An improved control strategy of virtual synchronous generator under symmetrical grid voltage sag

Abstract

The virtual synchronous generator (VSG) which mimics the synchronous generator enables the operation of inverter-interfaced distributed generations (DGs) more effectively. However, the insufficiency in the low-voltage ride-through (LVRT) capability of conventional VSG limits its extensive application during the voltage sag condition. In this paper, firstly the performance of conventional VSG under the condition of symmetrical voltage sag is elaborated, and then based on the reference dynamic compensation an improved VSG control strategy is proposed, where the new voltage reference generation algorithm is designated to accommodate to different degrees of voltage sag, avoiding controller switching. Then, a fault current limitation scheme is introduced to guarantee the transient peak current of VSG within an acceptable range, especially during serious system disturbances. When the grid voltage restores to normal, an additional phase angle control loop is implemented to facilitate the smooth and rapid transfer of VSG strategy while the voltage reference is switched back to the rated value. Furthermore, the details of design for the VSG algorithm and the phase angle controller are provided through the formulation of small-signal dynamic model, improving the overall system performance during grid voltage sag. Finally, the validity and effectiveness of the proposed strategy are verified by the hardware-in-the-loop (HIL) simulation results.