Impact of Voltage-Loop Feedforward Terms on the Stability of Grid-Forming Inverters and Remedial Actions

This paper reveals the impact of voltage control loop feedforward terms on the stability of grid-forming inverters (GFMIs) connected to a grid. By deriving a mathematical model, this paper shows that unless the GFMI current control loop is fast enough, the feedforward terms in the voltage control loop may threaten the GFMI small-signal stability. To this end, a necessary and sufficient condition for GFMI stability is proposed, provided that the grid impedance at the point of connection is known. Furthermore, two easy-to-implement yet effective remedial actions are proposed to enhance stability. Aiming to preserve the control structure, the first remedial action re-tunes the voltage controller to satisfy the obtained stability condition, while the second remedial action incorporates simple gains to attenuate the feedforward terms. Moreover, to demonstrate the effectiveness of the second remedial action, the obtained necessary and sufficient stability condition is modified to include the proposed attenuation gain. Eventually, the accuracy of the stability analysis and the effectiveness of the proposed remedial actions are evaluated in simulation and experiment.