A Transient Stability-Enhanced VSG Technique with Improved Voltage Support Capability

The virtual synchronous generator (VSG) is an effective grid-forming control technique for power converters that are required to provide inertia, frequency support, and voltage support. This work focuses on balanced three-phase operation under symmetrical voltage sags, where a three-phase grid-forming converter may simultaneously face transient power-angle instability, large fault-current peaks, and insufficient voltage support. In this work, a transient-stability-enhanced VSG technique with improved voltage support capability is proposed for balanced three-phase operation. First, the relationship between the converter voltage and the grid current is derived, and the converter voltage reference is calculated to use the available voltage support capability under the specified steady-state current limit. Second, an extended state observer (ESO) is designed from the VSG swing dynamics to adaptively adjust the active power reference and attenuate the fault-induced frequency deviations. Third, a short-duration virtual impedance is introduced to suppress transient fault-current peaks at fault transitions. Experiments under 0.6 p.u. and 0.3 p.u. symmetrical voltage sags maintain the periodic fault current below 1.3 p.u., reduce the measured transient peaks from 2.54 and 3.73 p.u. to below 2 p.u., and improve the PCC voltage from 0.76 and 0.47 p.u. to 0.86 and 0.60 p.u., respectively. Complementary simulations at 0.45 and 0.75 p.u. further confirm stable fault-period operation, periodic-current regulation, voltage support, and post-fault recovery over a wider range of balanced voltage-sag depths.