Investigation of Thermal Performance of Transformer Insulation Oil Under Nigerian Tropical Service Temperature Conditions

This study investigates the thermal ageing performance of transformer insulation oil samples sourced from distribution transformers in service under Nigerian tropical temperature conditions, with the aim of establishing locally calibrated ageing rate parameters for transformer lifetime estimation models. Existing transformer ageing models based on the IEC 60076-7 loading guide were developed using data from temperate climate environments and may not accurately represent the accelerated ageing experienced by Nigerian distribution transformers routinely operating at ambient temperatures 10 to 15 degrees Celsius above the assumed design baseline. Oil samples were collected from 30 transformers representing five age bands (0, 5, 10, 15, and 20 years of service) from distribution networks in Lagos, Kano, Enugu, and Bauchi to ensure geographic diversity. Thermal ageing testing was conducted by subjecting oil samples to accelerated ageing in an oxidation bomb apparatus at temperatures of 100, 110, 120, and 130 degrees Celsius for 500 hours. Oil characterisation tests performed before and after ageing included dielectric breakdown voltage per IEC 60156, acid number per ASTM D664, interfacial tension, colour, moisture content, and dissolved gas analysis. Results show that Nigerian service-aged oil samples exhibit higher initial acidity and lower dielectric breakdown voltage than the corresponding age-band benchmarks derived from temperate climate data, confirming that tropical operation accelerates ageing. The Arrhenius ageing constants derived from the accelerated tests show a 23 percent higher activation energy than the IEC 60076-7 default, necessitating revised loading guidelines for Nigerian transformers. Keywords: transformer oil, thermal ageing, insulation degradation, tropical conditions, Nigeria.