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Dielectric Breakdown Voltage Testing - Part 7

The IEC method from Standard 60156 including values used in IEC standards for unused and in-service mineral oil uses electrodes that are similar geometrically to the VDE electrodes used in ASTM D1816. The spherical electrodes are spaced 2.5 mm apart, and the rate of voltage increase is 2,000 volts per second. The method in Standard 60156 allows the optional use of an impeller, operating in similar fashion to the one described for D1816, except that it operates at 250 to 300 rpm. The IEC method also allows use of a magnetic stirrer operating at a similar rate, if there is no significant chance that magnetic particles will be removed from the oil. The presence of magnetic particles would affect dielectric breakdown in the transformer, so removal of those particles by the stirrer during the analysis would yield unrepresentative values.

Dielectric Breakdown Voltage Testing - Part 6

D1816 Dielectric Breakdown Voltage Uses VDE Electrodes to evaluate new oil. The minimum D1816 values that are acceptable are 20 kV for the 1 mm gap setting and 36kV for the 2 mm gap setting. D1816 oil results that do not meet the minimum for the selected gap should be rejected.

Dielectric Breakdown Voltage Testing - Part 5

When D1816 test results are outside of the acceptable range, the first consideration is to cross reference the results to moisture, liquid power factor, and liquid screen tests to identify possible causes.

Dielectric Breakdown Voltage Testing - Part 4

The D1816 dielectric breakdown voltage is a more sensitive method of dielectric breakdown voltage testing and is generally more useful than the D877 testing method under many circumstances. This 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. 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.

Dielectric Breakdown Voltage Testing - Part 3

The ASTM D1816 standard method of measuring dielectric breakdown voltage uses spherical VDE electrodes. This method is run at one of two gap setting: 1 mm or 2 mm. Because of the greater sensitivity, the rate of voltage rise is lower. Also, the D1816 test cell has a motor driven agitator that runs during the test to cause the oil to flow between the electrodes, carrying suspended particles into the gap between the VDE spheres where they can affect the breakdown voltage.

Dielectric Breakdown Voltage Testing - Part 2

There are three methods for dielectric breakdown voltage testing. The oldest, the flat disk method, is not very sensitive to the presence of moisture and not sensitive to changes in moisture content, unless the percent saturation of the oil is greater than 60 percent. The method is also not sensitive to the aging and oxidation of the oil. 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. Therefore, SDMyers continues to recommend the test for purposes of providing that information.

Dissolved Gas Analysis - Part 2

Some defects may provide initial symptoms during the first ten months of transformer installations without causing or revealing more obvious indications until after the warranty period has expired. The timing of the first interval at ten months, and running the complete recommended package of tests, will serve to establish a diagnostic baseline.

Dissolved Gas Analysis - Part 1

New transformers may have defects that can lead to failure. Frequently, such defects will leave signature dissolved gases in the oil. A timely Dissolved Gas Analysis may catch the fault as it begins, and before it advances far enough to do permanent damage.

DGA Recommendations for New Units

A combination of dissolved gas levels and comments on the Rainbow Report provides DGA recommendations for new units. For newly installed transformers for of a small to medium size and class, SDMyers recommends retesting the dissolved gasses in three months, to obtain baseline data. For the larger, more expensive, and higher maintenance units, such as furnace transformers and generator step-ups, we recommend starting the DGA retest at one month.

How should the Corrosive Sulfur test be performed? - Part 3

Method A is the older method, which uses conditions of 19 hours at 140°C. Method B is the newer method, which uses conditions of 48 hours at 150°C
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