The second harmonic component of magnetic inrush currents in transformer differential protection is analyzed. The analytical result shows that the second harmonic component of symmetric inrush current is big enough for blocking the differential protection. Hence a new criterion is proposed to intensify the second harmonic with the contribution of DC component. A second new criterion proposed is a constant-increase-of-slop restraint characteristic which replaces the old dual-slop percentage restraint characteristic. Analysis

tests and operation experiences prove the validity and feasibility of the proposed criteria.