Design of a digital multi-curve time-overcurrent relay

The author presents an algorithm for implementing a digital time-overcurrent relay on a single 8 bit microprocessor which accommodates more than one time-current curve within the same system. Integer arithmetic 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. Using this approach, as many as five or six curves, each with its own digitally adjustable settings and independent output, can be processed simultaneously. Use is made of a low-cost current-measuring circuit with a response time as short as one half-cycle and a broad dynamic range