Central Controller for Photovoltaic Power Plants With Hybrid Energy Storage Systems Providing Automatic Frequency Restoration Reserve

Large-scale photovoltaic (PV) power plants have been increasingly used in ancillary service markets, including services with very different time scales. These plants require additional flexibility to provide different services simultaneously and the solution is often sought in application of hybrid energy storage systems (HESSs). In HESSs, the power demand is shared between the storage units to maximise their performance and lifetime. However, this requires a detailed model of all storage elements that must be taken into account by the central controller of the PV plant. This aspect has not been yet studied in detail in the literature. In this paper, a central controller is proposed for a PV power plant with a HESS. This controller allows the PV plant to participate simultaneously in the day-ahead and the secondary reserve markets, providing both energy and power reserve services. First, the assets are scheduled in an optimal way, considering a horizon of 24 hours and taking into consideration weather forecasts and the expected balancing energy requests. Then, a short-term rolling-horizon optimisation with a time resolution of 30 minutes is used to consider the real operating conditions of the PV power plant and state of the storage units. The efficiency of each of the storage elements is modelled in detail for the short-term optimisation. The proposed controller is applied to a test case consisting of a 20 MW PV power plant with a HESS based on a lithium-ion battery and a redox-flow battery, each with 5 MW power and 10 MWh capacity. The main contributions are verified by using numerical simulations performed in MATLAB/Simulink for the electrical system and Gurobi for the optimisation procedures.