  Operational flexibility is an important property of electric power systems
and plays a crucial role for the transition of today's power systems, many of
them based on fossil fuels, towards power systems that can efficiently
accommodate high shares of variable Renewable Energy Sources (RES). The
availability of sufficient operational flexibility in a given power system is a
necessary prerequisite for the effective grid integration of large shares of
fluctuating power in-feed from variable RES, especially wind power and
Photovoltaics (PV). This paper establishes the necessary framework for
quantifying and visualizing the technically available operational flexibility
of individual power system units and ensembles thereof. Necessary metrics for
defining power system operational flexibility, namely the power ramp-rate,
power and energy capability of generators, loads and storage devices, are
presented. The flexibility properties of different power system unit types,
e.g. load, generation and storage units that are non-controllable, curtailable
or fully controllable are qualitatively analyzed and compared to each other.
Quantitative results and flexibility visualizations are presented for intuitive
power system examples.
