Research on Fault Analysis of Fully-Sealed Distribution Transformers Under Sudden Ambient Temperature Drop

A rural power grid in a southwestern region of China experienced multiple failures of 10 kV pole-mounted fully-sealed distribution transformers within a short period. These failures occurred under moderate rain conditions, with the ambient temperature plummeting from 37 ℃ to 18 ℃, significantly reducing power supply reliability and impacting agricultural production and daily life. Comprehensive analyses, including visual inspection, core lifting inspection, transformer testing, and oil sampling inspection, revealed that the sudden temperature drop caused contraction of the insulating oil volume inside the transformers, generating negative pressure. This negative pressure induced thermal deformation gaps in the high/low-voltage bushing sealing gaskets due to temperature differentials, allowing external rainwater or moisture to infiltrate the transformer. The intrusion of moisture led to insulation moisture absorption in the windings, triggering partial discharges, inter-turn short circuits, or insulation breakdowns. Ultimately, these cumulative effects resulted in winding burnout and transformer failure.