GlobeCore / Instruments / Oil service for power transformer

Oil service for power transformer


Power transformers are the basis of the modern electric power industry. Their reliable operation guarantees the delivery of electricity to large industrial facilities and ordinary consumers. In this article we will consider the oil servicing – as an important component of the transformer insulation maintenance.

We shall begin with functions. Oil in transformer isolates individual parts and removes heat, which forms in surplus when working at elevated temperatures. Oil takes on some of the negative effects of operation directed at transformer. Therefore, oil condition should be regularly checked and, if deviations are detected, take appropriate measures.


An external inspection of a transformer checks the temperature of an upper oil layer. Technically, it is provided by thermal sensors and thermometers. The operational temperature is 95 ° C. When oil temperature is higher  it is recommended to reduce the existing load.

The simplest and least labor-intensive way of testing transformer oil – is to determine the electrical strength, also visually assess the presence of mechanical impurities and moisture and in addition measure  the breakdown voltage.

The next in complexity of implementation is an oil screen test. It includes the previous tests plus the tests determining flash point and acid number. The latter parameters determine the degree of aging, since its process is always associated with an increase of acidic components. Oil screen test is recommended after the completion of transformer maintenance.

Complete oil test includes the maximum set of parameters and characteristics. All new and regenerated oils are subjected to this analysis.


Test results provide information on the current state of oil. The standard oil quality parameters are proposed in the guidelines and instructions. When oil test results do not exceed the specified limits, the oil continues to perform its functions well, protecting a transformer from overheating and breakdown. Otherwise the oil is subjected to regeneration or replaced with a new one.

If there are no serious structural changes in transformer oil, it can be restored through cleaning and drying in an UVM unit. If oil is saturated with aging products, it can not do without regeneration in a CMM-R unit. Regeneration equipment from GlobeCore allows you to process oil directly in an energised transformer. In practice, it is possible to restore oils with acid number below 0.4 mg KOH / g. Regeneration that is done on-time prolongs the life of oil, and also the life of transformer.

A regular oil maintenance consists of methods that prevent moisture and oxidation. To reduce the amount of moisture and air, the transformer is equipped with additional thermosiphon and air filters, as well as special protection – a nitrogen membrane system.

Adding oil additives is also an important part of the service process. Fresh oil  has resins, which at first serve well as antioxidants. But during the life their quantity decreases, requiring special additives to increase the antioxidant stability.


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