The Frequencies of Natural Power Oscillations in Interconnected Generating and Distribution Systems

Electric generating and distribution systems are capable of performing natural oscillations of frequency and of power, superposed upon the ordinary operation and sometimes disturbing the regular behavior of the system or its parts. These oscillations are due to the interlinked cooperation of the prime movers with their speed-governing mechanisms, the synchronous generators, the ramified network, and the numerous induction motors, and similar power consumers. Since most of the machines and network branches usually are different in size and in individual design, a multitude of natural oscillations will develop, the frequency spectrum of which can be divided into three groups. In perfect condition of the system, a single very low frequency develops, which is determined by the inherent properties of all speed governors and servomotors together, and by the inertias of the rotating masses of all the generators and motors together. This joint natural oscillation has a period around 10 to 20 seconds and causes an inphase movement of all mechanical parts of the system, which can be of considerable magnitude, while the electric power may oscillate in phase opposition within the various parts of the system. The next group of frequencies contains as many natural oscillations as there are different synchronous machines in the system and is defined by the faster electromechanical oscillations occurring between each synchronous generator and the entirety of all the other synchronous and induction machines.