In Japan, electric utilities have applied various kinds of special protection systems (SPS) to prevent major outage. Recently, some foreign countries such as USA or Italy experienced major blackouts, which made us realize again that SPS had important roles to keep power systems stable.
Overload relay (OLR) system is a sort of SPS. Every facility in the power system is operated under its capacity in a normal condition. However, if one or some of facilities are shut down due to a fault, the power flow takes a detour through remaining facilities and this could cause overload. Without appropriate actions, overload could cause a major outage due to cascading trips. To prevent a cascading outage, OLR system detects the overload of a facility and takes remedial actions to limit the power flow below its capacity, by increasing, reducing or shutting down the generations or tripping the loads, etc.
The Institute of Electrical Engineers of Japan (IEEJ) formed “Investigating R&D Committee on Overload Protection Relay Technology” with protective relay specialists in universities, institutes, manufacturers and utilities and made a survey on OLR systems from August 2004 to July 2006. This report presents the result of the survey, including what is overload, how overload is caused, capacity calculation of facilities, methods for detecting overload, remedial actions against overload, structures of OLR systems and examples of the application of OLR systems, etc.