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Abstract:Hydropower plays a key role in the actual energy market due to its fast response and regulation capacity. In that way, hydraulic turbines are increasingly demanded to work at off-design conditions, where complex flow patterns and cavitation appear, especially in Francis turbines. The draft tube cavitation surge is a hydraulic phenomenon that appears in Francis turbines below and above its Best Efficiency Point (BEP). It is a low frequency phenomenon consisting of a vortex rope in the runner outlet and draft tube, which can become unstable when its frequency coincides with a natural frequency of the hydraulic circuit. At this situation, the output power can significantly swing, endangering the electrical grid stability. This study is focused on the detection of these instabilities in Francis turbines and their relationship with the output power swings. To do so, extensive experimental tests for different operating conditions have been carried out in a large prototype Francis turbine (444 MW of rated power) within the frame of the European Project Hyperbole (FP7-ENERGY-2013-1). Several sensors have been installed in the hydraulic circuit (pressure sensors in the draft tube, spiral casing, and penstock), in the rotating and static structures (vibration sensors, proximity probes, and strain gauges in the runner and in the shaft), as well as in the electrical side (output power, intensity, and voltage). Moreover, a numerical Finite Element Method (FEM) has been also used to relate the hydraulic excitation with the output power swing.

Pumped storage hydropower plants are of paramount importance for the stability of the electrical grid. They can store huge amounts of energy by pumping water from a lower to a higher reservoir (pump operation) converting the surplus of electrical energy into potential energy [1]. At peak hours or in case of emergency this potential energy is converted again into electrical energy and delivered to the grid.The machines used in these power plants are generally reversible pump-turbines (RPT). RPT are high performance machines that have to change operation from pump to turbine mode (reversing runner rotation and flow direction) in a short time. Due to their special design characteristics (large power concentration) and operating conditions, they generate large dynamic forces when in operation. In the last years, due to the massive entrance of wind power, the number of start/stop cycles of RPT has increased dramatically as well as the operation at off-design conditions subjecting the machine to large dynamic forces. The RPT rotating train and structure has to resist all these forces for a lifespan of several decades. In this new scenario, more cases of damage have been reported and a more advanced and effective vibration monitoring is necessary.Compared with a conventional hydro turbine these machines have less number of blades (6 to 9) and higher rotating speeds operating at high pressure. The main excitation phenomenon is the well-known rotor-stator interaction (RSI) [2][3][4][5]. This excitation occurs in other kind of turbomachinery [2], but is critical in RPT due to the low number of blades and to the high head (difference in altitude between the upper and the lower reservoir). In some studies [3, 6, 7], RSI is supposed to be the cause of the failures found.In this paper the vibration monitoring in RPT is analysed. The characteristics of the main excitation forces and the procedure used to select the monitoring parameters (spectral bands) are presented. An analysis of the vibration signatures measured in different machines as well as the main types of damage found after several year of monitoring are introduced and discussed. 2.Vibration based condition monitoring system Description of the monitoring systemSince 1992 several pump-turbine units have been monitored. A reversible pumpturbine unit is a vertical shaft machine with a hydraulic turbine in the bottom and an electrical generator in the top. Typical RPT have three radial bearings, one in the turbine and two in the electrical generator, as well as one axial thrust bearing. For monitoring, sensors were located in all the bearings and in other positions depending on the machine (Figure 1).The signals from accelerometers and pressure sensors were acquired as well as the signals related to the operating conditions of the machine (head, distributor opening, pump/turbine operation).In the beginning off-line monitoring using data collectors were used. In 1998 remote on-line monitoring systems were installed in many of them (Figure 2). Acquisition systems MVX from...

To accurately determine the dynamic response of a structure is of relevant interest in many engineering applications. Particularly, it is of paramount importance to determine the Frequency Response Function (FRF) for structures subjected to dynamic loads in order to avoid resonance and fatigue problems that can drastically reduce their useful life. One challenging case is the experimental determination of the FRF of submerged and confined structures, such as hydraulic turbines, which are greatly affected by dynamic problems as reported in many cases in the past. The utilization of classical and calibrated exciters such as instrumented hammers or shakers to determine the FRF in such structures can be very complex due to the confinement of the structure and because their use can disturb the boundary conditions affecting the experimental results. For such cases, Piezoelectric Patches (PZTs), which are very light, thin and small, could be a very good option. Nevertheless, the main drawback of these exciters is that the calibration as dynamic force transducers (relationship voltage/force) has not been successfully obtained in the past. Therefore, in this paper, a method to accurately determine the FRF of submerged and confined structures by using PZTs is developed and validated. The method consists of experimentally determining some characteristic parameters that define the FRF, with an uncalibrated PZT exciting the structure. These parameters, which have been experimentally determined, are then introduced in a validated numerical model of the tested structure. In this way, the FRF of the structure can be estimated with good accuracy. With respect to previous studies, where only the natural frequencies and mode shapes were considered, this paper discuss and experimentally proves the best excitation characteristic to obtain also the damping ratios and proposes a procedure to fully determine the FRF. The method proposed here has been validated for the structure vibrating in air comparing the FRF experimentally obtained with a calibrated exciter (impact Hammer) and the FRF obtained with the described method. Finally, the same methodology has been applied for the structure submerged and close to a rigid wall, where it is extremely important to not modify the boundary conditions for an accurate determination of the FRF. As experimentally shown in this paper, in such cases, the use of PZTs combined with the proposed methodology gives much more accurate estimations of the FRF than other calibrated exciters typically used for the same purpose. Therefore, the validated methodology proposed in this paper can be used to obtain the FRF of a generic submerged and confined structure, without a previous calibration of the PZT.