Generation of furanic compounds in transformer oil under accelerated thermal and electrical stress

Abstract

High voltage transformers use Kraft paper to insulate the electrical windings present in the core, which is then subsequently immersed in oil. In service, the temperature of the windings will increase to typically 80degC. If the transformer is loaded to beyond its design ratings, the temperature can exceed 100degC causing the cellulose chains in the paper to cleave at an accelerated rate, resulting in the degradation of mechanical strength and performance of the insulation. If unchecked, this can lead to catastrophic failure of the transformer and accompanying disruption to electricity supply and large economic losses to the operating utility. Furfuraldehyde (FFA) is a chemical by-product, which is released into the oil by the thermal degradation of the paper winding. The concentration of FFA within the oil has been directly related to the condition of the paper insulation. Therefore, efforts are made to find the degradation of paper by evaluating the furfural compounds under accelerated thermal and electrical stress and results are correlated with degree of polymerization and tensile strength of paper. The samples are tested at 120degC, 140degC and 160degC and at electric stress 2.5 kV for 360, 720, 1080 and 1440 hours. It was observed that the Furan analysis was the best method for assessing the condition of paper insulation, as this method required the transformer oil to be sampled out and not the paper from the transformer during on-line operation of the transformer