  The massive deployment of inverter-based generation poses several challenges
to system operators but also offers new opportunities. In the context of grid
recovery, inverter-based generation is expected to take over new
responsibilities, as conventional generation that are blackstart capable are
being decommissioned. Inverter-based facilities, being very flexible and
controllable, have the ability to contribute during grid restoration, although
it is still unclear how to deal with the limited ratings of batteries, PV, etc.
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.
