Dielectric Breakdown Voltage Testing - Part 2

Dielectric Breakdown Voltage Testing Part 2 – ASTM D877

In this article, we will start looking at the three standard methods for dielectric breakdown voltage. We will be examining the applications for each method, conditions that cause results of each method to change, and the limitations of each method.

We will start with D877, the flat disk method, which is the oldest. Industry groups have been deemphasizing the application of D877 for several years. The method is not very sensitive to the presence of moisture, and, since that was the reason that most people were running the dielectric breakdown voltage, the use of the test decreased as this limitation became more widely understood. The D877 test is not sensitive to changes in moisture content, unless the percent saturation of the oil is greater than 60%. Transformer owners in the United States typically do not operate transformers that wet. The method is also not sensitive to the aging and oxidation of the oil. When IEEE revised C57.106 in 2002, they eliminated the values for the D877 from their evaluation criteria for in-service oil in transformers.

SD Myers has continued to provide results for the D877 dielectric breakdown voltage in all of our standard packages of tests that include the liquid screen tests. In fact, after the changes made by IEEE, we fielded criticism from some of our competitors for continuing to run this test routinely. In spite of the fact that the application of this method may be limited in mineral oil filled transformers with regard to moisture increases and oil aging, D877 dielectric breakdown voltage determinations continue to yield important information for a wide variety of equipment types and insulating liquid types - including oil filled transformers. We continue to recommend the test for purposes of providing that information.

D877 dielectric breakdown voltage is sensitive to high moisture levels and to certain types of contamination, particularly conductive particles. In mineral oil filled transformers, some of these types of contamination may show up first in the D877 determination, particularly if other tests such as the liquid power factor are not performed. For mineral oil-filled circuit breakers and switches, moisture levels are frequently high enough to depress D877 dielectric breakdown voltage values. In load tap changes, moisture and contamination levels may be high enough so that the lowered D877 values may indicate a need for inspection and maintenance. This is especially true in cases where the desiccant breathers may need to be serviced or may be improperly installed or operated.

For fluids other than mineral dielectric fluid, the D877 method has other valuable and specific applications. Chlorinated dielectric fluids such as askarel, chlorinated benzenes, and perchloroethylene may form hydrochloric acid when the fluid breaks down. The acid may then be neutralized by reaction with bare metal in the transformer. The resulting inorganic compounds and any residual acid have the effect of lowering the D877 dielectric breakdown voltage. In silicone fluid filled units, an electrical discharge has the effect of lowering the D877 value, as well. In any liquid filled electrical equipment, contamination by any incompatible dielectric fluid, free water, or materials other than a dielectric fluid may also be indicated first by a decreased value for the D877 determination.

Any D877 value that falls outside the acceptable range should be confirmed by retest and investigated as to the cause. This investigation may require that some non-routine or troubleshooting testing also be performed. The values that we use for evaluating D877 values for in-service transformers are indicated by the following table (values in kV):

The D877 dielectric breakdown is also frequently used as an acceptance test for new oil received from a supplier in either bulk loads or containers. Acceptable D877 are necessary (but not sufficient by themselves) to ensure that the oil was stored and transported properly. A minimum value of 30 kV is typically specified.

Generally speaking, D877 values for new oil and for in-service applications in transformers are higher than the minimums indicated above. A trend downward in the D877 value over time may also indicate the beginning of an abnormal condition or a contamination problem even if the values continue to be acceptable. Some care in interpreting the results should be taken. The precision of the D877 test is such that large fluctuations in the values may be reported. This is generally not a significant problem, as long as the results continue to be acceptable and there is no clear trend downward.

To repeat some information from the last article, and as indicated above, ASTM D877 is an older method. The electrodes resemble flat coins, and they are spaced 0.1 inches (2.54 mm) apart. Prior to testing using the D877 device it is imperative to inspect the electrodes for cleanliness, pits, or signs of corrosion. Any such conditions will cause an artificially low D877 value. If dirt, corrosion, pits, or other problems are noted, the laboratory technician will clean and polish the electrodes as needed in accordance with the standard method. The edges of electrodes should be checked on a regular basis using a standard gauge to ensure that the edges are sharp and have not become rounded off. Rounded edges have the opposite effect of dirty or corroded electrodes, and may artificially increase the dielectric breakdown voltage so that it is less likely that abnormal conditions may be noted. Finally, prior to use, the technician should confirm the gap setting using either round or flat gauges according to the standard method. The rate of AC voltage rise across the electrodes during the determination of dielectric breakdown voltage by the D877 method is 3,000 volts per second.

In the next few articles, we will talk about capabilities and sensitivities of the D1816 standard methods.