  Future active distribution grids (ADGs) will incorporate a plethora of
Distributed Generators (DGs) and other Distributed Energy Resources (DERs),
allowing them to provide ancillary services in grid-connected mode and, if
necessary, operate in an islanded mode to increase reliability and resilience.
In this paper, we investigate the ability of an ADG to provide frequency
control (FC) in grid-connected mode and ensure reliable islanded operation for
a pre-specified time period. First, we formulate the operation of the grid
participating in European-type FC markets as a centralized multi-period optimal
power flow problem with a rolling horizon of 24 hours. Then, we include
constraints to the grid-connected operational problem to guarantee the ability
to switch to islanded operation at every time instant. Finally, we explore the
technical and economic feasibility of offering these services on a balanced
low-voltage distribution network. The results show that the proposed scheme is
able to offer and respond to different FC products, while ensuring that there
is adequate energy capacity at every time step to satisfy critical load in the
islanded mode.
