k Measurements of Transformer Board Usin Wave1 en g t lh i e I ct ct ro m et ry Sensor

Ab~fr~~b.' The spatial distribution of dielectric permittivity and conductivity of a medium affects the quasistatic distribution of the electric field. Multiple wavelength interdigital sensors using the imposed frequency-wavenumber (0-k) dielectrometry technique can measure complex permittivity distributions in dielectrics. Measurements of oil-free and oil-impregnated pressboard using a three-wavelength sensor with changes in ambient moisture concentration are presented. With a calibrated mapping relating conductivity and permittivity to moisture concentration in &impregnated pressboard, the spatial distribution of moisture can be measured. The presence of moisture in transformer oil and oilimpregnated pressboard has a detrimental effect on insulation life by lowering electrical breakdown strciigtli and thermal endurance [l]. When the transformer is in operal.ion, the conventional way to estim,ate the moisture in the solid insulation is to measure the rnoisture content in the oil and to use moisture equilibrium curvcs to estiauate the iiioisturc level in the pressboard [2]. However, since it may take a long time for the system to reach equilibrium. using the curvcs under non-equilibrium conditions may result in errors. The thrccwavelength sensor is designed to monitor the dielectric rcsponse of the insulation in real time and to calculatc the moisture distribution in the pressboard by using a calibratcd mapping relating dielcctric permittivity and Conductivity to moisture coriccntratiort [a]. Preliniinary moisturc dynamic expcnments havc bccn donc with oil-free pressboard at room tempcraturc [4.5] We rcpcated the nicasurernciits alt 70°C to increase the moisture diffusion spccd Measurements of oil-impregnated pressboard at 70°C, a more typical case for the transfornicr systcni, are also presented