  Nowadays, power system inertia is changing as a consequence of replacing
conventional units by renewable energy sources, mainly wind and PV power
plants. This fact affects significantly the grid frequency response under power
imbalances. As a result, new frequency control strategies for renewable plants
are being developed to emulate the behaviour of conventional power plants under
such contingencies. These approaches are usually called 'virtual inertia
emulation techniques'. In this paper, an analysis of power system inertia
estimation from frequency excursions is carried out by considering different
inertia estimation methodologies, discussing the applicability and coherence of
these methodologies under the new supply-side circumstances. The modelled power
system involves conventional units and wind power plants, including wind
frequency control strategies in line with current mix generation scenarios.
Results show that all methodologies considered provide an accurate result to
estimate the equivalent inertia based on rotational generation units directly
connected to the grid. However, significant discrepancies are found when
frequency control strategies are included in wind power plants decoupled from
the grid. In this way, authors consider that it is necessary to define
alternative inertia estimation methodologies by including virtual inertia
emulation. Extensive discussion and results are also provided in this study.
