HCB Songo HVDC Converter Station Reabsub Brownfield Phase 1 & 2

HCB (Hidroeléctrica de Cahora Bassa), which owns and operates the Cahora Bassa hydroelectric power station on the Zambezi River near Songo in Mozambique, contracted WSP in Africa’s transmission and distribution team to provide expertise to the process of rehabilitating and replacing equipment at the Songo HVDC converter station in order to improve its reliability, availability and maintainability (RAM).

Design challenges

Mozambique has the largest power generation potential of all Southern African countries and is capable of generating as much as 187 gigawatts of power from its coal, hydro, gas, solar and wind resources. The Cahora Bassa system, commissioned between 1974 and 1979, is one of the most important power transmission systems in the region.

The majority of Cahora Bassa’s power is transmitted to South Africa, where it contributes more than 1000 MW to the South African grid. The Songo HVDC converter station is critical to this supply, exporting power from the power station to the Apollo inverter station in South Africa. The scheme is capable of dispatching up to 1,920 MW through the 1,414 km high voltage direct current (HVDC) 533 kV transmission lines between these two converter stations. 

“Ageing and unreliable equipment, resulting in poor performance of the Songo station, was the biggest challenge our client needed to address,” said Dinesh Buldoo, Director: Power - Transmission and Distribution, WSP in Africa. 

Buldoo indicated that working in a live yard meant outages had to be minimised during all the project phases undertaken. “Other challenges experienced on this project included the failure of the Pole 1 DC smoothing reactor during the procurement phase, which emphasised the urgency of replacing this equipment, and these pressures were further compounded by needing to negotiate with the contractor to expedite the replacement air-core DC smoothing reactor for Pole 1. And, unexpected ground conditions and rehabilitation works of the old converter transformers also contributed to making this a challenging project."

Phase 1 of the Brownfield Project consisting of two separate project packages prioritised the replacement of three single-phase converter transformers, air-core DC smoothing reactors, DC surge arresters and Direct Current Optical Current Converter Transformers (DCOCTs) as well as the design and installation of a third AC harmonic filter bank. These packages also included all associated control and protection equipment and civil works. 

Tarryn Garrun, Project Manager: Power, WSP in Africa, said “Our team prepared the Project Scope of Work, Technical Specifications and complete Bidding Enquiry Documents, assisted with contractor procurement and provided tender evaluation, an evaluation report and a recommendation of award.”

Garrun indicated that the WSP team also provided the techno-financial option analysis of “Greenfield” versus “Brownfield” rehabilitation of the HVDC converter station. “Our team conducted a feasibility study of the various rehabilitation options available and analysed the performance of the existing station to prepare a solid business case.” 

HCB selected to continue with the Brownfield option and the WSP team then provided a detailed performance analysis of the existing equipment to determine which replacements were most critical for an expedited project called Brownfield Phase 2. 

“We also developed the scope of work and technical specifications for the Brownfield Phase 3 Project, which will be undertaken once Phase 2 is completed,” added Garrun.

Phase 2 aims to improve the converter station performance in the short to medium-term, particularly of the HVDC converter transformers, and includes six packages. These packages cover the provision of a spare 400kV converter transformer, 220kV AC surge arresters, converter transformer refurbishment, an emergency diesel generator, a temporary workshop building and the refurbishment of the HVDC valve base electronics.

Despite Mozambique’s enormous potential for power generation, currently only 34% of its population has access to electricity. Added to this, Mozambique’s neighbours are also reliant on the Cahora Bassa system for power, making it clear that these projects at Songo are critical to enabling economies in Southern Africa. 

“Affordable and reliable access to electricity drives critical industries such as manufacturing, the provision of potable water, better healthcare, and education to name a few. Our team has been able to improve the Songo station’s performance in the medium-term, enabling HCB to evacuate the full output of Cahora Bassa more reliably - and thereby securing the system’s contribution to socioeconomic inclusivity and prosperity for Southern Africa. We were able to provide our client with a least-cost yet technically superior solution and enable the upgraded converter to transfer higher levels of power in the future,” concluded Buldoo.