DASP Blog: Darcy Cordell talks geomagetically induced currents in Alberta's power network

 In this DASP blog, lead author Dr Darcy Cordell (UofA) talks about their recent study published in Space Weather about modelling geomagnetically induced currents in the Alberta's power grid. Questions should be directed to dcordell@ualberta.ca.

Perturbations of the Earth’s geomagnetic field due to the solar wind induce an electric field at the surface of the Earth which depends on both the characteristics of the geomagnetic field as well as spatial variations in the Earth’s conductivity from the surface to depths of >100 km. During quiet times, the surface electric field is very weak (e.g. 0.1 - 0.01 V/km). However, during a geomagnetic storm, the surface electric field can increase significantly to peak values >1 V/km or even >10 V/km. This electric field on its own is still weak and not directly dangerous to people (e.g. a toaster produces an 80,000 V/km electric field at a distance of 30 cm). However, over very long distances the electromotive force (emf) of the weak electric field can be significant. For example, a 100 km transmission line that is grounded at both ends via a transformer neutral in a substation can result in an induced emf of 100s of Volts. This emf drives geomagnetically induced currents (GICs) through power networks. The presence of large currents (e.g. tens to hundreds of Amps) flowing through the grounded transformers in a substation can cause various problems including transformer saturation and overheating, voltage instability, reactive power losses, and—in the worst case—power outages. Thus, GICs in power networks present an important natural hazard requiring further investigation.

Our recent paper published in Space Weather details preliminary work in an ongoing collaboration between the space physics research group at the University of Alberta (PI: Ian Mann), and AltaLink, one of Alberta’s largest transmission companies. AltaLink installed GIC monitoring devices at five substations in the Alberta network which have been continuously recording since 2021. This dataset was used to correlate space weather events with measured network GICs during a geomagnetic disturbance on April 24, 2023. In addition, we modelled GICs across the full Alberta >240 kV network to estimate GICs flowing in 153 substation neutrals and 233 transmission lines. Comparing the modelled GIC to measured GIC is critical in improving and refining our network model to better understand and mitigate risks across the full network.

To accurately model the induced electric field during April 24, 2023, we leveraged two existing datasets in Alberta: (1) the relatively dense CARISMA magnetometer array along with NRCan and USGS magnetometers were used to characterize the spatial and temporal variation in the magnetic field during the event and (2) a relatively dense array of more than 500 surface impedance geophysical measurements collected primarily by Martyn Unsworth’s research group at the University of Alberta was used to characterize the spatial variability in subsurface conductivity. These two datasets allow for higher spatial resolution when calculating the induced geoelectric field across the province compared to other jurisdictions which often rely on only one or two magnetometers and use simple approximations of Earth’s conductivity. The electric field was then integrated along transmission lines and GICs are modelled in substation neutrals and transmission lines. The modelled results from April 24, 2023, were compared to the measured GIC provided by AltaLink L.P. at five substations. The figure below shows an example from the Keephills substation approximately 100 km west of Edmonton. The top panel shows the observed geomagnetic field at the nearby Ministik Lake CARISMA magnetometer, the middle panel shows the geoelectric field calculated near Keephills, and the lower panel shows the modelled and measured GIC at the Keephills substation. This site had peak measured and modelled GIC around 65 A and has the best model performance with a correlation coefficient of 0.79. Most importantly there is a clear correlation between variations in the magnetometer data and simultaneous spikes in both the calculated geoelectric field and the GIC. 

This work resulted in several important findings:

1. It showed for the first time clear and direct evidence that space weather events can lead to measurable GICs in the Alberta grid.
2. The levels of measured GIC during the moderate storm were higher than anticipated at several substations and this warrants further investigation
3. The model points to regions of Alberta that might be prone to larger GIC. This helps to identify potential locations for future monitoring equipment in northeastern Alberta and in the Edmonton region.
4. The preliminary network model performed reasonably well at some substations, but more work is required to refine the model and include more details about the network connections and resistances.

Since publication, this project has been expanded via an NSERC Alliance grant to include another transmission company (ATCO Electric) and the Alberta Electric System Operator (AESO) provincial regulator. Estimating and modelling the impacts of space weather events on a power system is an interdisciplinary problem involving space physicists familiar with Earth’s geomagnetic environment, geophysicists familiar with the electromagnetic induction in the conductive Earth, and electrical engineers familiar with power transmission and network analysis. This project brings together individuals from each of these disparate fields and plans are underway to expand monitoring efforts, improve model performance, evaluate worst-case scenarios, and continue to build interdisciplinary connections between industry and academia.

Figure 1: Geomagnetically induced currents at Keephills substation on April 24, 2023. (A) Geomagnetic disturbance at Ministik Lake, AB; (B) Calculated geoelectric field near Keephills; (C) Modelled and measured GIC at Keephills substation neutral.

Video: Calculated geoelectric field vectors (left) and modelled neutral-to-ground GIC values at each substation (right) are shown across Alberta during one of the peak phases of the storm from 06:50 - 08:00 UT on April 24, 2023. Blue (negative) GIC values denote current flowing out of the ground and red (positive) GIC values denote current flowing into the ground

Interested in writing a blog post for DASP about a recent paper? Email DASP at dasp.dpae@gmail.com, or daniel.billett@usask.ca.