  This paper presents a network hardware-in-the-loop (HIL) simulation system
for modeling large-scale power systems. Researchers have developed many HIL
test systems for power systems in recent years. Those test systems can model
both microsecond-level dynamic responses of power electronic systems and
millisecond-level transients of transmission and distribution grids. By
integrating individual HIL test systems into a network of HIL test systems, we
can create large-scale power grid digital twins with flexible structures at
required modeling resolution that fits for a wide range of system operating
conditions. This will not only significantly reduce the need for field tests
when developing new technologies but also greatly shorten the model development
cycle. In this paper, we present a networked OPAL-RT based HIL test system for
developing transmission-distribution coordinative Volt-VAR regulation
technologies as an example to illustrate system setups, communication
requirements among different HIL simulation systems, and system connection
mechanisms. Impacts of communication delays, information exchange cycles, and
computing delays are illustrated. Simulation results show that the performance
of a networked HIL test system is satisfactory.
