Variable-Speed Pumped Storage Bringing Flexibility to the South African Power System

Recent developments in power electronics have brought the possibility to operate ‘pumped hydro energy storage’ (PHES) units at variable speed. This feature enables to operate hydraulic machines over wider head ranges. For a given head, the variable speed allows power variation in pumping, but leaves the power range unchanged in turbine mode. Variable-speed PHES is thus able to provide primary frequency control both in pump and turbine modes. Operation ranges are limited to prevent undesired flow phenomena. Given a head, the studied pump-turbine’s allowed power range spreads 24 and 53 % of and from the maximum allowed power at the given head. South Africa has already several PHES plants, such as Ingula 1332 MW, Palmiet 400 MW, and Drakensberg 1000 MW, all with reversible fixed-speed Francis pump-turbines. In the future, these plants could be upgraded with the variable-speed feature, thereby extending the flexibility provided to the grid