Improving frequency control from Kaplan turbines to fulfill grid codes

New European [1], [2] and Nordic [3], [4] grid codes raise the requirements on frequency control, making it harder for hydropower plants to qualify for frequency control services. Field tests on a Swedish Kaplan unit showed how frequency containment reserve requirements can be difficult to fulfill for Kaplan units in particular. Can faster steering of the runner blades alone improve the power response sufficiently and help Kaplan units fulfill the frequency control requirements? We implemented various runner control improvement measures in a numerical turbine governor model to evaluate if faster runner control could help a Kaplan unit fulfill frequency control grid codes. We used field test data from a Swedish Kaplan unit and an existing Kaplan turbine model [5] to validate the governor model and to assess how the improvement measures would impact frequency control performance. The first improvement measure focused on improvements that can be achieved by only reprogramming the runner servo control loop, whereas the second test case also assumed improved servo characteristics. The final test case examined how the frequency control is improved if the Kaplan unit is always on-cam. Finally, we tested to what extent more system inertia can ease the Nordic frequency control stability requirements.