Investigation on Thermal Aging of Ester from Palm Oil and Kraft Paper Composite Insulation System for High Voltage Transformer

Mineral oil is widely being used as insulation for high voltage equipments such as transformer due to its excellent dielectric properties. Recently, due to environment consideration and the long term availability, organic liquid insulations from plantations are being introduced. However, the performance of such insulations in composite with paper should be investigated in order to maintain the performances of high voltage equipments in the electric power system. This paper reports the experimental results on the effects of thermal aging on the properties of natural ester from palm oil and Kraft paper composite insulation.The samples used in these experiments were natural ester from palm oil and kraft paper transformer insulation. The ester samples of 800 ml and 6 gram of kraft insulation paper were put together in heat-resistant hermetic bottles to simulate actual condition of transformator.All samples were subjected to thermal aging condition for duration of 336 hours to 672 hours at 120C and 150C in a controllable oven. The properties of new sample as well as thermally aged samples were investigated. Dielectric properties such as breakdown voltage, resistivity, water content were measured and dissolved gas analysis were conducted. The chemical element change was measured using JEOL 6510 EDS (energy dispersive X-ray spectroscopy) with accelerated voltage up to 30 kV. The experimental results showed that thermal aging process greatly affected the breakdown voltage and resistivity. At initial stage of thermal aging application the breakdown voltage increased significantly from 33.2 kV to 43.9 kV due to the reduction of water content. At the later time, however, the breakdown voltage reduced due to the drastic increase of oxidation by product which was indicated by the darker color. Resistivity reduced drastically from 8.5 T.cm to 1.3 T.cm after aging at 150C for duration of 672 hour. Dissolved gas analysis indicated that thermal aging in ester-paper composite insulation released, CH4 (Methane), C2H6 (ethane), and CO (Carbon Monoxide). The EDS analysis showed that during aging the C element increased while O element decreased. Tensile strength of paper reduced significantly with thermal aging. EDS data of Kraft paper aged at 120C for a duration of 4 weeks (672 hours) showed that the aged paper consists of 3 elements, C (75.23 %) , O (21.69 % ) and K (3.08%). C was observed at energy of 0.277 keV, O at energy of 0.525 keV while K was observed at energy of 3.312 keV. Experimental results revealed that C increased with aging while O decreased with aging. On the other hand oxygen reduced from 41.17 % to 21.69 % because the oxygen from the paper reacted with oil in an oxidation and release CO gas as confirmed by DGA (dissolved gas analysis). Similar behavior was observed for aging at 150C. Key-Words: -Palm oil, ester, Kraft paper, dielectric properties, tensile strength, thermal aging, high voltage transformer, DGA, EDS