Abstract
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.
Highlights
Dielectric materials are essential for a proper functioning of power transformers
Concerning the type of oil, the Breakdown voltage (BDV) of dielectric papers impregnated with methyl ester of palm kernel oil (MEPKO) is greater than the BDV of similar papers impregnated with mineral oil (MO), indicating a better preservation of paper when dipped in methyl esters
Breakdown strength (BDS) of Thermally Upgraded Kraft paper (TUK) dipped in MO is comprised between 55 and 54 kV/mm, while the BDS of TUK dipped in palm kernel oil alkyl ester fluid (PKOAE) is comprised between 78 and 75 kV/mm
Summary
Dielectric materials are essential for a proper functioning of power transformers. The insulation system is used to separate the primary and secondary windings of transformers. The dielectric materials help to evacuate the heat produced when the transformer is in operation [1]. Almost half of the transformers breakdowns observed are related to a failure of the insulation system [2] [3] [4]. Given the importance of electrical transformers in the energy transmission and distribution network, reliable and sustainable dielectric materials are essential. Any failure of the insulation system could induce a shortage of electricity, and thereby generating energy load shedding, shutdown of production in industries and a possible economic crisis
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