Epoxy encapsulation is widely used in dry-type transformer windings to improve insulation performance and mechanical robustness. However, significant thermo-mechanical residual stresses can be introduced during curing and cooling due to material property mismatch, leading to cracking and reliability concerns. This study aims to quantitatively analyze the evolution of thermal-mismatch-related residual stress in epoxy-encapsulated windings and to develop a reliability-oriented improved curing process. A representative encapsulated winding structure and a conventional industrial curing schedule are first modeled, and the evolution of the epoxy degree of cure is calculated based on curing kinetics. The obtained cure history is then coupled with a transient thermo-mechanical finite-element model that incorporates cure-dependent material properties to evaluate the residual stress distribution. The simulation results indicate pronounced stress concentration in specific regions of the encapsulation, which corresponds well with typical cracking locations observed in practice, demonstrating the validity of the proposed approach. Based on this model, several modified curing temperature profiles are further investigated to clarify the effects of temperature levels and dwell times on the development of residual stress. Finally, a reliability-oriented curing process improvement is identified, which effectively reduces stress concentration and mitigates cracking while maintaining adequate curing reliability.
Zhou et al. (Mon,) studied this question.