Kariba South powers on

Kariba South powers on

Early this year, the first of six refurbished and upgraded units at Kariba South hydropower plant in Zimbabwe will be recommissioned. The project will increase the output of the plant, as well as extend its lifetime, giving Zimbabwe`s generating plant a welcome boost.

In the mid-1980s, the Zimbabwe Electricity Supply Authority (ZESA) began to investigate the feasibility of refurbishing the Kariba South hydropower station. The reason was the deteriorating condition of the power station, in particular the electrical and mechanical equipment, with concerns over future availability and reliability. Kariba South provides approximately 30 per cent of electricity generation in Zimbabwe and is therefore vital to the country`s development.

ZESA commissioned engineering consultants Merz and McLellan in the 1980s to carry out feasibility studies and oversee project implementation. Now, the first of six refurbished and upgraded units is about to be recommissioned. When all six units are fully operational, the station`s output will have been increased by 84 MW, and its lifetime extended by 30 years.

Kariba South accounts for a large part of Zimbabwe`s 1946 MW of generating capacity. It is also the country`s only major source of hydropower and so makes an important contribution to fuel diversity. The fact that the power station could be uprated to 750 MW from 666 MW was therefore a major benefit, as was an increase in turbine efficiency.

The Kariba scheme

The Kariba hydroelectric scheme is on the River Zambezi, which forms the border between Zimbabwe and Zambia. The scheme comprises an arch dam 128 m high and two power station developments, one on each side of the dam. A gross head range of 85 m to 106.5 m is developed on the site. The Kariba dam impounds the third largest hydroelectric power reservoir in the world. It has a design volume of 180 billion m3 and measures 280 km in length.

The original scheme consisting of the dam and the Kariba South hydraulic system and power station in Zimbabwe, was constructed between 1955 and 1962. Kariba South power station is underground and houses six generating sets consisting of Alstom generators and Kvaerner turbines.

At Kariba South, there are three almost identical systems, each serving two turbines. Each turbine has an individual intake and pressure shaft on the upstream side, while downstream the draft tubes of the turbines discharge into individual gate chambers. There is a short transition to the surge chamber, where discharge from the two turbines joins together. A common tailrace tunnel leads to the outfall into the River Zambezi.

The six generating sets, each comprising a Francis turbine and a salient pole generator, are of vertical shaft arrangement of the “umbrella” type.

Feasibility study

ZESA engaged Merz and McLellan to investigate and report on the feasibility of uprating the plant and equipment at Kariba South. The question of how this could be achieved with minimal plant replacement was considered in detail, and tests were made on both the electrical and mechanical equipment to determine the existing performance, whether any degradation had occurred, and other factors which might affect plant life.

The original rating of the generating sets of 100 MW was increased shortly after commissioning to 111 MW without any major equipment changes, and short term operation at outputs approaching 120 MW was possible. Thus certain generating equipment had already been overloaded for prolonged periods. During later years of operation, set output was limited to 111 MW, giving a nominal station rating of 666 MW.

Costs and benefits

The decision to uprate and upgrade the turbines was made easy by the appearance of fatigue cracks in the runner blades. A close dialogue with Kvaerner revealed that their latest designs could significantly improve the turbine efficiency and provide an increase in output capability simply by fitting a replacement runner.

A cost-benefit analysis revealed that a tentative uprating to 125 MW would yield a benefit that exceeded many times over the cost associated with uprating and rehabilitation. Additional benefits would accrue due to the improvement in water turbine efficiency. The investigative work on the turbine performance concluded that replacement turbine runners could give a turbine output of 127.5 MW under a net head of 94 m consistent with a generating set output of 125 MW.

The increase in efficiency offered by new runners means that for the same water flow, energy production from the station can be increased. Weighted average efficiencies for the existing turbine and the uprated turbine were estimated based on the proportion of time the sets were run at different outputs, resulting in an improvement in weighted average efficiency of 1.7 percentage points. This improvement was compared with the fuel cost at Hwange coal-fired power station, yielding a capitalised saving over 30 years of Z$20.1 million ($0.464 million) at 1993 prices.

The feasibility study was refined by further work performed between 1986 and 1993. It was decided that the generator transformers required refurbishment, including new HV windings which, due to the employment of modern insulation techniques, increased the transformer rating to 140 MVA.

Overall, the cost-benefit analysis concluded that the principal benefit arises from the existing plant capacity made available for a further 30 years, with secondary benefits arising from installation of improved turbine runners.

The assessment of the additional capacity was based on an average uprated output, taking into account head variations, yielding a value of 75 MW. The value of this capacity calculated in 1993 was Z$337.5 million.

The cost of refurbishing and uprating Kariba South was estimated to be Z$244 million in 1993, giving a benefit to cost ratio of 1.47 (Z$358 million to Z$244 million). Financing for the project was provided by ZESA, EIB and contractors.

Project implementation

In 1993, ZESA appointed Merz and McLellan, in association with Gibb, to carry out Phase III consultancy services for the engineering, project management and supervision to implement the rehabilitation and uprating to 125 MW of Units 1 to 6 at Kariba South. Tendering for the contracts commenced in 1993.

The refurbishment work has largely been performed as optional work under the contract, with ZESA and Merz and McLellan reviewing the contractor`s recommendations to determine the level of work necessary to achieve the refurbishment objectives. While this can lead to extra costs, it has allowed ZESA to control the amount and cost of refurbishment work being done during the course of the contract.

The turbine refurbishment and uprating work on site was preceded by extensive efforts in terms of both mechanical and hydraulic design, the latter culminating in two laboratory tests of the model for the new Kariba turbine runner. Preliminary and general performance tests were carried out in the water turbine manufacturer`s own hydraulic laboratory before tests in an independent laboratory.

To achieve the efficiency guaranteed in the contract, modifications were required to both the stay vane and guide vane profiles, the former requiring air-arc gouging and grinding of the existing stay vanes while the latter required an extra milling operation to the existing guide vane trailing edges. The next stage was to ensure satisfactory replication between the model runner and the prototype runner.

In February this year, the first of the six refurbished and uprated units was due to be commissioned. The entire project will be complete at the end of 2000, when Kariba South will continue to account for a significant part of electricity production in Zimbabwe.

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Figure 1. Kariba is on the River Zambezi between Zimbabwe and Zambia

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Figure 3. Comparison between the original turbine performance and that expected from the replacement runners, based on the independent model test results

Africa`s Hydropower Renaissance

Hydropower is enjoying a renaissance in Africa, nowhere more so than on the continent`s longest river, the Nile, where numerous states are finally harnessing their natural water resources to fuel economic development.

Ethiopia is the scene of greatest activity with the state utility Ethiopian Electric Power Corp. (EEPC) developing five hydroelectric plants as part of an ambitious national plan to promote economic growth. Four new plants à‚– 305 MW Tekeze, 192 MW Gilgel Gibe, 150 MW Gojeb and 75 MW Tisabay à‚– will be completed over the next four years, and a 15 MW expansion of the existing 100 MW Fincha project is also in the pipeline.

Further upstream, Uganda also sees hydroelectricity as the cornerstone of its energy and economic plans. State utility Uganda Electricity Board (UEB) is close to completing an extension project that will increase generation capacity at the 180 MW Owen Falls scheme. The first phase, which will add 40 MW to the national grid, is planned to come on line in April 1999.

UEB is expected to appoint a consultant to design and oversee tenders for three additional 40 MW generating units to be installed at the 200 MW Owen Falls extension scheme. The project, which includes a dam and powerhouse with five 40 MW units, is under construction downstream from the original Owen Falls Dam on the Victoria Nile river at Jinja.

Downstream from Owen Falls I and II, two other projects are in advanced stages of preparation. Nile Independent Power (NIP), a US-Ugandan joint venture comprising AES Electric Ltd. and Madhvani International, is currently forging ahead with the development of the 290 MW Bujagali scheme. The Bujagali project will cost an estimated $450 million to build, financed in its entirety by AES Electric from its own capital resources. Construction is scheduled to be completed by 2001.

The $500 million, 350 MW Kalagala project is currently being studied by German consultants Lahmeyer International on behalf of Arab International Construction Co. (AIC) of Egypt. The Ugandan government and AIC have concluded preliminary negotiations towards a PPA and implementation agreement. UEB also announced that a Hydro Master Plan had identified four further hydro project locations on the Nile River as 180 MW Kamdini, 304 MW Ayago North, 234 MW Ayago South and 642 MW Murchison Falls.

UEB says the increased capacity resulting from all these schemes will not only allow it to meet growing domestic demand but also to increase electricity exports to neighbouring Kenya, Rwanda and Tanzania. UEB says that revenues from its currently limited power exports to Kenya earned it $10 million in 1997.

Construction will also begin in 1999 on the 60 MW Sondu-Miriu project in western Kenya, with contracts for the civil, hydro-mechanical and electro-mechanical works due to be awarded early this year. The $60 million run-of-river project is being funded by a loan from Japan`s Overseas Economic Cooperation Fund.

Despite continuing civil war, the Democratic Republic of Congo announced plans late last year to build a 3000 MW hydro plant, Inga 3, and a subsequent 40 000 MW plant to be known as Grand Inga. Operating capacity at the Inga complex is thus planned to increase from 1700 MW to more than 44 000 MW by 2010, to exploit more fully the potential of the Congo River.

The Congolese authorities envisage exporting electricity across Africa, in particular to South Africa, Egypt and west Africa. South African utility Eskom has already posted its interest in signing a contract for the export of electricity to other nations of the Southern African Power Pool (SAPP), to which Congo already belongs.

The ambitious plan depends however on a $7.5 billion expansion of the under-utilised Inga plant and an end to the country`s long-running conflict.

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