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The study determines the frequency–temperature dependence of the conductivity of a
moist solid insulation component of power transformers, impregnated with the innovative bio-oil
NYTRO® BIO 300X, manufactured from plant-based raw materials. The research was conducted for
six moisture levels ranging from 0.6% to 5% by weight, within a frequency range from 10−4 Hz to
5 · 103 Hz and measurement temperatures from 20 ◦C to 70 ◦C, with a 10 ◦C step. The conduction
model for both DC and AC, based on the quantum mechanical phenomenon of electron tunneling
between water nanodroplets, was used to analyze the obtained results. It was determined that the
frequency dependence of the conductivity of pressboard-bio-oil-moisture composites is influenced by
two factors as follows: the activation energy of conductivity and the activation energy of relaxation
time. For each moisture content, 16 values of the activation energy of the relaxation time and 16 values
of the activation energy of conductivity were determined. It was found that the values of activation
energy of conductivity and relaxation time are equal and independent of moisture content, frequency,
and temperature. Based on 192 residual activation energy values, the mean generalized activation
energy value for the relaxation time and conductivity was calculated with high precision, resulting in
∆E ≈ (1.02627 ± 0.01606) eV. The uncertainty of its determination was only ±1.6%. This indicates
that electron tunneling from the first nanodroplet to the second, causing AC conductivity, and their
return from the second nanodroplet to the first, determining the relaxation time, occur between the
same energy states belonging to the water nanodroplets located in the pressboard impregnated with
bio-oil. For each moisture content, the curves obtained for different measurement temperatures were
recalculated to a reference temperature of 20 ◦C using the generalized activation energy. It was found
that the shifted curves obtained for different temperatures perfectly overlap. Increased moisture
content shifts the recalculated curves toward higher conductivity values. It was established that for
all moisture contents in the lowest frequency range, conductivity is constant (DC conductivity). A
further increase in frequency causes a rapid rise in conductivity. The increasing period can be divided
into two stages. The first stage occurs up to about 100 Hz–101 Hz, depending on the moisture content.
In the second stage, the rate of conductivity increase is higher, and its value depends on moisture
content. The lower the moisture content, the faster the conductivity increases. Recalculation using
the generalized activation energy eliminated the effect of temperature on the curves. It was found
that the shapes of the recalculated curves and their position relative to the coordinates depend only
on the moisture content in the composite. The equality of the activation energy of the relaxation time
and conductivity established in the study, as well as their independence from frequency and moisture
content in the pressboard impregnated with NYTRO® BIO 300X bio-oil, allows for recalculatingthe curves of electrical parameters determined at any operating temperatures of the transformer
to a reference temperature, for example, 20 ◦C. Comparing the curve obtained for the transformer,
recalculated to the reference temperature, with reference curves determined by us in the laboratory
for different moisture contents, will allow for the precise determination of the moisture content of the
solid insulation component impregnated with NYTRO® BIO 300X bio-oil. This will contribute to the
early detection of approaching critical moisture content, threatening catastrophic transformer failure.
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