  The increasing penetration of renewable energy sources has been leading to
the progressive phase-out of synchronous generators, which constitute the main
source of frequency stability for electric power systems. In the light of these
changes, over the past years, some power systems started to exhibit an odd
frequency distribution characterised by a bimodal behavior. This results in
increased wear and tear of turbine governors and, in general, in degraded
frequency performances. This is a cause of concern for grid operators, which
have become increasingly interested in understanding the factors shaping
frequency distribution. This paper explores the root causes of unwanted
frequency distributions. The influence of some main aggregate system parameters
on frequency distribution is detailed. The paper also shows that the
implementation of the so-called synthetic inertia can lead to a robust unimodal
frequency distribution.
