Influence of board density on the conductivity of oil-impregnated pressboard for HVDC insulation systems

In HVDC (high voltage direct current) insulation systems, the conductivities of the insulating materials pressboard and oil are the relevant material parameters that determine the field distribution. The pressboard conductivity depends on various parameters, which can be categorized in service-parameters (e. g. time of energization, water content, impregnating fluid) and parameters during pressboard manufacturing (e. g pressboard density). Measurements of pressboard with different densities are performed by using a step response measurement in time domain. At first, the materials are impregnated with gas. It is found that electrical conductivity increases with an increase in pressboard density. The reason is that gas has a much lower conductivity than pressboard and therefore, a larger number of fibers per volume causes a higher conductivity than a smaller fiber number. Afterwards, pressboards with different densities are impregnated under vacuum with two types of mineral oil (a low-conductive oil and a high-conductive oil). For the low-conductive oil, the influence of the pressboard density is little, because conductivities and electric field stresses are similar, both for pressboard fibers and for oil. By impregnation with the high-conductive oil, the pressboard pores have a much higher conductivity than the surrounding pressboard fibers and thus the conductivity of the oil-impregnated pressboard depends on pressboard density again. In this case, a low-density pressboard has therefore a higher conductivity than a high-density pressboard corresponding to the different fiber volume and oil volume. The pressboard density can be modified during manufacturing and the designer can choose which pressboard density and which oil with a certain oil-conductivity should be used in a certain HVDC apparatus leading to desired conductivity values for the oil-pressboard system.