In the ELECTRA EU project, an innovative approach for frequency and voltage control is investigated, with reference to future power system scenarios characterized by massive amounts of distributed energy resources. A control architecture based on dividing the power system into a web of subsystems, the so-called cells, is proposed. Cells are individual control entities but also need to be coordinated together at system-wide level, in order to ensure secure and reliable overall operation (at Pan-European level). Task 5.4 in the ELECTRA project focuses on deriving novel observability concepts at system-wide scale. The methodology proposed in the task analyzes the system performance by investigating typical phenomena peculiar to each stability type and by developing observables necessary for the novel Web-of-Cells based control methods to operate properly at cell- and inter-cell level. Crucial aspects of angle, frequency and voltage stability are considered, according to the stability classification by CIGRE. In order to carry out the evaluations, a suitable test multi-cell grid model is developed. The paper aims at describing this reference model and at presenting the approach used in the task for assessing system stability in the developed WoC framework.