Experimental Assessment of Oil Regeneration Technique for Transformer Life Extension

  • Ramamoorthi Venkatasubramanian

Student thesis: Master of Philosophy


With a large population of ageing transformers, utility asset managers are becoming increasingly interested in techniques which can help prolong the life of the transformer and optimize its efficiency in terms of economic operation and long-term performance. The performance of a transformer is affected by the condition of the insulation, which in turn affects the reliability of the network. The ageing of oil and paper results in the generation of ageing products such as moisture and acids, which further accelerate the ageing process through cyclic reactions. Thus, these ageing by-products need to be removed from the system at an appropriate stage in order to ensure the long-term performance of the transformer by slowing down the ageing process. This can be achieved through several measures, of which oil regeneration is becoming popular.In order to study the long-term effect of oil regeneration, oil samples collected from an on-site regeneration exercise performed on a 132/33 kV transformer were utilised in a laboratory-accelerated thermal ageing experiment. Oil samples collected before and after the regeneration were aged alongside new oils for comparison. Key ageing indicators, namely Total Acidity Number (TAN), moisture, Low Molecular Weight Acid (LMA) content, and High Molecular Weight Acid (HMA) content for the oil, and moisture, Low Molecular Weight Acid (LMA) content, and Tensile Index (TI) for paper were measured regularly. It was found that performance of oil after regeneration is better than that of oil before regeneration, and is even comparable to that of new oils. Slower reduction of TI of paper aged in oil after regeneration than paper aged in oil before regeneration conforms to previous findings that oil condition plays a role in determining the ageing rate of paper, and supports the statement that oil regeneration can slow down paper degradation in transformers.Oil regeneration experiment was performed on a 6.4/0.4 kV 77-year-old retired distribution transformer. Oil regeneration was conducted in two stages (stage 1 and stage 2), with the first stage aimed at 'cleaning' the oil and the second stage targeted at 'cleaning' the paper. Oil samples were collected at regular intervals throughout the process, and paper samples were collected from the transformer before oil regeneration, after stage 1, and after stage 2 of oil regeneration. It was found that oil regeneration restores oil parameters, including moisture and acidity, similar to those of new oils. Analysis of paper samples indicated a reduction in paper moisture at the end of stage 2 by nearly 40%, while LMA in paper exhibited a reduction at the end of stage 2 by around 30% on average. Similar reductions of paper moisture and acid content in the laboratory-conditioned samples provided strong evidence to the notion that oil regeneration is capable of improving the paper condition, and subsequently reduce the ageing rate of the paper insulation. It was concluded that strong evidence has been presented to show the effects of oil regeneration techniques on improving the conditions of both oil and paper under the investigated conditions. Therefore, it could be an effective way of managing the ageing asset population.
Date of Award1 Aug 2015
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorQiang Liu (Supervisor) & Zhongdong Wang (Supervisor)


  • Reclamation
  • Ageing
  • Regeneration
  • Mineral oil
  • Asset management
  • Cellulose paper
  • Transformer

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