Online Diagnosis of Interturn Short-Circuits and Core Faults in Three-Phase Transformers Based on Electrical Indicators

This work presents a diagnostic strategy for transformers that analyzes the real and imaginary components of differential admittance. Two complementary indicators are defined based on this decomposition: one based on the calculation of differential conductance ˙ λ, which is sensitive to winding faults, especially for inter-turn short-circuits (ITSCs), and one based on the calculation of differential susceptance κ˙ , which detection core faults. Experimental validation on a 5 kVA transformer, under different fault severities and operating conditions, confirms the high sensitivity of the ˙λ indicator to ITSC. Additionally, the κ˙ indicator was evaluated in the presence of air gaps and inter-laminar short-circuits (ILSCs), demonstrating its ability to detect and diagnose these fault conditions. Finally, the proposed indicators were implemented in a 70 MVA step-up transformer at a wind farm substation. The indicators were monitored over time, and a safe operating zone was established. Deviations from this zone reliably indicate internal degradation. This strategy provides a non-invasive and robust solution for early detection of faults in power transformer windings and core.