Assessments of hydropower plants start-up sequences and equivalent runner damage under transient operation

With specific reference to the HPP start-up sequence, the conventional process requires to operate the runner in a particular operating regime called Speed-No-Load (SNL) that is detrimental for the runner. Thanks to last decade's improvements in MW-class power electronics, new generators' technologies have emerged providing an additional degree of freedom to HPPs control. In this respect, the control of the hydraulic turbine speed can be used to decrease the level of damage induced on the runner by choosing the appropriate trajectory for transient operations. Variable speed is a recent technology and its capability to improve the transient operation of hydraulic machines is not fully addressed in literature and, specifically, for start-up sequences. To fully take advantage of such an added degree of freedom, an in-depth experimental study has been performed at EPFL (PTMH) on a specific speed homologous reduced scale model of the unit 5 of Z'Mutt HPP equipped with a reversible pump-turbine. The paper presents a methodology to (i) investigate different transient sequences to start-up the hydraulic machine, which have been tested on reduced scale model, and to (ii) evaluate the equivalent damage impact on the runner. The results of this study highlight the advantages of leveraging the variable speed technology during the unit start-up.