Thermal Aging of Mineral Oil-Paper Composite Insulation for High Voltage Transformer.

High voltage transformer is one of the most important equipments in an electric power system. High voltage insulation plays important role in determining transformer performance and greatly determines the lifetime. In general, paper and oil composite is used as main insulations in a high voltage transformer. Due to thermal aging, the insulation performances of the insulation may be degraded. The degradation mechanisms are important to be clarified. This paper reports the experimental results on the effects of thermal aging on the properties of paper-oil composite insulation. The samples used were thermally upgraded kraft paper and mineral oil. They are widely used in high voltage transformers. Samples were conditioned to the same initial conditions through the heating at a temperature of 100 °C for 24 hours. Mineral oil samples with volume of 800 ml and 6 gram insulating kraft paper were put in hermetical bottles. The paper-oil ratio reflects the typical ratio of oil and kraft paper inside a real transformer. The bottles containing oil and paper samples were subjected to thermal aging at 120°C and 150°C in a controllable oven for a period up to 4 weeks. Dielectric properties of the oil such as breakdown voltage, resistivity, water content were measured after and before aging. Dissolved gas analysis (DGA) was used to identify the gasses released in the paper-oil composite insulation as the results of thermal aging process. The morphological aging of the kraft paper was investigated using SEM (scanning electron microscopy) while chemical element change was investigated using EDS (energy dispersive spectroscopy) with accelerated voltage of 0.3-30 kV. The results showed that the dielectric properties will decrease with the increasing duration and temperature of aging. From these experiments obtained CO gases, produced from thermal degradation of paper cellulose chain. H₂ and C₂H₂ gases are not generated in this experiment. The result was consistent since H₂ usually generated under partial discharges while C₂H₂ released under very high temperature such as partial combustion, electric arc which were not available in the experiment. Some hydrocarbon gas such as CH₄, C₂H₄ and C₂H₆ were detected. CO gas was obtained as the result thermal aging of paper insulation through oxidation process. The EDS analysis showed that during aging the C element increased while O element decreased. EDS data of krafft paper aged at 120°C for a duration of 4 weeks (672 hours) taken using JEOL 6510 at accelerated voltage of 10 kV with energy range of 0-20 kV showed that the aged paper consists of 3 elements, C, O and K elements with mass percentage of 75.23 % of C at energy of 0.277 kV, O of 21.69 % observed at energy of 0.525 kV and new element of K with mass percentage of 3.08 observed at energy of 3.312 kV. It is clearly observed that C increased from 58.83 % to 75.23 %. This is due to the migration from the ester into the paper 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 150°C. [ABSTRACT FROM AUTHOR]