Static relaying devices using magnetic cores and silicon-controlled rectifiers

THE BASIC PURPOSES of the relays associated with a power transmission system are: to detect any fault within the system, to localize the fault distinctly to a particular section of the system, and to cause that section to be disconnected without interfering with continued supply to the remainder of the system.
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In order to appreciate the relaying problem, the two familiar types of power system structure shown in Fig. 1 should be examined. In a radial system, a load fault can often be detected simply from the existence of an overcurrent; however, if the load contains transformers with high inrush currents on excitation or motors with high starting currents, it may be desirable to delay tripping the breaker, either by a finite time or by a time inversely dependent on the current. If a fault occurs in line section C-D of Fig. 1, all line relays will indicate an excessive current. But, since only the breaker in line C-D should operate, service to the other loads may be maintained. Thus, it is necessary to delay operation of the breaker in line B-C until the breaker in line C-D has had sufficient time to open, and provision must be made for the time-current co-ordination of the line relays.