  Traditionally, inertia in power systems has been determined by considering
all the rotating masses directly connected to the grid. During the last decade,
the integration of renewable energy sources, mainly photovoltaic installations
and wind power plants, has led to a significant dynamic characteristic change
in power systems. This change is mainly due to the fact that most renewables
have power electronics at the grid interface. The overall impact on stability
and reliability analysis of power systems is very significant. The power
systems become more dynamic and require a new set of strategies modifying
traditional generation control algorithms. Indeed, renewable generation units
are decoupled from the grid by electronic converters, decreasing the overall
inertia of the grid. 'Hidden inertia', 'synthetic inertia' or 'virtual inertia'
are terms currently used to represent artificial inertia created by converter
control of the renewable sources. Alternative spinning reserves are then needed
in the new power system with high penetration renewables, where the lack of
rotating masses directly connected to the grid must be emulated to maintain an
acceptable power system reliability. This paper reviews the inertia concept in
terms of values and their evolution in the last decades, as well as the damping
factor values. A comparison of the rotational grid inertia for traditional and
current averaged generation mix scenarios is also carried out. In addition, an
extensive discussion on wind and photovoltaic power plants and their
contributions to inertia in terms of frequency control strategies is included
in the paper.
