This article presents results on the DC conductivity of moist pressboard impregnated with insulating oil and its dependence on the moisture and temperature of the samples. The transfer of charges was found to take place because of electron hopping between potential wells produced by water nanodrops. The average dimension of the nanodrops was around 2.32 nm, forming approximately 220 water molecules.

The paper presents results of investigations of the effect of moisture content and temperature on DC conductivity of the impregnated pressboard. It has been found that the conductivity of the impregnated pressboard is a function of activation energy, moisture content and temperature. It has been found that moisture content ranging from 1.4% to 6.4% do not cause any changes in the conductivity activation energy whose value is ΔW _av = (0.853±0.071) eV. On the other hand water content strongly influences the value of DC conductivity. It has been also found that the conductivity is determined by electron hopping between the closest located potential wells formed by water molecules in the oil-impregnated cellulose structure. For the experimentally obtained dependences of conductivity vs. moisture content and temperature theoretical ones have been determined to describe the experimental results. Based on the research results, a localization radius of the water molecule valence electron that coincides with the O ^2- ion radius has been determined. The obtained results concerning DC conductivity vs. moisture level and temperature dependence of an electric insulation pressboard can make a basis for the identification of the amount of moisture accumulated in solid elements of a power transformer paper-oil insulation.

The paper presents research results of testing AC conductivity in a moist oilimpregnated electric insulation pressboard, depending on the moisture content level and temperature of the samples. A new method has been elaborated for converting experimentally obtained dependences from frequency-domain spectroscopy (FDS) to the reference temperature of 293 K (20 /dpl deg/C). Activation energy values have been determined for the conduction and relaxation time. It has been found the conductivity is realized by electron tunnelling between potential well formed by water molecules contained in the structure of moist cellulose.

The paper presents results of testing permittivity of an oil-impregnated electric pressboard containing water nanoparticles depending on AC frequency, moisture content level and the temperature of samples. A new method has been developed for converting experimentally obtained frequency dependences of permittivity determined with the frequency-domain spectroscopy (FDS) to the reference temperature of 293 K (20 °C) using exponential dependence of the relaxation time versus temperature. Activation energy of the permittivity relaxation time has been determined for a moist oil-impregnated electric insulation pressboard. It has been established that variations of the moisture content level in a composite of cellulose, mineral oil, and water nanoparticles do not cause any changes in the relaxation time activation energy value, neither in energy states of electrons in potential wells nor in the structure of water nanoparticles. It has been also found that the conversion of experimentally obtained permittivity versus frequency dependences determined with the FDS method to the reference temperature of 293 K (20 °C) eliminates temperature dependences that occur in characteristics based directly on measurement results. Once the relative permittivity is converted to the reference temperature it is only its dependence on the moisture content level that remains.

The paper presents an analysis of test results by Frequency Domain Spectroscopy (FDS) of the loss tangent (tgδ) in electrotechnical pressboard impregnated with insulating oil and containing nanoparticles of water depending on the frequency of alternating current, sample temperature and degree of moisture. We found tgδ reduction in areas of low and ultra-low frequency. It is associated with relaxation due to the hopping conductivity (tunnelling) of electrons between moisture nanoparticles occurring in the cellulose impregnated with insulating oil. In frequency areas close to the local minimum of tgδ and higher, relaxation does not depend on the moisture content and is associated with other polar molecules which are part of the cellulose. We developed a new method converting experimental frequency dependence of the loss angle tangent, measured by the FDS method to the reference temperature of 293 K (20 °C), using the exponential dependence of the relaxation time on the temperature, was developed. The activation energy of the relaxation time was determined based on the loss angle tangent of moist electrotechnical pressboard impregnated with insulating oil. It was found that the variation in moisture content in the composite cellulose–mineral oil–water nanoparticles do not cause changes in the activation energy of the relaxation time. The conversion of experimental frequency dependence of the loss angle tangent determined by the FDS method to the reference temperature of 293 K (20 °C) eliminates the temperature dependence in the areas of ultra-low and low frequencies occurring in the runs made directly on the basis of the measurement results. After calculating the frequency dependence of the loss angle tangent to the reference temperature, all that remains is its dependence on the moisture content.

The paper presents a model of hopping conductivity at direct and alternating current, developed on the basis of the quantum phenomenon of electron tunnelling between neighbouring potential wells. The tunnelling takes place between neutral potential wells, resulting in an electrical dipole and additional thermally activated polarisation. An important parameter of the model is the time of existence of the dipole (relaxation time). After this time, the electron with the probability p can hop to the third well, which causes a direct current flow, or return with the probability (1 − p) to the first well, thus causing the flow of high frequency current. The model shows that for direct or low frequency current, the current density and conductivity do not depend on the frequency. In the high frequency region, the current density does not depend on the frequency either. Low-frequency conductivity is 2p times smaller than high-frequency conductivity. In the transitional region there is a frequency dependence of the current density. A formula was derived for the dependence of the relaxation time on the distance between neighbouring potential wells and temperature. The dependence of the hopping probability on activation energy, potential dipole energy and temperature was determined. A model of hopping conductivity at direct and alternating current was used to analyse the experimental frequency dependence of conductivity and permittivity of the composite of electrotechnical pressboard, mineral oil and water nanoparticles. It was found that there is a high compatibility of the experimental course with the model. It was also found that in the composite of electrotechnical pressboard, mineral oil and water nanodrops there are at least two mechanisms of changes in low frequency and high frequency conductivity. For the low-frequency stage, the value of the probability of p₁ ≈ 0.2 and the relaxation time τ₁ ≈ 0.02 s. The probability of hops for the high-frequency growth stage is p₂ ≈ 0.0005 and the value of relaxation time τ₂ ≈ 2 × 10⁻⁷ s. The occurrence of two episodes of increase in conductivity is related to the presence in the composite of electrotechnical pressboard, mineral oil and water nanoparticles of differences in distances between neighbouring nanoparticles. For a large number of nanoparticles randomly distributed in the percolation channel, the distribution of the probability of the occurrence of the distance is in a good approximation normal. This means that in the percolation channel there are pairs of neighbouring wells, the distances between which are both much smaller than the average and much larger. A pair of wells, the distance between which is smaller than medium, participates in the conductivity on the high-frequency stage with a short relaxation time. On the other hand, a pair of neighbouring wells for which the distance is greater than average causes a high resistance to DC or low-frequency current flow as well as a higher relaxation time. On the basis of the dependences designated in laboratory tests of low-frequency conductivity (10⁻⁴ Hz) from the moisture content, using the elements of quantum mechanics, the dimensions of water nanoparticles in electrotechnical pressboard impregnated with mineral insulating oil were determined. It was found that nanoparticles contain on average about 200 molecules of water, and their diameters are about 2.24 nm. This approach allowed to calculate the dielectric permittivity of cellulose and to determine that water nanoparticles are located inside the cellulose fibres.