The dielectric breakdown voltage test is a measure of an insulating oil’s ability to withstand higher than rated voltage. Typically it is desirable that the breakdown voltage is high rather low. The DBV test can indicate the presence of moisture in oil.
Moisture In Oil
In the presence of oxygen, moisture can pose extreme hazards to transformer insulation. Typically, when moisture presence in insulating oils are doubled, the life of an insulating system is reduced by half.
As insulating oil ages, the same is contaminated by oxidized by-products of oil and paper insulation. Particles on top of water extend across the water and oil interface line which weakens the surface tension between two liquids. Particles in oil weaken interfacial tension and lower interfacial tension number.
When the cellulosic insulation materials within a transformer undergo degradation some of the products formed are derivatives of the aromatic compound called furan. The five most prevalent derivatives of furan that arise from the degradation of the cellulose and that are soluble in the oil to an appreciable degree are the following: 2-Furaldehyde ; Furfuryl alcohol ; 2-Acetylfuran ; 5-Methyl-2-furaldehyde ; and 5-Hydroxymethyl-2-furaldehyde. The five furanic materials normally analyzed in this procedure are aromatic compounds that arise from the degradation of the cellulosic materials within a transformer either by normal aging or from being involved with an incipient fault. Thus the amount of these products present in the oil might be a good indication of the condition of the cellulosic insulation.
Corrosive sulfur and the effect that it has in transformer systems can be significant. The extent of the corrosion damage caused by sulfur, if left unchecked can cause failure of the apparatus. Sulfur is found in many materials of transformer construction including the copper, paper insulation, gaskets and oil. Not all sulfur is considered corrosive but the tendency to operate transformers at substantially higher temperatures can aggravate an already present corrosive sulfur condition or convert stable compounds into reactive ones that will cause damage.