Interaction of pressboard with impregnating fluids in HVDC insulation systems

Pressboard is one of the key components in insulation systems for HVDC (high voltage direct current) equipment. The conductivity of pressboard is one of the most important properties of these systems. It is dependent on various parameters, e. g. time, temperature, electrical field strength, water content and impregnating fluid, which are varied in this paper. Pressboard manufacturing is described and results of conductivity measurements of pressboard according to the PDC method (step response measurement in time domain) are presented. In order to come to a physical understanding of pressboard conductivity in transformer insulation systems, conductivity of dry pressboard is examined under vacuum. Low electric field strength leads to a restriction in parameter variation (Paschen's law) and therefore, pressboard impregnated with dry air is investigated. No significant influence of impregnating air on pressboard conductivity is found. Moreover, pressboard is impregnated with mineral oil and the change in pressboard conductivity by impregnating with different fluids is shown. Polarization and conduction processes in insulation systems are taken into account. With an understanding of these processes, transformer design can be improved.