## 1 Introduction  
Nowadays, hydropower plays an important role in the energy market since it is the only renewable source which is able to compensate for the non-constant energy production of wind or solar power [1; 2; 3]. This means that hydro units regulate their power depending on the other renewable sources' production to ensure the stability of the grid and to satisfy the energy demand. However, when hydraulic turbines regulate their power below and above their best efficiency point (BEP), they present different dynamic problems [4; 5; 6; 7] which affect the useful life of the mechanical parts of the turbine. One of the most important problems that always appear in hydraulic turbines is cavitation [8; 9]. Cavitation is the formation of vapor cavities in a liquid due to rapid changes in pressure [10]. These cavities can grow, collapsing implosively and emitting large-amplitude shock-waves [11]. This cavitation phenomenon is very important in Francis or reversible pump turbines [8], since their runners are designed to have the maximum efficiency for a given head and flow rate, hence, cavitation appears out of this design point. In Kaplan turbines, cavitation is less likely to occur because they can regulate the angle of their runner blades depending on the head and the flow rate, ensuring a high performance in the whole operating range. The cavitation phenomenon in hydraulic turbines has been studied for many years, especially in laboratory models [12; 13; 14; 15; 16]. Different types of cavitation can appear in hydraulic turbines [8; 9]. The most important are: leading edge cavitation ((1) in Figure 1), travelling bubbles cavitation ((2) in Figure 1), draft tube swirl ((3) in Figure 1), inter-blade vortex cavitation ((4) in Figure 1), von Karman vortex cavitation ((5) in Figure 1), and tip vortex cavitation (only in Kaplan turbines). Of all types of cavitation, the maximum erosion occurs when clouds of bubbles collapse on the runner surface (cloud cavitation). The generation of these cavitation clouds may occur naturally or it may be the response to a periodic pressure fluctuation like the RSI (rotor stator interaction) in the hydraulic turbine. The cavitation phenomenon may be different in a real prototype than in the laboratory model because it depends on the operating conditions, therefore, its detection on-site in the power plant is necessary [17; 18]. In the past, different studies [9; 17; 18; 19; 20; 21; 22; 23; 24] were published about the experimental detection of cavitation in hydraulic turbine prototypes. The idea was to detect the shockwaves generated by the collapse of the cavitating bubbles, which are transmitted through the hydraulic turbine structure, by means of analyzing high-frequency vibrations. To do so, accelerometers and acoustic emission (AE) sensors were placed in different parts of the machine. The advantages and drawbacks of every location tested are summarized in the following points:  
* Detection with accelerometers in the guide vanes [9; 17; 19; 20; 22]: It is a direct measurement but it is only useful for entrance cavitation. It is difficult to separate from erosive and non-erosive cavitation. * Detection with accelerometers and AE sensors in the turbine bearing [9; 17; 22]: Only erosive cavitation is detected, but it is an indirect measurement and the transmission characteristics from the runner to the bearing are unknown and filtered. * Detection with accelerometers in the draft tube [17; 22]: Only for draft tube swirl cavitation. It is non-representative of the erosion suffered by the runner. * Detection with accelerometers in the shaft [21]: Direct path from the runner, but more complicated measurement with the possibility of having also generator excitations.