  Distributed energy resources are an ideal candidate for the provision of
additional flexibility required by power system to support the increasing
penetration of renewable energy sources. The integrating large number of
resources in the existing market structure, particularly in the light of
providing flexibility services, is envisioned through the concept of virtual
power plant (VPP). To this end, it is crucial to establish a clear methodology
for VPP flexibility modelling. In this context, this paper first puts forward
the need to clarify the difference between feasibility and flexibility
potential of a VPP, and then propose a methodology for the evaluation of
relevant operating regions. Similar concepts can also be used to modelling
TSO/DSO interface operation. Several case studies are designed to reflect the
distinct information conveyed by feasibility and flexibility operating regions
in the presence of "slow" and "fast" responding resources for a VPP partaking
in provision of energy and grid support services. The results also highlight
the impact of flexible load and importantly network topology on the VPP
feasibility (FOR) and flexibility (FXOR) operating regions.
