Nonlinear model predictive control of variable speed hydropower for provision of fast frequency reserves

This paper presents the development of a nonlinear model predictive controller (MPC) for controlling variable speed hydropower (VSHP) plants. The MPC coordinates the turbine controller with the virtual synchronous generator (VSG) control of the power electronics converter to optimize the plant's performance. The main objective is to deliver a fast power response to frequency deviations by utilizing the kinetic energy of the turbine and generator. This is made possible by allowing the turbine rotational speed to deviate temporarily from its optimal speed. In addition, the efficiency should be maximized while keeping the electric and hydraulic variables within their constraints. The simulation results show that the proposed MPC is also able to damp power oscillations in the grid, reduce water hammering in the penstock and improve the estimation of turbine head, turbine power and turbine flow. This ensures that the turbine head does not exceed its limits and that the overshoot in the turbine speed after a disturbance is reduced. Besides, the VSG converter control enables a fast power response by utilizing the rotational energy of the turbine and generator. Thereby, the VSHP can provide a significant amount of fast frequency reserves (FFR) to the grid.