Transformer insulating fluid acts as an insulator for electric and thermal capacities for many electrical equipment. Measurement of dielectric breakdown is a major way to measure the capability of a fluid to withstand electrical stress. In measuring dielectric breakdown, an increasingly high voltage is applied across the fluid via electrodes. The breakdown occurs when the leakage current leads to a spark or arc between the electrodes, indicating breakdown.
Contaminants such as moisture, sediment, particles, non-insulating fluid, and high dissolved gas content can reduce the dielectric strength. How far the strength is reduced depends on which contaminants or mixtures of contaminants are present. Different dielectric breakdown methodologies also differ in the effects of these contaminants.
The IEEE lists 2 ASTM methodologies in C57.106 -2015. They are ASTM D877 and ASTM D1816. They differ in the type of electrodes used, the gap between those electrodes, and the voltage ramp applied to testing.
The D877 uses flat disk electrodes at a 2.54 mm gap between and a 3000 volt/second ramp. This method is recommended in the field for acceptance tests on unprocessed fluids received from suppliers in tank cars, totes, or drums. It is not recommended for testing filtered, degassed, dehydrated oil prior to and during the energizing process. It is also not recommended for testing in-service oil in transformers but still can be used in LTC, OCB, Switches, and other contacting equipment.
The D1816 used hemispherical VDE (mushroom-shaped) electrodes at a 1 mm or 2 mm gap between and a 500 volt/second ramp. Additionally, this method also includes the use of a 2-bladed impellor to mix the fluid during testing. This fluid testing method can be used for the commissioning process as well as tank cars, totes, and drums. It is specifically recommended for in-service acceptance testing for transformers and related equipment as specified in IEEE C57.106-2015.
The significant difference between the methods is sensitivity to contaminants in the oil. The D877 using flat electrodes with edges and no mixing can allow for the settling of contaminants. This procedure is also not overtly sensitive to moisture content in the fluids.
The D1816 electrodes are smooth-rounded and more akin to components within transformers and related equipment. This allows a greater sensitivity to moisture. The stirring mechanism allows for a continuing homogenized sample and thus better sensitivity to any particulate contamination that would be present. The gap choice of 1 mm or 2 mm may be an evolution of more robust testing equipment. Early dielectric test sets would not read a breakdown over 60KV. Therefore, a 1 mm breakdown average would easily fall within the operating range. The newer test sets do not have this limit and the 2 mm gap can be performed.
The 2 mm gap does provide for a more consistent value over time as the increased gap isn’t as easily affected by stray particulates mixing with the sample if present.
