Hybrid frequency control strategies based on hydro-power, wind, and energy storage systems: Application to 100% renewable scenarios

Over the last two decades, variable-speed wind turbines (VSWTs) have gradually replaced conventional generation. However, the variable and stochastic nature of wind speed may lead to large frequency deviations, especially in isolated power systems with high wind energy integration, where this integration causes a lack of inertia. This paper proposes a hybrid hydro-wind-flywheel frequency control strategy for isolated power systems with 100% renewable energy generation, considering both variable wind and a generator tripping. VSWTs and flywheels include a conventional inertial frequency control. The frequency control strategy involves VSWTs rotational speed and State of Charge (SOC) of flywheels variations that may affect the wear and tear of mechanical elements and reduce the efficiency of the frequency control action. The hydro-power controller also tracks the VSWTs' rotational speed deviation and the flywheel SOC to modify the generated power accordingly. This hybrid frequency strategy significantly reduces frequency excursions, the rotational speed deviations of VSWTs and the SOC of flywheels. To reduce the hydro-power plants' wear, an additional control strategy is proposed by the authors and evaluated. Results from a case study based on an isolated power system located in El Hierro (Canary Islands, Spain) are included, and extensively discussed in the paper.