World's First Fully Rated Direct ac/ac MMC for Variable-Speed Pumped-Storage Hydropower Plants

This article presents in-depth insights about the development and successful introduction in the field of the first direct ac/ac modular multilevel converter (MMC) as static frequency converter (SFC) for a converter-fed synchronous machine (CFSM) variable-speed pumped-storage hydropower plants (PSHPs). Thanks to a fully rated SFC, the grid dynamics are completely decoupled from the machine dynamics, enabling a straightforward variable-speed operation with high torque over the entire speed range, starting from standstill. Over the whole speed range, the synchronous machine (SM) can be controlled at unity power factor, resulting in a smaller machine compared to a fixed-speed PSHP. The pump start without water blow-down in the turbine chamber means faster and simpler mode transition between turbine and pump operation. Thanks to the wide variable-speed range, the hydraulic efficiency of the turbine can be finely optimized. This results in increased partial-load efficiency and never seen cycle efficiency numbers. The possibility to provide balance energy also in pump operation offers great advantages to the plant's owner. A comprehensive measurement set acquired during the commissioning of the Malta Oberstufe plant in Austria asserts the suitability of a direct ac/ac MMC as SFC for a CFSM variable-speed PSHP.