A new technique of three phase overcurrent relay is described in this paper. The performance of the new design shows the ability to obtain any shape of characteristic using exponential type model with high accuracy. Also the new design used more accuracy, fast measurement for peak detection with error less than 2%. The relay tested for normal and very inverse characteristics, also tested for line-to-line fault the results show the ability of discrimination the fault current and isolate the section of the system affected by a fault so that the remaining system can continue to operate normally.
Key-words: overcurrent relay, Peak detector, time coordination margin (TC), time multiplier setting (TMS), Programmable Peripheral Interface 8255 (PPI).