  In order to correctly model the impacts of geomagnetically induced current
(GIC) flows, the generator step-up (GSU) transformer status must be properly
modeled. In power flow studies, generators are typically removed from service
without disconnecting their GSU transformers since the GSU transformer status
has little to no impact on the power flow result. In reality, removing a
generator from service involves also removing its GSU transformer from service.
This difference presents a discrepancy between simulated behavior and system
observations during geomagnetic disturbance (GMD) events. This paper presents
GMD case studies on 2000-bus and 24,000-bus systems in which reactive power
losses and geomagnetically induced currents are compared across scenarios in
which the GSU transformers for the disconnected generators are either
in-service or out-of-service. The results demonstrate a 3.2 to 15.5 percent
error in reactive power losses when the GSU status is modeled incorrectly.
Discrepancies of up to 95 A per phase for branch GIC flows and 450 A for
transformer neutral GIC flows are also observed and visualized.
