Divergent Voltage Dependence of Frequency Dielectric Response in Natural Ester and Mineral Oil-Impregnated Press-Board

Frequency dielectric spectroscopy (FDS), a nonintrusive diagnostic tool for oil-paper insulation systems, shows voltage-dependent nonlinear behaviors that differ significantly between natural ester and mineral oil impregnants. This study compares these nonlinear characteristics in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1-\text{mm}$</tex> Weidmann T4 press-boards impregnated with natural ester oil and 25# mineral oil. FDS measurements (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.001 \text{Hz}-1 \text{kHz}, 20^{\circ} \mathrm{C}$</tex>) using a three-terminal electrode system revealed distinct responses: natural ester insulation exhibited a pronounced <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.01-0.1 \text{Hz}$</tex> distinct dielectric loss enhancement and nonlinearity below 1 Hz, whereas mineral oil displayed nonlinearity only below 0.1 Hz. Notably, mineral oil's dielectric loss showed stronger voltage sensitivity. The results demonstrate that natural ester insulation manifests voltage-amplitude-related nonlinearity at higher frequencies, while mineral oil is more vulnerable to voltage variations.