a special solution to create the long distance power import capability it required to meet power demand, create grid stability and add a new connection in the Southern Africa Power Pool.
Nigel Blackaby, Associate Editor
|An aerial view of the Gerus converter station Source: ABB|
Namibia is a power hungry African nation. Its healthy level of economic growth, estimated at 4.4 per cent in 2010, together with its mineral wealth has put pressure on the national power utility, NamPower, to find innovative ways to meet increasing demand. Perhaps its most creative to date has been the use of one of the world’s most modern power transmission technologies to import electricity and, at the same time, to stabilise its grid. This is the story of how Swiss-based ABB was asked by NamPower to bring its HVDC Light technology to Africa, deploying it with overhead transmission lines for the very first time.
Long distances are quite the norm in Namibia. Apart from the capital city of Windhoek and the deepwater port of Walvis Bay, settlements are few and very far between in a country that is twice the size of Germany. It is one of the key appeals of this southern African country that it is blessed with an abundance of wild animal species, spectacular scenery and fine weather. The ambitious plans to bring electricity generated in Zambia’s hydroelectric power stations along the Caprivi Strip, through difficult terrain and down into central Namibia, involved the erection of a transmission line 950 km long, which even by Namibian standards was always going to be a challenge.
The demand conundrum
To understand the need for an interconnector of this length in this part of Africa, it is first necessary to explore the issues facing the local power industry. The responsibility for power generation, transmission and distribution in Namibia is almost exclusively the remit of NamPower. It owns and operates a mixture of power generating units including coal fired, diesel and hydroelectric and is responsible for 32 544 km of transmission lines. The limited number of generating units and small grid system leaves the network extremely weak. The challenge for NamPower in recent years has been the fast growing demand for electricity within the country, easily outpacing its ability to supply from domestic resources. This has meant reliance on imported electricity for 54 per cent of demand.
Imported power comes from the Southern Africa Development Community (SADC) region, which itself faces a critical power shortage. Namibia’s demand for electricity continues to increase. In 2011, total load demand stood at just over 600 MW. But NamPower forecast this rising to 700 MW by 2015 (assuming a medium growth scenario) and as much as 870 MW by 2025, on the same basis. A high growth forecast would see demand at around 950 MW by 2025. Consequently, the country is increasingly reliant on imports in terms of its power supply requirements, a situation that the government sees as unsustainable in the long term.
Much of the new demand for power is coming from the burgeoning mining industry in the Erongo Region, which is growing at an unprecedented level and is in turn creating considerable wealth for Namibia. Uranium and zinc deposits could transform the economy and Namibia is keen to ensure that economic growth from mining ventures is unconstrained by power shortages. Mining operations make large demand (step loads) on the national power grid for their period of operation. But they may only operate for a few years and will be closed once the economically viable deposits have been extracted. NamPower is therefore looking to build a degree of flexibility into its power supply system. It is building new power generation and transmission capacity, such as the 22.5 MW Anixas emergency diesel power station at Walvis Bay, opened in November 2011, the Hwange coal fired rehabilitation project and the West Coast 220 kV transmission reinforcement programme.
NamPower’s options for meeting growing demand were either to build new generation or to expand power import capacity. NamPower was mindful of the risk of being left with a stranded asset if it built new capacity for a transient mining community, and that this would be a more costly alternative. It elected to expand its import capacity from the northwest and at the same time create an important link in the eventual connection of the Southern Africa Power Pool. With extra import capacity, Namibia will reduce its reliance on coal for power generation during the dry season when the main Ruacana hydropower plant cannot operate at full capacity
The Southern African Power Pool (SAPP) is a crucial supplier of power to Namibia and other participating counties. Power trading is mostly through long-term power supply agreements. NamPower has agreements ranging from annual to 15-year duration with Eskom (South Africa), EDM (Mozambique), ZESA (Zimbabwe), ZESCO (Zambia) and SNEL (Democratic Republic of Congo). A new interconnection would provide a second north-south interconnector within the SAPP and contribute to the vision of interconnecting all the participating countries.
The solution chosen was therefore the Caprivi Link Interconnector. The route brings power from Zambia, through the far northeast of Namibia, down to the central part of the country. It runs through the Caprivi Strip, a panhandle squeezed between Zambia and Botswana created through a treaty in 1890 between Germany (represented by Chancellor Leo von Caprivi) and the UK, in which Germany sought to create a route from Namibia (then known as German South-West Africa) to the Zambezi River. The intention was to gain access through the Zambezi to Africa’s east coast. But, in the event, the river later proved to be unnavigable.
A conventional AC power line of nearly 100 km would have resulted in unacceptable levels of power loss and this, together with the high capital cost, would have cast doubt on the viability of the scheme. The alternative was to opt for a more efficient high voltage direct current (HVDC) transmission system, designed for transmitting large amounts of power over long distances, which would have a much smaller footprint on the landscape but was a much more high-tech solution.
|Rows of semiconductor units for converting current at the Gerus substation of the Caprivi Link Source: ABB|
The Caprivi Link is designed to move electricity in either direction, depending on the respective requirements of the two connected countries. In practice, the likelihood is that Namibia will look to import cheap power from Zambia’s hydroelectric generation plants and Zimbabwe’s coal fired plants more often than the reverse situation, but the technology has been built and tested to operate in either direction. Due to its nature of settling power flows, the Caprivi Link will add to the energy trading potential within the region and improve the dynamic stability of the SAPP transmission network.
The Caprivi Link Interconnector Project was commissioned at Katima Mulilo in the Caprivi Strip on 12 November 2010. At an estimated cost of N$2.9 billion ($360 million), it was the second largest individual investment in Namibia. NamPower looked to the European Investment Bank, the French Development Bank and Germany’s KfW for long-term funding. Each institution provided about N$350 million to the project, with an additional N$150 million interest rate subsidy from the EU-Africa Infrastructure Trust Fund. A contribution of N$60 million came from the Development Bank of Namibia with NamPower providing the balance.
The project involved constructing two converter stations, both capable of converting power from AC to DC. The Zambezi converter station is in the Caprivi Strip, close to the Zambian border, while the Gerus station is on the site of an existing sub-station near Otjiwarono in Namibia’s Otjozondjupa region. A 350 kV HVDC transmission line connecting the two converter stations is designed initially to transmit up to 300 MW and eventually 600 MW on completion of Phase 2 of the project.
The Zambezi station is connected through conventional transmission lines to major generating sources at Kafue Gorge and Victoria Falls. NamPower’s managing director, Paulinus Shiamba, describes the Caprivi Link’s objective as to, “provide an asynchronous link between Namibian and Zambian/Zimbabwean electricity networks in order to ensure a reliable power transfer capability between the east and west and an additional north-south route of the SAPP”. Other benefits include better utilisation of the Livingstone–Zambezi 220 kV line constructed by ZESCO and NamPower and the creation of an alternative wheeling path for SAPP electricity trades. Regionally generated electricity is already transported via South Africa but the link will allow power to be imported into Namibia that has been produced in Zambia, Zimbabwe, Mozambique and the DRC.
Under phase 1 of the project, completed in June 2010, a 300 MW HVDC system was built featuring 350 kV DC monopole converters using Voltage Sourced Converter (VSC) technology connecting to 330 kV AC at the Zambezi converter station and 400 kV at the Gerus converter station. The 950 km between the two substations were linked by overhead line with both pole conductors installed. The choice of a DC connection reduced the footprint of the transmission line and therefore the environmental impact. Phase 1 took 32 months and involved subcontractors from Namibia, South Africa and India for the civil aspects and HVDC line.
The converter stations were designed and supplied by ABB, the Swiss-based power and automation specialists with a southern Africa business and manufacturing base in Johannesburg. ABB used its own proprietary HVDC Light technology and was also responsible for system engineering and the installation of the two converter stations and earth electrodes. The project extends the voltage rating for HVDC Light to 350 kV and marks the technology’s first use with overhead transmission and for connecting of two inherently weak power networks, giving importance to the technology’s power quality features.
The AC networks at both ends have short circuit power levels of around 300 MVA and long AC lines that connect to remote generator stations. The AC networks are therefore also exposed to a risk of 50 Hz resonance. ABB studied the crucial AC and DC fault cases and verified that the dynamic performance of the HVDC Light was in line with the client’s requirements.
HVDC transmission is the preferred option for long-distance bulk power transfer. Unlike AC transmission, where the direction of current is constantly reversed back and forth, DC is based on direct flow of electric charge through a circuit, resulting in minimal losses (about 3 per cent per 1000 km) and allowing for better control of power flow. HVDC systems allow higher transmission capacity at lower capital costs and require less space.
HVDC Light is a more recent development of the technology widely used in offshore wind power integration, cross border interconnections, power supply to islands, shore supply to offshore platforms, and embedded DC in AC to strengthen AC networks. It allows for independent control of active and reactive power, enables black start capability and connection to passive loads, and its controllability enables enhancements of AC networks.
The second phase of the project has yet to be initiated but would involve upgrading the 300 MW monopolar scheme to a 600 MW bipolar scheme. A new converter station hall would be needed, but provision for that has been made in the site design. NamPower says Phase 2 will be implemented “if and when the need arises”. That need appears not to be immediately evident as the link is currently only scheduling a power flow of 50 MW out of a possible 300 MW. The 50 MW flow from Zambia to Namibia is under a ‘take or pay’ power purchase agreement.
|The Caprivi Link runs along a strip of land between Zambia and Botswana|
Expanding capacity and the ability to transmit larger power flows in the opposite direction will depend on removing constraints on the 330 kVAC lines in Zambia/Zimbabwe and on constructing a 285 km 400 kV AC line from Gerus to Auas, on the outskirts of Windhoek. It would also require developing the Zizabona project – a $225 million transmission project to build transmission lines to link Zimbabwe, Zambia, Botswana and Namibia. These countries have, in principle, agreed to develop a multi-million dollar power line that would allow an additional 600 MW to be transmitted around the region and help decongest the existing central transmission corridor that currently passes through Zimbabwe. Power utilities in these countries have signed an inter-utility Memorandum of Understanding for co-operating in the investment of new transmission infrastructure. The Zizabona Project is planned to be commissioned in two phases, namely the Hwange/Livingstone (Phase 1) and the Victoria Falls- Pandamatengo-Zambezi transmission stations (Phase 2). Implementation of this project would enable the full utilisation of the Caprivi Link.
The Caprivi Link interconnector is the first connection between the Caprivi region of Namibia and the rest of the country and can supply the region even if normal supplies from Zambia are disrupted. Even larger islanded parts of the Namibian and Zambian grids can be supplied by the DC link, which maintains frequency control and therefore avoids power outages.
Using HVDC Light technology has made it possible to stabilise two weak networks and opened up the possibility of power trading in a wider region of Africa. It is likely to usher in several similar connections in a region that remains heavily interdependent for electricity and where distances are challenging for conventional transmission solutions.
In Namibia, uranium mine developers continue to ask NamPower to create grid connections. If these developers start operating, the utility will need more 220 kV lines and substations to reinforce supply to the Erongo Region. In the meantime, NamPower can reflect on having built Africa’s first HVDC Light project, its first commercial overhead line project and the first in the world at the 350 kV level.
Chancellor von Caprivi’s vision of connecting East and West Africa by river may not have been realised, but 120 years later his name has been put to another sort of connection that may prove just as valuable.
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