Dynamic Thermo-Mechanical Degradation and Evolution of Hotspots in EPDM T-Branch Cable Joint Insulation

This paper presents a quantitative non-invasive diagnostic framework for assessing early-stage degradation of 35 kV ethylene propylene diene monomer (EPDM) T-branch cable joint insulation using dynamic infrared thermography under controlled pulse-heating (50% load equivalent). Unlike conventional steady-state methods, a reproducible 660 seconds heating pulse followed by natural cooling isolates geometric thermal delays from aging-induced impedance changes. Experiments on new and one-year field-aged specimens reveal a geometry-dominated thermal response, with thick insulation regions exhibiting a 14-minute delay in peak temperature attainment compared to thin layers. However, service aging introduces a distinct temporal signature: aged specimens demonstrate a quantifiable hysteresis in cooling dynamics and a reduced heat dissipation rate compared to new samples. Accelerated aging tests at 125-150°C confirm that oxidative crosslinking increases tensile strength (TS) by ~28.6% while reducing elongation at break (EAB) by up to 60.5%, directly correlating mechanical embrittlement with the observed increase in thermal resistance. These quantifiable dynamic metrics enable early detection of insulation degradation before electrical failure, offering a practical tool for reliability enhancement in offshore wind and industrial networks.