Transformer oil management overview

by J De Bruto, Saftronics


Published in: Transformers and Substations Handbook: 2014

Chapter 4: Maintenance - page 77

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Transformer oil maintenance and management are important factors in the performance of oil-filled transformers. Many transformer owners periodically make use of companies that service their transformers and submit oil test certificates prior to and after oil purification.

Oil in transformers plays an important role in transformer reliability and life expectancy. The main functions of oil in transformers are to:

o Provide dielectric strength
o Provide heat transfer for cooling
o Protect the transformer paper insulation
o Test as a diagnostic tool for condition of equipment

Oxidation is damaging to oil in transformers and is increased by the following factors:

o Heat owing to load conditions
o Oxygen content
o Presence of metal catalysts (iron, copper and aluminium)
o Electrical stress cellulose
o Oxidation products

By-products of oxidation include alcohols, acids, ketones, peroxides, etc. They act on each other and the oil to form sludge, within the cellulose and this is aggressive towards the insulation paper.

Oxidation not only shortens the life of the insulation paper, but it also restricts heat transfer and this can lead to overheating.

To control oxidation it is necessary to control four of the factors mentioned:

o Oxygen content
o Moisture content
o Heat
o Oxidation products


Take note

  • The insulating system is the weakest link and therefore the most important part of the transformer to maintain.
  • Of all transformer failures, 85 % are attributable to failure of the insulating system.
  • No single test is consistently adequate for pinpointing a transformer problem.