  Battery energy storage systems (BESS) are proving to be an effective solution
in providing frequency regulation services to the bulk grid. However, there are
several concerns for the transmission/distribution system operators (TSO/DSO)
with the frequent dispatching of the distribution-connected fast-responding
storage systems. Unfortunately, the existing decoupled models for transmission
and distribution (T&D) simulations are unable to capture the complex
interactions between the two systems especially concerning frequency regulation
problems due to rapidly varying distribution-connected distributed energy
resources (DERs). In this paper, an iteratively coupled T&D hybrid
co-simulation framework is developed to facilitate the planning studies for the
system operators to help integrate distribution-connected BESS in providing
frequency regulation services in response to highly variable DERs such as
photovoltaic generation (PVs). Specifically, the proposed framework helps
evaluate: (1) the effects of distribution-connected DERs/PVs on the response of
the system's automatic generation control (AGC) response, and (2) highlights
the use of BESS in providing frequency regulation services using integrated T&D
model. The proposed framework is demonstrated using the IEEE 9-bus transmission
system model (operating in dynamics mode) coupled with multiple EPRI Ckt-24
distribution system models (operating in quasi-static mode). It is shown that
the proposed co-simulation framework helps better visualize the system AGC
response and frequency regulation especially in the presence of high-levels of
DER generation variability requiring frequent dispatch of BESS.
