Comprehensive Condition Assessment of 46 In-Service Liquid-Immersed Transformers Using Oil Diagnostic Parameters (2025)

Condition monitoring of in-service transformers is essential for maintaining power system reliability and preventing unexpected failures. In this study, the operational health of 46 distribution transformers, each rated at 2 MVA and 6.6 kV, is systematically evaluated, considering service periods ranging from 10 to 30 years. Insulating oil samples are collected at specified service intervals of 10, 15, 20, 25, and 30 years and subjected to comprehensive diagnostic investigations, including breakdown voltage (BDV), moisture content, acidity, interfacial tension (IFT), flash point, tan delta, electrical resistivity, dissolved gas analysis (DGA), and furan compound analysis. The test results are classified according to transformer age and compared with fleet-level averages to examine patterns associated with progressive aging. The findings reveal a consistent reduction in the dielectric strength and chemical stability of the insulating oil with increasing operational duration. These findings highlight the critical role of oil diagnostic techniques in accurately assessing transformer condition and generating valuable insights for implementing condition-based maintenance practices, ultimately enhancing equipment reliability, operational safety, and the overall resilience of the power infrastructure.