The interfacial breakdown between two dielectric surfaces has been reported to represent one of the principal causes of failure for power cable joints and connectors; thus, a better understanding of interfacial breakdown mechanisms is vital. The primary purpose of this paper is to investigate the influence of the surface roughness and interfacial pressure on the tangential AC breakdown strength (BDS) of solid-solid interfaces experimentally. The three-dimensional surface texture parameters are utilized to characterize the morphology of the surfaces. Experiments were performed using samples made of cross-linked polyethylene (XLPE) at three different contact pressures. The surface roughness was varied by polishing the surfaces using four different sandpapers of different roughness. Each surface topography was then assessed using a 3-D optical profilometer. Next, the samples were assembled under ambient laboratory conditions. The experimental results showed a good correlation between the tangential BDS and the surface roughness. The results suggested that reducing the surface roughness resulted in decreased mean height of the surface asperities by nearly 97% and increased the real contact area of the interface considerably. As a result, the tangential BDS rose by a factor of 1.85 -2.15 with increasing pressure. Likewise, the increased contact pressure yielded augmented tangential BDS values by a factor of 1.4 -1.7 following the decrease of the roughness.

Morphology of the contact area between solid insulation materials ultimately determines the long-term electrical properties of the complete insulation system. The primary purpose of this paper is not only to propose a statistical model to scrutinize the real area of contact between solid dielectric surfaces but also to verify and correlate the model outputs with experiments. The model computes real area of contact, number of contact spots and average cavity size at the interface as a function of elasticity, contact force and surface roughness. Then, using the average cavity size and the Paschen's law, cavity discharge inception field (PDIE) is calculated. In the experiments, AC breakdown strength (BDS) testing of solid-solid interfaces was carried out, where cross-linked polyethylene (XLPE) samples with four different surface roughnesses were subjected to various contact pressures.Following the increased contact force, the calculated average cavity size decreased by a factor of 4.08 − 4.82 from the roughest to the smoothest surface, that in turn yielded increased PDIEs by a factor of 2.01 − 2.56. Likewise, the experimentally obtained BDS values augmented by a factor of 1.4 − 1.7 when the contact pressure was elevated from 0.5 MPa to 1.16 MPa.A linear correlation between the PDIE and BDS was assumed, yielding a correlation coefficient varying within 0.8−1.3. When the 90% confidence intervals were considered, the range reduced to 0.86 − 1.05. This close affinity suggests that interfacial breakdown phenomenon is strongly governed by the cavity discharge. Hence, the proposed model is verified with experiments.

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link ABSTRACT This position paper, prepared by the IEEE DEIS HVDC Cable Systems TechnicalCommittee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests.

We study positive streamers in air propagating along polycarbonate dielectric plates with and without small-scale surface profiles. The streamer development was documented using light-sensitive high-speed cameras and a photo-multiplier tube, and the experimental results were compared with 2D fluid streamer simulations. Two profiles were tested, one with 0.5 mm deep semi-circular corrugations and one with 0.5 mm deep rectangular corrugations. A non-profiled surface was used as a reference. Both experiments and simulations show that the surface profiles lead to significantly slower surface streamers, and also reduce their length. The rectangular-cut profile obstructs the surface streamer more than the semi-circular profile. We find quantitative agreement between simulations and experiments. For the surface with rectangular grooves, the simulations also reveal a complex propagation mechanism where new positive streamers re-ignite inside the surface profile corrugations. The results are of importance for technological applications involving streamers and solid dielectrics.

We investigate the propagation of positive streamers along a profiled dielectric surface in air at atmospheric pressure. Results from experiments and two-dimensional planar low-temperature plasma fluid simulations are presented and analysed. The test object consists of a disk-shaped high voltage electrode and a dielectric slab with 0.5 mm deep corrugations. The corrugated surface has a 47% larger surface area than the smooth reference surface. The experiments and simulations are performed at voltage levels that lead to either gap-bridging or arrested streamers. In both experiments and simulations, the streamers take a longer time to reach the ground electrode when propagating along the profiled surface than along the smooth reference surface. Also, arrested streamers stop closer to the high voltage electrode when a profiled surface is used. Streamers propagate closely along the surface profile in the simulations, which suggests that the observed surface profile effect is mainly a result of elongated streamer channels. Compared to the streamers propagating along the smooth surface, the elongated streamers on the profiled surface have less residual voltage at the streamer front to drive the streamer advancement.