Protective relay for an electric power system including decision-making

Abstract

A protective relay for use in deciding an operating condition of an electric power system with a digital computer and protecting equipment connected to the electric power system. Basically, the protective relay is required to operate under an internal fault and not to operate under an external fault. The protective relay has a first comparison and decision element for determining whether the fault is an internal fault or an external fault by comparing a ratio between a vector summation (actuating quantity) and its maximum value (restraining quantity) in respect to an instantaneous electric current in the system. A second comparison and decison element decides whether the system is in a fault condition or not by detecting the actuating quantity in reference to its level with an output of the second comparison and decision element being inhibited by the output of the first comparison and decision element. The external fault is instantaneously judged by the first comparison and decision element and if a current transformer becomes saturated under the external fault and an excessive actuating quantity is generated, the relay is constructed with a return timer or a memory of the restraining amount in such a way as to continue inhibiting the second comparison and decision element to inhibit an erroneous operation of the protective relay.