Plasma Interface Modification Endows Bilayered Polypropylene Insulation with Suppressed Space Charge and Enhanced Breakdown Strength for Submarine Cable Joint

High-voltage direct current submarine cables are essential for developing cross-sea transmission and clean energy production. However, due to the limited length (<35 km) of a single submarine cable, cable joint is necessary for realizing cable splicing. Within the submarine cable joint, the interface between cable insulation (CI) and reinforcing insulation is inevitably formed, where charges tend to accumulate, eventually leading to insulation failure. Herein, this paper constructed an Ar-driven dielectric barrier discharge system, in which the treatment time and the H 2 O precursor concentration are adjusted to modify the physicochemical state of the interface. Results show that the 7 min treatment with H 2 O concentration of 1.0% yields the most significant performance improvement. Following this treatment, the protruding spikes on the CI surface are effectively etchedaway, thereby reducing microdefects during the interface welding process. This reduction in microdefects suppresses heterocharge accumulation by 66% and increases breakdown strength by 8.2%. Furthermore, the incorporation of H 2 O introduces foreign oxygen-containing groups (hydroxyl group –OH and methoxy group –OCH 3 ) to the interface, significantly increasing trap depth (1.06 to 1.15 eV) and the density of deep traps in the interface region. Density functional theory calculations and interface modeling elucidate charge behavior in the interfacial region and demonstrate that a uniform interface morphology reduces electric-field distortion and weakens charge injection into the interface region. In addition, the oxygen-containing groups act as charge-capture centers that hinder the migration of heterocharges across the interface, thereby further lowering interfacial charge density and mitigating electric-field distortion. Overall, the interface-modification strategy for PP bilayered insulation proposed in this paper can help extend submarine cable joint lifetime and provide a key reference for the manufacturing of high-performance bilayered insulation interfaces.