A Comparison of Peak Electric Fields and GICs in the Pacific Northwest Using 1‐D and 3‐D Conductivity

Abstract

Geomagnetically induced currents (GICs) are a result of the changing magnetic fields during a geomagnetic disturbance interacting with the deep conductivity structures of the Earth. When assessing GIC hazard, it is a common practice to use layer‐cake or one‐dimensional conductivity models to approximate deep Earth conductivity. In this paper, we calculate the electric field and estimate GICs induced in the long lines of a realistic system model of the Pacific Northwest, using the traditional 1‐D models, as well as 3‐D models represented by Earthscope's Electromagnetic transfer functions. The results show that the peak electric field during a given event has considerable variation across the analysis region in the Pacific Northwest, but the 1‐D physiographic approximations may accurately represent the average response of an area, although corrections are needed. Rotations caused by real deep Earth conductivity structures greatly affect the direction of the induced electric field. This effect may be just as, or more, important than peak intensity when estimating GICs induced in long bulk power system lines.

Geoelectric Hazard Maps for the Pacific Northwest

“…Our maps of geoelectric amplitude do not resemble those of Gannon et al (). They report (their section 2.2) selecting tensors having quality ratings of 5 (the highest standard) and that the tensors they use are those shown in their Figure 3.…”

“…Long‐term collection of 10‐ and 1‐s magnetometer time series only commenced at most magnetic observatories relatively recently; they are not available from either NEW or VIC for the 1989 storm, for example. The NERC () 10‐s resolution benchmark time series, derived by scaling the amplitude of a magnetometer time series of the March 1989 magnetic storm recorded at the Ottawa (OTT), Ontario, magnetic observatory, is sometimes treated as a hypothetical (or scenario ) representation of geomagnetic field variation in evaluating geoelectric hazards at locations far removed from Ottawa, including for the Pacific Northwest (e.g., Gannon et al, ). However, what is unclear is how well the NERC time series represents the details of geomagnetic field variation at locations other than Ottawa.…”

“…In light of Figure , we do not use 1‐D surface impedance tensors derived from simple depth‐dependent models of Earth conductivity. Those developed for North America (Blum et al, ; Fernberg, ; Ferguson & Odwar, ) have been used to estimate benchmarks for NERC and in numerous related studies (e.g., Marti et al, ; Nikitina et al, ; Ngwira et al, ; Pulkkinen et al, ; Trichtchenko et al, ; Wei et al, ); indeed, 1‐D models are sometimes treated as first‐order representations of the Earth (e.g., Boteler, ; Butala et al, ; Gannon et al, ; Marti et al, ). Despite this, several studies have shown that the use of some 1‐D models (e.g., Fernberg, ) can give estimated geoelectric amplitudes that are erroneous by more than an order of magnitude (in some cases, the error exceeds 2 orders of magnitude; e.g., Bedrosian & Love, ; Cuttler et al, ; Love, Rigler, et al, ; Lucas et al, ).…”

A 100‐year Geoelectric Hazard Analysis for the U.S. High‐Voltage Power Grid

“…In the northwest United States, several geoelectric hazard analyses have been conducted with the nearby Victoria (VIC) and Newport (NEW) geomagnetic observatories (e.g., Gannon et al, ; Love, Lucas, Kelbert, et al, ), but separate extrapolation techniques have been applied to analyze a potential extreme event. Gannon et al () scaled the 1989 Ottawa magnetic observatory time series, whereas Love, Lucas, Kelbert, et al () generated a long time series and used a statistical extrapolation method equivalent to that employed in this paper. The northwest region does not produce the large amplitudes seen in the eastern United States, but the region reveals how polarization of the electric fields produce more pronounced geoelectric hazards along the east‐west transmission lines.…”

“…In the northwest United States, several geoelectric hazard analyses have been conducted with the nearby Victoria (VIC) and Newport (NEW) geomagnetic observatories (e.g., Gannon et al, 2017;, but separate extrapolation techniques have been applied to analyze a potential extreme event. Gannon et al (2017) scaled the 1989 Ottawa magnetic observatory time series, whereas Love, Lucas, Bedrosian and Love (2015) and . A hotspot can be seen around the Denver metropolitan of Colorado, which should be interpreted with caution, and is analyzed further in Figure 10 and the associated discussion.…”