Kaplan Turbine Model Validation for Large Grid Frequency Disturbances

The grid frequency stability is decreasing in many AC grids due to less inertia in these systems, and their stability is commonly evaluated through simulation. Hydropower often delivers frequency control, yet research on hydropower model validation for large grid frequency disturbances is scarce. We performed frequency control tests on a Kaplan unit and evaluated three models (called the standard grey-box, Kaplan grey-box, and white-box models) against validation data. The grey-box model parameters were identified with grey-box system identification, whereas the white-box model used index test data to relate guide vane opening and runner blade angle to net head, flow, and efficiency. All models accounted for steering mechanism backlash. The results showed that the white-box model was overall more reliable than the two grey-box models. However, all models had difficulties in simulating the output power the first few seconds after a large change in frequency. Moreover, backlash sizes had considerable performance impacts on all models. The uncertainties in hydropower model response during the first few seconds after a disturbance emphasize the importance of sufficient stability margins in grid frequency stability simulations.