Water hammer influences the life cycle of hydraulic passages and may even cause catastrophic structural failures. Several catastrophic failures of hydraulic power units have been reported in the literature due to the effects of transient regimes. The objective of the study is to highlight the global trend in water hammer assessment and to quantify the effect of factors influencing overpressure in hydraulic passages during load rejection in different hydropower plants. A brief and concise literature review is conducted to document the parameters associated with the water hammer phenomenon and to thereby identify the necessary prerequisites to validate theoretical and numerical results against experimental data. The purpose of the analysis is to identify extreme transient loads on hydraulic passages in order to properly adapt hydropower unit operation, to make recommendations for design and industry, and to guide the progress of adapted models and numerical simulations to capture complex phenomena. Empirical correlations are determined based on the experimental data that are transferable from one unit to another, even if a deep flow analysis is performed. The experimental results confirm that the rapid closure rate of the guide vanes has a significant impact on the phenomenon. A third order polynomial equation is applied to capture the general overpressure trends. Equation parameters change from case to case depending on the type of hydraulic power unit, closing rate and the type of hydraulic passage. The results confirm also that overpressure values depend significantly on other factors, some of which are not usually taken into account (e.g., runner speed). Experimental correlations make it possible to understand the water hammer phenomenon, which could help not just assessing and optimizing loads, but also verifying and validating more complex physical models, to ensure that hydraulic passages are reliable. A well-documented analysis also makes it possible to optimize equipment design, improve and adapt maintenance programs and to recommend appropriate operating parameters to increase equipment lifespan, while preventing incidents.

In the dynamic landscape of renewable energy, the primary goal continues to be the enhancement of competitiveness through the implementation of cutting-edge technologies. This requires a strategic focus on reducing energy costs and maximizing system performance. Within this framework, the continuous online monitoring of assets is essential for efficient operations, by conducting measurements that describe the condition of various components. However, the execution of these measurements can present technical and economic obstacles. To overcome these challenges, the implementation of indirect measurement techniques emerges as a viable solution. By leveraging measurements obtained in easily accessible areas, these methods enable the estimation of quantities in regions that would otherwise be inaccessible. This approach improves the monitoring process’s efficiency and provides previously unattainable information. Adopting indirect measurement techniques is also cost-effective, allowing the replacement of expensive sensors with existing infrastructure, which cuts down on installation costs and labor. This paper offers a detailed state-of-the-art review by providing an in-depth examination and classification of indirect measurement techniques and virtual sensing methods applied in the field of renewable energies. It also identifies and discusses the existing challenges and limitations within this topic and explores potential future developments.