Electrical conductivity of oil and oil-impregnated pressboard dependent on aging byproducts

The estimation of material parameters is an essential step during the design of high voltage direct current (HVDC) insulation systems. The key role of insulation coordination in power transformers is to provide the required insulation components among different voltage classes to prevent dielectric breakdown. The design of a feasible insulation for transformers requires understanding of electrical field distributions. The DC field stresses are determined by means of insulating material conductivity which is normally a combination of dielectric liquid, paper and pressboard. One of the parameters which has an influence on the electrical conductivity is the aging of solid and liquid insulations. During converter transformer operation time, there is a normal degradation of all insulation materials. Aging byproducts like various acids increases the electrical conductivity of the system. This can lead to a variation of the field distribution over the operation time of the converter transformer. In this contribution, the influence of different carboxylic acids on electrical conductivity of oil and impregnated pressboard is investigated. The results compare the effects of adding different amounts of hydrochloric acid to mineral oil and natural ester liquid during electrical conductivity measurement. The conclusion illustrates different behavior of electrical conductivity comparing short and long-chain acids content of specimens.