LAUSR

Abstract The second harmonic restraint method is the most common technique used by relay manufacturers to discriminate between inrush and fault currents. Unfortunately, many relays in the Egyptian distribution networks are old versions digital overcurrent relays that don't contain the second harmonic restraint method. In developing countries like Egypt, replacing a large number of conventional relays at one time may not be possible due to difficult economic conditions. This paper proposes a novel inexpensive solution by incorporating an additional circuit with the old version over-current relays. The proposed circuit applies a second harmonic restraint function for detecting the inrush currents as in the modern digital relays. The modified overcurrent relay is applied to a test system picked from North Delta Electric Distribution Company (NDEDC) Network in Egypt. The results demonstrate the successful operation for the proposed circuit along with the overcurrent relay in normal operation, switching cases, permanent faults, and in case of faults during energization. Furthermore, the proposed circuits integrated with old version overcurrent relays are successfully tested and verified in the presence of grid-connected Distributed Generation (DG) for both switching and fault operation modes. Also, it provides a cost-effective solution for the old version digital overcurrent relay in NDEDC.