Improving GIC modelling and validation with high-quality information on power network parameters

Space weather can affect the operation of high voltage AC transformers in power grids by applying an offset DC current during periods of heightened geomagnetic activity. Modelling GIC requires knowledge of magnetic field variation, the response of the local subsurface geoelectric field (related to conductivity) and a representation of the connections between transformers and various resistance parameters of the power network. Presently, in Britain, the largest uncertainty in this chain applies to the resistance parameters of the network, as these values come from open-source data which are known to have many approximations.

Recent work with a transmission network operator in the UK has provided us with an improved dataset of resistance parameters of transformers, power lines and substation grounding. The grounding resistance at electrical substations has not been known before and so historically was set at 0.5 Ω in our models. The new dataset of 110 sites around central Scotland reveals substation grounding resistance varies from 0.04 Ω to 11.7 Ω with a mean of 0.54 Ω but a median of 0.2 Ω. Combined with line and transformer resistance information, we have created an improved representation of the power grid in Scotland.

Using GIC measurements from three sites (Torness, Strathaven and Neilston) for the largest geomagnetic storms in the past 25 years (October 2003, September 2017 and May 2024), we are able to validate the new model, demonstrating its improved accuracy.

The new model demonstrates that our previous assumptions of grounding resistance were too high but our estimate of line resistance was too low, thus balancing out the overall GIC magnitude on average. However, in detail, some locations show large differences in GIC compared to the original model. This highlights the importance of using accurate resistance information to correctly capture GIC.