Instability phenomena in interconnected power systems caused by current limitation of grid-forming converters

Abstract

Inertia and voltage source behavior are essential for power system stability. At present, these essential features are provided by synchronous generators (SG). Grid-forming voltage source converters (VSC) can emulate these essential properties of SG and make interconnected power systems with up to 100% converter based generation possible. However, VSC lack an important property of SG: overcurrent capability. Up to now, current limiting has not been considered holistically in most grid-forming control concepts of VSC, although it has a massive impact on their dynamic behavior when active. In this paper, it is shown that this becomes particularly evident in the analysis of transient stability. By means of simulations, the stability behavior of current-limited VSC is compared to the unlimited case. By a synthetic study case and subsequent EMT simulations of a simple transmission system model, we show that there are significant differences and that the transient stability reserve is reduced due to current limitation. If current limiting is not considered in the grid-forming control concept, faults that are easy to cope with in a SG dominated system may lead to instability in power systems with a large share of such grid-forming VSC.