On the damping of power systems during inverter-based blackstart

Grid recovery has been traditionally performed using large conventional power plants. With the massive deployment of inverter-based generation, these new units are expected to take over new responsibilities, as conventional generation that are blackstart capable are being decommissioned. Grid-forming converters, being very flexible and controllable, have the ability to contribute during grid restoration, although it is still unclear how to deal with their limited ratings and how to address nonlinear effects such as inrush currents or harmonics. In this direction, the requirements of grid-forming inverters to comply with black start capabilities are still to be defined. During the first steps of a grid restoration process the dynamics of the grid largely vary from the grid behaviour under normal operation. In particular, the grid is nearly unloaded and therefore poorly damped. Indeed, overvoltages and resonances are a common concern during the energization of long transmission lines. In this work we analyse the spectral properties of unloaded transmission lines, and describe the impact of voltage controllers on the damping of the system. As a byproduct, this analysis can be leveraged to propose high level requirements for grid-forming devices, in order to provide adequate damping during the early stages of a grid restoration process.