The invention is related to the electricity sector with the aim of quick and efficient prevention of the collapse of synchronous stability in a single synchronous power line with one or more parallel transmission lines and, if the failure of stability is not prevented, disconnecting the only synchronous power line. The invention is based on the approach of measurement of an angle between two modelled voltages: the first voltage (Ú1) is towards the equivalent voltage (É1) after the simulated resistance (ZK1) and the second voltage (Ú2) is towards the equivalent voltage (É2) after the simulated resistance (ZK2). From the side of small association of power systems the application of the invention for a single transmission line will ensure in the first case [((Ú2) falls behind (Ú1))] the disconnection of generating capacity, and in the second case [((Ú2) leads ahead (Ú1))] the disconnection of consumer capacity or the start-up of hydro-generators in the small power system association.