Avoidance distance analysis based on HVDC transient magnetic field and grounding current additional magnetic field

High Voltage Direct Current (HVDC) transmission disturbs geomagnetic observations not only through overhead line currents, but also via the additional Magnetic Fields (MF) introduced by the transient MF and the grounding current. Mechanistic calculation reveals that the additional MF generated by the transient MF contributes to interference both in the vertical and horizontal directions, while the additional MF generated by grounding current primarily affects the horizontal direction. Both interference sources exhibit lower amplitude compared to the direct MF generated by overhead line currents. Practical case studies indicate that the interference of the additional MF generated by the grounding current can be mitigated through spatial avoidance: to reduce the interference amplitude below 0.5nT, an observation station should be positioned at least 30km from the grounding point, while complete absence of interference requires a minimum distance of 60km.