Moisture estimation of transformer pressboard by micro-strip ring resonator at GHz frequencies

Abstract

Increased moisture content (MC) is considered as one of the main factors for the failure of transformer insulation. This paper presents a method of estimating the MC of a transformer pressboard insulation based on the relative permittivity and loss tangent measurements performed at GHz using a multi-layer micro-strip ring resonator. It was fabricated on a material of known dielectric properties and the sample to be tested (i.e. pressboard) was placed on the ring structure. The dielectric properties, namely loss tangent and permittivity, of the sample were then estimated by measuring the S 21 transmission characteristics of the ring structure using a vector network analyzer. The technique was validated using a test sample of known dielectric properties. Additionally the results were compared with traditional frequency domain dielectric spectroscopy (FDS) measurements conducted at 1 mHz −1 kHz. The measurements were performed on oil impregnated and non-impregnated pressboard samples having different MCs conditioned by drying and/or wetting processes. The actual MC of the pressboard samples were obtained by weight measurements. It was found that the resonance frequency increased when the MC was reduced. Furthermore, a positive non-linear relation could be observed between the MC and both the relative permittivity and the loss tangent. It can be concluded that the proposed method could be used to estimate the MC of pressboards.