Distribution Protection as Used on the Portland General Electric Company System

The Portland General Electric Company distribution system is protected with phase and ground relays set relatively fast and selective with all sectionalizing devices. Phase relays are set to trip at about 2.25 times the individual circuit ratings, disregarding actual loads. Maximum circuit loading is normally limited to 1.5 times circuit rating for 1/2-hour peaks. Higher loadings will probably be used in the future and phase relays will be desensitized to meet cold load requirements. The ground relays are set as sensitive as possible, yet must be selective with the largest sectionalizing devices. Operating experience in terms of service outages and the number of high resistance ground faults not detected appear to justify the general scheme of operation. In short, it is thought that the protective scheme must be chosen to support all other effort to serve customers. Engineering, construction, and service department efforts are directed toward the common goal of supplying continuous electric service to the greatest number of customers possible. The protective relay schemes must support this effort. The protection must be fast, to limit burndowns; and, selective to limit outages, and speed service restoration.