Siemens overhauls 15 converter transformers at Cahora Bassa HVDC link in Mozambique

6 September 2017

Siemens is modernizing 15 converter transformers at the Cahora Bassa dam in Mozambique for the operator Hidroeléctrica de Cahora Bassa (HCB). The transformers form part of the HVDC transmission link built in the 1970s to transmit electrical power from the hydropower plant over a distance of 1,450 kilometers. The order volume is about €26 million, and the project as scheduled by Siemens will take three years.

The HVDC transformers manufactured in the 1970s are installed in the Songo converter station. Siemens will perform a comprehensive service retrofit of the transformers, which are of various voltage series and construction types. This includes maintenance and induced repair of the tap changer and replacing the bushings and several winding sets. For the overhaul, the transformers will be brought to the assembly hall that was built in the converter station when construction began on the HVDC transmission link. Although the hall doesn't offer the same installation conditions of a transformer factory, it has a crane with the necessary lifting capacity and it provides a clean work environment. After the overhaul, the Siemens specialists from Transformer Lifecycle Management will test the modernized transformers with a mobile high-voltage testing facility and hand them over to HCB for operation.

"Projects like this illustrate the increasing importance of service over the entire lifecycle, and they also show that customers today need more than just reactive service. It highlights the fact that we not only understand the traditional maintenance business but we also support our customers with practical and sometimes also unconventional service solutions," said Volker Mailänder, Head of the Customer Services Segment at the Siemens Energy Management Division.

The HVDC transmission link has a capacity of 1,920 megawatts (MW) and is crucial to leverage Mozambique's position as one of the major electrical power exporters in the region, namely to South Africa's power system. It's been in operation since 1997 following an interruption of many years during the civil war in Mozambique, and has been operating at full capacity since 1998. During the war, numerous high-voltage towers along the long-distance transmission line were destroyed. The HVDC link was therefore unusable for some years, and the transformers have seen limited operational service since 1974. However, the windings have been subjected to the same stress they would normally have been exposed to after 30 years of use. This was caused by the large number of faults-to-ground and short circuits that occurred during the initial operating phase as a result of unscheduled switching operations. The retrofit was essential in order to enable the transformers to continue operating reliably.