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.

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.

Epoxy nanocomposites, with inorganic oxide nanoparticles as filler, can exhibit novel property combinations, such as enhanced mechanical strength, higher thermal conductivity, increased dielectric breakdown strength, and reduced complex permittivity. Therefore, they have interesting applications in nanodielectrics, such as high‐voltage insulation materials or in microelectromechanical systems. The primary challenge in the processing of nanocomposites is achieving a homogeneous dispersion of the nanoparticles. The dispersion quality affects the interfaces between the organic and the inorganic components, which can determine the final properties of the nanocomposite. Here, the processing methods and the resulting dielectric, mechanical, and thermal properties of epoxy nanocomposites with inorganic oxide fillers are presented. Functionalization of the nanoparticle generally improves the dispersion of the particles in the polymer matrix. Different oxide fillers are observed to have similar effects on the properties of the nanocomposites. Epoxy‐based nanocomposites exhibit improved dielectric breakdown strength and lower complex permittivity with inorganic oxide nanoparticles at low filler contents, compared to conventional composites with micrometer‐sized particles. While there are some inconsistencies in the findings, which may be attributed to differences in the dispersion quality, an improved understanding of the nanoparticle–epoxy interfaces in nanocomposites will enable tailoring of the desired properties, opening new avenues for application.

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.

No 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.

No abstract

Detection of partial discharges (PDs) is widely used as a condition assessment tool for high voltage equipment. Application of low frequency test voltage is often preferred in the case of test objects with a large capacitance. The question addressed here is how results from PD-measurements performed at low frequencies correspond to that occurring at 50 Hz power frequency. Different theoretical models for void voltage were examined and compared to experiments performed on laboratory samples of mica/epoxy, including embedded cylindrical voids. All test objects were preconditioned at 10 kV and 50 Hz for 5 min before partial discharge inception voltage (PDIV) testing by stepwise increasing the test voltage from 0 to 10 kV. The PDIV test was first completed at 300 Hz before being repeated at decreasing frequencies down to 0.1 Hz. The temperature was varied in the range of 20° to 155°C. The results at high frequencies showed that a pure capacitive model fits well to the measurements. Measured dielectric response in mica/epoxy explained the decreasing PDIV at low frequencies and high temperatures. A high PDIV was measured at a combination of low temperatures and low frequencies. This was correlated with a reduced void resistance of the electrically stressed void sidewalls caused by the PD activity during the preconditioning period. This indicates that the effect of PD by-products decays faster at higher temperatures. Values of PDIV are, therefore, expected to be dependent on both temperature and frequency.