Investigation of Threshold Electric Field Characteristics in Polypropylene-Laminated Paper and Oil-Impregnated Paper Insulation

Currently, polypropylene laminated paper (PPLP) is one of the primary insulation materials employed in high-voltage direct current (HVDC) oil-filled cables. In this study, traditional oil- impregnated paper and PPLP insulation were selected as test objects; their conductivity and threshold electric field characteristics were comparatively analyzed under high electric fields across a wide range from 0.1 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$80 \text{kV} / \text{mm}$</tex>. The results indicate that the current density of PPLP remains below <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10^{-7} ~\mathrm{A} / \mathrm{m}^{2}$</tex> in the low-field region, while exhibiting conductivity approximately one order of magnitude lower than that of oil-impregnated paper insulation even under elevated electric fields. A distinct conduction threshold is observed at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$34.4 \text{kV} / \text{mm}$</tex> for PPLP, in contrast to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10.5 \text{kV} / \text{mm}$</tex> for oil- impregnated paper insulation, which reveals the superior charge injection suppression capability and electric field endurance of PPLP. These advantages are attributed to the low polarization loss of the polypropylene layers and the effective blocking of carrier transport at material interfaces, making PPLP a highly promising candidate for application in ultra-high-voltage DC insulation systems.