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This paper reviews microprocessor based protective relay (MBPR) systems with emphasis on differential equation algorithms. In the present, the application of protection relaying in power systems, using MBPR systems, based on the differential equation algorithm are valued more than the protection relaying based on any other algorithms, because the algorithm has the advantages of accuracy and easy implementation. MBPR differential equation approach can tolerate some errors caused by power system abnormality such as DC offset. It is widely implemented in the protections for lines, transformers, buses, motor, and other equipment in power systems. However, the parameters for system description algorithms are obtained from power system current i(t) or voltage v(t), which are abnormal values under fault or distortion situations. So the error study for the algorithm is considered necessary.