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.

The combination of two solid dielectrics (interface) increases the risk of formation of microscopic cavities reducing the breakdown strength (BDS) of the interface considerably, particularly when the electric field has a tangential component. The main purpose of this paper is to investigate the impact of the applied contact pressure and composite elastic modulus on the tangential ac BDS of the solid-solid interfaces experimentally. In the experiments, three different contact pressures were applied using different mechanical loads with two different materials having different elastic moduli, i.e. cross-linked polyethylene (XLPE) and silicon rubber (SiR). Two rectangular prism shaped samples were placed between two vertical Rogowski shaped electrodes either in air or oil. The type of the interface (air/oil) is highlighted duly upon showing the results. Increase in contact pressure caused relatively higher increase in the tangential BDS of dry SiR-SiR (assembled in air) than that of XLPE-XLPE, revealing that elastic modulus facilitated significantly to reduce the mean void size in SiR that in turn improved the tangential BDS. Likewise, the tangential BDS of hybrid interfaces formed by XLPE-SiR specimens increased by 43% compared to that of XLPE-XLPE interface at the same pressure. Additionally, the same set of experiments assembled in oil reveals that the presence of oil enhanced the tangential BDSs around 2-3 times for all three-interface cases. Moreover, with the increase of applied pressure the tangential BDS of air-filled and oil-filled cavities tended to get significantly higher.

This paper assesses the feasibility of three-phase pulse-width modulated (PWM) three-level voltage-source converters (VSC), namely neutral-point-clamped (3L-NPC) and T-type (3L-T) as alternatives to two-level converter (2L-VSC) for low-voltage multimegawatt renewable energy grid-connected converter applications. For this purpose, a novel design algorithm that takes switching frequency, capacity factor, modulation index, PWM scheme(s), and converter topology(ies) as input is unveiled to provide the design steps and performance benchmark for the specified topologies and modulation methods. Design of the LCL filter is also addressed, and equivalent output ripple performance criteria is used to benchmark the converter performances for a set of selected filter parameters. Main contribution of this algorithm is that it covers cost and operating factors of the design during the hardware design phase by introducing/modifying the terms of total cost of ownership (TCO) and return on investment (ROI) time considering the major energy markets. The design is detailed for 1-MVA system, and the results are summarized for 0.5 and 2-MVA systems. The study shows that 3L topologies perform more efficiently than 2L-VSC (T-type being the most prominent-with the shortest ROI), whereas the difference becomes much more prominent at the multimegawatt range, provided that all three yield equivalent outputs fulfilling the stringent grid codes.

Solid-solid interfaces between insulating materials dictate the long-term electrical properties of the complete insulation system. This paper presents theoretical and experimental investigations aiming to address the impact of the material elasticity on tangential AC breakdown strength (BDS) of interfaces between polymers. Four different polymers with different elastic moduli were tested using: Cross-linked polyethylene (XLPE), filled epoxy resin (EPOXY), polyether ether ketone (PEEK) and silicone rubber (SiR). The interfaces were formed between identical specimens and were breakdown tested at various contact pressures. It was found that elastic modulus and contact pressure had pronounced effects on the BDS of interfaces. Higher elastic modulus correlated with decreased BDS by a factor of 1.6 at the same contact pressure. On the other hand, the increase of contact pressure by a factor of 3 elevated the interfacial BDS by a factor of 1.4 in the case of the lowest elastic modulus (SiR-SiR) whereas that for the highest modulus (PEEK-PEEK) was about 2.4 times higher. Using the proposed theoretical approach, we postulated that discharged cavities govern the interfacial BDS at the interface together with the electrical tracking resistance of contact area between the cavities. Although the electrical tracking resistance increases with a higher modulus, local field enhancements due to discharged cavities also increase significantly. Therefore, the observed reduction of the BDS with the increase of the elastic modulus is ascribed to the larger cavity size and hence the smaller contact area. It is concluded that increased elasticity reduces the dominance of the discharged cavities over the interface breakdown and increase the governance of the electrical tracking resistance of the contact spots.