Variable speed pumped hydro based on modular multilevel converters: modeling, control and analysis of pumping-mode operation

Abstract

This work deals with the behavior of a Modular Multilevel Converter (MMC) driving synchronous machines in variable-speed pumped hydro storage plants. This MMC-based topology has been proposed as one capable of implementing large-scale energy storage with fast dynamics and ancillary services while being possibly realized via retrofitting of existing fixed-speed pumped hydro storage units. The pumping mode of this application presents important differences in comparison to mature MMC applications, such as High-Voltage Direct Current links. For example, the low-speed operation of pumped hydro storage units during start-up and the control of an electrically excited salient pole machine. This dissertation presents a detailed PSCAD/EMTDC model and the results of time-domain simulations of this relatively new application. Some contributions are made to the start-up procedure technique based on common-mode voltage injection. Considerations on the main circuit design and a detailed description of the control system design are presented. Furthermore, the implementation of synthetic inertia into the pumping mode operation of this large-scale energy storage technology is studied.