On the changes in dynamic behavior produced by the hydraulic turbine runner damage

Resumen

Hydropower plays a very important role in the world electricity generation nowadays. Hydropower is one type of renewable energy and is the only renewable energy source that can provide a wide range of power regulation with fast response, which is very important for the electricity grid stability. Hydraulic turbines are the key equipment of hydropower plants. The power concentration in hydraulic turbines is increasing very fast in the past years. As a consequence, heads and fluid velocities are higher, and the hydraulic excitation forces on the turbine runner increase. On the other hand, to improve the efficiency of hydraulic turbines, the thickness and weight of the runner have been decreased as much as possible, which also increases the stresses in the runner. Furthermore, the operation range of hydraulic turbines is widened in order to satisfy the end-users’ demand of larger regulation capacity. This operation at extreme off-design conditions leads to even larger forces. Due to these reasons, many fatigue failure cases have been reported in the literature. Some fatigue failure cases showed very large cracks, which also indicates the challenge of crack monitoring during operations. To monitor the cracks in hydraulic turbines, it is imperative to study the effect of a crack on the dynamic behavior of hydraulic turbines. The dynamic behavior of hydraulic turbines has been studied extensively during the past decade. However, most of these studies were focused on Francis turbines and pump turbines, and the dynamic behavior of other types of hydraulic turbines, e.g., Kaplan turbines, have still been studied limitedly. Moreover, all of these studies were conducted on runners without cracks, and the effect of a crack on the dynamic behavior of hydraulic turbines has still not been studied before. In the present thesis, the effect of a crack on the dynamic behavior of Kaplan turbines and Francis turbines has been studied in detail. The research emphasis is laid on Kaplan turbines. This is divided into two steps. First, the dynamic behavior of an intact Kaplan turbine runner is studied. Then, based on the dynamic behavior of intact turbine runners, the effect of a crack on one blade is investigated. A systematic approach has been used for study. The research start from numerical models, and then, the numerical results are validated by experiments. The studies on the numerical models are conducted step by step from simplified blade models to single blade models and continuously to whole turbine models. The knowledge obtained on Kaplan turbines is also applied to a Francis turbine runner, whose dynamic behavior was previously studied