Investigation of the Temperature Dependence of Dielectric Properties of Local Transformer Oil Blends

The electrical insulation performance of power transformer oil deteriorates with increasing temperature, a process critical to managing transformer ageing and preventing catastrophic dielectric failure. Understanding the temperature dependence of dielectric properties in transformer oil blends used by Nigerian power utilities is essential for maintenance scheduling and life assessment. This study investigates the temperature dependence of dielectric permittivity, dissipation factor, and dielectric breakdown voltage of three transformer oil blends in service in the Abuja and Lagos distribution networks, including standard mineral oil, a mineral-vegetable oil blend (70:30 mineral to castor oil), and a reclaimed oil sample. Measurements are conducted at temperatures of 25, 40, 60, 80, and 100 degrees Celsius following IEC 60247 procedures for permittivity and dissipation factor and IEC 60156 for breakdown voltage. Dissolved gas analysis is performed to characterise ageing markers. Results indicate that dissipation factor of all three oils increases exponentially with temperature, with the reclaimed oil showing dissipation factor values 3 to 5 times higher than new oil across the measured temperature range. The mineral-vegetable blend maintains higher breakdown voltage than mineral oil at temperatures above 80 degrees Celsius, suggesting improved thermal stability. Dissolved gas ratios indicate incipient thermal fault in one mineral oil sample from the field. Keywords: transformer oil, dielectric properties, temperature dependence, breakdown voltage, dissipation factor