  Dispatching a large fleet of distributed energy resources (DERs) in response
to wholesale energy market or regional grid signals requires solving a
challenging disaggregation problem when the DERs are located within a
distribution network. This manuscript presents a computationally tractable
convex inner approximation for the optimal power flow (OPF) problem that
characterizes a feeders aggregate DERs hosting capacity and enables a realtime,
grid-aware dispatch of DERs for radial distribution networks. The inner
approximation is derived by considering convex envelopes on the nonlinear terms
in the AC power flow equations. The resulting convex formulation is then used
to derive provable nodal injection limits, such that any combination of DER
dispatches within their respective nodal limits is guaranteed to be AC
admissible. These nodal injection limits are then used to construct a realtime,
open-loop control policy for dispatching DERs at each location in the network
to collectively deliver grid services. The IEEE-37 distribution network is used
to validate the technical results and highlight various use-cases.
