Oil Acidity Analyses for Condition Assessment of Transformers in a Distribution Network

A good understanding of the accumulation of measured ageing by-products can help to make proper decisions for transformer maintenance. Analyses were performed on the oil acidities of nearly 700 operating 33/11(or 6.6) kV transformers, which were manufactured within the time span from 1936 to 2011. The trend of acidity growth versus age is characterised into three stages—the early stage with the slow acidity growth which is dominated by intrinsic acidity fluctuations (acidity drops<0.05 mgKOH/g), the transitional stage, followed by the stage of exponential increase. In the early ageing stage, the lack of acidity records before 30 years' age seems of no significance for a normal transformer which does not experience the fast acidity growth (acidity>0.15 mgKOH/g, age<30 years). The acidity…