Transformer insulation system evaluation

Many engineers are familiar with the early ratings of materials as defined in A.I.E.E. #1, the present emphasis on systems, and the determination of system life by approved test procedures. The material definitions were found wanting in transformers, since the life of oil insulated transformers operated under full load continuously was found to be very short as long ago as 1925. At the same time, the life was found to be lengthened very greatly by using nitrogen above the oil and by the careful use of materials considering both their solubility in oil and compatibility with each other. At a later time, similar considerations were found to exist for dry type transformers. Enclosure and the exclusion of oxygen, the choice of material used depending on its actual operating temperature and function, and the avoidance of materials adjacent to each other which were chemically incompatible, were all found to increase life. In other words, defining the materials used is not now adequate, and as a result, some method of test to show that the assembly is both mechanically and chemically stable is required. For the purposes of this test, a model or the actual equipment is required. The assembly is now termed an insulation system, and test procedures for these must be established, according to present recommendations. Tests on test tube samples, although useful in comparing materials, are definitely not acceptable in place of a system test.