Virtual Friction: Experimental Validation in a Microgrid of 3 Virtual Synchronous Machines

Virtual synchronous machines (VSMs) are a promising technology to integrate distributed energy sources and storage into power grids. The VSM is a power converter that emulates the behavior of a synchronous machine, providing grid services which are necessary to operate a power system in a stable manner. When more VSMs are connected to the same grid, sub-synchronous oscillations between them (and between VSMs and other generators) may occur if damping coefficients and inertias are not properly tuned. For this purpose, virtual friction (VF) has been proposed to provide high damping without a strong coupling of frequency deviation and power output, unlike for frequency droop. VF applies a damping torque to the virtual rotor of the VSMs, proportional to the deviation of the rotor frequency from the center of inertia (COI)-frequency of the grid. To the best of the authors' knowledge, this technique has only been validated theoretically and in simulations for isolated microgrids. The goal of this paper is to demonstrate the effectiveness of VF both in isolated microgrids and in grid connected operation by experiments on a 45kVA setup and their theoretical assessment.