Investigation in the Effects of GICs on Power System Harmonics

The primary objective is to assess how GICs influence harmonic distortion and to identify the harmonic orders and system locations most affected during geomagnetic disturbances. A further aim is to understand which system conditions, such as transformer saturation, grid topology and loading, exacerbate harmonic levels. By clarifying these relationships, the study supports the development of targeted mitigation strategies that enhance grid resilience and maintain power quality during periods of elevated geomagnetic activity. The methodology combines a data-driven analysis of field measurements with GIC modelling.

High-resolution harmonic data from multiple Irish substations were collected for the May 2024 storm period and its surrounding baseline to characterise changes in total harmonic distortion and in individual harmonic orders. Modelled GIC values, derived from a detailed network and

Earth-conductivity representation, were used to estimate the quasi-DC currents induced in the transmission system, with peak values between 69 and 80 A at the most exposed locations.

Pearson correlation coefficients were calculated between hourly GIC values and harmonic components to quantify the statistical link between geomagnetic activity and harmonic behaviour.

The results show a clear association between GIC activity and increased harmonic distortion, with correlation coefficients reaching 0.91 at some sites and orders. Even-order harmonics, particularly the 4th and 8th, together with selected higher orders such as the 10th and 11th, exhibited the strongest sensitivity, with peak increases of several hundred percent compared to pre-storm levels at certain substations. Load conditions influenced the severity of distortion, as higher load periods tended to amplify resonance effects and intensify voltage waveform distortion.

Spatial patterns were also evident. Substations connected to long high-voltage lines and those in regions with higher modelled geoelectric fields experienced the largest harmonic excursions.

Proximity to heavily loaded, GIC-susceptible transformers further increased local vulnerability.

These findings underline that GIC-driven harmonic impacts are strongly dependent on network configuration, transformer design and regional Earth conductivity, in addition to the intensity of the geomagnetic disturbance itself.

Although the case study focuses on the Irish system, which is relatively small, coastal and largely radial, the observed phenomena are consistent with reports from larger systems in New

Zealand and Canada. The generation of significant even-order harmonics during geomagnetic storms appears to be governed more by transformer characteristics, soil conductivity structure and magnetic latitude than by grid size alone, suggesting that the mechanisms and vulnerability patterns identified here are transferable to other networks.

The paper concludes with recommendations for enhancing preparedness, including wider deployment of real-time GIC monitoring (for example, Hall-effect probes on transformer neutrals), improved harmonic monitoring during space-weather events, and further investigation of adaptive harmonic filtering and operational measures to limit the risk of resonance and equipment overstress during severe solar storms.