Flexible inverse overcurrent relaying using a microprocessor

The fundamentals of inverse time overcurrent relaying, recent changes in transformer characteristic curves, and a flexible microprocessor-based relay that has wide application possibilities are described. Insight into the process of selecting a particular time-current operating characteristic shape for a given application is provided. Guidelines are presented for the selection of a setting for an instantaneous-trip device based upon its transient reach tendencies. It is concluded that the availability of a microprocessor-based inverse time overcurrent relay that has selectable characteristics built in will greatly relieve the application difficulties that have been associated historically with their fixed-characteristic electrochemical counterparts.<
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