Stable Operation and Small-Signal Analysis of Multiple Parallel DG Inverters Based on a Virtual Synchronous Generator Scheme

For a high penetration level of distributed energy resources (DERs) in the grid, virtual synchronous generator (VSG) control is applied to the power electronic converters to mimic the rotating mass and damping property of a conventional synchronous generator (SG), which can support virtual inertia and damping for the power system. For VSG control, a phase locked loop (PLL) is needed to estimate the angular frequency of the point of common coupling (PCC); however, the deviation of PLL will affect the accuracy of the active power reference, and even the VSG stability control. From this perspective, an enhanced active power controller without PLL was proposed for VSG control. Furthermore, an accurate small-signal model of the multiple parallel VSGs system that considers the dynamic characteristics and the changing of a steady state operation point was derived for system analysis and parameter design. Based on this model, the influence rules of the eigenvalues by droop and virtual inertia were acquired. The simulation and experimental results are presented to verify the validity of the proposed active power controller and parameter design rules.