Fatigue investigations of a converter-fed variable speed pump-turbine for flexible power generation

Climate neutrality goals envisaged by many nations on the globe imply substantial deployment of intermittent renewable energy sources in the electrical power system that increases the need for grid stabilizing elements. One key asset to respond to these emerging requirements are pumped storage hydropower plants that are capable of injection, absorption and storage of large amounts of electrical energy. New flexibility technologies aimed at facilitating the integration of renewable energy sources have recently been demonstrated throughout the XFLEX HYDRO European project. Variable speed technology for hydroelectric units is one of the options considered for flexibility enhancement. Pumped storage hydropower plants with adjustable speed are considered key for various applications in today's electrical supply networks. A 5 MW reversible pump-turbine unit is selected as one of the seven XFLEX HYDRO demonstrators to investigate a fully fed converter solution able to provide rotational speed regulation from -100% to +100%. Modern converter technologies can provide such capabilities for power scales of hundreds of MW and their particular advantage lies in the high reactivity with respect to provision and absorption of electrical energy. To ensure safety, profitability and consequently feasibility of advanced ancillary services, the potential impacts on the reliability of hydroelectric units subjected to new dynamic operating schemes must thoroughly be studied. In this context, the thesis focus is set on the assessments of structural degradation related to flexible power generation backed by full-size converter solutions. Numerical and experimental methodologies are developed to harvest relevant quantities from the pump-turbine demonstrator to perform runner fatigue calculations. One-way coupled fluid-structure interaction simulations are carried out for the entire turbine operating range reachable by the full-size converter during start-up and partial load. Thereby, sophisticated methodologies like critical plane analyses and new stress extrapolation methods are considered to open new perspectives for the understanding of fatigue mechanisms in hydraulic runners. Extensive instrumentation deployed on the pump-turbine prototype ultimately provides the evidences regarding structural impacts in advanced grid regulation modes. The numerical fatigue models disclose that structural degradation is significantly mitigated at partial load conditions by adjusting the rotational speed and even improved with respect to the nominal conditions. In contrast, the fixed speed scenario predicts damage rates increased by several orders and important efficiency losses in the partial load regime. Moreover, various variable speed start-up protocols tested on the prototype demonstrate active power provision five times faster compared to fixed speed technology. The analyses reveal improved runner life expectancy applying these new start-up procedures. The structural damage is a