The most recent flood-related disruptions to the transmission line from Mozambique’s Cahora Bassa hydropower scheme to South Africa were unrelated to the major reactor failure of July 2012 that halved imports for four months and led to the deferment of critical Eskom power-station maintenance. But Eskom and Hidroelectrica de Cahora Bassa (HCB), which have joint responsibility for the scheme, have disclosed that there have been six incidents of power reduction since the repair. These have had various causes, attributed primarily to the age of the equipment on the network.
The reactor failure, which reduced imports materially between mid-July and mid-November, has been described as an “abnormal incident”, triggered by an electrical short circuit.
Eskom immediately offered a replacement unit, but transportation of the spare became a “major challenge” as bridges along the route had to be reinforced, and temporary bridges erected, to carry its the 130 t weight.
The unit was installed and commissioned under the supervision of the original-equipment manufacturer, Siemens, and the replacement unit remains in working condition.
HCB, which is responsible for the infrastructure on the Mozambican side of the border, has ordered three new reactors to replace those currently installed. The new, modern units, which should be delivered by May, are lighter than those originally deployed and should, therefore, be easier to transport.
However, prior to the January disruption, caused by the flooding of the Limpopo river crossing Mozambique, five other disruptions were also experienced.
Eskom says controlled power reductions were implemented earlier in January to deal with the loss of auxiliary supply, owing to the failure of an inverter card. This repair was completed in about three-and-a-half hours.
For the flood-damaged line, HCB and Eskom are working on a repair solution. The damage reduced the capacity available from Cahora Bassa to 650 MW from 1 300 MW.
Eskom spokesperson Hilary Joffe says the import disruptions have negatively affected Eskom's generation maintenance planning, leading to delays in the maintenance schedule.
“Therefore the pace of catching up on the maintenance backlog has slowed down. This is partly the reason we had to enter into buy-back arrangements,” she reports.
Eskom is communicating with HCB on improving the performance of the line, which is in need of refurbishment.
“In Mozambique, most of the equipment is old but there is a plan for a complete refurbishment of the equipment. Unfortunately, refurbishment will take some time and until then we remain vulnerable,” Joffe reports.
“In South Africa, we have already replaced most of the problematic equipment and have doubled up on critical spares for the equipment not replaced.”
The disruptions have not, however, dissuaded Eskom from pursuing the prospect of additional imports from the Southern African region.
Any scaling-up of such imports are also likely to involve high-voltage, direct current (HVDC) schemes, but will incorporate newer, more reliable technology than is being used for the Cahora Bassa scheme.
The Cahora Bassa HVDC scheme is currently the only such scheme in Southern Africa.
However, Eskom is investigating the feasibility of using the technology to transmit bulk power from generating sources, mostly other hydropower schemes, to load centres in the future.
“The reliability of the transmission networks that will be required to connect South Africa to Southern African generation resources is only one of many considerations when determining the level of future imports. As indicated in the Integrated Resource Plan, South Africa has a vision to import power, particularly clean hydropower that will be developed in the region,” Joffe concludes.