Transformer Oil Tests
7.1 Dielectric Strength
This test measures the voltage where the oil electrically breaks down. The test gives a good indication of the amount of contaminants (water and oxidation particles) in the oil. DGA laboratories typically use ASTM D-1816. Using the D-1816 test, the minimum oil breakdown voltage is 20 kV for transformers rated less than 288 kV and 25 kV for transformers 287.5 kV and above. If the dielectric strength test falls below these numbers, the oil should be reclaimed. Do not base any decision on one test result or on one type of test; instead, look at all the information over several DGAs and establish trends before making any decision. The dielectric strength test is not extremely valuable; moisture in combination with oxygen and heat will destroy cellulose insulation long before the dielectric strength of the oil has given a clue that anything is going wrong .
The dielectric strength test also reveals nothing about acids and sludge. The tests explained below are much more important.
7.1.1 Interfacial Tension
This test (ASTM D-791-91 ), is used by DGA laboratories to determine the interfacial tension between the oil sample and distilled water. The oil sample is put into a beaker of distilled water at a
temperature of 25 ºC. The oil should float because its specific gravity is less than that of water (specific gravity of water is one). There should be a distinct line between the two liquids. The IFT number is the amount of force (dynes) required to pull a small wire ring upward a distance of 1 centimeter through the water/oil interface. (A dyne is a very small unit of force equal to 0.000002247 pound.) Good, clean oil will make a very distinct line on top of the water and give an IFT number of 40 to 50 dynes per centimeter of travel of the wire.
As the oil ages, it is contaminated by tiny particles (oxidation products) of the oil and paper insulation. Particles on top of the water extend across the water/oil interface line that weaken the surface tension between the two liquids. Particles in oil weaken interfacial tension and lower the IFT number. The IFT and acid numbers, together, are an excellent indication of when the oil needs to be reclaimed. It is recommended that the oil be reclaimed when the IFT number falls to 25 dynes per centimeter. At this level, the oil is very contaminated and must be reclaimed to prevent sludging, which begins around 22 dynes per centimeter. See FIST 3-5 . If oil is not reclaimed, sludge will settle on windings, insulation, cooling surfaces, etc., and cause loading and cooling problems, as discussed earlier. This will greatly shorten transformer life. A definite relationship exists between the acid number, the IFT, and the number of years in service. The accompanying curve (figure 56) shows the relationship and is found in many publications. (It was originally published in the AIEE transactions in 1955.) Notice that the curve shows the normal service limits for both the IFT and the acid number.
Figure 56 – Interfacial Tension, Acid Number,
Years in Service 7. Transformer Oil Tests