**3. Computational models and experimental evaluation operational state of hydro turbines**

The main source of the most severe HPP accidents and disasters are damage and destruction of hydro turbines. Therefore, the problem assessing resource, diagnosing damage, optimizing the operating modes of hydro turbines, and timing of repair works takes a special place in ensuring HPP safety. Until recently, the hydro turbine resource received little attention, since it was assumed that the hydraulic turbines have sufficient strength for long-term safe operation. However, the statistics of failures of hydro turbines shows [6, 7] that large safety margins do not guarantee long-term safe operation of hydro turbines.

The hydro turbine resource must be justified taking into account the peculiarities of the loading modes and damage accumulation processes. With this in mind, the interest in assessment of the resource of hydro turbines is steadily growing. This is facilitated by the following circumstances [8]:


The main factors that reduce the life of hydro turbines are fatigue, corrosionfatigue and cavitation damage, degradation mechanical properties of materials, and redistribution of stress and strain fields in the most loaded local zones. Fatigue damages are caused by a complex loading spectrum of hydro turbines, containing components with different frequencies. Low-frequency loads (with a frequency below or equal to rotation frequency) are dangerous the high amplitudes that cause formation and development of cracks in the most loaded zones. High-frequency components have small amplitudes, but the number of cycles can reach 10<sup>9</sup> –1010, which ultimately also leads to the formation and development of cracks. A significant danger is represented by "start-stop" cycles, in which parasitic vortex structures, hydraulic shocks, and flow instability zones with nonoptimal flow around the blades arise. The most dangerous are the loads caused by water pressure pulsations
