  A widespread approach to characterise the aggregated flexibility of active
distribution networks (ADNs) is to estimate the boundary of the feasible
network operating areas using convex polygons in the P-Q space. However, such
approximations can be inaccurate under realistic conditions where, for example,
the nonlinear nature of the network is captured, and the behaviour of flexible
units is constrained. This letter demonstrates, using a small ADN example with
four flexible units and considering only nonlinearities from the network, that
reaching the full P-Q flexibility areas would require perfect coordination of
units and high ramping rates. Without these requirements, the P-Q areas become
nonconvex. Thus, if the effects of nonlinearities and nonconvexities are
ignored, existing approaches in the literature can result in overestimation of
ADN flexibility and give rise to impractical solutions, hampering coordination
between transmission and distribution system operators.
