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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