Improving Transients for Droop-Controlled Inverters

This paper analyzes different methods for improving the connection transients of inverters in an inverter-based micro grid. Three methods are evaluated: the phase limiter, virtual impedance and virtual inertia (or power filter) control techniques. The phase limiter imposes a limitation on the difference between the micro grid and inverter phase angles to avoid rush currents. The virtual impedance algorithm achieves further enhancement of the inverter's transients in terms of settling time and maximum rush current. Finally, the power filter is also evaluated showing the effect of under and over damping dynamics in power and voltage signals in the connection process. All of the methods are validated in an experimental setup of two grid-forming three-phase inverters. In the presented tests, the effect of the phase limitation in achieving a smoother connection is clearly shown. Also, the effect of introducing a virtual impedance to further improve the transients is discussed. Furthermore, the effect of the power filter of the droop control loop on the connection transients is presented.