Assessment of Insulation Aging Condition for Dry-Type Transformer Epoxy Resin Based on Dielectric Response and Activation Energy Analysis

The accurate assessment of the aging state of epoxy resin insulation is critical for the safe operation of cast resin dry-type transformers. This study investigates the evolution of activation energy during thermal aging and its correlation with insulation degradation. Accelerated aging experiments at 150 °C, 170 °C, and 200 °C were conducted, followed by frequency-domain dielectric spectroscopy and Havriliak–Negami (HN) model analysis. An improved method for calculating activation energy, incorporating temperature correction via an HN-based model, is proposed. The evolution of key HN parameters—relaxation strength (Δε), relaxation time (τ), and shape parameters (α, β)—serves as the criterion for identifying the dominant aging mechanism: crosslinking at 150 °C, competition between crosslinking and degradation at 170 °C, and degradation-dominated chain scission at 200 °C. Using 150 °C data as a baseline, the initial activation energy is determined to be 90.03 kJ/mol, increasing to 166.83 kJ/mol at the end of service life. A practical, graded insulation condition indicator based on the rate of change in activation energy (ΔEa) is established, providing clear guidance for maintenance decisions—from routine monitoring (ΔEa ≤ 20%) to prioritized inspection or replacement (ΔEa > 60%). The proposed method offers a non-destructive tool for insulation diagnosis, residual life prediction, and condition-based maintenance of dry-type transformers.