This review focuses on the use of polyolefins in high-voltage direct-current (HVDC) cables and capacitors. A short description of the latest evolution and current use of HVDC cables and capacitors is first provided, followed by the basics of electric insulation and capacitor functions. Methods to determine dielectric properties are described, including charge transport, space charges, resistivity, dielectric loss, and breakdown strength. The semicrystalline structure of polyethylene and isotactic polypropylene is described, and the way it relates to the dielectric properties is discussed. A significant part of the review is devoted to describing the state of art of the modeling and prediction of electric or dielectric properties of polyolefins with consideration of both atomistic and continuum approaches. Furthermore, the effects of the purity of the materials and the presence of nanoparticles are presented, and the review ends with the sustainability aspects of these materials. In summary, the effective use of modeling in combination with experimental work is described as an important route toward understanding and designing the next generations of materials for electrical insulation in high-voltage transmission.

Bipolar charge transport (BCT) models have been used for the past few decades to simulate and analyze charge transport and its associated phenomena in polymeric insulation materials. Whilst in this method primarily, electronic charge carriers are used, recent studies show that there has been a high interest in the addition of ionic charge carriers and their physical processes in the model. Among the byproducts in the crosslinked XLPE, acetophenone is speculated to have deep electron traps and therefore can contribute to the electrical conduction processes in XLPE insulation, leading to charge and field dynamics in XLPE insulation. In this paper, a new model is attempted considering acetophenone for modelling the ions originating from acetophenone molecules, which demonstrates the charge and field dynamics in the insulation. Using the BCT model, combining with the electronic species, the impact of the presence of the ions is discussed. The ionic charge carrier and the neutral molecule are modelled based on Marcus theory. The results reported in this paper draw attention to the charge and field distribution for different ionic concentrations, the effect of thermal gradient and mobilities of the ions. A comparison between the experimental conductivity values from various literature and the proposed model has been done.

Polyethylene (PE) is an excellent electrical insulator, and is common in high voltage cable applications, where it is chemically cross linked to improve thermal stability, leaving cross linking residues in the material. Acetophenone is one such residue, which is often studied due to its tendency to trap mobile electrons. In the present work the keto-enol tautomerization of acetophenone in the gas phase is studied using quantum chemistry methods. Activation energies, free energy change and Eyring rate constants are calculated for both reaction directions, for both neutral and ionized acetophenone. Ionization is shown to lower the activation energies of tautomerization, and the energetically favored enol to keto reaction has particularly high reaction rates for ionized molecules. Activation energies are similar to electron trapping energies of acetophenone in PE, and we postulate a trapping activated tautomerization mechanism, and discuss how the various modes of trapping, de-trapping and recombination for keto and enol acetophenone may establish a balance keto and enol tautomers over time.

Electrical conductivity measurements of polyethylene indicate that the semicrystalline structure and morphology influence the conductivity. To include this effect in atomistic charge transport simulations, models that explicitly or implicitly take morphology into account are required. In the literature, charge transport simulations of amorphous polyethylene have been successfully performed using short oligomers to represent the polymer. However, a more realistic representation of the polymer structure is desired, requiring the development of fast and efficient charge transport algorithms that can handle large molecular systems through coarse-graining. Here, such a model for charge transport simulations in polyethylene is presented. Quantum chemistry calculations were used to define six segmentation rules on how to divide a polymer chain into shorter segments representing localized molecular orbitals. Applying the rules to amorphous systems yields distributions of segments with mode and median segment lengths relatively close to the persistence length of polyethylene. In an initial test, the segments of an amorphous polyethylene were used as hopping sites in kinetic Monte Carlo (KMC) simulations, which yielded simulated hole mobilities that were within the experimental range. The activation energy of the simulated system was lower compared to the experimental values reported in the literature. A conclusion may be that the experimental result can only be explained by a model containing chemical defects that generate deep traps.

Experimental results indicate that electrical conductivity in polyethylene depend on the morphology. In order to simulate mobility in polyethylene the morphology need to be included implicitly or explicitly. To include the morphology explicitly in atomic scale simulation of mobility the charge transport along the chain need to described. Marcus theory can be used to describe the hopping process. Here a ghost atom model was used to calculate electronic coupling both along and between chains. A factor 10 difference was seen between intra-chain electronic coupling obtained here compared to previous results. However, the charge transport in amorphous polyethylene was primarily limited by the inter-chain electronic coupling.

Polyethylene is a model for semicrystalline polymers that provides the option to vary crystallinity within wide ranges and then to establish relationships between structure and mass and charge transport properties. Three different topics are covered: diffusion of n-hexane in polyethylene, extensive penetrant uptake kinetics, swelling and the design of a novel sensor, and finally electrical conduction in polyethylene, a field important to modern distribution of electric power (HVDC). This feature article presents past and ongoing studies at KTH Royal Institute of Technology using a variety of experimental methods and computer-aided simulation and modelling.

Electron traps in polyethylene (PE) origin from water has been calculated using density functional theory. Atomistic structure has been simulated using molecular dynamics (MD) and used to calculate the electronic structure. Densities comparable with experimentally determined values are obtained. Both single water molecules and cluster of water molecules are considered. For a single water molecule both electron and hole states are within the bands from PE. The electron state is close to the conduction band minima (CBM) and may coincide with the CBM depending on the local atomic density. The absence of electron traps due to water is contradictory to previous simulation results but can be explained by atomic density and relaxation of conduction states of PE. Systems with 10 water molecules relaxed to two pentamers in the MD simulations. Calculation of trap levels of the pentamers result in trap levels in the order of 0.1-0.3 eV. 

We demonstrate that the chemistry at the interface between nanoparticle and polymer matrix influence charge dynamics in polymer nanocomposite. Applying density functional theory, we investigate the influence of crystal surface termination, silicon treatment, and water and carboxyl defect on the electronic properties of interfaces in MgO-polyethylene nanocomposite. The band offsets between the nanofiller and base matrix materials show a strong dependence on the chemical composition at the interface. Based on the calculated electronic structure, we propose a band alignment model for charge dynamics in nanocomposite dielectrics. The model not only provides a mechanism of reduction of space charge and conductivity but also predicts an increase in thermal stress and susceptibility to the chemical additives. It is suggested that the suppression mechanisms of space charge and conductivity in nanocomposites can be inherently unstable and promote material aging. The results of the study show a need for long-term performance tests of nanocomposite dielectrics.

Electronic charge transport in polyethylene is via a hopping mechanism. Theoretically hopping transport in polymers can be studied using kinetic Monte Carlo and Marcus Theory. An essential input to the charge transport simulation is to describe how the electronic states localize. Holes in polyethylene localized along the polymer backbone. In the amorphous phase the electronic states are localized to segments of the polymers. No structural criteria is available that determine how electronic states will distribute along the polymer backbone. Here we have applied quantum chemistry to study conformations of linear alkanes in order to identify structural criteria for valence states localization. Two subsequent gauche formations is identified as one localization criterion.

Polymer nanocomposite dielectrics are promising materials for electrical insulation in high voltage applications. However, the physics behind their performance is not yet fully understood. We use density functional theory to investigate the electronic properties of the interfacial area in magnesium oxide-polyethylene nanocomposite. Our results demonstrate polyethylene conduction band matching with conduction bands of different surfaces of magnesium oxide. Such band bending results in long range potential wells of up to 2.6 eV deep. Furthermore, the fundamental influence of silicon treatment on magnesium oxide surface properties is assessed. We report a reduction of the surface-induced states at the silicon-treated interface. The simulations provide information used to propose a new model for charge trapping in nanocomposite dielectrics.