Moisture and space charge in oil-impregnated pressboard under HVDC

Transformer oil and oil-impregnated pressboard are major insulating materials in a HVDC (high voltage direct current) converter transformer which provides electrical power transformation between an AC and a DC system. However, the dc component on the valve side of a HVDC converter transformer gives rise to special conditions, regarding space charge for example, for the insulating materials. In this paper, the relationship between moisture and space charge in oil-impregnated pressboard was investigated under dc electrical fields at room temperature. Formation, accumulation and distribution of space charge were studied using a PWP (pressure wave propagation) technique with the moisture concentration varying between 0.3% and 1.7% by weight. Experiments have shown that moisture influences seriously space charge distribution and thus electric field distribution in oil-impregnated pressboard. The higher the moisture concentration, the faster the establishment of space charge and the deeper the charge penetration into the insulation. The influence of moisture on the distribution of space charge should be taken into consideration in the design of an insulating system under HVDC.