Humidity measurement in a very low moisture atmosphere was studied by using solid electrolyte film coated with porous electrodes,considering its application to the moisture monitoring of SF_6 gas-insulated high voltage instruments such as gas-insulated switchgear (GIS) and gas circuit breakers (GCBs).Compared to the AC impedance values measured in an ambient atmosphere where the moisture atmosphere is far higher than that in these instruments,considerably large impedance values were obtained in a very low moisture atmosphere.The impedance was systematically measured in accordance with the conditions of these instruments whose moisture contents were less than 1 000×10^(-6).A good correlation was obtained between the impedance values and the moisture contents.The frequency characteristics of the impedance were analyzed based on a conventional equivalent circuit where a number of CR circuits were connected in series.Considering the dielectric relaxation of each circuit,it was found that the frequency characteristics of the impedance can be explained by the equivalent circuit. Two semicircles were clearly obtained in the Cole-Cole impedance plot which are thought to reflect the impedance characteristics of the film and the electrodes.The AC impedance can be a good indicator of the moisture content of SF_6 gas-insulated high voltage instruments.
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