The Effects of Correctly Modeling Generator Step-Up Transformer Status in Geomagnetic Disturbance Studies

TL;DR

Properly modeling generator step-up transformer status is crucial for accurate geomagnetic disturbance impact assessments, as incorrect modeling can lead to significant errors in reactive power and GIC flow predictions.

Contribution

This paper highlights the importance of accurately modeling GSU transformer status during GMD studies and demonstrates the impact of incorrect modeling through detailed case studies.

Findings

Reactive power loss errors of 3.2% to 15.5% when GSU status is misrepresented.

GIC flow discrepancies up to 95 A per phase and 450 A in transformer neutrals.

Correct GSU modeling reduces discrepancies between simulation and observed system behavior.

Abstract

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.