  The electric power system is currently experiencing radical changes stemming
from the increasing share of renewable energy resources and the consequent
decommissioning of conventional power plants based on synchronous generators.
Since the principal providers of ancillary services are being phased out, new
flexibility and reserve providers are needed. The proliferation of Distributed
Energy Resources (DERs) in modern distribution networks has opened new
possibilities for distribution system operators, enabling them to fill the
market gap by harnessing the DER flexibility. This paper introduces a novel
centralized MPC-based controller that enables the concurrent provision of
voltage support, primary and secondary frequency control by adjusting the
setpoints of a heterogeneous group of DERs in active distribution grids. The
input-multirate control framework is used to accommodate the distinct
timescales and provision requirements of each ancillary service and to ensure
that the available resources are properly allocated. Furthermore, an efficient
way for incorporating network constraints in the formulation is proposed, where
network decomposition is applied to a linear power flow formulation together
with network reduction. In addition, different timescale dynamics of the
employed DERs and their capability curves are included. The performance of the
proposed controller is evaluated on several case studies via dynamic
simulations of the IEEE 33-bus system.
