Non-standard characteristic of overcurrent relay for minimum operating time and maximum protection level.

• The overcurrent relay settings are coordinated for minimum operating time. • The developed technique for relay operation is simulated in MATLAB software. • The simulation are confirmed and verified by experimental laboratory tests. • The settings of overcurrent relay are applied as backup of each other. • The model protects the system when one of the settings fails to operate. Industrial overcurrent relays such as MiCOM P123 (overcurrent and earth fault protection relay) often offer three individual options for separate settings. These settings can be separately tuned and such ability can be utilized to apply the relay as backup of itself. In this system, not only the other relays operate as backup of the current relay, but also settings of the relay operate as backup for themselves. In order to verifying such ability, this paper tunes the settings of MiCOM P123 overcurrent relay as backup of each other to make the non-standard characteristic for overcurrent relay. In the developed model, the first setting of the relay is set on normal inverse (NI) curve, the second setting on very inverse (VI) curve, and the third setting on the instantaneous definite minimum tome (DMT) type. The operating area of the relay is divided into two areas. In the first area, the VI curve operates as backup of NI and in the second area, the NI curve operates as backup of VI. The third setting also operates as backup of both the first and second settings. The system is implemented in the protection laboratory as well as it is simulated using MATLAB software. The simulation outputs are verified and confirmed by the experimental tests. The results confirm that the proposed model has two advantages. The first achievement is to reduce the tripping time by about 5%. And he second advantage is to increase the protection level when one of the settings fails to operate. [ABSTRACT FROM AUTHOR]