Pumped hydropower is gaining importance as a key technology for the integration of large quantities of renewable electricity, in particular from wind and solar sources. With the progress in power electronics, it has recently become possible to build frequency converters with a rated power in excess of 100 MVA, paving the way for a new variable speed pumped hydropower solution based on synchronous machines whose stator is driven with a variable frequency. This solution, refered to as converter fed synchronous machine (CFSM), is offering even higher flexibility and efficiency than DFIM (doubly-fed induction machine). A first CFSM plant with a rated power of 100 MVA has been installed in Switzerland and has been in productive operation since late spring 2013. As a further step a new CFSM power converter type based on modular multi-level converter (MMC) technology is proposed. This topology, powered by advanced IGCT (integrated gatecommutated thyristor) power semiconductor devices, enables a total power conversion efficiency greater than 98.5%. Very smooth waveforms and special starting algorithms make it possible to use standard generator and transformer insulation schemes. Due to a modular converter construction, the accessible range of rated power spans from approximately 50 MVA up to 500 MVA. Built-in redundancy enables a very high availability.