  Utilizing distribution grid flexibility for ancillary services requires the
coordination and dispatch of requested active and reactive power to a large
number of distributed energy resources in underlying grid layers. This paper
presents an approach to hierarchically dispatch flexibility requests based on
Online Feedback Optimization (OFO). We implement a framework of individual
controllers coordinating actors, contributing to flexibility provision, to
track a requested operating point at the interface between grid layers. The
framework is evaluated in terms of performance during coordination and possible
interaction between individual controllers, both central and distributed.
Results show high reliability and robustness of the OFO controllers as well as
an efficient dispatch of active and reactive power. Its computational
efficiency and capabilities in set point tracking during online grid operation
are making OFO a promising approach to the flexibility dispatch problem.
