Geomagnetic Disturbances in Electric Power Systems and Power Transformer Protection Against Geomagnetic Induced Currents

Geomagnetic storms (GMS) pose a real threat to electric power systems not only in the Russian Arctic zone but also in mid-latitude territories due to the long-term natural trend of the magnetic pole shifting from the territory of Canada towards the Russian sector of the Arctic. The main disadvantages of known resistive grounding systems for the neutral of a power transformer for protection against the effects of geo-induced currents (GIC) are identified, which consist of the need to use additional heterogeneous electrical equipment to protect the grounding resistor from single-phase short-circuit currents and isolate the neutral from overvoltages. Quantitative criteria are proposed that make it possible to select the resistance of the grounding resistor, ensuring the limitation of the GIC to a level that is safe for the power transformer and does not cause saturation of the magnetic system. Requirements for the resistive-thyristor grounding system of the neutral of a power transformer are formulated, considering the restrictions imposed by regulatory documents on grounding devices. A system of resistive-thyristor neutral grounding has been developed with an automatic transition to the resistive grounding mode when the magnetic system is saturated under the influence of the GIC and a return to the solidly grounded neutral mode after the termination of the GMS. The results of field tests of the GIC monitoring channel in the neutral of the 135 MVA/500 kV/13.8 kV power transformer of the Hydro Power Station (HPS) during the geomagnetic storm are presented.