Analysis of mode-switching of a contra-rotating pump-turbine based on load gradient limiting shutdown and startup sequences

Abstract The contra-rotating pump-turbine (CRPT) is investigated as a runner configuration enabling more efficient low-head pumped hydro storage (PHS). To enhance the flexibility of low-head PHS with a CRPT, it is important to examine mode-switching. This study examines two mode-switching sequences for the CRPT, one turbine-to-pump and one pump-to-turbine. The mode-switching sequences are made as a combination of shutdown and startup sequences of each mode. The goal is to avoid pump and turbine brake modes while maintaining smooth load gradients. The turbine shutdown of the turbine-to-pump mode-switching involves a multi-stage valve closure to reduce the rotational speed of the runners, followed by the pump startup based on a previously optimised sequence. The pump shutdown of the pump-to-turbine mode-switching involves a gradual valve closure and rapid speed down of the runners at the end of the sequence. This is followed by the turbine startup, where a valve opens to generate sufficient flow over the runners before increasing the rotational speed of the runners simultaneously. Both mode-switching sequences require approximately 5 s each, and the largest loads are experienced during the pump mode parts of the sequences. The analysis is carried out using transient computational fluid dynamics with the OpenFOAM open-source software.