Transient study of a variable-speed Pump-as-Turbine storage unit to characterise its ability to provide ancillary services

The evolution of the electricity generation landscape towards distributed and intermittent generation leads to new challenges for the grid management. Consequently, hydraulic pumped-storage power plants are most likely to be an important stake for the future. These units indeed present high energy storage and load-balancing capabilities. The development of power electronics allows to build control strategies for variable-speed hydro-power stations and enables them to compensate for fast power fluctuations in the grid. Nevertheless, these installations are usually expensive and on-site custom manufactured. Previous studies have shown that the cheap and readily available pumps can also work in turbine mode (Pump-as-Turbines). In the present master thesis, a model of a variable-speed PaT storage unit is suggested in order to study its ability to provide ancillary services to the grid operator. We start by establishing a review of primary frequency and voltage control reserves. Then, the complete installation model is developed as a general case, as well as its control strategy. Finally, the model is applied to the existing hydro-power plant of Negundo and tested regarding ancillary services provision and fault ride-through capacity. The results show that both primary frequency and voltage controls work in pump and turbine operations. Two different control strategies however needed to be implemented, one for each operating mode. Indeed, a direct power control via the electromagnetic torque, as achieved when pumping, is unstable in a part of the turbining working range. This instability is identified and demonstrated. The unstable behaviour is overcome by controlling the power via a speed regulator. Last of all, the simulation demonstrates the robustness of the plant to three-phase voltage dips in case of the application of a specific control strategy