Unexplained distribution transformer failures: Revealing the underlying mechanisms

Distribution-class transformers continue to experience a significant number of premature and so-called unexplained failures, even when operating within their nameplate voltage, loading, and insulation limits. These failures are frequently observed following temporary system disturbances such as earth faults, switching events, and lightly loaded operating conditions. Post-failure assessments, however, often fail to identify a definitive root cause. This points to a fundamental limitation in current transformer selection and assessment practices, which rely predominantly on nameplate ratings and insulation coordination while giving limited consideration to magnetic performance. This paper advances a magnetizing-curve-based perspective for understanding and preventing unexplained transformer failures. Using a case study, the paper shows that transformers designed for a maximum flux density of 1.6 T can be driven into severe saturation when phase-to-earth voltages rise to √3 times their nominal value during earth faults. The analysis indicates that limiting the rated flux density to approximately 1.0 T provides sufficient magnetic margin to prevent saturation under such conditions. The findings highlight the need for utilities to move beyond nameplate-based specifications and incorporate magnetizing curve criteria into transformer design.