Virtual Inertai Modeling of Variable Speed DFIM System

The variable-speed DFIM system is a next-generation energy storage technology in which a rotor is connected to a back-to-back converter to adjust the active and reactive power of the device to compensate for the output variability of renewable energy sources. However, general DFIM has less inertia than synchronous generators to support the system when a disturbance occurs in the system, and requires an active technology to assist it. In this paper, a steam turbine governor with droop characteristics was modeled to explain the power control of the existing variable speed DFIM and simulate the drop in system frequency, and a virtual inertia controller was used for the variable speed DFIM system. The current and active power of the DFIM and the frequency change is compared by distinguishing the before and after using the virtual inertia. DFIM, which operates as a variable load after using virtual inertia, reduces the rotor current and reduces the active power absorbed from the system from 1p.u to 0.78p.u by using the virtual inertial active power. Accordingly, it was confirmed that the rotor speed was variable, and it was confirmed that the virtual inertia loop using kinetic energy operated effectively. When disturbance occurs in the system frequency due to the dropout of the power source of the system, the frequency nadir point drops to 48.5Hz. When the system frequency inertia was supported using virtual inertia, the frequency nadir increased from 48.5Hz to 48.76Hz, confirming that the system frequency nadir was improved by mimicking the inertial characteristics of the synchronous generator even during pump operation.