What is hardware-in-the-loop (HIL) testing and what is needed to perform HIL testing?

“Hardware-in-the-loop testing connects real physical equipment to a simulated power system in a closed loop. Devices under test communicate with the simulator through analog, digital, or communication protocol interfaces. To perform HIL testing, engineers need a real-time simulator with processing hardware, simulation software, I/O devices, and the physical device under test.”