Resumen de Study of the dynamic behavior of pelton turbines
The future of hydropower is tied to the rapid increase of new renewable energies, such as photovoltaic and wind energy. With the growing share of intermittent electricity production, the operation of hydropower installations must be more flexible in order to guarantee the balance between supply and demand. As a result, turbines must increase their operating range and undergo more starts and stops, what leads to a faster deterioration of the turbine components, especially the runner. In the current scenario, condition monitoring constitutes an essential procedure to assess the state of the turbines while in operation and can help preventing major damage.
Pelton turbines are used in locations with high heads and low discharges. The runner is composed by a disk with several attached buckets, which periodically receive the impact of high speed water jets. Buckets must thus endure large tangential stresses that can lead to fatigue problems and, in case the natural modes of the runner are excited, this problem can be severely aggravated. Therefore, a deep comprehension of the modal behavior and dynamics of Pelton turbines is required in order to keep track of the runner condition with monitoring systems.
In this thesis, the dynamic behavior of Pelton turbines during different operating conditions has been studied in detail and the knowledge acquired has been used to upgrade the present condition monitoring. The first part of the document comprises the study of the modal behavior of Pelton turbines. A systematic approach has been followed with such purpose; first a single bucket has been analyzed, second the runner and then the whole turbine. With the help of numerical models and experimental tests the natural frequencies and mode shapes have been identified and classified. The effect of the mechanical design and the boundary conditions has also been discussed.
The second part of the thesis is focused on determining the transmission of the runner vibrations to the monitoring locations. It is proved that these can be detected from the bearings and that the transmission depends on the mode type.
In the third and last part, the analysis of Pelton turbines in operation is carried out. Two different machines have been studied during start-up and under different load conditions to determine which modes are excited, how the frequencies change in operation with respect to the still machine and how they are detected from different positions. The spectrum frequency bands corresponding to the runner modes and the overall vibration levels have been analyzed. Finally, the information obtained has been used to propose an upgrade of the current practice in condition monitoring. A case of damage has been analyzed with a numerical model and with historic data to illustrate the strategy.
