High-speed protection as an aid to maintaining electric service following system short-circuits

The interconnection of generating stations through overhead networks has not only increased the number of short-circuits but has spread their disturbing effect over a much wider area. The disturbances are of major importance when they affect rotating machines, and the degree of stability of a system immediately after a short-circuit may be the determining factor which governs the amount of power transferable over a given line. Attempts to increase this power limit by controlling the excitation of the synchronous machines were only partially successful because they did not cater for certain short-circuit conditions. It is now recognized that the mostsuccessful way of dealing with such conditions is to disconnect rapidly the faulty section and sominimize the development of angular displacement between machines in interconnected stations. The paper reviews the various factors determining total system stability, the importance of quickly relieving the system of short-circuits, and the order of time delay which can be tolerated if system stability is to be maintained. The effect of short-circuits on load stability, and means for preventing the loss of machine load following faults, are also discussed. After outlining the essential requirements of high-speed protection generally, descriptions of various methods and systems now in use are given. The method of calculating the degree of stability (or instability) of a system following a fault condition is outlined in the appendices.