Flywheel in the loop
In order to investigate the possible support of the primary frequency control in the power grid by a flywheel memory, a 60kW flywheel from Stornetic was connected as a test object to our 1MVA PHIL test field and a special adaptive inertial control was developed and implemented in this flywheel.
As network environment, the power network "Microgrid Benchmark" suggested in  was simulated on our Opal-RT 5700 real-time simulator with a time step of 24µs.
To connect the flywheel to the simulation in the real-time simulator, the common "Voltage-type Ideal Transformer Method" is used, where the real test object is represented in the simulation like a current source at the connection point. The simulated voltage at this point is sent digitally to our power amplifiers from Egston as a setpoint value. The amplifiers supply real busbars, to which the flywheel storage is connected, with this voltage. The resulting current at the connection of the flywheel is then measured and sent back to the simulator as a new current value for the simulated current source. In the next calculation step, this new current value is used to calculate the new target voltage.
Papathanassiou, Stavros, Nikos Hatziargyriou, and Kai Strunz. "A benchmark low voltage microgrid network." Proceedings of the CIGRE symposium: power systems with dispersed generation. CIGRE, 2005.