On the Tangential AC Breakdown Strength of Polymer Interfaces Considering Elastic Modulus , Frank Mauseth, and Erling Ildstad Department of Electric Power Engineering Norwegian University of Science and Technology Trondheim, Norway Sverre Hvidsten Electric Power Technology SINTEF Energy Research Trondheim, Norway Abstract—The interfacial breakdown between two dielectric surfaces was reported to represent one of the leading causes of failure for power cable joints and connectors, in which elastic modulus of the dielectric material plays a key role. The primary motivation of this paper is to study the influence of the elastic modulus of the polymer insulation on the tangential AC breakdown strength (BDS) of polymer interfaces experimentally. In the experiments, four different materials with different elastic moduli were employed under various contact pressures: polyether ether ketone (PEEK), cured end product of epoxy resin (EPOXY), cross-linked polyethylene (XLPE), and silicone rubber (SiR). The BDS of each interface increased as the contact pressure was augmented. As the contact pressure became threefold, the interfacial BDS rose by a factor of 2.4, 1.7, 1.8, and 1.4 in the case of the PEEK, EPOXY, XLPE and SiR interface, in a sequence following the decrease of the elastic modulus. Under the same contact pressure, it was observed that the lower the elastic modulus, the higher the BDS. Index Terms—Cavity, dielectric breakdown, epoxy, partial discharge, PEEK, polymer interface, silicone rubber, surface breakdown, void, XLPE. I. I NTRODUCTION Subsea cable connectors are vital components of oil and gas installations, future offshore wind and wave energy systems. Although materials and production technologies for subsea applications have gained a fair amount of experience over the years, cable connectors and joints where solid-solid interfaces emerge are still considered the weaker parts of complete cable systems [ ]–[ ]. One of the main reasons of a solid-solid interface being weaker than its intrinsic material is that an interface contains microscopic imperfections such as cavities (see Fig. ), pro- trusions, and contaminants. Such defects reduce the tangential AC electric breakdown strength (BDS) of the interface notably [ ], [ ]. Even in cases when the magnitude of the longitudinal electric field is much lower than the dielectric strength of the intrinsic insulation, the imperfections at the interface cause local electric field enhancements [ ]. They are, thus, likely to initiate partial discharges (PD), electrical treeing, and a complete flashover might eventually follow [ ]–[ ]. Study of insulating materials and BDS of applications for cables and accessories has been covered to a large extent in The authors acknowledge the financial support of the Research Council of Norway (project no. 228344) and the SUBCONN Project Consortium. the literature. Greenwood et al. [ ] and Bhusnan [ ] reported that total area of contact at an interfacial surface substantially increases in the cases when the elastic modulus is decreased, the contact pressure is augmented, or both. The interfacial breakdown between two dielectric surfaces was reported to represent one of the principal causes of failure for power cable joints and connectors, in which elastic modulus of the dielectric material plays a key role [ ], [ ], [ ]. There is; however, still a lack of knowledge on the correlation between the elastic modulus and the BDS of the interfacial surfaces. Therefore, the primary objective of this paper is to experimentally examine the influence of the elastic modulus on the longitudinal AC breakdown strength of dry-assembled solid-solid interfaces under various contact pressures. II. BACKGROUND When a polymer interface is assembled in dry conditions, interfacial cavities as illustrated in Fig. are filled with air. The applied voltage is then distributed along strings of the cavities and contact spots. Since the dielectric strength of air is much lower than that of the polymer insulation, the dielectric breakdown will first occur in the air-filled cavities, and then the complete flashover presumably takes place eventually [ ]. In the case of a homogeneous electric field, the correlation between the cavity size and the breakdown voltage (BDV) thereof is characterized by the Paschen’s curve for air [ ]. Referring to the left side of the Paschen’s curve for air (the left branch of the V-shaped curve), it can be inferred that as the cavity length increases, the BDV of the cavity reduces. In case of an elastic contact at the polymer interface, the interfacial cavities shrink as the contact pressure is increased, because afloat asperities come to contact and form smaller cavities with the pressure increase [ ]. Sizes of the inter- facial cavities changing as a function of the applied load can be determined by adopting the ”deterministic contact E y x Cavity Contact spot Fig. 1. An illustration of the air-filled cavities at the interface in two- dimensional profile.