Highly Flexible Indirect Modular Multilevel Converter for High Power Pumped Hydro Storage Plants

As fossil-fuel- and nuclear-based generating facilities are being phased-out in favor of intermittent renewable energy sources, the need for large scale, highly flexible balancing storage is imminent. By the generating capacity of readily available units, enormous potential lays in retrofitting of large pumped hydro storage plants to variable speed operation, increasing their flexibility to meet future power system demands. Indirect Modular Multilevel Converter is a highly scalable and inherently redundant topology that meets the power and voltage ratings of large pumped hydro generating units. Variable DC link converter operation, offering common-mode-free machine-side operation has been evaluated, considering constraints in retrofit of old machines, not designed for converter-fed operation. Performance evaluation over the entire operating range of the plant is performed. Grid support dynamics has been assessed. Finally, application-specific challenges in the design stage are pointed out.