Rapid degradation of transformer oil quality may occur not only due to insufficient chemical stability, but also due to the influence of construction and insulation materials used in a transformer.
The most active catalyst of transformer oil oxidation is copper. The opinions of researchers regarding the influence of iron, aluminum, nickel and zinc are divided. This can be explained, on one hand, by the difference of oil oxidation conditions, and on the other hand, by the different sensitivity of oil of varying hydrocarbon composition to different metals. Copper-phosphorus brazing alloy is known to have a very pronounced effect on oil oxidation. Other metals have little influence on oxidation. Catalytic properties of copper alloys (bronze or brass) increases proportionately to the content of copper.
Chromate steel and titanium alloys are inert as oxidation catalysts.
It should be noted that the dissipation factor is the most sensitive parameter to assess the influence of metals on oil oxidation; acid numbers of the oil after oxidation in the presence of metals with low catalytic properties are practically the same, while dissipation factors are notably different.
A copper wire tightly wrapped in several layers of cable paper causes less change of oil parameters than the same wire without paper insulation. This is caused by reduced diffusion of oil to the copper surface through the paper, as well as absorption of the oxidation products by the paper.
Copper catalytic properties depend on the condition of its surface. Thus, when assessing the influence of metals on transformer oil oxidation, specific conditions in the equipment must be taken into consideration.
It has long been believed that copper and iron, taken in certain surface proportion, as well as the oxides of these metals, accelerate oil oxidation significantly. However, research shows that this is only true for oils lacking purification.
The question of different catalytic properties of metals in the oxidation process does not yet have a definitive answer.
The higher catalytic properties of copper compared to iron is due to the former’s higher thermolability, making copper more likely to convert to soluble state, accelerating the interaction between copper and peroxides, further developing the oxidation process.
Besides, metal particles are likely to form soaps in contact with oil aging products. Metal derivatives, i.e. oxides and layers of organic acids – soaps are usually more likely to promote oxidation of oil than metals themselves. Accumulated small metal particles (5-10 micron) form nodes in the parts of transformer with the highest load, which may lead to premature aging of paper insulation, overheating and partial discharge on the particles.
Oil transformers contain various solid insulation materials. The amount of cellulose materials in oil transformers is quite significant.
Varnished fabric or paper are used to increase the mechanical strength of insulation of high voltage bushing and other high voltage parts of the transformer. Some parts are made of SRB paper laminate, plastic, wood or cotton tape.
When the oil ages in contact with oil-resistant rubber, the dielectric strength of the oil is reduced. Ample white sediment can be seen in the oil; the sediment contains zinc oxide, one of the ingredients of the rubber.
Cellulose materials inside the transformer also have an impact on oil aging. The higher density of a cellulose material, the higher its absorption ability. It is for this reason that oil quality changes more with dense paper than lighter one.
When oxidation is assessed by the amount of consumed oxygen, it is possible to clearly identify the accelerating impact of cellulose materials on oil oxidation. Besides, when oil is in contact with cellulose, fibers of that cellulose can be found in the oil. These fibers are the result of cellulose degradation. They contain absorbed water, increasing moisture content in the oil and decreasing its dielectric strength.
There are four types of water contamination of transformer oil;
- Dispersed water comes in the form of droplets and as the worst effect on the dielectric properties of oil and paper. Due to uneven distribution of the water inside the tank, the most intensive aging of oil and paper occurs, especially in contact with metals.
- Water absorbed by solid particles reduces the dielectric strength of the oil and other oil quality parameters. In case of uneven distribution, there is a risk of drift into the areas with maximum loads.
- Solved water, unlike dispersed and absorbed water, is evenly distributed throughout the volume and has minimal effect on the electrical insulation properties of the oil, but has pronounced effect on aging of both paper and oil insulation.
- Water in the main and aromatic rings, formed due to hydrogen lings has virtually no effect on oil quality.
Dispersed water in the electric field is attracted to the areas of highest loads. The droplets are elongated along force lines of the field. At a certain field strength, depending on actual water concentration, droplets begin to coagulate forming thin water channels. This results in strong deformation of the electric field, since the dielectric strength of polar liquid is much lower than that of the oil. The strongest reduction of critical strength occurs when the oil also contains fibers. The fibers intensively absorb water, and are drawn into the force lines, therefore, the formation of ‘bridges’ of conductivity occurs faster.
A breakthrough in liquid dielectrics with impurities under long term exposure to electric current is in essence a gas breakthrough. The impurities in the oil and forming a colloid solution or an emulsion, are drawn into the area between electrodes and drift in the direction of the field. A significant amount of heat due to the high heat conductivity of the dielectric, is consumed by heating the particles of the impurities. If the impurities are the reason for the high mean conductivity of the oil, they evaporate at low impurity boiling point, forming a gas channel, where the breakthrough occurs.
Beside desorption of water from the surface of cellulose fibers, gas desorption also occurs with consequent formation of gas channels if the gas content in the oil is over 6% by volume, depending on the temperature and nature of the gases formed.
With high gas concentrations, bubbles are formed in the oil, where the discharge, especially in its initial stages, occurs more easily than in the oil.
The influence of the dissolved gases defines the dependency of the dielectric strength on pressure. Gas solubility increases with pressure, the amount of bubbles is reduced, with the corresponding effect on the breakthrough voltage.
The amount of particles is influenced by carbonization of oil molecules, which occurs with partial electric discharges. The amount of particles formed correlates with the mass number of the base oil, and in this respect naphthenic oils have an advantage over paraffinic oils.
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