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Water hammer in hydropower plants (HPPs) is caused by the closing or opening of the turbine unit distributors, the operation of the safety shutoff valves, as well as unwanted turbine runaway. Careful water hammer control is essential to ensure reliable operation of hydropower plants. If this is not provided, severe problems may arise in operation, damage to individual components of the system may occur or, in the worst case, accidents with human casualties may happen [1] to [5]. Modelling and analysis of extreme hydraulic transients (plant emergency shut-down) in new or refurbished HPPs are of utmost importance because, in this way, extreme values of pressures that may occur during system exploitation can be determined. Based on these values, closing and opening times of the turbine units’ distributors are devised, as well as dimensioning of the system components is done. The objective of this paper is to investigate and discuss water hammer effects in Perućica HPP, Montenegro during the entire plant emergency shutdown, i.e., simultaneous closure of all seven Pelton turbine units. The units are installed in three parallel penstocks (each of about 2 km long) that are coupled to a concrete tunnel (about 3.3 km long) with a surge tank. Previously, measurements of the first and second Pelton turbines in one penstock only have been investigated [6]. This paper presents new results at much higher Reynolds number flows (higher than 107) than previously presented (in order of 106). In the first part of the paper, mathematical tools for solving water hammer equations are presented [7] and [8]. Friction losses in the plant’s penstocks are calculated with two different models: (1) standard quasi-steady (QSF) and (2) convolutionbased unsteady friction model (CBM) [9] and [10]. Turbine speed change during emergency shut-down is calculated, taking into account dissipative torques, including the shaft-bearing friction torque and ventilation losses in the turbine housing [6]. In the second part of the paper, comparisons of numerical Water Hammer Investigation of the Shut-Down of a High-Head Hydropower Plant at Very High Reynolds Number Flows Karadžić, U. – Bergant, A. – Starinac, D. – Božović, B. Uroš Karadžić1,* – Anton Bergant2,3 – Danica Starinac4 – Boško Božović5 1University of Montenegro, Montenegro 2Litostroj Power d.o.o., Slovenia 3University of Ljubljana, Faculty of Mechanical Engineering, Slovenia 4Jaroslav Černi Institute, Serbia 5Electric Power Supply Company, Montenegro