Polymers are widely used as insulating materials in high voltage power apparatus because of their excellent electrical insulating properties and good thermomechanical behavior. However, under high electrical stress, polymeric materials can get deteriorated which can eventually lead to the failure of the insulation and thereby the power apparatus. Electrical treeing is one such phenomena whereby dendritic paths progressively grow from a region of high electrical stress and branch into conducting channels in a solid dielectric. The propagation of electrical trees is of particular interest for the power industry as it is one of the major causes of failure of high voltage insulation especially in high voltage cables, cast resin transformers as well as rotating machines. To improve the life time of the electrical insulation systems there is a need to improve the electrical treeing resistance of the insulating material for high voltage application. With the development of nanotechnology, polymer nanocomposites containing nano sized particles have drawn much attention as these materials are found to exhibit unique combinations of physical, mechanical and thermal properties that are advantageous as compared to the traditional polymers or their composites. Literature reveals that significant progress has been made with respect to the mechanical, optical, electronic and photonic properties of these functional materials. Some efforts have also been directed towards the study of dielectric/electrical insulation properties of these new types of materials. Considering the above facts, the present research work focuses on utilizing these new opportunities which have been opened up by the advent of nanocomposites to develop tree resistant insulating materials for high voltage power applications. Electrical treeing is a common failure mechanism in most of the polymeric insulation systems and hence electrical treeing studies have been carried out on two types of polymers (viz. polyethylene used in high voltage cable and epoxy used in rotating machines and resin cast transformers) along with three different types of nano-fillers, viz. Al2O3, SiO2 and MgO and with different filler loadings (0.1, 1, 3, 5 wt%). Furthermore, considering the fact that electrical treeing is a discharge phenomenon, the partial discharge characteristics during electrical tree growth in polymer nanocomposites was studied. As morphological changes in the polymer influence the electrical tree growth, the influence of nano-particle induced morphological changes on the electrical treeing has also been studied. Above all, an attempt has also been made to characterize and analyze the interaction dynamics at the interface regions in the polymer nanocomposite and the influence of these interface regions on the tree growth phenomena in polymer nanocomposites. A laboratory based nanocomposite processing method has been successfully designed and adopted to prepare the samples for treeing studies. Treeing experimental results show that there is a significant improvement in tree initiation time as well as tree inception voltage with nano-filler loading in polymer nanocomposites. It is observed that even with the addition of a small amount (0.1 and 1 % by weight) of nano-particles to epoxy results in the improvement of electrical treeing resistance as compared to the unfilled epoxy. In fact, different tree growth patterns were observed for the unfilled epoxy and epoxy nanocomposites. Surprisingly, even though there is not much improvement in tree inception time, a saturation tendency in tree growth with time was observed at higher filler loadings. To understand the influence of nano-particles on electrical treeing, the interaction dynamics in the epoxy nanocomposites were studied and it was shown that the nature of the bonding at the interface play an important role on the electrical tree growth in epoxy nanocomposites. The results of electrical treeing experiments in polyethylene nanocomposites obtained in this study also reveal some interesting findings. An improved performance of polyethylene against electrical treeing with the inclusion of nano-fillers is observed. It is observed that there is a significant improvement in the tree inception voltage even with low nano-filler loadings in polyethylene. Other interesting results such as change in tree growth pattern from branch to bush as well as slower tree growth with increase in filler loading were also observed. Another peculiar observation is that tree inception voltage increased with increase in filler loading upto a certain filler loadings (3 % by weight) and then decreased in its value at high filler loading. The morphology of polyethylene nanocomposites was studied and a good correlation between morphological changes and treeing results was observed. Effect of cross-linking on electrical treeing has also been studied and a better performance of cross-linking of nano-filled polyethylene samples as compared to the polyethylene samples without cross-linking was observed. The partial discharge (PD) activity during electrical tree growth was monitored and different PD characteristics for unfilled and nano-filled polyethylene samples were observed. Interestingly, a decrease in PD magnitude as well as the number of PD pulses with electrical tree growth in polyethylene nanocomposites was observed. It is known that PD activity depends on the tree channel conductivity, charge trapping and gas pressure inside the tree channel. The ingress of nano-particles into the tree channel influences the above known phenomena and affects the PD activity during electrical tree growth. The observed decrease in PD magnitude with increase in filler loading leads to the slow propagation of electrical trees in polyethylene nanocomposites. In summary, it can be concluded that polymer nanocomposites performed better against electrical treeing as compared to the unfilled and the conventional micron sized filled polymer composites. Even with low filler loading an improved electrical treeing resistance was observed in polymer nanocomposites. An optimum filler loading and a suitable filler to inhibit electrical treeing in the polymers studied are proposed. This work also establishes the fact that the characteristics of the interface region and the induced morphological changes have a strong influence on the electrical treeing behaviors of nanocomposites. These encouraging results showed that epoxy and polyethylene nanocomposites can be used as tree resistant insulating materials for high voltage applications. These results also contribute to widen the scope of applications of polymer nanocomposites in electrical power sector as well as development of multifunctional insulation systems.

Nanocomposites of polyethylene and metal oxide nanoparticles have shown to be a feasible approachto the next generation of insulation in high voltage direct current cables. In order to reach an operationvoltage of 1 MV new insulation materials with reduced conductivity and increased breakdown strengthas compared to modern low-density polyethylene (LDPE) is needed.In this work polyethylene MgO nanocomposites for electrical insulation has been produced andcharacterized both from an electrical and material perspective. The MgO nanoparticles weresynthesized into polycrystalline nanoparticles with a large specific surface area (167 m2 g–1). Meltprocessing by extrusion resulted in evenly dispersed MgO nanoparticles in LDPE for the silane surfacemodified MgO as compared to the unmodified MgO. All systems showed a reduction in conductivityby up to two orders of magnitude at low loading levels (1–3 wt.%), but where the surface modifiedsystems were able to retain reduced conductivity even at loading levels of 9 wt.%. A maximuminteraction radius to influence the conductivity of the MgO nanoparticles was theoretically determinedto ca. 800 nm. The interaction radius was in turn experimentally observed around Al2O3 nanoparticlesembedded in LDPE using Intermodulation electrostatic force microscopy. By applying a voltage on theAFM-tip charge injection and extraction around the Al2O3 nanoparticles was observed, visualizing theexistence of additional localized energy states on, and around, the nanoparticles. Ptychography wasused to reveal nanometre features in 3D of electrical trees formed under DC-conditions. Thevisualization showed that the electrical tree grows by pre-step voids in front of the propagatingchannels, facilitating further growth, much in analogy to mechanical crack propagation (Griffithconcept). An electromechanical effect was attributed as possible mechanism for the formation of the voids. / Nanokompositer av polyeten och metalloxidpartiklar anses vara möjliga material att använda i morgondagens isolationshölje till högspänningskablar för likström. För att nå en transmissionsspänning på 1 MV behövs isolationsmaterial som i jämförelse med dagens polyeten har lägre elektrisk ledningsförmåga, högre styrka mot elektriskt genomslag och som kan kontrollera ansamling av rymdladdningar. De senaste årens forskning har visat att kompositer av polyeten med nanopartiklar av metalloxider har potential att nå dessa egenskaper. I det här arbetet har kompositer av polyeten och nanopartiklar av MgO för elektrisk isolation producerats och karaktäriserats. Nanopartiklar av MgO har framställts från en vattenbaserad utfällning med efterföljande calcinering, vilket resulterade i polykristallina partiklar med en mycket stor specifik ytarea (167m2 g-1). MgO-nanopartiklarna ytmodifierades i n-heptan genom att kovalent binda oktyl(trietoxi)silan och oktadekyl(trimetoxi)silan till partiklarna för att skapa en hydrofob och skyddande yta. Extrudering av de ytmodifierade MgO nanopartiklarna tillsammans med polyeten resulterade i en utmärkt dispergering med jämnt fördelad partiklar i hela kompositen, vilket ska jämföras med de omodifierade partiklarna som till stor utsträckning bildade agglomerat i polymeren. Alla kompositer med låg fyllnadsgrad (1–3 vikt% MgO) visade upp till 100 gånger lägre elektrisk konduktivitet jämfört med värdet för ofylld polyeten. Vid högre koncentrationer av omodifierade MgO förbättrades inte de isolerande egenskaperna på grund av för stor andel agglomerat, medan kompositerna med de ytmodifierade fyllmedlen som var väl dispergerade behöll en kraftig reducerad elektrisk konduktivitet upp till 9 vikt% fyllnadshalt. Den minsta interaktionsradien för MgO-nanopartiklarna för att minska den elektriska konduktiviten i kompositerna fastställdes med bildanalys och simuleringar till ca 800 nm. Den teoretiskt beräknade interaktionsradien kompletterades med observation av en experimentell interaktionsradie genom att mäta laddningsfördelningen över en Al2O3-nanopartikle i en polyetenfilm med intermodulation (frekvens-mixning) elektrostatisk kraftmikroskop (ImEFM), vilket är en ny AFM-metod för att mäta ytpotentialer. Genom att lägga på en spänning på AFM-kantilevern kunde det visualiseras hur laddningar, både injicerades och extraherades, från nanopartiklarna men inte från polyeten. Det tolkades som att extra energinivåer skapades på och runt nanopartiklarna som fungerar för att fånga in laddningar, ekvivalent med den gängse tolkningen att nanopartiklar introducera extra elektronfällor i den polymera matrisen i nanokompositer. Nanotomografi användes för att avbilda elektriska träd i tre dimensioner. Avbildningen av det elektriska trädet visade att tillväxten av trädet hade skett genom bildning av håligheter framför den framväxande trädstrukturen. Håligheterna leder till försvagning av materialet framför det propagerande trädet och förenklar på det sättet fortsatt tillväxt. Bildningen av håligheter framför trädstrukturen uppvisar en analogi till propagering av sprickor vid mekanisk belastning, i enlighet med Griffiths koncept. / <p>QC 20161006</p>