Variable Virtual Impedance-based Overcurrent Protection For Grid-forming Inverters: Small-Signal, Large-Signal Analysis and Improvement

Grid-forming inverters are sensitive to large grid disturbances that may engender overcurrent due their voltage source behavior. To overcome this critical issue and ensure the safety of system, current limitation techniques have be implemented. In context, variable virtual impedance (VI) appears as a suitable solution for problem. The design basically rests on static considerations, while, its impact system stability dynamics considering both small-signal large-signal aspects can significant. This paper proposes nonlinear power models assess parameters angle stability. Thanks proposed approach, it has been demonstrated ratio σXVI/RVI contradictory effect transient i.e., resistive results in well-damped response but very limited margin, while an inductive poorly-damped acceptable margin. Based that, concluded conventional cannot cope at once with dynamic performances constraint, Variable Transient Virtual Resistance (VTVR) additional degree freedom vary σXVI/RVI. It decreases damp increases quasi-static steady-state guarantee maximum margin allowed by impedance. effectiveness control proven through time-domain simulations.