  Distributed energy resources (DERs) such as responsive loads and energy
storage systems are valuable resources available to grid operators for
balancing supply-demand mismatches via load coordination. However, consumer
acceptance of load coordination schemes depends on ensuring quality of service
(QoS), which embodies device-level constraints. Since each device has its own
internal energy state, the effect of QoS on the fleet can be cast as fleet-wide
energy limits within which the aggregate "state of charge" (SoC) must be
actively maintained. This requires coordination of DERs that is cognizant of
the SoC, responsive to grid conditions, and depends on fast communication
networks. To that effect, this paper presents a novel real-time grid-and-DER
co-simulation platform for validating advanced DER coordination schemes and
characterizing the capability of such a DER fleet. In particular, we present
how the co-simulation platform is suitable for: i) testing real-time
performance of a large fleet of DERs in delivering advanced grid services,
including frequency regulation; ii) online state estimation to characterize the
corresponding SoC of a large fleet of DERs; and iii) incorporating practical
limitations of DERs and communications and analyzing the effects on fleet-wide
performance. To illustrate these benefits of the presented grid-DER
co-simulation platform, we employ the advanced DER coordination scheme called
packetized energy management (PEM), which is a novel device-driven,
asynchronous, and randomizing control paradigm for DERs. A fleet of thousands
of PEM-enabled DERs are then added to a realistic and dynamical model of the
Vermont transmission system to complete validation of the co-simulation
platform.
