The Influence of the Geoelectric Coast Effect on Geomagnetically Induced Currents

Geomagnetically induced currents (GICs) in power networks can damage transformers, cause voltage instability and lead to power outages. GICs are driven by an induced voltage in transmission lines due to the induced surface geoelectric field component parallel to the line. It is well-known that an electrically conductive ocean can increase the geoelectric field magnitude on the landward side of the coast. However, limited work has been done to elucidate how the adjacent ocean impacts network GICs. We model GICs using a well-known network model situated adjacent to an ocean. Contrary to the notion that GIC risk is higher in coastal areas, we show that the ocean can cause a decrease in the maximum possible GIC in coastal power networks relative to calculated GICs which exclude coast effects, while increases in GIC due to the ocean can be relatively modest. This is because the geoelectric field only increases in the component perpendicular to the coast but decreases parallel to the coast. Thus, transmission lines parallel to coastlines experience a net decrease in induced voltage along their entire length, while transmission lines perpendicular to coastlines experience an increase in induced voltage that is self-limited by the distance from the coast.