Research on UHV AC/DC power grid's harmonic coupling characteristic under the action of geomagnetically induced currents

· An equivalent model of the converter transformer is established under the influence of geomagnetically induced currents. · A GIC harmonic simulation model of the UHV AC/DC grid under hierarchical access mode is established, and the accuracy of the simulation model is verified by theoretical analysis. · The harmonic state space model under GIC is formulated, Euler's formula simplifies the calculation, and the accuracy of its calculation results is validated using the simulation model. The harmonic coupling characteristics in the UHV AC/DC power grid under GIC are analyzed. Geomagnetic-induced currents (GIC) caused by geomagnetic storms (GS) can lead to transformer core saturation and harmonic generation, which further couples with harmonics from ultra-high voltage (UHV) AC/DC transmission systems, complicating grid stability. Focusing on China's UHV DC hierarchical connection mode (HCM), this study establishes a harmonic coupling model integrating GIC effects and converter-transformer interactions. A harmonic state-space (HSS) framework combined with Euler's formula simplification enables efficient analysis of harmonic propagation in multi-voltage-level grids. Simulations using PSCAD/EMTDC reveal that the harmonic content generated by the coupling of the transformer and converter is higher than that generated by the transformer alone. The superimposed coupling of cluster harmonics exceeds national standards, with 2nd harmonic currents surpassing 1 % at the AC side and 3rd harmonic voltages reaching 4.86 % in 500 kV DC links. This work provides theoretical and practical foundations for enhancing UHV grid resilience against geomagnetic disturbances.