Adaptive Virtual Resistance for Postfault Oscillation Damping in Grid-Forming Inverters

Postfault oscillations in active power and voltage responses of grid-connected voltage source inverters (VSIs) have been reported in the literature. They are caused by nonideally tuned controllers and the implemented current limitations used for protecting the VSIs from overcurrents. These oscillations become even more significant when the VSI is connected to a weak grid, deteriorating the recovery process of the VSI. This article presents a method for damping the postfault oscillations by using an adaptive virtual resistor (VR). Even though these oscillations are observed with both grid-following and grid-forming inverters, this article focuses on droop-based grid-forming inverters. The proposed method dynamically integrates a VR into the VSI control and removes it in the normal operation mode of the VSI. This method is implemented in the synchronous reference frame, which is commonly used due to its decoupled active and reactive power control. The amount of virtual resistance used for oscillation damping is adaptive to the recovery rate of the VSI. Hence, the proposed method is robust against changes in grid strength. Finally, the performance of the method is evaluated in PSCAD/EMTDC and also experimentally validated.