  This paper presents the development of a real-time T&D co-simulation testbed
for simulating large grids under high DER penetration. By integrating bulk
power system, distribution feeders, and distributed energy resources (DER)
models into one simulation environment, the testbed enables the performance
analysis and validation of DER management systems (DERMS) algorithms. This work
proposes a co-simulation timestep sequence for the cross-platform data exchange
and time synchronization, with a communication framework based on MQTT
communication protocol. The proposed strategy is tested with a 5,000 buses
model of part of the North American bulk power system (BPS) and a 9,500 nodes
distribution feeder model obtained from a local utility. Simulations are
carried out to demonstrate the capability of the proposed framework to
propagate events between the transmission and distribution models. Results are
used to quantify how the co-simulation timestep size can affect the propagation
of dynamics between the models.
