Re: transformer di-electric strenght!!!
As long as no one elso took a shot at answering you post I did a bit of research and fould the following which may be of some help to you:
My reference from the Electrical maintenance hints as published one time by Westinghouse stated that " No one test is sufficient to reveal the condition of insulating oil in service. The composition evaluation of certain significant characteristics permits placing oils in classifications." For example: Group I- Oils that are satisfactory for continued use, Group II-Oils that require only minor reconditioning, etc. Group III- Oils in poor conditions that may be wither reclaimed or disposed of, and, Group IV- Oils that must be discarded.
Besides the dielectric test that you have referred to there is also a Neutralizations No. mgKOH per gr. oil (the number of milligrams of potassium hydroxide required to neutralized the acid in one gram of oil), Interfacial tension in dynes per cm (the inerfacial tension between insulating oil and water is a measure of the molecular force between the two substances), Power factor 60hz 25C percent(determines the ration of the power dissipated in the insulating oil in watts), and moisture content PPM.
I also found this reference on the US Dept. of the Bureau of Reclamation:
"Dielectric Strength. This test measures the voltage at which 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 Test Method No. D-877 or D-1816. The acceptable minimum breakdown voltage is 30 kV for transformers 287.5 kV and above, and 25 kV for high voltage transformers rated under 287.5 kV. 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 [5]. The dielectric strength test also reveals nothing about acids and sludge."
When looking at a small 15kva transformer it may be less expensive just to replace the oil than to do all of these additional tests. However, one must also consider if there are other contaminates that may have deteriorated the winding insulation as noted above.
Also, it was interesting to note that Westingshouse considered that 28kv was the value for dielectric breakdown in the manual.
