Hydro-electricity plays an important role in the safe, stable and efficient operation of electric power systems. Frequency stability of power systems refers to the ability to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load. In order to suppress power grid frequency fluctuations, generating units change their power output automatically according to the change of grid frequency, to make the active power balanced again. This is the primary frequency control (PFC). PFC of electrical power grids is commonly performed by units in hydropower plants (HPPs), because of the great rapidity and amplitude of their power regulation.
A hydropower generation system is a complex nonlinear power system including hydraulic, mechanical, electrical and magnetic subsystems. Nowadays, the size of HPPs and the structure complexity of systems have been increasing, especially in China. The proportion of electricity generated by intermittent renewable energy sources have also been growing. Therefore, the performance of HPPs in terms of frequency control is more and more important. The research on control strategies and dynamic processes of HPPs is of great importance. The frequency stability of hydropower units is a critical factor of power system security and power quality. The power response time for evaluating the frequency regulation quality, is also a key indicator.
In recent years, there is a tendency that the new turbines experience fatigue to a greater extent than what seem to be the case for new runners decades ago, due to more regulation movements caused by increasingly more integration of intermittent renewable energy sources. In some countries, as in Sweden, PFC is a service that the transmission system operator buys from the power producers. In other countries, as in Norway and China, there is also an obligation for the producers to deliver this service, free of charge. However, there are costs related to this, e.g. due to design constraints and auxiliary equipment when purchasing a new unit or system, and due to wear and tear which affects the expected life time and maintenance intervals. Hence the specific research on wear and tear of hydro units due to PFC is exceedingly necessary.