The increasing penetration of variable renewable energy sources in the Nordic Power System is causing frequency quality degradation and has increased the importance of primary frequency control provided by hydropower plants. Hydropower is the world’s largest renewable energy source. Its reliability, controllability and dispatchability along with its large storage volume makes it the most important source for providing frequency regulation in the Nordic Power System. Many hydropower plants offering regulating power have Kaplan turbines which have complex mechanical systems. Furthermore, the frequent and fast mechanical movements of the Kaplan turbines, providing frequency regulation causes the problem of wear and tear in the guide vanes and runner blades of the turbines. Kaplan turbines are suitable for stable operation.   To mitigate this problem, a solution of hybrid hydropower plants combined with  battery energy storage systems is investigated in this thesis, where batteries can take care of fast frequency deviations, allowing for a more stable operation of the turbines. The analysis is based on the FCR-N service offered by hydropower plants, because FCR-N is identified as one of the services that requires very fast changes in the output power of the hydropower plant. Modelling and simulation, data analysis and on-site measurement are adopted as main study methods in this thesis.  The simulation models of a hydropower plant and a hybrid hydropower plant are developed for the analysis. The simulation model of the hydropower plant is validated using data from a typical Swedish hydropower plant. Quantification of wear and tear is the main focus of the study. The performance of the hydropower plant and hybrid hydropower plant are compared in terms of wear and tear of turbines, speed of the response of plants to frequency deviations and number of directional changes during the mechanical movements of the turbine. Finally, it is concluded that, addition of batteries with hydropower plants will reduce wear and tear of the turbines as well as improve the frequency quality in the Nordic Power System.