Christer Törnkvist

Electronic structure of polyethylene — Crystalline and amorphous phases of pure polyethylene and their interfaces

For understanding electron and hole transport, it is important to know the electronic structure of the material. The density of states and particularly the energy band gap are two important properties characterizing the electronic structure. With the help of density functional theory the electronic structure of can be calculated. Three different cases are studied: the crystalline phase, the amorphous phases, and crystalline-amorphous interfaces. The density of states, band gaps, and the generals shapes of the electronic states are calculated. Some improvement of the band gap of crystalline polyethylene (PE), from 6.0 to 6.7 eV, by including van der Waals forces, which generally are not treated in density functional theory. The band gap of the amorphous phase was found to be 6.2 eV, and a physically realistic interface has a band gap of 5.9 eV, which is 0.8 eV lower than for the crystalline phase. These results indicate that for semicrystalline materials, an understanding of conduction behaviour must consider three phases: crystalline, amorphous and interface regions. The consequent conjecture that the interface phase with its lowest band gap might provide the main contribution to conduction is in accordance with literature.

Modeling and Measurements of Electric Fields in Composite Oil/Cellulose Insulation

In this paper we report on basic studies of the electric fields under DC stress in a composite insulation consisting of transformer oil and oil-impregnated pressboard, typical for HVDC converter transformers. The time dependent electrical fields in our model geometry are measured using electro-optic Kerr method, and theoretically studied with an ion drift-diffusion conduction model. The measured and calculated electrical fields are compared and analyzed.

Electrode influence on DC conductivity measurements of low density poly ethylene

In high voltage DC applications one of the most critical parameters of insulation materials is the conductivity. It is however challenging to measure conductivity accurately, since it is influenced by composition, morphology, sample thickness, temperature, electric field strength and electrode material. There is no standard electrode material used for electrical characterization of insulation in research. It is known that different electrode materials give rise to different conduction currents and different space charge inside a polymer sample. It is also believed that oxide layers on metal electrodes influence the activation energy of the interface between the polymer sample and the metal electrode. In the present work, a pure non-stabilized LDPE was used to investigate different electrode materials. The following materials were investigated: brass, copper, aluminum, gold, silver and semi conductive polymer. The method of application of the electrodes onto the polymer samples was also varied. The results indicate that the contact material and the method how to connect the material to the polymer sample had an influence on the time behavior of the DC conductivity during 24h of polarization. It could be shown that the applied electrical field influenced the crystal structure of the sample.

Differences in streamer initiation and propagation in ester fluids and mineral oil

The properties of ester fluids and mineral oils differ in many cases in a qualitative way. This means that these liquids may look like very similar in some experiments while in others they behave very differently. These differences must be understood and taken into account in the design of ester filled transformers to avoid unexpected failures. Classic models such as the two-parameter Weibull statistics of breakdown voltage are in some respects too simple to properly catch the underlying physics. We discuss the importance of segregating initiation and propagation processes to properly interpret results of experiments with esters and mineral oil. In transformer design it is in some parts more important to avoid initiation while in others it is more important to stop propagating streamers. Furthermore, to obtain low probability statistics from high probability breakdown experiments, both average and spread of breakdown voltages are required. The difference in spread of initiation and propagation voltages is discussed. Esters and mineral oils behave in a qualitatively similar way regarding initiation while they differ substantially in the propagation process. In this paper we compare some different experiments in terms of initiation and propagation.

Molecular scale simulation of hole mobility and current densities in amorphous tridecane

The hole mobility of amorphous tridecane (a model of amorphous polyethylene) is simulated using a parameter-free approach which combines density functional theory, molecular dynamics and kinetic Monte Carlo methods. We observe large variations of the current density in the samples, typical to materials with large energetic disorder. The obtained mobility values are of the same order of magnitude as the highest experimentally reported values. By introducing carbonyl groups, we assess the effect of material oxidation and find that the mobility is reduced by an order of magnitude already at moderate concentrations of these groups.

Space charges and deep traps in polyethylene — Ab initio simulations of chemical impurities and defects

In the framework of Density Functional Theory (DFT), we calculate the electronic structure of an amorphous polyethylene (PE) structure including defects and impurities. In particular, we provide tables with trap levels for defects like carbonyl groups at different chain locations, various double and triple bonds, hydroxyl and vinyl groups, and for impurities relevant for insulation applications, like acetophenone, alpha-methylstyrene, cumene, cumyl-alcohol, and water. It turns out that conjugated double-bonds and carbonyl groups yields generally the deepest traps. Shallow traps are seen for single double bonds in the PE chain, hydroxyl groups, and water. Traps originating from water, vinyl or hydroxyl groups have depths similar to the band gap of the amorphous-crystalline interface phase of PE; thus, these later defects could at high enough concentration bridge the interface phases yielding a sub-band in the band gap of PE.

Geometry impact on streamer propagation in transformer insulation liquids

Lightning impulse voltage breakdown tests were made in needle/plane, needle/sphere and U-type electrode systems and compared between ester fluids and mineral oil. Results show that the geometry of electrode has significant impact on both breakdown voltage and the velocity of streamer. The more divergent the electric field, the lower the breakdown voltage. The lowest breakdown voltage was found in needle/plane geometry having the most divergent field. In needle/sphere geometry the breakdown voltage was twice as high. In the U-type geometry the breakdown voltage was further increased. At breakdown voltage, streamer propagation velocity in needle/plane geometry was about 2 km/s and in the U-type geometry, the streamer velocity was about 10 km/s. Less difference was found between ester fluids and mineral oil in the U-type geometry for both streamer velocity and the values of breakdown voltage at an oil gap length of 35 mm, which is closer to the field distribution in a real transformer. The impact of electrode geometry dominated over the difference between liquids, which made the ester fluids and mineral oil more similar in the more homogeneous electric field of the U-type electrode system.

Breakdown voltage of polypropylene laminated paper (PPLP) in plain samples and a full scale cable

Polypropylene laminated paper (PPLP) has been used as ac power cable insulation for a number of years. However, reports on the use of PPLP in high voltage direct current (HVDC) cables are sparse. In this paper we report dc breakdown strength of impregnated PPLP samples. Experimental breakdown data from identically treated samples both from Kraft paper and PPLP have been analyzed assuming a Weibull distribution. The results suggest that PPLP insulation is superior to paper insulation on a lab scale but more sensitive to volume effects than paper insulation. A full scale mass-impregnated PPLP cable has been type-tested successfully according to CIGRE recommendations. Additional impulse tests on the cable suggest a higher Lightning Impulse Withstand Level (LIWL) compared with a similar mass-impregnated paper cable. The performance ratio of PPLP vs. paper is substantially higher for dc breakdown tests on plain lab samples than for impulse breakdown test on full scale cables.

Influence of carbon black in polyethylene on space charge accumulation

In several studies regarding space charge accumulation in peroxide crosslinked polyethylene (XLPE) it is identified that the products from the crosslinking process give rise to heterocharge accumulation in a DC-field. Several ways to reduce this accumulation have been proposed. We have studied polyethylene compounds containing 1 wt% carbon black of different types. It is found that compounds containing carbon black with very large surface area have somewhat lower accumulation of space charges in samples that are not degassed. One possibility may be increased adsorption of low molecular weight species formed at the crosslinking process. Measurements show that the addition of a low amount of carbon black gives higher resistivity but lower DC breakdown strength compared to unfilled XLPE.

Ultra high speed camera studies of paper puncture in oil-cellulose insulation systems under LI testing

This paper aims at presenting results related to electrical breakdown of oil paper insulation systems by puncturing of paper insulation. In the investigations the used experimental setup is a needle-plane geometry with sheets of paper inserted in the oil gap. The voltage applied to the needle is a Lightning Impulse (LI) of positive polarity. The propagation of streamers was studied by an ultra-high-speed frame camera. The streamers were always initiated at the needle tip and were classified as 2nd mode streamers. In some cases the streamer spread out along the top of the paper without puncturing it. In other cases the streamer punctures the paper and continues to propagate below the paper. Differences between the two cases are discussed and analyzed. It is found that the paper puncture voltage coincide with the voltage at which the Laplace field below the paper sheet reach the critical field for streamer initiation.