Usage of CLPU curve to deal with the cold load pickup problem

Restoration of load after an extended power outage, whether planned or unplanned, raises various practical concerns. Previous studies have shown the cold load condition lasts between 2 to 60 minutes, or longer, depending on the duration of the outage, type of load and the local weather. The theory presented in this paper is applicable to all microprocessor-based overcurrent protective devices used at the distribution level. The example used in the paper is to provide a possible method to handle cold load pickup problems at line reclosers installed on the distribution system. The new approach is to control the pickup of the phase element of the recloser as a function of time. The pickup of the recloser starts at a preset value after an extended outage and slowly decreases and returns to the normal pickup in a predetermined manner. The relationship of the pickup level and time could be determined by considering the maximum cold load current and its gradual decay with time.