Operational condition assessment of in-service distribution transformers

Oil tests are crucial for condition assessment of transformers. Moisture, acidity, breakdown voltage, dissolved gas analysis and furans analysis are common oil tests carried out by utilities. IEC 60422 and IEC 60599 have specified some limit values for detecting any abnormalities based on oil tests. However, these limit values might not be suitable for all transformers at different ages or under different operating conditions and relying too much on these values could be costly for transformer utilities. Trend analysis of oil condition monitoring data is useful in providing additional information for the detection of any abnormalities of transformers. In this paper, analysis on the oil condition monitoring data of 132 kV distribution transformers is presented. Typical percentile values are calculated for moisture, acidity, breakdown voltage and furans levels whose results are compared with the limit values provided in IEC 60422 and IEEE C57.106. Trend analysis of moisture, acidity, furans, Carbon Monoxide (CO) and Carbon Dioxide (CO₂) are carried out on a transformer population. Acidity is found to be strongly related to transformer age and a model is proposed to represent ageing of transformers based on acidity.