The present invention relates to a method for assessment of a power system and an early warning system for a power system, the method being based on aperiodic small signal instabilities. The power system has a plurality of generators comprising one or more synchronous machines and is represented by a plurality of nodes of power injection, injecting power into a network having a plurality of nodes and branches. The method for assessment of the power system comprises determining a present state of the power system, calculating system Thevenin impedances from each node of power injection in the power system on the basis of the determined present state of the power system, calculating injection impedances from each node of power injection in the power system on the basis of the determined present state of the power system, determining a stability boundary for each generator based on the calculated system Thevenin impedances and the calculated injection impedances. Also a method of early warning for a power system
