Among the various requirements that high voltage direct current (HVDC) insulation materials need to satisfy, sufficiently low electrical conductivity is one of the most important. The leading commercial HVDC insulation material is currently an exceptionally clean cross-linked low-density polyethylene (XLPE). Previous studies have reported that the DC-conductivity of low-density polyethylene (LDPE) can be markedly reduced either by including a fraction of high-density polyethylene (HDPE) or by adding a small amount of a well dispersed, semiconducting nanofiller such as Al2O3 coated with a silane. This study demonstrates that by combining these two strategies a synergistic effect can be achieved, resulting in an insulation material with an ultra-low electrical conductivity. The addition of both HDPE and C-8-Al2O3 nanoparticles to LDPE resulted in ultra-insulating nanocomposites with a conductivity around 500 times lower than of the neat LDPE at an electric field of 32 kV/mm and 60-90 degrees C. The new nanocomposite is thus a promising material regarding the electrical conductivity and it can be further optimized since the polyethylene blend and the nanoparticles can be improved independently.

Power transmission over long distances by using high voltage direct current (HVDC) cables is important for the transition from fossil energy to using renewable energy sources, e.g. wind, solar and water. Higher operating voltages enable longer transmission lines but better insulation materials with a much lower conductivity than today´s crosslinked polyethylene (PE) are required to reach the goal of 1 MV by 2030. Nanocomposites consisting of small fractions of metal oxide nanoparticles in PE are promising insulation materials, showing ca. 100 times lower conductivity. The reasons for the better insulating properties are however not fully understood.

The properties of PE and inorganic nanoparticles were studied in this project to evaluate the influence of different material parameters on the conductivity of the cable insulation material. For pristine PE, the polymer morphology and oxidation were found to have a significant impact on the conductivity. For PE nanocomposites, the particle/polymer interface was shown to adsorb polar molecules, which are present in PE cable insulation. A suggested hypothesis is that the adsorption on particle surfaces results in cleaning of the bulk polymer from impurities, which in turn contributes to decreased nanocomposite conductivity. Since the particle interface is believed to be decisive for the nanocomposite properties, the role of particle terminations was investigated in detail. Oxygen dominated particle terminations resulted in 2 times higher composite conductivity than with zinc dominated surfaces, while fully oxygen covered surfaces showed 10 times higher conductivity. Composite systems with micro-sized particles allowed for evaluating parameters independently, which is not possible for nanocomposites. Terminations of ‘PE-like’ hydrocarbon chains lowered the conductivity and these trends could also be transferred to similar zinc oxide nanocomposite systems.

Distribution av elektrisk energi över långa avstånd genom att använda högspänd likström (HVDC) blir allt viktigare för att ställa om till en förnyelsebar energiproduktion (t.ex. solkraft, vindkraft och vattenkraft). Med ökad driftspänning kan längre kabelsystem användas på grund av minskade förluster, men detta ställer högre krav på isoleringsmaterialet. Nya koncept med bättre isolerande egenskaper (t.ex. lägre konduktivitet) än dagens tvärbundna polyeten (PE) måste utvecklas för att kunna uppnå målet med en driftspänning på 1 MV till 2030. Kompositer bestående av nanopartiklar i PE är ett lovande alternativ som är ca. 100 gånger mer isolerande än PE men kunskapen om varför kompositer uppvisar bättre isolerande egenskaper är inte komplett.

Egenskaper hos PE och inorganiska nanopartiklar studeras i detta projekt för att utvärdera vilken betydelse olika parametrar har för DC konduktiviteten. För ren PE så påverkade polymerens morfologi och oxidation konduktiviteten signifikant. För nanokompositer är gränsytan mellan partikel och polymer viktig för kompositens egenskaper och det visades att polära molekyler som finns i kabelisolering av PE adsorberades på partikelytorna. Det föreslogs att adsorptionen bidrar till en renare polymer i kompositerna, vilket i sin tur minskar konduktiviteten. Termineringar på zinkoxidpartiklar undersöktes i detalj och partiklar med en majoritet av syre på ytan ökade kompositens konduktivitet 2 gånger jämfört med dominerande termineringar av zink. Ytor helt täckta av syre ökade konduktiviteten 10 gånger. Påverkan av funktionaliteten på partikelytan kunde studeras oberoende av andra parametrar genom att använda större mikropartiklar, vilket inte är möjligt för nanopartiklar. Slutsatsen att partikelytor med kolväten som liknar PE sänkte konduktiviteten jämfört med syredominerande ytor kunde även bekräftas för kompositer med nanopartiklar.

Destruction of the spherulite structure in low-density polyethylene (LDPE) is shown to result in a more insulating material at low temperatures, while the reverse effect is observed at high temperatures. On average, the change in morphology reduced the conductivity by a factor of 4, but this morphology-related decrease in conductivity was relatively small compared with the conductivity drop of more than 2 decades that was observed after slight oxidation of the LDPE (at 25 degrees C and 30 kV mm(-1)). The conductivity of LDPE was measured at different temperatures (25-60 degrees C) and at different electrical field strengths (3.3-30 kV mm(-1)) for multiple samples with a total crystalline content of 51 wt%. The transformation from a 5 mu m coherent structure of spherulites in the LDPE to an evenly dispersed random lamellar phase (with retained crystallinity) was achieved by extrusion melt processing. The addition of 50 ppm commercial phenolic antioxidant to the LDPE matrix (e.g. for the long-term use of polyethylene in high voltage direct current (HVDC) cables) gave a conductivity ca. 3 times higher than that of the same material without antioxidants at 60 degrees C (the operating temperature for the cables). For larger amounts of antioxidant up to 1000 ppm, the DC conductivity remained stable at ca. 1 x 10(-14) S m(-1). Finite element modeling (FEM) simulations were carried out to model the phenomena observed, and the results suggested that the higher conductivity of the spherulite-containing LDPE stems from the displacement and increased presence of polymeric irregularities (formed during crystallization) in the border regions of the spherulite structures.

The best commercial high-voltage insulation material of today is (crosslinked) ultra-pure low-density polyethylene (LDPE). A 100-fold decrease in electrical conductivity can be achieved by adding 1–3 wt.% of well-dispersed inorganic nanoparticles to the LDPE. One hypothesis is that the nanoparticle surfaces attract ions and polar molecules, thereby cleaning the surrounding polymer, and thus reducing the conductivity. LDPE-based nanocomposites with 1–12 wt.% octyl-coated aluminum oxide nanoparticles were prepared and the sorption and desorption of one polar compound (acetophenone, a crosslinking by-product) and one non-polar compound of a similar size (limonene) were examined. Since the uptake of acetophenone increased linearly with increasing filler content, whereas the uptake of limonene decreased, the surface attraction hypothesis was strengthened. The analytical functions for predicting composite solubility as a function of particle size and filler fraction were derived using experimental solubility measurements and Monte Carlo simulations.

The effects of particle surface termination by zinc or oxygen were evaluated for composites containing micro-sized ZnO particles with rod shapes (17% oxygen terminations) or ball shapes (67% oxygen terminations), and it was found that the rods gave a conductivity (1.2 x 10(-16) S m(-1)) half that given by the ball-shaped particles (2.4 x 10(-16) S m(-1)). Both composites containing the micro-sized particles showed a conductivity almost two orders of magnitude lower than that of the LDPE reference material (1.2 x 10(-14) S m(-1)). When a 5 nm thick silica coating was applied to the particles, the silica encapsulation eliminated the difference between the particles and resulted in both cases in an increase in conductivity by an order of magnitude to ca. 2 x 10(-15) S m(-1). The conductivity was still lower than that of the pristine polyethylene polymer. It was concluded that neither the particle morphology nor the inter-particle distance (1 mu m for rods and 8 mu m for balls) had any effect on the conductivity of the composites for identically terminated particles, while demonstrating that the conductivity of these materials relies uniquely on the particle surface terminations. In contrast, a markedly reduced conductivity was observed for composites containing the same particles but terminated with aliphatic hydrocarbon tails, the conductivity for both rod-shaped and ball-shaped particles (1 x 10(-16) S m(-1)) being reduced to even lower values than for the pristine particles without surface modification. The same trend was observed with the 25 nm ZnO nanoparticles, showing a record low conductivity of 1 x 10(-17) S m(-1) for 3 wt% nanoparticles with aliphatic hydrocarbon tails. In practical applications, this would permit higher operation voltages than currently employed HVDC cable systems by controlling the resistivity of the composite insulation for various electric fields and temperatures and making it possible to tailor the dielectric design of cable components.

Promising electrical field grading materials (FGMs) for high-voltage direct-current (HVDC) applications have been designed by dispersing reduced graphene oxide (rGO) grafted with relatively short chains of poly (n-butyl methacrylate) (PBMA) in a poly(ethylene-co-butyl acrylate) (EBA) matrix. All rGO-PBMA composites with a filler fraction above 3 vol.% exhibited a distinct non-linear resistivity with increasing electric field; and it was confirmed that the resistivity could be tailored by changing the PBMA graft length or the rGO filler fraction. A combined image analysis- and Monte-Carlo simulation strategy revealed that the addition of PBMA grafts improved the enthalpic solubility of rGO in EBA; resulting in improved particle dispersion and more controlled flake-to-flake distances. The addition of rGO and rGO-PBMAs increased the modulus of the materials up to 200% and the strain did not vary significantly as compared to that of the reference matrix for the rGO-PBMA-2 vol.% composites; indicating that the interphase between the rGO and EBA was subsequently improved. The new composites have comparable electrical properties as today's commercial FGMs; but are lighter and less brittle due to a lower filler fraction of semi-conductive particles (3 vol.% instead of 30-40 vol.%).

The authors wish to make the following corrections to their paper [1]: i. On pages 13 and 14, the numbering of references from 17 to 30 is incorrect. References 17 to 30 should be renumbered from the original order below: 17. Liu, Z.; Liu, Q.; Wang, Z.D.; Jarman, P.; Krause, C.; Smith, P.W.R.; Gyore, A. Partial discharge behaviour of transformer liquids and the influence of moisture content. In Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), Bled, Slovenia, 29 June–3 July 2014. 18. Yamashita, H.; Yamazawa, K.; Wang, Y.S. The effect of tip curvature on the prebreakdown streamer structure in cyclohexane. IEEE Trans. Dielectr. Electr. Insul. 1998, 5, 396–401. 19. Dumitrescu, L.; Lesaint, O.; Bonifaci, N.; Denat, A.; Notingher, P. Study of streamer inception in cyclohexane with a sensitive charge measurement technique under impulse voltage. J. Electrostat. 2001, 53, 135–146. 20. Pourrahimi, A.M.; Hoang, T.A.; Liu, D.; Pallon, L.K.H.; Gubanski, S.; Olsson, R.T.; Gedde, U.W.; Hedenqvist, M.S. Highly efficient interfaces in nanocomposites based on polyethylene and ZnO nano/hierarchical particles: A novel approach toward ultralow electrical conductivity insulations. Adv. Mater. 2016, 28, 8651–8657. 21. Li, J.; Du, B.; Wang, F.; Yao, W.; Yao, S. The effect of nanoparticle surfactant polarization on trapping depth of vegetable insulating oil-based nanofluids. Phys. Lett. A 2016, 380, 604–608. 22. Aljure, M.; Becerra, M.; Pallon, L.K.H. Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration. In Proceedings of the 2016 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Toronto, ON, Canada, 16–19 October 2016; pp. 687–690. 23. Primo, V.A.; Garcia, B.; Albarracin, R. Improvement of transformer liquid insulation using nanodielectric fluids: A review. IEEE Electr. Insul. Mag. 2018, 34, 13–26. 24. Jin, H.; Andritsch, T.; Morshuis, P.H.F.; Smit, J.J. AC breakdown voltage and viscosity of mineral oil based SiO2 nanofluids. In Proceedings of the 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Montreal, QC, Canada, 14–17 October 2012; pp. 902–905. 25. Jin, H.; Morshuis, P.; Mor, A.R.; Smit, J.J.; Andritsch, T. Partial discharge behavior of mineral oil based nanofluids. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 2747–2753. 26. Du, Y.; Lv, Y.; Li, C.; Chen, M.; Zhong, Y.; Zhou, J.; Li, X.; Zhou, Y. Effect of semiconductive nanoparticles on insulating performances of transformer oil. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 770–776. 27. Dung, N.V.; Høidalen, H.K.; Linhjell, D.; Lundgaard, L.E.; Unge, M. Effects of reduced pressure and additives on streamers in white oil in long point-plane gap. J. Phys. D Appl. Phys. 2013, 46, 255501. 28. McCool, J.I. Using the Weibull Distribution; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012. 29. Lesaint, O.L.; Top, T.V. Streamer initiation in mineral oil. part I: Electrode surface effect under impulse voltage. IEEE Trans. Dielectr. Electr. Insul. 2002, 9, 84–91. 30. Becerra, M.; Frid, H.; Vázquez, P.A. Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane. Phys. Fluids 2017, 29, 123605. to the following, corrected numbering: 17. Dumitrescu, L.; Lesaint, O.; Bonifaci, N.; Denat, A.; Notingher, P. Study of streamer inception in cyclohexane with a sensitive charge measurement technique under impulse voltage. J. Electrostat. 2001, 53, 135–146. 18. Liu, Z.; Liu, Q.; Wang, Z.D.; Jarman, P.; Krause, C.; Smith, P.W.R.; Gyore, A. Partial discharge behaviour of transformer liquids and the influence of moisture content. In Proceedings of the 2014 IEEE 18th International Conference on Dielectric Liquids (ICDL), Bled, Slovenia, 29 June–3 July 2014. 19. Yamashita, H.; Yamazawa, K.; Wang, Y.S. The effect of tip curvature on the prebreakdown streamer structure in cyclohexane. IEEE Trans. Dielectr. Electr. Insul. 1998, 5, 396–401. 20. Becerra, M.; Frid, H.; Vázquez, P.A. Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane. Phys. Fluids 2017, 29, 123605. 21. Pourrahimi, A.M.; Hoang, T.A.; Liu, D.; Pallon, L.K.H.; Gubanski, S.; Olsson, R.T.; Gedde, U.W.; Hedenqvist, M.S. Highly efficient interfaces in nanocomposites based on polyethylene and ZnO nano/hierarchical particles: A novel approach toward ultralow electrical conductivity insulations. Adv. Mater. 2016, 28, 8651–8657. 22. Li, J.; Du, B.; Wang, F.; Yao, W.; Yao, S. The effect of nanoparticle surfactant polarization on trapping depth of vegetable insulating oil-based nanofluids. Phys. Lett. A 2016, 380, 604–608. 23. Aljure, M.; Becerra, M.; Pallon, L.K.H. Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration. In Proceedings of the 2016 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Toronto, ON, Canada, 16–19 October 2016; pp. 687–690. 24. Primo, V.A.; Garcia, B.; Albarracin, R. Improvement of transformer liquid insulation using nanodielectric fluids: A review. IEEE Electr. Insul. Mag. 2018, 34, 13–26. 25. Jin, H.; Andritsch, T.; Morshuis, P.H.F.; Smit, J.J. AC breakdown voltage and viscosity of mineral oil based SiO2 nanofluids. In Proceedings of the 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Montreal, QC, Canada, 14–17 October 2012; pp. 902–905. 26. Jin, H.; Morshuis, P.; Mor, A.R.; Smit, J.J.; Andritsch, T. Partial discharge behavior of mineral oil based nanofluids. IEEE Trans. Dielectr. Electr. Insul. 2015, 22, 2747–2753. 27. Du, Y.; Lv, Y.; Li, C.; Chen, M.; Zhong, Y.; Zhou, J.; Li, X.; Zhou, Y. Effect of semiconductive nanoparticles on insulating performances of transformer oil. IEEE Trans. Dielectr. Electr. Insul. 2012, 19, 770–776. 28. Dung, N.V.; Høidalen, H.K.; Linhjell, D.; Lundgaard, L.E.; Unge, M. Effects of reduced pressure and additives on streamers in white oil in long point-plane gap. J. Phys. D Appl. Phys. 2013, 46, 255501. 29. McCool, J.I. Using the Weibull Distribution; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2012. 30. Lesaint, O.L.; Top, T.V. Streamer initiation in mineral oil. part I: Electrode surface effect under impulse voltage. IEEE Trans. Dielectr. Electr. Insul. 2002, 9, 84–91. ii. On the last paragraph of page 9, the last sentence should be changed from: However, the results in [11] also show the consistent increase in the initiation voltage of prebreakdown phenomena in both polarities, as reported in Figure 11. to the following, corrected version: However, the results in [26] also show the consistent increase in the initiation voltage of prebreakdown phenomena in both polarities, as reported in Figure 11. iii. On the last paragraph of page 10, the third sentence should be changed from: Even though the existing hypotheses of the dielectric effect of NPs [8–10] were proposed for blunter electrodes (where charge generation before streamer initiation is less important [30]), they should still apply under the experimental conditions here reported. to the following, corrected version: Even though the existing hypotheses of the dielectric effect of NPs [5,6,16] were proposed for blunter electrodes (where charge generation before streamer initiation is less important [30]), they should still apply under the experimental conditions here reported. The authors would like to apologize for any inconvenience caused to the readers by these changes. The changes do not affect the scientific results. The manuscript will be updated and the original will remain online on the article webpage, with a reference to this Correction.

This work introduces two measurement techniques for precision determination of ultra-low conductivity and accurate characterization of dielectric frequency response for the assessments of polymeric materials used in HVDC insulations. To demonstrate the proposed methods, electrical properties of four different low density polyethylene (LDPE) specimens, obtained by different manufacturing processes were characterized. Results obtained from dc conductivity measurements revealed clear separation of the conduction current levels in the studied specimens. Dielectric losses obtained from the frequency response measurements agrees well with the differences observed in the measured conductivities. This study concludes that the different manufacturing processes have a significant impact on materials electric properties and these parameters can be characterized with precision beyond the existing instruments' specification by using the proposed methods.

Positive and negative streamer inception voltages from ultra-sharp needle tips (with tip radii below 0.5 m) are measured in TiO2, SiO2, Al2O3, ZnO and C-60 nanofluids. The experiments are performed at several concentrations of nanoparticles dispersed in mineral oil. It is found that nanoparticles influence positive and negative streamers in different ways. TiO2, SiO2 and Al2O3 nanoparticles increase the positive streamer inception voltage only, whilst ZnO and C-60 nanoparticles augment the streamer inception voltages in both polarities. Using these results, the main hypotheses explaining the improvement in the dielectric strength of the host oil due to the presence of nanoparticles are analyzed. It is found that the water adsorption hypothesis of nanoparticles is consistent with the increments in the reported positive streamer inception voltages. It is also shown that the hypothesis of nanoparticles reducing the electron velocity by hopping transport mechanisms fails to explain the results obtained for negative streamers. Finally, the hypothesis of nanoparticles attaching electrons according to their charging characteristics is found to be consistent with the results hereby presented on negative streamers.

A protocol for the aqueous synthesis of ca. 1-mu m-long zinc oxide (ZnO) nanorods and their growth at intermediate reaction progression is presented, together with photoluminescence (PL) characteristics after heat treatment at temperatures of up to 1000 degrees C. The existence of solitary rods after the complete reaction (60 min) was traced back to the development of sea urchin structures during the first 5 s of the precipitation. The rods primarily formed in later stages during the reaction due to fracture, which was supported by the frequently observed broken rod ends with sharp edges in the final material, in addition to tapered uniform rod ends consistent with their natural growth direction. The more dominant rod growth in the c direction (extending the length of the rods), together with the appearance of faceted surfaces on the sides of the rods, occurred at longer reaction times (>5 min) and generated zinc-terminated particles that were more resistant to alkaline dissolution. A heat treatment for 1 h at 600 or 800 degrees C resulted in a smoothing of the rod surfaces, and PL measurements displayed a decreased defect emission at ca. 600 nm, which was related to the disappearance of lattice imperfections formed during the synthesis. A heat treatment at 1000 degrees C resulted in significant crystal growth reflected as an increase in luminescence at shorter wavelengths (ca. 510 nm). Electron microscopy revealed that the faceted rod structure was lost for ZnO rods exposed to temperatures above 600 degrees C, whereas even higher temperatures resulted in particle sintering and/or mass redistribution along the initially long and slender ZnO rods. The synthesized ZnO rods were a more stable Wurtzite crystal structure than previously reported ball-shaped ZnO consisting of merging sheets, which was supported by the shifts in PL spectra occurring at ca. 200 degrees C higher annealing temperature, in combination with a smaller thermogravimetric mass loss occurring upon heating the rods to 800 degrees C.