Effect of Including Transient Virtual Impedance in Droop-Controlled Microgrids

Abstract

Droop control has shown promising results for decentralized power sharing in microgrids. However, the basic scheme can suffer from unequal reactive power sharing power due to differing line impedances between converters and due to the predominantly resistive nature of low voltage networks. Virtual impedances are able to mitigate these drawbacks, and are mostly implemented using a quasi-stationary approach. This approach replicates an impedance in steady state, but transients of the load current are only partly reproduced. This paper discusses the effect of including the transient term of the virtual impedance. A state-space small-signal model is derived in order to show the effect of including the transient virtual impedance. Simulations of a two-inverter microgrid are also shown to test the model and offer a numerical example. Both modal analysis on the small-signal model and the results from the simulation indicate improved damping when including the transient virtual impedance.