Combined Hydro-Wind Frequency Control Scheme: Modal Analysis and Isolated Power System Case Example

Wind speed fluctuations and the lack of Variable Speed Wind Turbines (VSWTs) synchronous inertia make difficult grid frequency control, mainly in isolated power systems with high penetration of wind energy. To solve this drawback, a combined wind-hydro frequency control scheme is proposed and evaluated. The VSWT rotational speed variations are included as an additional input of the hydro-power governor units, determining the hydro-power frequency control response in accordance with such VSWT rotational variations and grid frequency oscillations. This new combined control scheme is introduced in two cases: VSWTs providing or not inertial control. A modal analysis based on a linear reduced-order model is used to discuss the oscillation modes, participation factors and power system stability. An isolated power system located in El Hierro (Canary Island, Spain) is proposed as case example. Results, discussion and comparative to previous frequency control approaches based on twenty-two different schedules are included in the paper. The proposed combined frequency control scheme reduces both VSWTs rotational speed and frequency deviations, regardless VSWTs frequency contribution. Additionally, VSWTs inertial control becomes more effective under imbalances, mainly due to the minor rotational speed excursions