ASTM D7150 for Determination of Gassing Characteristics of Insulating Liquids
ASTM D7150 Standard Test Method for the Determination of Gassing Characteristics of Insulating Liquids Under Thermal Stress at Low Temperature
4. Summary of Test Method
4.1 Method A - Insulating liquid is filtered through a mixed cellulose ester filter. A portion of the test specimen is sparged for 30 min with dry air. A test specimen is then placed into a glass syringe, capped and aged at 120 ± 2°C for 164 h. The test is run in duplicate. The other portion of the test specimen is sparged for 30 min with dry nitrogen. A test specimen is then placed into a glass syringe, capped and aged at 120°C ± 2°C for 164 h. The test is run in duplicate. After, the test specimens have cooled, dissolved gas-in-oil analysis is then performed according to Test Method D3612.
4.2 Method B - Insulating oil is passed through a heated (60 to 70°C) attapulgite clay column at a rate of 3 to 5 mL per minute. The insulating liquid is contacted with the attapulgite clay at a ratio of 1 g clay to 33 mL (range: 30 to 35 mL) of insulating liquid (0.25 lb clay: 1 gal of insulating liquid). The insulating liquid is collected and subjected to the testing as outlined in 4.1.
5. Significance and Use
5.1 Generation of combustible gases is used to determine the condition of oil-filled electrical apparatus. Many years of empirical evidence has yielded guidelines such as those given in IEEE C57.104, IEC 60599 and IEC 61464. Industry experience has shown that electric and thermal faulted in oil-filled electrical apparatus are the usual sources that generate gases. Experience has shown that some of the gases could form in the oil at low temperatures or as a result of contamination, without any other influences.
5.2 Some severely hydro-treated transformer oils subjected to thermal stress and oils that contain certain types of contamination may produce specific gases at lower temperatures than normally expected for their generation and hence, falsely indicate abnormal operation of the electrical apparatus. Some new oils have produced large amounts of gases, especially hydrogen, without the influence of other electrical apparatus materials or electrical stresses. This renders interpretation of the dissolved gas analysis more complicated.
5.3 Heating for 164 h has been found to be a sufficient amount of time to reach a stable and characteristic gassing pattern.
5.4 This method uses both dry air and dry nitrogen as the sparging gas. This is to reflect either a electrical apparatus preservation system that allows oxygen to contact the oil or one that is sealed from the outside atmosphere. Oils sparged with air generally produce much more hydrogen as a percentage of the total combustible gas content as compared to oils sparged with nitrogen as these produce more hydrocarbons in relation to hydrogen.
