PhD thesis defense to be held on September 22, 2023, at 11:00 (e-presence)

Picture Credit: Apostolos Papakonstantinou

Thesis title: Control of Autonomous Electrical Systems with High Penetration of Renewable Energy Sources and Electricity Storage

Abstract: This dissertation investigates frequency control in autonomous power systems operating under high renewable energy sources (RES) share. Power regulation challenges faced in such grids, that are examined in this dissertation, include their low inertia, the need for reserve provision by stochastic RES, batteries with energy reserves limitations, pumped-hydro storage stations with limited ramping capabilities, as well as system operation supplied entirely by inverter-based resources. The investigation presented addresses two main topics, fast reserves provision and secondary frequency regulation.
As regards fast reserves, a control strategy is introduced for a photovoltaic (PV) generator, allowing operation at a specified upward reserve level, along with primary frequency regulation and inertia emulation. Simulation results demonstrate the effectiveness of the proposed method in supporting an island grid subjected to major active power equilibrium disturbances. In the absence of storage within the PV station, underfrequency support can only be provided when the station operates in curtailed output mode, depending on system conditions.
The operation of autonomous systems under 100% inverter-based resources (batteries and stochastic RES) is also examined. In such conditions, the operation of some converters in grid forming mode is required. Simulation results from a non-interconnected island grid show that applying a droop-based grid-forming control scheme in battery converters achieves effective sharing of fast reserves amongst participating units, including PVs adopting the proposed control method.
In addition, a comparative assessment is presented of solutions allowing the provision of primary reserves by hybrid stations, consisting of pumped-hydro storage and wind turbines, operating in island grids in the absence of thermal units (100% RES share). Assets utilized for reserve provision include Pelton hydro turbines, regulated via their water jet deflectors, variable speed pumps, wind turbines and batteries. Two novel control methods are introduced; the 1st utilizes the hydro units’ deflectors for fast reserve provision, while in the 2nd method batteries support the pumping facility of the hybrid station in tracking wind power variations. Simulation results demonstrate that the proposed deflector control supports the active power equilibrium, albeit at the detriment of energy efficiency. Battery storage is the most effective solution, especially when implementing the proposed wind farm – pumping station – battery storage coordinating control scheme.
A novel three-level power control method is also introduced, allowing the effective participation of batteries in secondary reserves provision, in parallel with thermal units. The 1st control layer is implemented locally, at individual battery station level, ensuring operation within acceptable state of charge limits, while also providing primary reserves and inertia emulation. The 2nd control level is a central automatic generation control system that allocates required secondary reserves, limiting the contribution of batteries whose available energy reserves are diminishing. State of charge restoration is achieved via the economic dispatch algorithm of the 3rd level of control. In addition, a secondary frequency control method for 100% RES conditions is introduced. The effectiveness of the proposed control method is demonstrated via time-domain simulations of an island grid under conditions resulting in battery energy reserves depletion, while providing secondary reserves.

Supervisor: Professor Stavros Papathanasiou

PhD Student: Apostolos Papakonstantinou