This paper presents a design of non-directional overcurrent
relay micro-controller-based having the advantages of low
cost, high performance and can be totally manufactured by
local manufacturers. The proposed overcurrent relay can
be employed to protect feeders of distribution systems for
overcurrent and earth fault protection. Also, the proposed
relay is considered a high speed overcurrent relay, thus it
can be used for restricted earth fault applications. The main
hardware platform is designed and tested with pickup
accuracy not more than 0.001In and total scan cycle not
more than 1ms which make this relay very accurate and
adequate for fast tripping applications. . The relay has the
capability to treat the incoming analogue signals of
voltages and currents via the Discrete Fourier Transform
(DFT) through the designed main hardware platform. In
this way if the relay protects a distribution power
transformer, it can detect the inrush conditions via the
second harmonic detection. Also it can detect the over
excitation via the fifth harmonic calculations to block the
abnormal tripping for more reliable and secure distribution
systems.