Variable Speed Hydropower for Provision of Fast Frequency Reserves in the Nordic Grid

The main purpose of this article is to demonstrate the grid support capability of variable speed hydropower (VSHP). The hypothesis is that active power control of VSHP plants can improve both the frequency stability and the transient rotor angle stability by contributing to virtual inertia (VI), fast frequency reserves and power oscillation damping. Fast active power control is made possible by power electronic converters obtaining the energy from the rotating masses of the turbine and generator while the rotational speed is restored by control of the guide vane opening of the hydro turbine. A detailed dynamic simulation model is developed to simulate the VSHP within the Nordic 44-bus grid model. The VSHP controller includes a moving horizon estimator (MHE), model predictive control (MPC) with inertia emulation realised by the frequency converter of the VSHP plant. The results show that VSHP can contribute significantly to reduce the maximum frequency deviation following a sudden loss of generation. In addition, the amplitudes of power oscillations are reduced and the critical fault clearing times are generally improved. The VSHP should, therefore, be regarded as one of the technically viable options for provision of fast balancing system services.