Advanced Transformer Differential Protection Under GIC Conditions

This paper presents an advanced differential protection scheme for protecting transformers during geomagnetic disturbance (GMD) conditions and the flow of geomagnetically induced current (GIC) in the transformer. In this study, the inaccuracy of the existing harmonic blocking (HB) methods of the differential protection is investigated under the GIC conditions. It is shown that there are scenarios in which the HB-based differential relays fail to clear the internal faults since they are blocked due to the GIC-related harmonic currents. By detecting both GIC conditions and internal short-circuit faults, a solution scheme is proposed which overrides the harmonic inhibition signals issued by the HB module and makes the differential relay capable of tripping the transformer. Extensive time-domain simulations within the EMTP-RV verify the effectiveness of the proposed scheme in (i) unblocking the transformer differential protection for the internal faults during the GIC, (ii) preventing the malfunction for the external faults, (iii) distinguishing the transformer inrush and sympathetic inrush currents from the GIC conditions, and (iv) immunity to current transformer (CT) saturation. Furthermore, the simulation results prove that the differential protection equipped with the proposed approach outperforms the existing HB methods under the GIC conditions.