  Harnessing flexibility from distributed energy resources (DER) to participate
in various markets while accounting for relevant technical and commercial
constraints is essential for the development of low-carbon grids. However,
there is no clear definition or even description of the salient features of
aggregated DER flexibility, including its steady-state and dynamic features and
how these are impacted by network constraints and market requirements. This
paper proposes a comprehensive DER flexibility modelling and characterisation
framework that is based on the concept of nodal operating envelope (NOE). In
particular, capacity, ramp, duration and cost are identified as key flexibility
metrics and associated with different but consistent NOEs describing
capability, feasibility, ramp, duration, economic, technical and commercial
flexibility features. These NOEs, which conceptually arise from a Venn diagram,
can be built via optimal power flow (OPF) analysis, visualised in the
active-reactive power space, and used by different stakeholders. Results from a
canonical test system and a real distribution system illustrate the value and
applicability of the proposed framework to model and characterise provision of
flexibility and market services from DER for different use cases.
