Early ageing of 33KV transformers in a distribution network

A good understanding of the accumulation of measured ageing by-products can help to improve the maintenance strategy of transformers. Among the different ageing byproducts, an increasing rate of acid in oil is considered to be a reliable ageing indicator according to IEC 60422. The previous work on oil acidity analysis identifies some transformers with accelerated or abnormal acidity growth with values over 0.15 mgKOH/g before 30 years. This paper continues the previous work by focusing on the accelerated acidity growth identified from an oil database containing twenty years´ worth of oil data. Results show that transformers manufactured after 1980 tend to have an accelerated acidity growth and that the underlying cause of this phenomena is unlikely to be associated with indirect influencing factors such as the environment, load or transformer design. As a result, a fourth indirect influencing factor, oil chemistry is suspected as a potential cause. Whatever the reason, it is clear that changes made in the mid1980s have led to accelerated acidity growth, faster transformer ageing and require extra efforts of asset managers to maintain transformer assets.