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The ability of unused mineral insulating oil to withstand oxidation under thermal stress, whilst in the presence of oxygen and a copper catalyst, is called oxidation stability. To increase oil resistance towards oxidation an inhibitor additive can be added to the oil. In transformer oils it is mainly the phenolic type that is used, with the common and generally accepted compounds being 2,6-di-tert-butyl-paracresol (DBPC) and 2,6-di-tert-butyl-phenol (DBP). The efficiency, or inhibitor response, of added inhibitors will vary according to the chemical composition of the base oil. Hence the selection of an inhibited oil should be made with consideration of the refining degree of the base oil.
Inhibited oils have a different oxidation pattern when compared to uninhibited oils. At the beginning of service life, the synthetic inhibitor is consumed with little formation of oxidation products. This is referred to as the induction period. When the level of inhibitor is reduced, the oxidation rate increases.
Apart from decrease of inhibitor content, decrease of interfacial tension (IFT), increase of dielectric dissipation factor (DDF) and change in colour in inhibited oils may also be an early indication of the initial formation of oxidation products.
The inhibitor content should be monitored at regular intervals, the frequency of which will depend upon operational temperature and load levels. It is recommended that inhibitor concentration is measured initially to establish a baseline to compare future changes to that and when concentration falls below 60-40% of the initial concentration a common measure is to top up with inhibitor to prolong useful oil lifetime.
Carl has a BSC(eng) in Electrical Engineering from the University of Witwatersrand, South Africa. He joined Nynas in 2015 and has a background working for the South African power utility Eskom where his focus was power transformers and other high voltage equipment. Carl has experience in specifications, maintenance, condition monitoring and failure investigations of power transformers as well as high voltage insulation.
He currently holds the position of Development Engineer where he works developing insulating oil products as well as liaising with and supporting the industry.
With greater demands on power grids and ever-tighter load loss regulations, it’s vital to reduce the energy loss from transformers. Choosing the right transformer oil is a quick and cost-effective first step.Read more about Energy loss: Do you have room for improvement?