Influence of Moisture on Broadband Dielectric Properties of the Impregnated Transformer Paper Insulation

The actual life of a transformer is determined by ageing ofthe cellulose insulation such as Kraft paper. The presence of moisture in the insulation system decreases the electrical strength of paper and accelerates the aging. An efficient way to monitor the moisture content in paper insulation is to measure the dielectric frequency response of the paper. The moisture dependent permittivity of impregnated transformer paper below 1 MHz has been widely investigated. The frequency response of the paper between 1 kHz and 1 MHz doesn’t show significant differences with different moisture contents. Higher frequency dielectric spectroscopy (>1 MHz) is needed to understand the influence of moisture better. The higher frequency range upto GHz is investigated by the Transmission/Reflection technique. Paper insulation is placed in a coaxial line and the scattering parameters are obtained by the modern Vector Network Analyzer. Full wave analysis is utilized to calculate the permittivity from the obtained S-parameters due to its high accuracy. It is proved that moisture in paper makes the response curves shift towards higher frequencies over a wide frequency range. Due to the dimension limitation of the standard coaxial line, the starting frequency is 100 MHz instead of 1 MHz.QC 20211008