Effects of thermal aging on paper characteristics in paper-mineral oil composite insulation

Thermal aging is one of the most important phenomena affecting the lifetime of a high voltage transformer. This paper reports investigation of thermal aging of transformer oil-kraft paper in mineral oil. Kraft paper sample was put in a hermeneutical bottle and subjected to an accelerated thermal aging in a controllable oven under aging temperature of 120°C and 150°C for duration of p to 672 hours in mineral oil. Effects of thermal aging on the properties of kraft paper was conducted by measuring the tensile strength and degree of polymerization. Energy Dispersive X-ray Spectroscopy or EDS JEOL type JSM6510 LA was utilized to investigate the change of chemical components in the paper before and after aging. DGA analysis was done to clarify the gases released in oil during thermal aging. Experimental results showed that under the aging at 120°C and 150° the generated combustible gases were CH4, C2H6 and C2H4. CO was also detected. This indicated the degradation of kraft paper during the thermal aging. EDS results showed that thermal aging increased the C content in the paper due to migration of C from the transformer oil. On the other hand the thermal aging reduced significantly the element of O. This was due to the oxidation of the paper and released gas CO that dissolved into the transformer oil as confirmed by the DGA results. Sulphur element was found in the paper at later stage of the thermal aging. This corrosive element may be the cause of corrosion found in transformers.