Influence of spatial variations of the geoelectric field on geomagnetically induced currents

Abstract

-The geoelectric field driving geomagnetically induced currents (GIC) has complex spatial variations. It follows that different patterns of the field vectors in a given area having the same regional mean can produce very different GIC. In this study, we consider a few power grid models and calculate GIC due to a modelled geoelectric field with a regional mean magnitude of 1 V/km. Altogether 8035 snapshots of the electric field are included. We also assume two different ground conductivity models, of which the simpler one consists of two layers across the whole region, and another model has four different two-layer blocks. As a measure of GIC, we use the sum of currents at all substations of the power grid. We also consider the distribution of GIC at a single site. For a given grid, differences between the two ground conductivity models are small. When comparing different power grid models, the sum of GIC varies relatively more for a grid with a small number of substations and transmission lines. When the area and the number of substations increase, the relative difference between the smallest and largest GIC sum decreases. In all cases, assuming a spatially uniform electric field leads to a reasonable estimation of the GIC magnitudes, but it does not produce the largest GIC sum. For a single substation, there is a large variety of GIC values due to different electric field configurations.

On the Regional Variability of dB/dt and Its Significance to GIC

“…It is shown that spatial‐scale geomagnetic variations on the order of hundreds of kilometers can result in up to a 60% difference in GICs across a transmission line of 200‐km length. Viljanen and Pirjola (2017) explicitly investigated the effects on similar scales, but from variations in the geoelectric field vector direction instead of magnitude; even so, their results converge on a similar conclusion to ours. That is, accurate modeling of GICs requires that the spatial structure of the geomagnetic variations should be taken into account with the scale of the network.…”

Geolectric field measurement, modelling and validation during geomagnetic storms in the UK

“…Fernberg, 2012;North-American Electric Reliability Corporation, 2016). However, research over the past decade has shown that this assumption is too simplistic and will cause a poor estimate of GIC during even modest storms (Beggan et al, 2013;Viljanen and Pirjola, 2017;Love et al, 2018;Blake et al, 2018;Sun and Balch, 2019). Hence, both the spatial and temporal pattern of the magnetic field and the underlying conductivity structure must be accounted for in order to produce realistic geoelectric field models over a wide region (Lucas et al, 2020).…”

The Role of Global/Regional Earth Conductivity Models in Natural Geomagnetic Hazard Mitigation

“…Spatial variability of geoelectric fields is determined by non-uniformity of the geomagnetic field, combined with a nonlinear dependence on the 3D Earth's electrical conductivity structure. Even in the case of a 1D ground model, spatial variations of the magnetic field can produce a strongly non-uniform electric field (e.g., Viljanen et al 1999;Viljanen and Pirjola 2017). Vice versa, a realistic 3D ground conductivity model makes the electric field strongly non-uniform even when a plane-wave magnetic field is assumed (e.g., Bedrosian and Love 2015).…”

“…A range of possible approaches may be applied to obtain the (a, b) parameters. The most common approach is to estimate these linear coefficients based on the power grid model (e.g., Viljanen et al 1999;Beggan et al 2013;Beggan 2015;Viljanen and Pirjola 2017). Other methods, particularly applicable in the absence of such information, but when selected GIC measurements are available, involve least-squares parameter estimation (e.g., Pulkkinen et al 2007;Ngwira et al 2008).…”