Mechanical Deterioration Mechanism of Inter-Turn Insulation in Dry-Type Air-Core Reactors

Polyester (PET) inter-turn insulation of dry-type air-core reactors endures long-term compressive stress induced by internal pre-tightening force and temperature fluctuation during service, which easily induces microcrack propagation and accelerated insulation breakdown. Existing studies mainly focus on thermal, electrothermal and hygrothermal aging of PET films, while few investigations reveal the mechanical degradation laws under various compressive stresses sustained during reactor operation. To fill this research gap, this paper develops a continuous compressive loading test platform matched with an exponentially damped oscillating voltage (EDOV) test system. Thermal- and damp-heat-pretreated PET specimens are prepared via accelerated aging methods to simulate actual operating environments. Combined with impulse withstand tests and dielectric loss characterization, the micro-damage evolution and insulation performance decay rules under multi-gradient compressive stress are analyzed. The results indicate that under sustained compressive stress, progressive microcrack growth and property decay are observed, with a more pronounced degradation for specimens subjected to prior thermal or damp-heat aging treatment. The findings can provide quantitative support for reactor on-site insulation assessment, condition monitoring and maintenance strategy formulation.