ANALYSIS OF THERMAL AGEING EFFECTS ON TRANSFORMER OIL DIELECTRIC PROPERTIES

Abstract

This study investigates the thermal ageing effects on transformer oil dielectric properties by analyzing three key parameters: breakdown voltage (BDV), operating temperature, and load in oil-immersed distribution transformers. Transformer oil samples were randomly collected from operational units rated at 200, 300, and 500 kVA and subsequently tested in the laboratory. The dielectric strength was measured using an FS 2080 dielectric oil tester, while transformer load was monitored with a 1000-ampere digital clamp meter. Analysis of the data revealed that approximately 60% of the transformers exhibit a degraded BDV, with values reduced below the America standard for testing and measurement (ASTM) minimum standard of 30 kV from the 60–70 kV typical of new oil. Elevated operating temperatures were also observed, exceeding manufacturer nameplate ratings by an average of 10°C for 200 kVA, 6°C for 300 kVA, and 12°C for 500 kVA units, which significantly accelerates the oxidation process in the oil. This thermal stress, often resulting from excess load, localized internal heating, and issues such as loose contacts, low oil level, and water ingress, enhances oxidation reactions that lead to the release of hydrocarbon gases and formation of acidic compounds. These chemical changes degrade the oil’s dielectric strength, thereby increasing the risk of sudden transformer failure. The findings underscore the critical need for real-time temperature monitoring and control to mitigate the rapid degradation of insulation oil and extend transformer life.