Abstract
The reference characteristics of complex permittivity of the transformers insulation solid component were determined for use in the precise diagnostics of the power transformers insulation state. The solid component is a composite of cellulose, insulating oil and water nanoparticles. Measurements were made in the frequency range from 10−4 Hz to 5000 Hz at temperatures from 293.15 to 333.15 K. Uncertainty of temperature measurements was less than ±0.01 K. Pressboard impregnated with insulating oil with a water content of (5.0 ± 0.2) by weight moistened in a manner maximally similar to the moistening process in power transformers was investigated. It was found that there are two stages of changes in permittivity and imaginary permittivity components, occurring for low and high frequency. As the temperature increases, the frequency dependencies of the permittivity and imaginary permittivity component shifts to the higher frequency region. This phenomenon is related to the change of relaxation time with the increase in temperature. The values of relaxation time activation energies of the permittivity ΔWτε′ ≈ (0.827 ± 0.0094) eV and the imaginary permittivity component ΔWτε″ = 0.883 eV were determined. It was found that Cole-Cole charts for the first stage are asymmetric and similar to those described by the Dawidson–Cole relaxation. For stage two, the charts are arc-shaped, corresponding to the Cole-Cole relaxation. It has been established that in the moistened pressboard impregnated with insulating oil, there is an additional polarization mechanism associated with the occurrence of water in the form of nanodrops and the tunneling of electrons between them.
Highlights
Insulation of power transformer windings has been made from cellulose and mineral oil
The moisture is dissolved in oil, which supplies it to the cellulose
The moisture is absorbed by the cellulose, where its content gradually increases over time
Summary
Insulation of power transformer windings has been made from cellulose (in the form of paper and pressboard) and mineral oil. The moisture content of the cellulose during the long-term operation of transformers gradually increases from an initial level of about 0.7–0.8% by weight to as much as 5% by weight and more. This is a limit value, exceeding which can lead to a catastrophic failure of the transformer [11,12,13]. Non-destructive methods are used to determine the moisture content These are primarily electrical methods whose application requires the determination of so-called reference characteristics. The estimation of insulation moisture levels is based on a comparison of transformer insulation electrical parameter measurements results and reference characteristics, simultaneously taking into account geometrical dimensions of transformer solid insulation and oil channels
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