Mineral oil is the most popular insulating liquid for high voltage transformers due to its function as a cooling liquid and an electrical insulator. Kraft paper has been widely used as transformer solid insulation for a long time already. The degradation process of transformer paper due to thermal aging in mineral oil can change the physical and chemical structure of the cellulose paper. Fourier transform infrared (FTIR) spectroscopy analysis was used to identify changes in the chemical structure of transformer paper aged in mineral oil. FTIR results show that the intensity of the peak absorbance of the O–H functional group decreased with aging but the intensity of the peak absorbance of the C–H and C=O functional groups increased with aging. Changes in the chemical structure of the cellulose paper during thermal aging in mineral oil can be analyzed by an oxidation process of the cellulose paper and the reaction process between the carboxylic acids in the mineral oil and the hydroxyl groups on the cellulose. The correlation between the functional groups and the average number of chain scissions of transformer paper gives initial information that the transformer paper performance can be identified by using a spectroscopic technique as a non-destructive diagnostic technique.

Mineral oil is widely used as liquid insulation in high voltage equipment. Due to environmental considerations, recently natural esters have been considered as naturally friendly liquid insulation candidates for high voltage transformers. In this experiment, transformer insulation paper was subjected to get accelerated aging test with copper strip in natural ester in a hermeneutical heat-resistant glass bottle at temperatures of 120 °C and 150 °C for 336 h, 672 h and 1008 h. The experimental results of Fourier transform infrared spectroscopy (FTIR) showed that the intensity of the absorbance peak of the O–H functional group decreased with aging, while the intensity of the C–H and C=O functional group absorbance peaks have increased with aging and the intensity of the C–O functional group absorbance peak has a tendency to increase with aging. The energy dispersive X-ray spectroscopy (EDS) experimental results showed that the weight percent of the element C increased with aging and the weight percent of the element O has decreased with aging. The experimental results show a good correlation between the degree of polymerization (DP) and the weight percent of O element. This indicates that EDS may be used as a new method for estimating the DP of transformer insulation paper.

Transformer insulation paper especially kraft paper is extensively used as transformer solid insulation since a long time ago. Natural ester from palm is considered as a candidate of naturally friendly liquid insulation for high voltage transformers. Accelerated thermal aging can change the structure of transformer insulation paper in natural ester. In this paper, transformer insulation paper get accelerated aging test in natural ester at temperature 150°C for varying durations. This research was conducted by soaking transformer insulation paper in natural ester which was thermal aged at temperature 150°C from 336 hours, 672 hours and 1008 hours. The object of experiment on this research is copper which is wrapped around by transformer insulation paper placed in natural ester. The results of the experiment showed the structural changes of transformer insulation paper under accelerated thermal aging with fourier transform infrared spectroscopy (FTIR). Microstructure on the surface of transformer insulation paper changing during accelerated thermal aging was observed by using scanning electron microscope (SEM). The FTIR results of transformer insulation paper showed the intensity of peaks absorbance of O-H functional group (at wavenumber 3273 cm ^-1 and 3325 cm ^-1 ), the intensity of peaks absorbance of C-H functional group (at wavenumber 2855 cm ^-1 and 2922 cm ^-1 ), the intensity of peak absorbance C=O functional group (at wavenumber 1743 cm ^-1 ) and the intensity of peak absorbance of C-O functional group (at wavenumber 1159 cm ^-1 ). The SEM results of transformer insulation paper showed the paper surface morphology. Analysis of structural changes with FTIR can be explained by aging mechanisms of transformer insulation paper in natural ester.

Sealed bottle accelerated aging studies demonstrates a correlation between degree of polymerization and tensile strength. Experimental was done by soaking transformer insulation paper in natural ester which is thermal aged from 2 weeks, 4 weeks and 6 weeks. The amount of temperature used is 120 degree Celsius. There are two object on this research which are transformer insulation paper which has not been applied pretreatment (non-drying samples) and transformer insulation paper which has been applied pre-treatment (drying samples). The experiment was done by conduct several test such tensile strength test and degree of polymerization test. The results of the experiment shows that there are correlation between degree of polymerization and tensile strength. Both of samples show that the decline rate of degree of polymerization is bigger than the decline rate of tensile strength and the smaller DP is, the smaller tensile strength will become. Differences characteristics also shown when sealed bottle is applied with different pre-treatment. Lifetime estimation showed that under same temperature, non-drying sample will have shorter lifetime than drying sample.

Due to environmental consideration, natural ester was developed as liquid insulation for high voltage transformer. This paper reports the Dissolved gas Analysis (DGA) and Fourier Transform Infrareed (FTIR) Spectroscopy of Thermally-Aged Natural Ester-paper composite. The sample used were natural ester from palm oil and kraft paper insulation. The samples used in these experiments were natural ester from palm oil and kraft paper transformer insulation. The 200 gr of ester samples, 10 gr of kraft insulation paper, and 17.5 gr copper were put together with copper conductor in heat-resistant hermetic bottles to simulate actual condition of transformator. The samples were subjected to a thermal stress in a controllable oven at temperature of 120°C and 150°C. The thermal aging was conducted for duration of 2 weeks, 4 weeks and 6 weeks. The dissolved gas in the ester after a certain aging temperature and period was determined using a DGA testing. The gas species and the concentration of each gas under various aging conditions were analyzed. Dissolved gas analysis indicated that thermal aging in ester-paper composite insulation released, CH ₄ (Methane), C ₂ H ₆ (ethane), and CO (Carbon Monoxide). FTIR measurement was conducted for the thermally-aged ester insulation. The comparative analysis of DGA and FTIR results was done to clarify the origin of the gas released in the thermally-aged ester paper composite insulation.

The accuracy of conventional DGA interpretation methods can be different when each of these methods are used in different places or different circumstances. Rogers Ratio Method (RRM), IEC Ratio Method (IRM) (Basic Gas Ratios Method), GB/T 7252 (National Standard of the People's Republic of China) are popular conventional methods for interpreting the possible faults indicator of transformer in Indonesia and China. This research proposes artificial intelligence to interpret DGA by combining conventional method and artificial intelligence method using weighting factor. The artificial intelligence method which are used in this research is fuzzy logic. DGA practical data which used as refer data for data mining in this research were taken from China and Indonesia. This research also uses Thompson tau method to filter the data from outlier and fuzzy c means clustering to cluster the data to make sure the data are used is valid and good enough to be used to build artificial intelligence through data mining. The output of this research is to create an artificial intelligence and the combination between artificial intelligence which have been built with conventional method to interpret DGA whether there is any fault in transformer or not.