Geoelectric field modelling for GIC hazard evaluation in Germany's power grid

Abstract

The hazard posed by geomagnetically induced currents (GICs) to ground-based conductors arises from the induced geoelectric field (GEF) under geomagnetically disturbed conditions. Here we report on our latest results of modelling the GEF that drives GICs in Germany's high-voltage transmission grid. We model the GEF from horizontal geomagnetic disturbance (GMD) vectors and co-located 3-D surface impedance matrices under the plane wave assumption on a grid covering Germany. GMDs are obtained from measurements of INTERMAGNET geomagnetic observatories. These are freed from station-specific baselines before Spherical Elementary Current System basis functions are fitted to the residuals. The baselines capture "nonstorm" geomagnetic variability, which is identified considering 1-minute observatory time series since the year 2000. We also identify the most disturbed days to facilitate the selection of suitable events for GEF modelling. The surface impedance is calculated from our dedicated 3-D electrical conductivity model "Eu-Sigma". It particularly takes care of the top layer, which includes land and marine sediments as well as ocean/sea water and provides realistic impedance near the coast. We perform pointwise comparisons to independent magnetic and electric field measurements and/or magnetotelluric transfer functions at a few locations. Based on this initial evaluation of the performance of our GEF model, we hope to improve its calibration in the future to produce reliable GIC hazard maps.