Abstract One of the most widely used approaches for transformer protection is the low impedance differential relay, which can be adversely affected by inrush currents. Although the conventional inrush current detectors, i.e. the gap detection and second harmonic criteria, can block the differential relay in most of such cases, they are severely prone to maloperation in the case of large inrush currents. This paper presents a new strategy to enhance the security of the differential relay. The suggested approach exploits an intrinsic feature of large inrush currents in three-limb three-phase transformers with the widely used star-delta connection. Based on the feature, some novel criteria are proposed to improve the inrush current detection scheme. To evaluate the performance of the proposed approach for both inrush current and internal fault phenomena, a real 230/63-kV power transformer is modeled based on the time-based transient simulation. Extensive simulation studies and also evaluation using real data reveal that the proposed approach results in a more secure inrush current discrimination method.