Transformer-level modeling of geomagnetically induced currents in New Zealand's South Island

During space weather events, geomagnetically induced currents (GICs) can be induced
in high-voltage transmission networks, damaging individual transformers within substations. A common
approach to modeling a transmission network has been to assume that every substation can be represented
by a single resistance to Earth. We have extended that model by building a transformer-level network
representation of New Zealand’s South Island transmission network. We represent every transformer
winding at each earthed substation in the network by its known direct current resistance. Using this network
representation significantly changes the GIC hazard assessment, compared to assessments based on the
earlier assumption. Further, we have calculated the GIC flowing through a single phase of every individual
transformer winding in the network. These transformer-level GIC calculations show variation in
GICs between transformers within a substation due to transformer characteristics and connections.
The transformer-level GIC calculations alter the hazard assessment by up to an order of magnitude in some
places. In most cases the calculated GIC variations match measured variations in GIC flowing through the
same transformers. This comparison with an extensive set of observations demonstrates the importance
of transformer-level GIC calculations in models used for hazard assessment.