Ten years of progress in relaying

This is the second of a series of three related articles which together present a review of the progress made in the past decade in the protection of electric power-system circuits and equipment. This article, concerned with protective relaying, has been prepared under the auspices of the AIEE committee on protective devices. The first article of the series, concerned with circuit-interrupting devices, was published last month; the third, dealing with lightning-surge protective devices and the protection of systems against lightning, is now in preparation and is scheduled for publication in a later issue. JUDGING from the external appearance of relays, one might conclude that protective relaying was an unromantic and prosaic subject. Here are a lot of little devices that spend most of the time sitting still, with not even the little eye-appeal of the moving pointer of an instrument or the passing black spot on the disk of a watt-hour meter. Yet compacted within these small cases is the fruit of the labor of many minds, in such form that, when called upon to function, relays take a set of observations, perform a lightning calculation, and act, all within 1/20 of a second. Put in more technical terms, the general objectives of protective relaying are: (a) the limitation of damage to electric equipment, and (b) the maintenance of adequate service with reasonably low investments in generation, transformation, transmission, and distribution equipment. In order to obtain these objectives, the protective-relay system must detect positively the occurrence of a short circuit, determine enough about its location to permit selection of the breakers which must be opened, and control the opening of only those breakers necessary to remove the fault from the system — all in a time short enough to limit equipment damage and reduce system disturbances to a reasonably low value.