Damping Effect of Virtual Synchronous Machines Provided by a Dynamical Virtual Impedance

Virtual synchronous machines (VSMs) emulate the swing equation for grid synchronization and inertia provision. Usually, a large damping coefficient is necessary in VSMs to provide equivalent damper winding effect, which is, however, not aligned with the fact that the physical swing equation of synchronous generators (SGs) contains only a small mechanical friction factor. This large damping coefficient could result in undesired droop characteristics. In this letter, we use damping torque analysis to reveal the damping mechanism of SGs, and show that the damping effect actually comes from the dynamics of the transient/subtransient reactances aggregated as a dynamical impedance. Based on this finding, we elaborate on how to get rid of the large damping coefficient in VSMs by using a dynamical virtual impedance (DVI) to emulate SG's damping mechanism.