  This paper proposes a framework to assess the flexibility of active
distribution networks (ADNs) via P-Q area segmentation, considering the
reliability of flexible units (FUs). A mixed-integer quadratically constrained
programming (MIQCP) model is formulated to analyse flexible active and reactive
power support at the interface with transmission networks, explicitly capturing
the contributions and reliability of FUs that provide flexibility services
within an ADN. The numerical simulations performed for a real 124-bus UK
distribution network demonstrate the optimal flexibility provision by different
FUs, as well as the corresponding reliability and the impact of network
reconfiguration. Distribution system operators (DSOs) can use the proposed
framework to identify critical units, select an adequate combination of
flexibility volumes, and manage its reliability.
