Analysis of alternative frequency control schemes for increasing renewable energy penetration in El Hierro Island power system

El Hierro, island declared as a biosphere reserve by the UNESCO in 2000, aims to become self-sufficient in energy and 100% free of greenhouse gas emissions. This isolated power system consists of diesel units and a hybrid Wind- Pump Storage Hydropower Plant (W-PSHP), equipped with Variable Speed Wind Turbines (VSWTs), Pelton turbines and a pump station with both fixed- and variable-speed pumps. During last years the annual average renewable energy participation is increasing, especially due to the improvements in the frequency control strategies in PSHP including the operation in short circuit mode. This performance involves an important reduction of the system efficiency but allows PSHP to regulate frequency deviations when available wind power is higher than power demand and the Diesel units are disabled. In this paper different alternative frequency control schemes are proposed so that Pelton units support to the frequency control can be substituted, avoiding energy losses owing to short-circuit operation. This way renewable energy participation would be increased. The control schemes are developed using pumping station regulation capacity, the proper kinetic energy of the VSWTs rotors and a new Flywheel Energy Storage System connected to the grid by means of power electronics. Nine different control cases have been presented, including hydraulic short circuit operating mode. Different simulations have been carried out and they confirm that proposed control schemes fulfil the initial research objectives and enable to improve the global energy efficiency of the system.