Impact of moisture on the frequency–temperature characteristics of the complex permittivity of pressboard composite impregnated with an innovative bio-oil for power transformer solid insulation

Environmental concerns and the need to reduce climate warming are driving efforts to eliminate petroleum-based mineral oils in power transformer insulation, replacing them with plant-based bio-oils. This transition necessitates new diagnostic standards tailored to cellulose–bio-oil insulation systems. A key parameter in such diagnostics is complex permittivity, consisting of real and imaginary components. This study examined six pressboard samples containing 0.6–5 wt% moisture and impregnated with NYTRO® BIO 300X bio-oil. Measurements revealed two distinct stages in the real permittivity response. Stage I occurs below 2 wt% moisture, while Stage II appears at higher moisture levels and shifts toward higher frequencies as water content increases. Both stages are strongly moisture-dependent. Dielectric relaxation times were analyzed, and an average activation energy of ΔE(ε,τ)≈ (1.057 ± 0.0371) eV was calculated. The unusually high susceptibility (28–36) and long relaxation times (tens of seconds) at Stage II—compared to theoretical values for liquid water—confirmed the absence of free water in the composite. The frequency–temperature dependence of dielectric loss (ε″) was also determined. The activation energy for lossiness matched that of conductivity, ΔE(ε”,τ)≈ (1.02627 ± 0.01606) eV. Temperature-normalized loss curves for different moisture levels overlapped precisely, validating a unified approach to diagnostic modeling. Moisture increase shifted the curves toward higher frequencies and loss values. These findings support the development of reliable, moisture-sensitive diagnostics for next-generation transformer insulation systems using bio-oils.