Cahora Bassa comes back to life

It was October 1995 when the first proposals were made to recover Cahora Bassa. The dam and power plant had been in existence for twenty years, but had only operated for a fifth of that time. The long shutdown was caused by the civil war that raged in Mozambique throughout the 1980s.

When the plant went into operation in 1976 almost all of its 2000MW output was destined for South Africa, the region’s biggest demand centre. The electricity sale was an important foreign currency earner for Mozambique’s Frelimo government, but as Eskom’s Peter Adams explains, in the civil war of the 1980s the arrangement was highly vulnerable. ‘The dam and its installation were well guarded,’ he says, ‘but there was no way that Frelimo could maintain an effective watch over 900km of twin power lines from the dam to the South African border’. The anti-government Renamo forces at first used explosives to blow up the transmission towers, and eventually dismantled the towers bolt by bolt, carrying away the steel sections. The power plant went out of action in 1980.

Bringing the plant back on line has required a joint effort from the various parties involved. A permanent joint committee (PJC) set up to plan the rehabilitation of the project has representatives from the governments of South Africa, Mozambique and Portugal, as well as from Eskom and the Portugal-based plant managers Hidroeletrica de Cahora Bassa (HCB).

While the dam and power house were guarded during the war years, the effort extended on maintenance was minimal, and was possible only for easily accessible parts of the plant. To bring it back to life, PJC turned to the plant’s original builder, Portugal-based Sorefame, now part of ABB Portugal. The company took up the challenge in October 1995, and following a two-month inspection of the hydromechanical equipment the company put forward proposals to recover the plant.

While the five 400MW generating units had not been brought into operation during the shutdown, the auxiliary generators — there were five, associated with the main generators, each rated at 20MW — had been used occasionally by one or other group. Keeping the main plant out of action, and operating the low-power generators, have both, over time, had deleterious effects. ABB Portugal found an extensive upgrading project when it undertook the inspection. The main points:

•The five water intake caterpillar gates were working but they had been badly corroded, having been under water without maintenance for nearly 20 years. They had been used from time to time but had to be closed for long periods of time. The chains and watertight seals were no longer usable and had to be replaced.

•The distributor had been kept at an intermediate position — open by around 10% — to allow low power operation. This had badly damaged the distribution servometers and they needed to be repaired.

•The turbines were in relatively good condition.

•The draft tube caterpillar gates (two associated with each turbine) were in very bad condition and subject to frequent jamming. In the worst case, one gate element had broken and fallen from its position (a designed-in effect that reduces damage to the turbine). All the chains and watertight seals had to be replaced.

•The spillway steel linings had suffered substantial damage. Some 2000m3 of water should pass through the turbines but during low power operation this volume had been constantly discharged through the spillway gates and corresponding steel linings. Watertight seals on the radial gates needed replacing and the gates required new coatings. The servomotors also needed some refurbishment.

•The stoplogs were operative, but all of them needed new wheels and watertight seals.

HCB asked for a quick quote on the refurbishment and a tight schedule for the work, so ABB submitted refurbishment proposals in December 1995 and began work at the site in February 1996. The first phase of the refurbishment (the water intake and draft tube) was completed in March 1997. Seven ABB specialists worked with 20 members of HCB to carry out the first phases of the repair. According to ABB Portugal, the refurbishment was as follows:

•Each water intake and draft tube gate was removed, cleaned, refurbished and painted, chains and seals were replaced and each gate was put back in place and tested. As there was only one stoplog, water intake gates were worked on one gate at a time. For the draft tube gates, the watertight enclosure had to be disassembled first, and its sealings were also replaced.

•The broken gate element was removed and a new one was fabricated and installed.

•The distribution servo-motors were disassembled and completely refurbished, and the seals were replaced. They were then put back in place and tested.

•All the gate servomotors were thoroughly tested and repaired where necessary.

•Stoplogs were refurbished and painted, and the wheels and watertight seals were replaced.

•For some gates the embedded components also required refurbishment because they had presented misalignments that could cause jamming of the gates and stoplogs.

•The hydraulic system was refurbished.

•Head loss measuring equipment was replaced and upgraded.

•A new system was installed in the draft tube gates to limit lateral vibrations, improving reliability and extending the life of the equipment.

This extensive refurbishment has allowed the plant to be brought on line and has prepared the hydromechanical equipment for at least its next 20 years of operation, according to ABB Portugal. The company now expects a request from HCB to refurbish the spillway.

The extensive work required to bring Cahora Bassa back on line has made it economic to upgrade some parts of the plant, for example the vibration blocking system for the draft tube gates described above. The control system has also been upgraded by Siemens, and similarly control of the high voltage DC transmission line is being upgraded with the introduction of Siemens’ Grid Master Power Controller (GMPC) system.

From dam to customer

The transmission line between Mozambique and South Africa runs via two converter stations, Songo in Mozambique and Apollo in South Africa. Rebuilding the twin 900km lines has cost around US$125M, and taken more than two years.

Once financing was in place the first step toward repair was taken by HCB in May 1995, when a contract was signed with South Africa-based ABB Feralin to manufacture the transmission towers. This was soon followed by contracts for the cable manufacture, which were awarded to Quintas & Quintas and Solidal, both based in Portugal, and finally for construction and mine removal, which was awarded to two Italian companies — Italia 2000 and ENEL. Construction of the first of the twin lines was completed in November 1997 and of of the second in February 1998.

Before the link goes into operation the new transmission management system must be completed. There are two systems in the transmission line, a three phase and an HVDC link. Both are operated from one busbar which is supplied by all the power station’s five generators. This means that the coupler circuit breaker is closed, and the result during the previous operating period was a series of stability problems during operation, especially so in the event of a fault on one or other of the lines.

GMPC is intended to solve these problems. It controls the voltage angle between Songo and Apollo with the aid of braking resistors. In the event of critical system states the coupler circuit breaker can divide the busbar; one section is then responsible for the three-phase link and one for the HVDC transmission.

The HVDC line is expected to be back in operation this year, when the system is finalised, and when negotiations between Mozambique and South Africa over energy purchase have been completed.

Further expansion potential

Power supply in southern Africa is increasingly becoming an international affair, and the rebirth of Cahora Bassa fits well into long term plans to develop interconnections between the region’s electricity grids.

The plant is indeed an example of the international approach. Originally planned to supply South Africa and Mozambique, since it reopened the dam’s output has been transferred to two other countries. The first is Zimbabwe, which in early 1998 obtained from Eskom a cession of part of its supply contract with the dam. A transmission line has now been built from the dam to the Zimbabwe capital Harare and some 500MW is being transferred. The contract will be in force until 2002.

Under a second agreement, also signed this year, the dam will supply power to Mozambique’s northern neighbour Malawi. The agreement is initially for 50,000kW for five years. In the following five years the export will rise to 100,000kW per year.

As piecemeal upgrading projects gradually turn the Southern African electricity grid into a reality, the need for more power has once again raised the question of the capacity of Cahora Bassa.

Plans have been mooted in the past that will mirror the five generating units already in existence at the dam with five more on the opposite bank, doubling the plant’s generating capacity. However, such ambitious expansion is unlikely to be more than a ‘might have been’ in the foreseeable future. Doubling capacity is widely thought to be beyond the potential of the Zambezi at that point, and in any case the immediate priority is likely to be to ensure that Cahora Bassa as it stands is a reliable and economically sound electricity supplier.