T-equivalent zero-sequence impedances of transformers
with a tertiary delta winding obtained from test data Mohammad Shafiepour | Robert Southey | Simon Fortin | Farid Paul Dawalibi

When computing short-circuit (i.e., fault) currents using full-wave electromagnetic or circuit-based methods that are not reduced to sequence components only, equivalent Thevenin source impedances are required at substation terminals or at the end of truncated transmission lines that explicitly define self and mutual parameters between all modelled conductors. This information is typically provided as sequence component equivalents, including zero-sequence components. For three-phase, three-winding transformers that have a tertiary delta winding, zero-sequence tests are typically performed with the delta winding in place. Results from such tests account for the circulating currents in the delta winding. To use the resulting impedances in a computer model based on a T-equivalent circuit, the test data must be transformed accordingly. In this article, formulae for such transformations are derived from the typical zero-sequence test setup vs. the desired zero-sequence test setup. The methodology is applicable to three-winding transformers with a tertiary delta winding such as star-star-delta and auto-star-delta configurations.

Published in: Electric Power Systems Research, Vol. 251, February 2026, Article 112211.

Electromagnetic Fields of Cloud-to-Ground
Lightning Strikes in Heterogeneous Soil Regions Mohsen E. Nazari | Mojtaba Dehmollaian | Rouzbeh Moini | Simon Fortin | Farid P. Dawalibi

This paper investigates the electromagnetic fields produced by cloud-to-ground (CG) lightning in a heterogeneous soil region. The lightning channel is modeled as a wire monopole antenna. An antenna-theory (AT) approach with a distributed current source along the channel is used to represent the CG lightning channel. Phase shifts and amplitude variations between consecutive current sources in the frequency domain account for the propagation speed and attenuation of the current wave along the channel. Soil heterogeneities are represented through an equivalent surface integral equation. The full-wave formulation, which combines the surface integral equation for the heterogeneous soil inside a multilayer ground structure and the AT lightning channel model is solved with the Method of Moments (MoM) to evaluate lightning-induced electromagnetic fields over a wide frequency range. The results suggest that soil heterogeneity could influence the amplitude and distribution of lightning-generated electromagnetic fields, underlining the importance of employing realistic ground models in lightning interaction studies.

Published in: 38th International Conference on Lightning Protection (ICLP 2026), Sapporo, Japan, 2026.

Advanced Magnetic Sensing for High-Resolution
Mapping of Substation Earthing Grids Using a Rover Houda Rebhi | François Grange | Alexandre Seller | Sébastien Journet | Farid Paul Dawalibi

The integrity of substation earthing systems is essential for personnel safety and reliable equipment operation, yet verification of buried earthing layouts remains challenging in existing installations due to undocumented modifications and heterogeneous soil conditions. This paper presents a non-invasive electromagnetic mapping system for detecting and reconstructing buried earthing conductors in electrical substations. It combines low-frequency current injection with tri-axial magnetic field measurements collected by a remotely operated rover equipped with centimeter-level GNSS RTK positioning. A dedicated processing algorithm generates georeferenced maps of conductor locations and orientations. Experimental validation was performed on a 63 kV substation with an unknown earthing grid topology. The reconstructed earthing layout was independently cross-validated using surface potential gradient measurements. The developed solution offers an efficient and portable alternative to conventional intrusive investigation methods.

Published in: 26th International Conference on Environment and Electrical Engineering (EEEIC), Lisbon, Portugal, June 29 – July 2, 2026.