Transformer Oil Degradation Analysis

Transformer oil Degradation

Transformer oil degradation is a process caused by a number of factors. It can be the result of thermal, mechanical stresses and a few other things. Aging of transformer oil leads to many possible failures. That is why proper transformer maintenance is important.

Transformer oil maintenance

Maintenance of transformer oil should include several important phases, which should not be omitted. First of all, every transformer and its contents needs regular control and analysis. Impurities or other suspicious changes must be found as soon as possible. Purity of transformer oil is crucial. When problems in the system are neglected, transformer oil degradation will lead to serious consequences.

The next phase of transformer oil maintenance should be changing or purifying it. This can be done in several different ways. The simple basic oil change can be a long and a rather expensive procedure. The biggest  problem is the raw material cost. Due to the fact that oil change must be performed regularly (with varying frequency), transformer maintenance can be worth a fortune. However, there are technologies that allow to clear up used transformer oil. This way, maintenance costs can be reduced significantly. Some companies, such as GlobeCore, design special equipment for automatic oil purification, pumping, dehydration etc.

The frequency of transformer oil analysis depends on transformer specifics. Some transformers require annual service, while others must be serviced quarterly or even monthly.

Why transformer oil analysis is important

Transformer oil functions as an electrical insulator, coolant etc. This makes its condition very important for correct and reliable operation of transformers.

With time, transformer oil deteriorates due to oxidation and moisture ingress. Transformer oil should be tested properly and frequently.  Changes in the oil have a significant impact on economic performance, transformer reliability and other factors. So utility companies have a vested interest in periodic oil testing. Through such testing, transformer life can be substantially increased, delaying the need to invest into replacement of the equipment. To avoid trouble, any suspicious changes in the oil must be addressed immediately.

Transformer Oil IFT test

The purpose of transformer oil interfacial tension test is to estimate the level of transformer oil degradation.

Various types of impurities may be found in the oil: oxidation byproducts, acids etc.  The presence of these hydrophilic materials in the insulating fluid can affect the chemical (acidity), electrical (dielectric strength), and physical (interfacial tension) properties of the fluid. This analysis  measures the surface tension of oil against water, which is highly polar. The higher the difference in polarity between the two liquids, the lower the index of the interfacial tension between them. Therefore, the higher the concentration of hydrophilic materials in the insulating fluid, the lower the interfacial tension of the oil is. There are several methods of interfacial tension tests against water:

  • Measuring the size of a water droplet

It is a simple method, in which a droplet of water below the surface of the oil is analyzed.

  • The ring method

The ring method of transformer oil degradation analysis (IFT) against water is as follows: a special device, called tensiometer is used. It measures interfacial tension based on Du Nouy principle. A sample of oil is carefully floated on top of water. Then the force necessary to pull a platinum ring upward from below the water through the oil is measured with the help of a calibrated torsion wire. The force is measured at the point at which the ring breaks free of the water as it is being pulled upward through the oil layer. The platinum ring is made to precise dimensions. Since this test is very sensitive to trace contaminants, one must be very thorough in handling the sampling device, the sample, and the instrument. There are correction factors that have to be taken into consideration associated with the dimensions of the ring and the densities of the water and the oil sample. The results are given in units of dynes/cm or the numerically equivalent units of milliNewtons/meter (mN/m).

Transformer oil Features

  • Transformer oil density

The density of transformer oil is one of its most important physical characteristics. Oil density has a large impact on the operation of your electrical power transformers. It is therefore important to pay close attention to oil density when filling or refilling your electrical transformers with dielectric insulating oil.

The specific density of transformer oil varies based on the manufacturer and the region or location where the oil is primarily used.  Despite the variables that determine the density of the oil, it must not exceed 900 kg/m3 at +20°C/+68°F.

Transformer oil density is defined as the ratio of the mass of the substance to the volume of the substance.  Simply stated, it is the ratio of the weight of the oil to the volume of the oil. Transformer oil temperature also has an effect on the density of transformer oil.  As the temperature increases, the density of the oil decreases.

  • Transformer Oil Color

The color and appearance of transformer oil are useful for comparative evaluation: rapid darkening or dark color of the oil is a sign of oil ageing. Oil color is assigned a number ranging from 1 to 8: the darker the oil, the higher the color index.

Undesirable contaminants can be detected by analysing the appearance of the oil. Turbidity or sediment indicate free water, insoluble sludge or dust particles. If these are present, breakdown voltage and/or dissipation factor, as well as other parameters, are likely out of specs and corrective measures are necessary.

Transformer Oil Degradation Test by GlobeCore

Transformer oil degradation analysis can be made much simpler: GlobeCore offers a special laboratory scale unit to assist with oil analysis, the UVR-L. This unit will help in the analysis of not only transformer, but also turbine, diesel and other oils. You can order this GlobeCore device or, alternately, request oil sample testing at our facilities.

 

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