ASTM standard (USA) for insulating oil dielectric breakdown voltage testing
There are two standard test methods from ASTM International. The first is ASTM D877-02 (latest edition 2002) - Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes. The second is ASTM D1816-04 (latest edition 2004) - Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes. Although this is essentially an American standard, it borrows from VDE, a German standards organisation.
ASTM D877
ASTM D877 is an older standard, and is generally not very sensitive to the presence of moisture. For that reason it is not widely used for in-service applications. In 2002 the IEEE revised C51.106, Guide for the Acceptance and Maintenance of Insulating Oil in Equipment the values from D877 were removed from their criteria for evaluating in-service oil in transformers. Generally ASTM 877 is recommended only for acceptance testing of new oil received from a supplier, either in bulk loads or containers, to ensure the oil was correctly stored and transported. Typically a minimum breakdown value of 30 kV is specified.
The ASTM D877 standard specifies the use of disc shaped electrodes that are 25.4 mm (1 inch) is diameter and at least 3.18 mm (0.125 inch) thick. These electrodes are made of polished brass and are mounted to have their faces parallel and horizontally in line in the test vessel. The edges are specified to be sharp with no more than a 0.254 mm (0.010 inch) radius. The sharp edges should be regularly inspected to ensure that they have not become too rounded. Excessively rounded edges will have the effect of falsely raising the breakdown voltage, possibly passing oil that should have failed the test. It is also important that the electrodes are kept very clean, with no pitting or signs of corrosion, otherwise breakdown values can be falsely low. Instrument users should regularly inspect electrodes, cleaning and polishing as required.
ASTM D1816
ASTM D1816 has become widely used over the years, even being used outside the standard's stated application of petroleum origin insulating oils and viscosity limits. D1816 is more sensitive than D877 to moisture, oil aging and oxidisation, and is more affected by the presence of particles in the oil. When the IEEE revised C51.106 in 2002, breakdown voltage limits for new and in-service oil using D1816 were added.
ASTM D1816 specifies the use of mushroom-shaped electrodes 36 mm in diameter. As with D877, the electrodes are made of brass must be polished to be free of any etching, scratching, pitting, or carbon accumulation. The oil is stirred throughout the test sequence, and a two-bladed motor-driven impellor is specified. The standard prescribes the impellor dimensions and pitch as well as the operating speed, which must be between 200 rpm and 300 rpm. To prevent air coming into contact with the circulating oil, the test vessel must have a cover or baffle.
The D1816 standard, although generally accepted as more useful than D877, has got one significant limitation: when testing in-service oil this test method is very sensitive to dissolved gases. Excessive amounts of gas in the oil can lower the test results to the point that a perfectly good sample of oil, with low moisture and particle content, will fail the test. It is important to bear this in mind when testing oil from small gas blanketed transformers and even, in some cases, from free-breathing transformers.
The insulating fluid manufacturer will usually quote typical breakdown values for both new and in-service fluid in the insulating fluid data sheet. In addition the test standards will refer to another oil condition standard that will provide guidance as to what is acceptable. Test houses will also provide guidance as to what is acceptable.
