Organizing Projects and Minimizing Downtime

The operator of the Cahora Bassa hydropower plant in Mozambique wants to tackle two major modernization projects: replacing the generators in the power plant and modernizing the Songo HVDC converter station, via which most of the generated electricity is exported to South Africa over a distance of 1,420 km. This will inevitably involve downtime for plant components, but this can be minimized by carrying out the two projects at the hydropower plant and the converter station in parallel, thus also minimizing economic losses. The interface management required to achieve this is being handled by Fichtner. Our team is also assisting with the tendering process for modernization of the Songo station and with selection and contracting of the HVDC supplier.

Hydroelectric Power – An Important Source of Income for Mozambique

Since the late 1970s, a 165-meter-high dam has been impounding the Zambezi, Mozambique’s largest river, in its lower reaches. The resulting reservoir, which is around 270 kilometers long, feeds the Cahora Bassa power plant, whose five generators together produce more than two gigawatts (GW) of electrical power. Most of the electricity is sold to South Africa. For the power plant operator Hidroeléctrica de Cahora Bassa (HCB), which is majority-owned by the Republic of Mozambique, the export of electricity is an important source of income.

To ensure minimal electrical losses along the 1,420-kilometer route to South Africa, the electricity is transported via a 533-kilovolt high-voltage direct current (HVDC) transmission system. Today, HVDC technology is widely used to transport large amounts of electricity over long distances, but it was still a rarely used technology back when the Cahora Bassa power plant was built. In fact, the connection between the Songo HVDC converter station in Mozambique and the Apollo HVDC converter station in South Africa was one of the first in the world to use thyristors – which are controllable power semiconductors, i.e. electrical valves – to convert alternating current into direct current.

Many Important Installations in Need of Extensive Renovation

Five decades or so will take their toll, even on power plant equipment and power grids. The HVDC line was repaired after damage caused by the civil war, and modernization projects were carried out in Mozambique and South Africa that addressed its aging. Among other things, the oil-cooled thyristors at the Apollo station in South Africa were replaced with new, water-cooled thyristors just over 15 years ago.

The first-generation thyristors are still in use in Songo. Due to their age, the risk of failure is high and replacement is urgently recommended, which also applies to the control system and parts of the DC and AC switchgear. In addition, the generators in the hydropower plant are now due for replacement. Both are projects that will lead to production restrictions or temporary shutdowns.

Interface Management Keeps Export Losses to a Minimum

HCB’s goal is to keep losses of electricity exports as low as possible. Therefore, the modernization works on the hydropower plant and those on the HVDC converter station should not be performed at different times. Instead, downtime during the modernization should be kept to a minimum, and works on the power plant and the Songo station should be carried out in parallel. In 2020, HCB awarded the contract for interface management to us.

Establishing the Conditions for Project Launch

The goal of completing the extensive works within the same timeframe appeared to be in jeopardy in early 2024, however, as no contract had yet been signed for the replacement of the oil-cooled thyristors with new, water-cooled ones. HCB therefore contracted us to assist with the award of the contract for modernizing the Songo HVDC converter station. Our experts reviewed the available documents, made adjustments where necessary, and assisted with contract negotiations between HCB and the potential plant manufacturer. The contract is to be concluded as soon as possible, by summer 2025 at the latest. This will allow for the planning and final coordination of the modernization at Songo station and the generator replacement at the hydropower plant to go ahead soon.

As is customary in the industry, the fact that Fichtner is playing a key role in the contract award and negotiation phase means we will not be involved in supervising the subsequent execution of the Songo retrofit project. Our team is therefore currently assisting HCB in its search for a suitable owner’s engineer.

June 2025