Geomagnetically induced currents (GICs) in long high-voltage transmission lines cause half-cycle saturation of transformers, malfunction of protective relays, reactive power loss, and voltage instability. This study describes the design for a GIC effects mitigation device (GEMD) that can be installed at a power system autotransformer common node connection to block the flow of GIC DC flux at the cores of the autotransformers. The devices consist of variable resistors, which are controlled by a wide-area method based on particle swarm optimisation algorithm. The effectiveness of the proposed protective electrical devices is investigated and validated through simulation studies under many operational cases. The test results obtained from 648 cases verify the high performance of GEMD. The proposed method can mitigate the reactive power loss due to GIC in the power system with average improvements by 7.9 and 7.09% over resistive and capacitive blocking devices, respectively.
