Design of a digital multi-curve time-overcurrent relay

An algorithm for implementing a digital time-overcurrent relay on a single 8-b microprocessor is presented. Integer arithmetics is found to provide an adequate overall error performance. The relay responds to the mean value of the fault current, as do most available solid-state overcurrent relays. An extra feature has been devised in this application: the possibility of processing more than one time-current curve concurrently at low additional cost, thus offering a multioutput relay, with each output corresponding to a particular curve. As many as five or six curves, each with its own digitally-adjustable settings and independent output, can be processed simultaneously. A low-cost current-measuring circuit with response time as short as one-half cycle and a broad dynamic range is used