Overcurrent relay with unconventional curves and its application in industrial power systems

•An inverse time (IT) overcurrent relay with dynamic dial able to generate non-conventional IT curves is presented.•The proposed model has greater flexibility for the creation of time curves than conventional relay models.•The proposed overcurrent inverse time relay model allows the incorporation of independent functions of the time lever.•The digital IT relay model proposed, results in time curves that can be designed for any specific coordination problem.

This paper presents a generalized formulation of an inverse time overcurrent relay that can generate non-conventional inverse time curves. The proposed model considers a variable time dial position as a function of the fault current; the interaction of two dynamics, the digital representation of the movement of the induction disc (such as conventional relays) and the time dial result in time curves that can be designed for any specific protection coordination problem. The proposed relay does not require more input data that conventional relay, only the fault current in the relay location is required. The proposed model has greater flexibility for the creation of time curves than conventional relay models because it allows the incorporation of independent functions of the time lever that will result in several time curves depending on the defined application criteria. A comparison of the curve fitting between the proposed model and the curve standard model was evaluated using two cases of an industrial power system, for which the non-conventional curves allow the reduction of operation times, mechanical stress and thermal effects. Furthermore, this approach could also prevent damage to the primary equipment.