This paper is devoted to a comparative analysis of thermal ageing tests of Thermally Upgraded Kraft (TUK) and Nomex-910 papers dipped in palm kernel oil methyl ester (MEPKO), at 110, 130, and 150 °C, for 96 h each. After ageing, paper breakdown voltage (BDV), oils’ dissipation factor, dielectric constant and oxidation stability were determined. Nomex’s BDV was greater than TUK BDV. However, the dissipation factor of the Nomex samples is slightly higher than that of TUK, and the dielectric constant has the opposite behaviour. In addition, the decay content of Nomex/MEPKO was greater than with TUK/MEPKO. These experimental findings indicate that Nomex can be mainly used in high voltage, and TUK in medium voltage power transformers respectively, to allow judicious use of their individual characteristics and money savings.
The purpose of this work is to determine the impact of thermal aging on the dielectric and physicochemical properties of the oil/paper mixed insulation. We performed a comparative analysis of dielectric paper dipped in two cooling fluids: palm kernel oil methyl ester (MEPKO) and mineral oil (MO). Two types of dielectric paper were used: Thermally Upgraded Kraft paper (TUK) and Nomex-910 paper (NP-910). An accelerated aging test was realized at 110˚C during a total of 96 hours. Samples of oil and paper were collected after 0, 48, 72 and 96 hours for analyses purposes. The analyses performed included the measurement of the Breakdown voltage (BDV) of the dielectric papers, the Total Acid Number (TAN) and the Decay Dissolved Products (DDP) of the liquid dielectrics. The BDV of NP-910 is greater than the BDV of TUK. Concerning the type of oil, the BDV of dielectric papers impregnated with MEPKO is greater than the BDV of similar papers impregnated with MO, indicating a better preservation of paper when dipped in methyl esters. The analyses of TAN and DDP revealed that Nomex-910 improves the oxidation stability of MO, but reduces the oxidation stability of MEPKO. These results prove that methyl esters can be used as a substitute to replace mineral oils in power transformers. Furthermore, they show that NP can be used mainly in areas of transformer where solid insulation is subjected to high thermal and electrical stress, and TUK other places where solid insulation is required. Such combination could assure money savings and a better preservation of the oil viscosity.
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