Increasing grid flexibility – Implementation and testing of improved start and mode change procedures in a PSPP with double-fed induction machines and reversible pump-turbines

Abstract Besides being a carbon-free and highly efficient energy storage technology, pumped-storage power plants (PSPP) offer operational flexibility which in turn eases the integration of volatile energy sources such as wind and solar energy into the electric grid. The flexibility to bring the power unit quickly to generation mode and thus to inject active power to the transmission grid involves various process transitions. In specific, this paper describes the implementation and respective site tests of the following mode changes: a) from standstill to generation mode by simultaneously accelerating the power unit to synchronization speed with the turbine and the voltage source inverter of a double-fed induction machine and b) from pump mode to generation mode. This work was part of the activities of the EU-funded H2020 project XFLEX HYDRO. The demonstration plant is the Frades 2 PSPP in Portugal, which is equipped with two reversible Francis-type pump turbines, each coupled to a 420 MVA double-fed induction machine. Based on recorded process values and additional measurements such as a turbine runner strain gauge measurement and shaft torque measurement, a detailed comparison of the different mode changes in terms of mechanical stress and vibrational behaviour was conducted. To a large extend, the obtained results and the applied methodology are valid as well for reversible pump-turbine sets with synchronous generators and start-up inverter technology. The outlook comprises a discussion about further improvements for the transition from pump mode to generation mode specifically for double-fed induction machines.