Aging of cellulose at transformer service temperatures. Part 1: Influence of type of oil and air on the degree of polymerization of pressboard, dissolved gases, and furanic compounds in oil

A power transformer is expected to operate up to 40 years. With current cost-benefit calculations, the user is forced to perform maintenance that will bring a further extension of transformer life. The maintenance can only be based on on-site and off-site monitoring with extended analytical and electrical tests, which can define the service condition of the transformer and predict its further life expectancy. The life of a transformer is mainly dependent on the life of its solid insulation, and the life-limit is determined by the thermal degradation of the winding paper. The Kraft paper decreases in tensile strength with the progress of aging and, at some point, can no longer withstand the short circuit stresses. The monitoring of the complex permittivity of transformer oils, as a function of frequency and temperature, may provide information concerning the state of the insulation. In this article, we show the influence of air and oil type on the aging of pressboard under the influence of a considerable amount of moisture. The rate of the degree of polymerization, the development of furanic compounds, as well as the gas-in-oil analysis in comparison with the aging of the pure oil under the same conditions have been investigated.     

Unusual cases of gassing in transformers in service

This paper presents some unusual cases of gassing in transformers in service without an actual fault. These cases reveal that hydrogen production caused by stray gassing may be due to strongly hydro-treated oils, but is also very strongly influenced by transformer materials. Stray gassing with overproduction of saturated hydrocarbons with no production of hydrogen and no increased rates of hydrogen, ethylene, and other fault gases has probably always existed and is possibly related to oxidation. The overproportional development of methane and especially ethane proceeds in the first years of service and reaches a plateau after considerable consumption of oxygen. Possibly, this process is also catalytically triggered by transformer materials. Catalytic phenomena and stray gassing can be distinguished from a transformer fault through regular checks not only of the absolute DGA values but also the rates of gas increases.     

Aging of cellulose at transformer service temperatures. Part 2. Influence of moisture and temperature on degree of polymerization and formation of furanic compounds in free-breathing systems

This paper examines the influence of moisture and temperature on the degree of polymerization (DP) and furanic compounds. At normal service temperatures, the average moisture content in the solid insulation of transformers has a much stronger influence on the development of furanic compounds and degradation of the insulation than temperature itself. The influence of the initial moisture in the solid insulation on aging under normal transformer service temperatures of 75, 85, and 95 /spl deg/C has been investigated experimentally. Wet transformers have usually higher levels of furans, inferior oil properties, and lower DP values of paper than dry ones under similar service conditions. The development of furan concentration and moisture content are very important aging indicators for the evaluation and life assessment of transformer in service. Further work is necessary to combine these important aging characteristics into a valuable tool for the assessment of a large aging transformer population.