For a country that has been enjoying the benefits of hydroelectric power for almost 80 years and is currently generating 96% of its electricity from the Zambezi river basin, Zambia is an unlikely contender to be in the final stages of commissioning its first coal-fired thermal generating plant.
But the Zambian economy has been growing in recent years, leading to the expansion of energy-intensive industries. This has increased base-load requirements to the point where demand for electricity is outstripping the available hydroelectric resources. The situation is compounded by climate change, which is making the country’s hydroelectric power less reliable.
Improving the network
The new 300MW coal-fired power plant is at Maamba in the Zambezi basin, in the south of the land-locked country. But the additional power is needed hundreds of kilometres to the north, in Zambia’s capital, Lusaka, one of the fastest growing cities in southern Africa.
ZESCO, Zambia’s electricity utility, appointed Mott MacDonald as project manager to improve the capacity of transmission network infrastructure along the Kafue-Muzuma-Victoria Falls corridor to enable the grid to cope with the power generated at the new coal-fired plant.
The project involves upgrading the existing 348km transmission line from Kafue Town through Muzuma to the Victoria Falls from 220kV to 330kV. The work also includes building a new substation and control centre at Livingstone, a few kilometres from the falls, and upgrading existing substations at Muzuma and Kafue, south of Lusaka.
The main challenge is to maintain supply to the Lusaka region at the same time as the work is done. In a highly interconnected grid system, a single line can be shut and power transmitted through an alternative. But in this case, there is no other route.
We initially considered live-line working, but this is not allowed under Zambian regulations. Instead, our eningeers decided to use a series of by-pass circuits to de-energise sections of the line, with work undertaken sequentially. It is essentially the method used when there is an emergency and supply must be restored to a power line before the problem can be fixed. However, it has not been used before on such a scale and certainly not as a means of keeping a long line, with individual bypass lengths of 30–40km, live during restringing, says Mott MacDonald project manager John Clarke.
Bypasses are created using removal steel monopoles rather than conventional lattice steel towers. The single poles are electrically safe and designed with the same clearances as a permanent line. Two restringing teams will be employed, leapfrogging each other along the line.
The temporary bypasses run within the existing power line easement, so there are no major challenges over access, land use and permissions. However, advance teams will clear African bush ahead of the work.
Security of supply across southern Africa
The need to maintain consumer supplies has also influenced the plans to refurbish two existing substations to cater for the 330kV voltage. In what is an extremely demanding logistical operation, we plan to perform the work in sections, using local bypass circuits to isolate work areas while maintaining sufficient step-down capacity to supply consumers.
In terms of switchgear, our design is an interesting mix of old and new technology. Traditional air-insulated switchgear will be installed at the new Livingstone substation and the upgraded Mazuma facility, but the refurbishment of Kafue includes gas-insulated switchgear, a first in Zambia. This is because the Kafue substation is next to a chemicals factory and acid emissions from the plant had previously affected equipment, leaving it in a very bad condition. Our proposal is to shrink the size of the substation, and using gas-insulated switchgear enables us to dramatically reduce the dimensions of the plant.
The upgrade will form an important component in a proposal to establish interconnectors between the Zambian grid and those in Namibia, Zimbabwe, Botswana and South Africa, increasing the security of power supply across the region.