Relative Permittivity Assessment of Oil-Impregnated Cellulose Insulation

Insulating system of a power transformer consists of an insulating liquid (mineral oil or ester liquid) and cellulose-based materials (paper and pressboard). Dielectric properties of an insulating material may be expressed in terms of relative permittivity and material conductivity. Material conductivity influences electric fields in power transformers exposed to DC voltages (e. g. HVDC transformers), whereas relative permittivity is more relevant in power transformers subjected to power frequency (50 or 60 Hz). It is well known that a higher discrepancy between relative permittivity of liquid-impregnated material and insulating liquid increases electric field in insulating liquid. It is therefore essential to know values of relative permittivity of cellulose-based materials in order to properly evaluate electric field strengths in the transformer. This paper presents both theoretical and practical approach to relative permittivity assessment of two types of cellulose material in two different impregnation states.