State-owned power utility Eskom has invested R35-billion in improved electricity transmission infrastructure – boosting the utility’s capacity to provide electricity – as part of its new build programme.

The infrastructure upgrade project aims to strengthen South Africa’s national power grid as new power plants, such as Medupi, in Limpopo and Kusile, in Mpumalanga, are linked to the network.

In addition to linking these two coal-fired plants and the pumped-storage hydroelectric power station Ingula, on the border between the Free State and KwaZulu-Natal, Eskom’s new build programme includes several initiatives to strengthen the grid by installing additional transmission lines and upgrading existing ones.

To date, Eskom has spent R21-billion on transmission projects, which will increase to R34.5-billion by the time the new build programme is completed.

Since the start of the programme, Eskom has installed 5 200 km of new transmission lines, as well as substations with a transfer capability of more than 24 000 MVA.

Eskom acting executive project manager for 765 kV power delivery projects Ben Herbst says, while most of these high-voltage lines are rated 400 kV, a significant portion of the new lines can carry higher capacity, at 765 kV, which provides improved transfer capability of bulk power over long distances, such as to KwaZulu-Natal and the Western Cape.

The integration of the new Medupi power station is mainly through the 400 kV network to Rustenburg, in the North West, and Polokwane, in Limpopo. “However, one of the lines is built to carry 765 kV, but will be operated at 400 kV in the interim. This gives Eskom the flexibility of upgrading to 765 kV in future,” he says.

The Kusile power station and the Ingula pumped-storage scheme will be linked to the grid mainly through existing 400 kV networks, as both are in close proximity to established networks and load centres.

“Since 2007, more than 1 560 km of 765 kV power line has been constructed and handed to the various transmission grids to maintain and operate,” says Herbst.

He adds that most of the line is operated at 765 kV, but there are a few exceptions, such as the link between Majuba and Umfolozi, in KwaZulu-Natal, where the line is operated at 400 kV until the rest of the 765 kV infrastructure is in place.

By 2016, the 765 kV network will stretch from the power stations in Mpumalanga to the Western Cape, says Herbst. “The 765 kV installation team is working on the section of transmission lines between Beaufort West and Ceres, both in the Western Cape, which will be completed in the current financial year.”

Construction on the remaining portion from Ceres to Cape Town will take two years to complete.

“These projects all have their own individual challenges,” he adds, explaining that in the Klerksdorp area, the erection of towers is complicated by undermining, as the area is dotted with shallow coal mines, some of which are about 100 years old, for which there are no mining maps. Between Orkney and Parys, the team encountered extensive waterlogged areas, while in the Northern Cape, they found huge amounts of shallow rock.

“The installation team is working on the challenges posed by the mountainous terrain of the Western Cape as the team is planning the completion of the 765 kV transmission line to Cape Town. Traversing the mountains is difficult for construction vehicles and getting heavy cranes and concrete trucks to tower positions will be hugely problematic,” says Herbst.

To date, the team has built more than 3 000 towers along the various routes carrying the new transmission lines. The materials used include nearly 79 000 t of structural steel and more than 100 000 t of concrete foundations.

“While rolling out 765 kV power lines at an unprecedented rate, Eskom has maintained a safety record of which it is proud,” he boasts. Since 2009, there has been a consistent improvement in the lost-time injury rate, owing to continuous management interventions, and Eskom has maintained its low-incident record for a high-risk industry such as line construction.

In addition, the installation team has, on several occasions, achieved three-million man work hours without a lost-time incident.

Curbing Theft
Eskom’s line engineering depart- ment team has introduced numerous innovations that have not only reduced the capital cost of Eskom’s new build programme, but also helped to counter the theft of steel from the pylons supporting high-voltage transmission lines.

With the installation of the new transmission lines from Medupi, several changes to standard tower design were implemented, which have become standard practice in Eskom’s transmission line design and construction.

Two new anti-theft measures were implemented to curb the theft of tower steel, which can cause towers to collapse, threatening network reliability. These included the use of a new anti-theft bolt for the structures as well as marking the steel support beams with the Eskom name at 300 mm to 500 mm intervals. The new anti-theft bolts cannot be removed using conventional hand tools.

Eskom has also developed a cross-rope tower structure – which uses less steel and reduces costs – on its 400 kV lines and the utility is developing a cross-rope structure for the 765 kV lines to achieve the same benefits.

The cross-rope towers, as well as the guy cable stablised V-shaped towers, will have saved more than 15% of capital cost over the standard self-supporting structures. They are not only expected to perform better and withstand extreme weather conditions, but are also considered more visually appealing, fading into the background when viewed from a distance.

The line cost a kilometre in South Africa is among the lowest in the world. “A recent study, done as part of an International Council on Large Electric Systems workgroup exercise, found that the Medupi 400 kV lines were the cheapest a kilometre, taking into account the total cost of items such as materials, construction and land cost,” concludes Herbst.