Dielectric properties measurements of transformer oil, paper and pressboard with the effect of moisture and ageing

Dissipation factor (tan delta) and capacitance measurement at power frequency (50 Hz) has been commonly used to diagnose the insulation integrity and condition of transformers. The main drawback of this technique is that the measurement results obtained represent the status of the "complete" transformer insulation systems - a combination of the bulk of oil, paper insulation and pressboard. The contribution of each individual insulation material to the end result is not well known, and the degradation status of the whole insulation structure and each material are difficult to separate. In order to improve comprehension of the measurement results of this technique, the basic relationship between tan delta and each insulating material has to be established first. Hence, in this paper, experimental investigation has been done by studying the dielectric properties changes (tan delta and dielectric constant, epsiv_tau) of mineral oil, cellulose paper and pressboard with the affecting parameters, namely moisture and ageing. A simulation model of the combined transformer insulation has been used to correlate the tan delta of each insulating material to the measured combined value of a transformer.