Novel differential protection with double restraint coefficient

In this paper a novel differential protection scheme is proposed to overcome the shortcoming of common approaches. Conventionally the percentage differential criterion have just only one coefficient in all instances. Therefore the contradiction of the sensitivity in the inner faults and the security in the outer ones is unavoidable. After comparative investigation on existed differential protection schemes, an universal mathematics expression about differential protections is drawn out at first. Discussions on the impact of the TA (transformer amplifier) saturation on the differential and restraint currents are then presented. Based on the universal expression, a novel non-linear resistant criterion with double resistant coefficient is proposed. According to the mechanism of the differential imbalance current in external faults, this new criterion can auto-regulate the resistant coefficient by the amplitude-phase relationship between the two side currents. This proposed criterion provides a novel means to promote the use of the non-linear resistant differential protection in field. Simulations and lab tests show that this novel scheme is effective in the inner faults and reliable in external faults.