Dielectric Breakdown Voltage Testing - Part 4

Dielectric Breakdown Voltage Testing Part 4 – ASTM D1816 for In-service Insulating Liquids - The Fundamentals

For in-service insulating liquids, D1816 values are expected to fall within acceptable ranges listed below. These AC ranges are the same as those contained in the IEEE standard. These values are indicated for both gap settings and for the three voltage classes indicated by the standard. We include also our own questionable and unacceptable ranges (all values are in kV):

While the D1816 Dielectric Breakdown Voltage is more sensitive and is also generally more useful than the D877 method under many circumstances, D1816 has a different, but still important, limitation. The method is sensitive to dissolved gases in the oil. As a result, high dissolved gas content in the insulating oil sample may depress the D1816 value to the point where it is outside the acceptable range. This may occur in insulating liquids where the moisture content is acceptably low, where the aging of the liquid has not become advanced, and where there are no problems with abnormal particle counts or levels of outside contamination. In short, oil that is acceptable in every respect that affects its performance in electrical equipment may still "fail" the D1816 determination because of dissolved gas content. It is our experience that this can happen in gas blanketed transformers and even occasionally in free breathing transformers.

A "good" D1816 result indicates that there is nothing seriously wrong with the insulating liquid with regard to moisture, contamination, or aging/oxidation products. A "bad" D1816 result, however, does not always indicate that there is something wrong with the oil. Acceptable levels of the other materials to which the test is sensitive can be detected along with a normal amount of dissolved gas to depress the D1816 result so that it is no longer in the acceptable range. For this reason, in particular, relying on the D1816 to provide meaningful results for smaller industrial and distribution class equipment that is gas blanketed is usually not terribly useful.