Multiphysics Modelling and Experimental Validation of Thermomechanical Fatigue in Nigerian Distribution Transformer Windings Under Cyclical Loading Conditions

This research develops and experimentally validates a multiphysics model of thermomechanical fatigue in Nigerian distribution transformer windings subjected to the cyclical load current and ambient temperature variations characteristic of Nigerian distribution system operation, making original contributions to the coupled thermal-electrical-mechanical modelling of transformer insulation ageing. Distribution transformers in Nigeria experience daily load cycles with peak-to-minimum ratios substantially higher than those in temperate climate countries due to the coincidence of an evening demand peak with elevated ambient temperatures and the absence of space heating loads that flatten demand in colder climates. These severe cycles induce repeated thermal expansion and contraction stresses in the winding copper conductors, paper insulation, and press board insulation structures that accumulate as mechanical fatigue damage, reducing insulation dielectric strength and ultimately precipitating turn-to-turn fault failures. The multiphysics model couples a 3D finite element thermal model of the transformer active part, an electromagnetic model for loss density distribution, and a continuum mechanics model for thermal stress calculation using viscoelastic constitutive relations appropriate for cellulose insulation materials, all co-simulated in COMSOL Multiphysics. Original contributions include calibrated cellulose insulation viscoelastic material parameters derived from dynamic mechanical analysis of insulation samples aged under controlled conditions replicating Nigerian thermal profiles, and a novel fatigue damage accumulation law incorporating both thermal ageing of insulation mechanical properties and the cyclic stress amplitude history. The model is validated against winding short-circuit withstand test results from 10 distribution transformers withdrawn from Nigerian service. Keywords: thermomechanical fatigue, transformer winding, multiphysics modelling, insulation ageing, Nigeria.