Modular Multilevel Converter for Variable Speed Operation of Pumped Storage Hydropower Plants

Abstract:
This paper presents the Modular Multilevel Converter topology as a potential alternative for the full-size converter fed synchronous machines in range of 100 MVA in a pumped storage power plants. The suitability of the converter has been analyzed based on the torque it can provide at standstill to drive the turbine in pump mode with water in the turbine casing. The basis of analysis is the conduction and switching losses incurred in the semiconductor devices at standstill compared to the rated operation to determine the possible output current from the converter, which consequently transforms to torque availability. The analysis also takes into account the ripple in the submodule capacitor voltages occurring at low or zero frequency operation.