The Costabrunella Hydropower Plant (HPP), owned by Dolomiti Energia since 2008, is located in the Lagorai region, a significant natural area within the Dolomitic alpine environment.
The plant comprises two generating units:
- Unit 1 is a horizontal-axis ternary unit consisting of a two-jet Pelton turbine rated at 4.4MW, a synchronous motor-generator, and a multistage pump rated at 6MW. The pump can be mechanically coupled to the motor-generator through a dedicated clutch system (see Figure 1). Pump flow regulation is achieved by means of an axial control valve equipped with a needle-type flow control element. The upper reservoir, Costabrunella Lake, serves as the headwater basin for both turbine and pumping operation and is located approximately 750m above the powerhouse elevation. During pumping operation, water is transferred from the lower reservoir, Sorgazza Lake, which is situated approximately 160m above the powerhouse level. The ternary unit is connected to the Italian 60kV high-voltage transmission network through a dedicated transformer.
- Unit 2 is a vertical-axis generating unit composed of a five-jet Pelton turbine coupled to a synchronous generator with a rated power output of 1MW. This unit is connected to the Italian 20kV medium-voltage distribution network through a dedicated transformer.
The Costabrunella HPP was originally constructed by Società Industriale Trentina (SIT) and entered commercial operation in 1941, at a time when the Italian power system operated at a nominal frequency of 42 Hz. The national grid frequency was subsequently standardized to the current value of 50 Hz.
Over the years, both generating units, together with their automation systems and auxiliary equipment, have undergone several refurbishment and modernisation interventions. These upgrades have ensured compliance with current technical standards while maintaining reliable operation in both generating and pumping modes. In addition, the plant has been remotely monitored and controlled since 1970.
Failure of the suction-side penstock
In January 2024, following normal operation in pumping mode, an anomaly occurred that activated the plant’s automatic protection system and triggered an emergency shutdown of the storage pump. Subsequent inspection revealed damage to a section of the suction-side penstock, specifically at the bifurcation connecting the pump suction line and the inlet branch of Unit 2’s turbine.
This bifurcation had previously been replaced in 2014. Figure 2 shows the newly installed replacement component.
A detailed failure investigation concluded that the damage was caused by a localised pressure transient significantly exceeding all previously recorded pressure events at the facility. The bifurcation had originally been designed to withstand both normal operating pressures and the maximum internal overpressures experienced during approximately 85 years of plant operation. However, the pressure levels associated with this exceptional event exceeded the design envelope of the component.
Analysis of the event recordings further demonstrated that the emergency shutdown sequence of the pump did not operate according to the intended control logic, thereby contributing to the occurrence of the abnormal transient conditions.
To restore generation capability in the shortest possible time, the damaged bifurcation was removed, allowing Unit 2 to resume operation independently of the pumping system. In parallel, a new bifurcation was designed with enhanced structural margins to withstand not only conventional operating and transient loads but also the extreme pressure conditions associated with rare and exceptional events such as the one experienced in January 2024.
Hydraulic and mechanical transient analysis: comparison with the original 1939 study
The available recordings of the transient behaviour of Unit 1 operating in pumping mode did not provide sufficient information to fully explain the causes of the failure. Consequently, a complete numerical model of the hydropower plant was developed and implemented using a modern transient simulation tool in order to accurately predict the hydraulic and mechanical response of the system during an emergency shutdown of the storage pump.
The historical archives of the plant provided valuable reference material for the validation of the new model, including the original transient analysis carried out in 1939 by the Italian-Swiss manufacturer of the turbine and storage pump, De Pretto Escher Wyss, now part of Andritz Hydro (Figure 3). The original study contains detailed predictions of pressure and rotational speed transients during pump shutdown events, although the surge shaft located upstream near Costabrunella Lake was not included in the calculations.
The new investigation considered the complete hydraulic system, including the surge shaft, the entire waterway system, the hydraulic characteristics of the generating unit, and the operating characteristics of the pump axial valve. Emergency shutdown scenarios of the storage pump were simulated for both minimum and maximum gross head conditions. To ensure consistency with the historical analysis, the simulations adopted the same rotating inertia value used in the original calculations, i.e. GD2=11300 kpm2. The results corresponding to the maximum gross head condition are presented in Figure 4.
The results obtained from the current simulations showed a strong agreement with the historical calculations. In particular, the predicted pressure peak exceeding 22 bar in the suction-side penstock was confirmed by the new model.
To mitigate the magnitude of the pressure surge in the suction penstock, a dedicated oil-actuated pressure relief valve was designed with the objective of limiting the maximum pressure to 20 bar. Furthermore, the hydraulic power unit (HPU) oil circuit responsible for actuating the axial valve was redesigned to improve system reliability and ensure correct operation during emergency shutdown sequences.
Applied approach for the safe recommissioning of the unit with risk mitigation measures
The ternary unit was originally designed according to safety standards that differ substantially from current engineering practice. Modern pumped-storage installations typically incorporate a redundant arrangement of pump isolation and protection valves, generally consisting of two main valves. In contrast, the Costabrunella unit is equipped with a single axial valve, which performs multiple functions including plant protection as well as the control of pump start-up and shutdown sequences.
The axial valve is actuated by an internal single-acting servomotor during opening, whereas closure is achieved through the action of penstock pressure. The opening and closing manoeuvres are governed by a dedicated hydraulic control system (Figure 5).
The investigation of the January 2024 incident indicated that the existing hydraulic control system associated with the axial valve may have contributed to the failure event. Consequently, it was decided to replace the original hydraulic system with a new design incorporating improved reliability, enhanced safety features, and modern control functionality.
The mechanical integrity and functionality of the axial valve components were thoroughly inspected and verified. In addition, partial operational testing of the valve was performed on site under dry conditions using external equipment specifically developed to reproduce the hydraulic forces generated by the penstock pressure during valve closure. This approach enabled a realistic simulation of the valve closing sequence without requiring operation under live hydraulic conditions.
A dedicated recommissioning procedure was subsequently developed to minimize operational risks during the return to service of the pumping unit. The procedure included detailed operating instructions for the newly installed pressure relief valve during the commissioning tests and was based on a step-by-step analysis of the historical transient recordings and of the failure event data.
Description of the new pressure relief valve
The new pressure relief valve is an independent active protection system designed to discharge water from the bottom section of the suction penstock when a predefined pressure threshold is reached, thereby protecting the system against excessive pressure peaks.
A commercially available axial valve (see Figure 6), actuated by an oil-operated servomotor, has been installed and tuned on site to achieve an opening time of less than 2 sec once the selected pressure threshold is exceeded. The valve has been sized to discharge approximately 280 l/sec at a pressure of 22 bar, with a maximum valve opening of 45mm.
In principle, the relief valve operates similarly to a conventional turbine relief valve, but it is configured to open only when the pressure reaches the activation threshold of 23 bar.
In addition, during commissioning activities, the valve can be used to reduce and mitigate overpressure conditions and any associated pressure fluctuations.
Recommissioning of the storage pump
The storage pump was successfully recommissioned in late spring 2025. Particular attention was devoted to the commissioning strategy and testing procedures, with emphasis on the tuning and verification of the hydraulic power unit (HPU) controlling the multistage pump axial valve, as well as on the calibration and validation of the newly installed pressure relief valve.
The initial commissioning tests revealed that the actual total rotating inertia of the unit was significantly higher than the value assumed in the original transient calculations performed in 1939. Consequently, the numerical model was updated using a more representative value of GD2=20600 kpm2. The revised simulations (Figure 7) showed excellent agreement with the measurements recorded during the commissioning tests (Figure 8).
The comparison between field measurements and updated simulations demonstrated that the storage pump can safely withstand emergency shutdown events without requiring activation of the pressure relief valve, even under the most severe operating conditions considered for the plant. Nevertheless, the pressure relief valve has been retained as an additional safety feature, providing increased protection against unforeseen transient events and enhancing the overall operational robustness of the installation.
In conclusion, the adopted recommissioning strategy proved fully successful, resulting in improved reliability, increased operational safety, and enhanced resilience of the Costabrunella pumped storage unit.
Author information
Paolo Caretti graduated in Mechanical Engineering from the Politecnico di Milano in 1987. In 1991, he joined Riva Hydroart (Voith Hydro Milano from 1993), where he worked in the Engineering Department as a hydraulic and mechanical engineer. In February 2016, he established his own hydro-mechanical consultancy, HMSS of Paolo Caretti, and has since been operating as an independent consultant in the field of hydropower machinery.
Mattia Roccabruna graduated in Mechatronics Engineering from the University of Trento in 2013. Between 2012 and 2018, he worked as an R&D Engineer in Renewable Energy Technologies at Fondazione Bruno Kessler in Trento. Since 2018, he has been working at Dolomiti Energia within the Electromechanics and Automation Division, with a particular focus on mechanical and hydraulic systems.
Danilo Dalmonego graduated in Industrial Engineering from the Buonarroti Institute in Trento in 1977. In 1987, he started his career in the hydropower sector with ENEL, focusing on hydroelectric power plant design, operation, and maintenance, eventually assuming full responsibility for several hydroelectric facilities. Since 2019, he has been serving as Head of the Electromechanics and Automation Division at Dolomiti Energia.
