Improved method for measuring moisture content of mineral-oil-impregnated cellulose pressboard based on dielectric response

Abstract

The moisture content in transformer insulation pressboards is an important index to evaluate the insulation aging in the transformer. The dielectric-response technique based on frequency domain spectroscopy is used to accurately measure the moisture content in transformer insulation pressboard. In this study, according to the distribution of water molecules in oil-impregnated cellulose pressboard and the polarization behaviour of water molecules at different frequencies, a theoretical calculation method for moisture content in a composite model of cellulose pressboard, mineral oil, and moisture based on a typical transformer structure is proposed. To verify the effectiveness of the numerical iterative calculation method further, a typical structure model of a transformer with different proportions was prepared under laboratory conditions of natural moisture absorption and accelerated aging. Three groups of comparison tests were done. In the stage temperature rise test (40–90 °C), the moisture contents are compared and analysed by the method proposed in this research and the IDAX-300 software fitting method. It was found that the fitting of the dielectric loss factor curve is more accurate in the high-temperature and low-frequency region. In a room temperature (25 °C) test, the moisture contents obtained by the method proposed in this study, the IDAX-300 instrument test method, and the traditional Karl–Fischer titration method are compared and analysed. After testing the XY model with different moisture absorptions at 25 °C, it was found that the accuracy of the algorithm is higher. Testing the XY model at different degrees of aging at 25 °C, showed the effect of the aging of the oil-impregnated cellulose pressboard. The selected fitting frequency range should be (10−1–103) Hz, and the accuracy of the numerical iterative calculation method is high.