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Converter transformers in HVDC stations are subject to both AC and DC stresses. With new oils entering the market it is important that these are characterized both for AC and DC. This paper focuses on DC characterization of one such oil, which is an isoparaffinic oil produced using a gas to liquid production process. The results are compared to mineral oil as reference. The isoparaffinic oil is shown to have a very high resistivity. In fact, the resistivity is so high that it is more resistive than impregnated pressboard with low moisture content. This is a dielectric risk which must be considered if the oil is to be used in converter transformers.

Results of the measurements of air ion concentrations performed under different conditions including various sources of ions (natural factors and external corona discharge), applied DC electric fields and relative humidity of air are reported. It is shown that ion concentrations correlate with radon activity at zero field and a strong sweep out effect is observed due to the applied field. It is noticed that an increase of air humidity leads to a significant increase of the amount of negative ions in air. Possible reasons for the observed effects are discussed.

Charge injection from metallic surfaces into mineral oil is investigated utilizing transient currents recorded in coaxial cylindrical electrode systems with different degrees of inhomogeneity of electric fields. The currents obtained with single polarity and reverse polarity methods demonstrate differences in their behavior depending on the geometry and type of the test voltage. The data show clear relation between the sign of the current and the polarity of the applied voltage, which is attributed to the polarity dependence of the ion injection from oil-metal interfaces. The results of the experiments are discussed in terms of relaxation time of the system and time of flight (drift time) of ions in the oil gap.

The present study aims at the understanding of charge injection from metal electrodes into mineral oil under DC stress. The work focuses on the field dependence of the charge injection. In a previous article the electrical field in a blank oil gap with parallel electrodes is measured with a so called Kerr electro-optic technique. The data show that the electrical field is close to symmetric for low fields (~ 0.1 kV/mm) but strongly asymmetric at higher fields (~ 1 kV/mm). In the present study an ion drift model is used to analyze the data. We show that an ion drift model including a field dependent injection term can reproduce the measured results fairly well. Furthermore, the comparison of measured and simulated results shows that the charge injection is stronger from an electrode with positive polarity compared to one of negative polarity.

For the design of the electrical insulation of an UHVDC (Ultra High Voltage Direct Current) converter transformer the resistivity of the pressboard and mineral oil has to be known. For solid insulation the resistivity is virtually constant, however, in liquids it is dependent on, e.g., applied electric stress and time of electrification. Several methods are discussed in the standards. One way to characterize the oil is the measurement of the equilibrium resistivity, in contrast to an apparent resistivity for oil under high electric stress. Measurements using a low voltage triangular shaped voltage form allow calculation of equilibrium resistivity. The oils investigated show equilibrium resistivity in the range 5E11 Ohm·m to 5.96E12 Ohm·m, which are typical values for transformer oil. There is good agreement between these results and reference measurements using dielectric response

The knowledge of the behavior of electric conductivity in mineral oils for possible applications in HVDC converter transformers is of paramount importance for proper design of their insulation system. This study presents the results of measurements of dielectric properties of various transformer oils by means of frequency response technique as well as time domain measurements, including measurements of ion mobility using the reversal polarity method. Influences imposed by varying the measurement voltage level and temperature are investigated. Some important parameters of the investigated oils, e.g. their conductivities, ion mobilities, ionic concentrations and effective ionic radiuses are compared and discussed. The results show that despite of similarities in various physical parameters of insulating mineral oils available on the market, the dielectric behavior and especially ionic conduction vary greatly between different oil types. Changes of these properties with temperature are characterized by different activation energies.

Aromatic molecules are central components of model systems for molecular electronics, with C-60 one of the most studied. Upon adsorption on (metallic) substrates a splitting of the frontier orbitals is commonly observed, with a strong dependence on substrate material, but little dependence on substrate structure. We report the detailed photoelectron angle dependence of C-60/A1(110) over a wide range of energy, finding a strong remnant molecular character. In particular, certain HOMO-derived suborbitals couple strongly, and others weakly, with the metal, which results in final state charging for those weakly coupled. C Is data correlate well with the assignments made on this basis, as does the comparison of ground state partial densities-of-states (PDOS) to photoelectron spectra. Detailed analysis of the PDOS supports a rough division into surface-near and surface-far components, in agreement with the molecular picture. The component spectral widths are attributed to intramolecular vibrational coupling, which is suggested to aid in the electronic decoupling of certain suborbitals from the substrate, facilitating the observed final state charging.

Scanning tunneling microscopy (STM) studies of monolayer of metal-free phthalocyanine (H2Pc) adsorbed on Au(111) have shown ordered arrangement of the molecules on the surface. Evaporation of H2Pc onto the Au(111) surface and post annealing of the sample to 670 K results in a densely packed structure of the molecules. The monolayer is characterized by molecules adsorbed with the molecular plane parallel to the substrate surface in a square adsorption unit cell. Furthermore, the high resolution images revealed the orientation of individual molecules. The H2Pc/Au(111) system has also been doped by rubidum and compared to the undoped layers. The Rb affects the molecular adsorption geometry, and a hexagonal unit cell is found for the coadsorption of H2Pc and Rb. Upon doping, highly ordered Rb-induced protrusions are observed at the benzene site of adsorbed molecules.

We present an electron spectroscopy study of phase-pure AC(60) thin films (A = Rb, Cs) in their monomer (face-centred cubic (fcc)) and polymer phases. A surface electronic reconstruction is observed in polymeric RbC60, analogous to that reported for the fcc phase. As for pristine C-60, the occupied electronic states of AC(60) fullerides are not dramatically affected by polymerization. The energy separation between the leading feature in photoemission and inverse photoemission is similar in both stable AC(60) phases. These observations suggest that electron correlation effects are similar in the two phases, and that their different electronic behaviour is mainly related to the reduction of degeneracy of the polymer frontier states. Photoemission and electron-energy loss spectroscopy data show that the thin-film form of the RbC60 polymer is metallic at room temperature, and that it undergoes a metal-insulator transition at around 100 K. This transition temperature is much higher than that reported for the corresponding bulk phase and signals a poorer screening of Coulomb interactions at the film surface.

We compare photoelectron spectra (PES) and theoretical densities-of-states of C-60/Al(111), C-60/Al(110) and C-60/Al(100). The splitting observed on all three surfaces is attributed to final state charging. This splitting can also be used to improve estimates of the charging energy U of adsorbed molecules and as a criterion for the existence of strong covalent bonding to the electrode. Au exhibits weak bonding in available data, whereas Ag(100), with measurable charge transfer, appears qualitatively similar to Al. (C) 2009 Elsevier B. V. All rights reserved.

The growth of iron phthalocyanine (FePc) on InSb(001) c(8 x 2) at submonolayer coverage has been investigated with scanning tunneling microscopy (STM). FePc adsorbs flat centered on the In rows both at 70 K and at room temperature (RT). However, the shapes of the two-dimensional molecular islands are fundamentally different; while the RT growth results in chainlike structures along the [I 10] direction, as already observed for other Pc's adsorbed on the same surface, the islands are prolonged along [110], i.e., perpendicular to the substrate rows, at 70 K. These observations are explained on the basis of a recently observed new surface phase at low temperature, resulting in structural domains on the surface. The molecular growth front follows the propagating domain boundary that freezes at low temperature.

We demonstrate the growth of crystalline Li(4)C(60) films. The low-energy electron diffraction pattern of the films indicates the formation of polymer chains in the plane of the surface, consistent with the reported crystal structure. Electron energy loss and photoemission spectra identify the Li(4)C(60) polymer as a low band gap semiconductor, with a relatively strong coupling of electrons to low-frequency stretching modes of the polymer bonds and alkali phonons. No evidence is found for hybridization between the Li- and fullerene-derived electronic states. Instead, a partial charge transfer takes place, which is the same for different Li concentrations. This result rationalizes the stability of the polymer phase over a wide range of stoichiometries.

Zinc phthalocyanine (ZnPc) adsorbed on the InSb(0 0 1)-c(8 × 2) surface has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Coverages from sub-monolayer to monolayer (ML) have been investigated. The molecules form ordered structures on the reconstructed rows of the surface with the molecular plane parallel to the surface. A change in the electronic structure between the sub-ML and higher coverages has been observed. Moreover, in order to study the influence of annealing on the electronic and geometric structures, the samples have been heated to elevated temperatures (about 640 K). In addition, multi-layer ZnPc films have been characterized by XPS measurements.

NaxC60 thin films with Na concentration 0 ⩽ x ⩽ 3 were investigated using angle-dependent photoelectron spectroscopy. For low doping we observed two distinct fulleride phases coexisting with regions of pristine C60. One of these fulleride phase is predominantly formed close to the surface and disappears after annealing and further addition of Na. At higher doping a phase with stoichiometry of x = 3 is formed.

The immobilisation of thiol-derivatized cobalt porphyrins on gold surfaces has been studied in detail by means of combined scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). S-thioacetyl has been used as a protective group for the thiol. Different routes for deprotection of the acetyl groups were performed in acidic and in basic conditions. The results show the formation of monolayer films for the different preparation schemes. The immobilisation of the molecules on the gold surface takes place through the thiol-linkers by the formation of multiple thiolate bonds. In the case of layers formed with protected porphyrins approximately 60% of the linkers are bonded to the gold surface whereas for deprotected layers the amount of bonded linkers is increased up to about 80%. STM measurements revealed that the molecules arrange in a disordered overlayer and do not exhibit mobility on the gold surface. Annealing experiments have been performed in order to test the stability of the porphyrin layers. Disordered patterns have been observed in the STM images after annealing at T = 400 °C. XPS revealed that the sulphur content disappeared completely after annealing at T = 180 °C and that the molecules did undergo significant modifications.

In aromatic molecules characterised by extended π-systems valence and core excitons can have very similar binding energies. This is shown for a number of compounds, and in each case the core exciton binding energies are analysed in terms of the Coulomb screening and rehybridisation contributions. The latter semiquantitatively explain the binding energy differences between the valence and core excitons. This suggests that core and valence excitons have a similar spatial extent in such systems. In favourable cases core level techniques can thus be used to elucidate valence excited state properties.

We present photoelectron (PE) and x-ray absorption spectra (XAS) of macroscopically aligned multiwalled carbon nanotubes. We identify the peaks in the valence PE spectra with regions of high density of states through comparison to calculations for graphite. Finally, we outline and illustrate a simple method for using XAS to determine the average alignment of a nanotube sample.

We show that Cr subcoatings in beamline optical elements can introduce an additional structure in C K-edge X-ray absorption measurements, which somewhat resembles the well-known losses produced by carbon contamination. The photon throughput variations induced by Cr and C require different treatments when correcting absorption spectra. The potential difficulties in unravelling the two effects could be avoided with efficient filtration of higher-order light.

Even under corona-free conditions, insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated by background radio activity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 ions/m3 which leads to conductivities ranging from 10-16 to 10-13 S/m. An experiment has been conducted where the end of a terminated high voltage XLPE cable was used as an insulating structure. The field distribution around the stripped cable end was measured with an applied DC voltage of -140 kV using a rotating field probe. The measurements are compared to simulations, which do not specifically take into account space charge effects. A big discrepancy is observed, showing the relevance of taking into account space charge effects when modeling HVDC insulation components subjected to air.

Corona currents in air measured in a large scale coaxial geometry (d outer = 1 m, d inner = 0.26 mm), under both DC and AC voltages of a triangular shape with frequencies in the range (1–50) Hz are analyzed. A computational model describing dynamics of positive and negative ionic charges is employed to examine the results of the measurements. The performed simulations show a good overall agreement with the experimental data.

The aim of this paper is to develop a theoretical foundation of the reversed polarity method for measuring the mobility of ions in transformer oil. The results of the measurements of transient currents in oil, showing very distinct transient current peaks appearing after polarity reversal, are evaluated by computer simulations using an ion drift model. It is found from the analysis that the peaks in the measured currents occurred at instants much shorter than the so-called transit time that is the time for an ion to cross the oil gap between electrodes. A relation between the transit time and the current peak position is found that can be used to extract the ion mobility from data obtained with an experimental set up in which the transit time is shorter than the dielectric relaxation time of the liquid. On the other hand, for a setup providing the dielectric relaxation time shorter than the transit time, the current peak position strongly depends on the former and no simple correlation between the current peak position and transit times can be established.

Conduction process in dielectric liquids usually occurs due to ion migration and is therefore characterized by concentration and mobility of the ionic charge carriers. For this reason accurately determining these parameters and especially ion mobility in insulating oils for HVDC applications needs to be done precisely. In this report various methods for determining ion mobility are described, including single polarity, reversal polarity methods in time domain as well as frequency domain based measurement of dielectric response. The advantages and disadvantages of each of them are presented and illustrated with the results of measurements on various oils using two different test cells. The dependence of ion mobility on measuring voltage level and temperature are analyzed and compared.

The electrical insulation system of power transformers is a combination of mineral oil and pressboard materials. We have measured the influence of forced oil flow on the electric field stress in oil pressboard insulation system with applied DC potential using Kerr electro-optic effect. The data show a variation in the electric field strength in flow direction, depending on velocity, time and applied voltage. The observations are explained by variation in space charge distribution on the pressboard.

The oxygen content in YBCO thin films can be changed locally by scanning a focused laser beam across the film surface at defined oxygen partial pressure. In contrast to annealing experiments in a furnace where the oxygen concentration remains homogeneous in the direction of the film layer, the local heating with a laser beam leads also to a lateral oxygen diffusion in the thin film. By investigating the laser written structures a deeper understanding of the patterning process itself and the oxygen diffusion in YBCO thin films could be obtained. For the detection of laser written structures and natural defects in HTSC thin films the magneto-optical method is highly qualified. It allows to visualize the magnetic screening behaviour with a high lateral resolution in the range of micrometers. In the past this technique was used to investigate the magnetic flux distribution in smaller samples at temperatures far below the boiling point of liquid nitrogen. In this work the magnetic resolution of the method was optimized so that much weaker magnetic field contrasts at 77 K can still be detected. In particular the presented apparatus allows the characterization of double-sided 3” HTSC Wafers even under conditions where cooling with liquid helium or closed-cycle refrigerators is not available.

A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied Ti 3d orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant d(y)(2)-z(2) character. The analysis of the Ti L(2,3) x-ray absorption spectra confirms the complete splitting of the Ti 3d shell. X-ray absorption and resonant photoemission at the O 1s edge provide direct evidence for hybridization between the transition metal orbitals and the O 2p levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex TiOX phase diagram.

We present C 1s x-ray absorption, x-ray emission, and resonant inelastic x-ray scattering (RIXS) spectra of single-phase crystalline K3C60. The comparison to valence-band photoelectron spectra from the same sample facilitates identification of the contribution from surface and bulk electronic states in the latter. Bulk-sensitive techniques show that the valence bands of K3C60 and pure C60 are characterized by spectral features of similar width, in agreement with the predictions of band-structure calculations. Symmetry selectivity in the RIXS process allows us to assign peaks in the C 1s absorption spectrum, demonstrating a close correspondence with pure C60 also in the conduction band. The symmetry selectivity is as pronounced in K3C60 as in pure C60, indicating that the local C60 symmetry is not appreciably affected by the K doping, either in the ground state or intermediate state, on the time scale of 6 fs.

We investigated (C59N)2 multilayer films, intercalated with different amounts of potassium, by electron energy loss spectroscopy and by synchrotron radiation techniques. For low K exposures, valence band photoemission shows, apart from a rigid shift towards higher binding energies of the whole spectrum, two new LUMO-derived peaks, indicative of a mixture of phases. One of the doped phases still preserves the dimer structure. At K concentrations, the (C59N)2 dimer breaks up, as seen by HREELS, but the dimer phase can be recovered by distillation. C1s and K2p photoemission spectra show two K sites up to about K3C59N and one K site for higher K concentrations.

The electronic properties of Sc2@C84 deposited on Au(110) were investigated by X-ray absorption spectroscopy, valence band and core-level photoemission spectroscopy. We compared the bulk spectra of Sc2@C84 to those of a monolayer produced by annealing the thick film to about 750 K. Strong evidence is found for chemisorption of the first layer of Sc2@C84: the C1s photoemission spectrum shows a shift in binding energy and an asymmetric broadening towards higher binding energies pointing to metallic screening and/or an increase in the number of non-equivalent carbon sites. The valence band spectra present a broadening and non-rigid shifts indicating hybridization of the molecular electronic states with those of the substrate. The X-ray absorption spectra recorded at the Sc 2p edge does not change with film thickness. We can therefore conclude that the scandium electronic structure is not influenced by the chemisorption of the endofullerene.

We report transverse-field muon spin rotation studies of the magnetic field distribution n(B) in the vortex state of the high temperature superconductor Bi2Sr2Ca1Cu2O8+δ (Bi-2212) and present data on three sets of overdoped, nearly optimized and underdoped single crystals which to our best knowledge provide the first evidence that a two-stage melting transition of the vortex matter occurs under equilibrium conditions. In addition, we compare the TF-μSR results with those from DC-magnetization measurements and magneto-optic experiments on the same crystals.

Using a magneto-optic technique we have investigated the magnetic flux propagation into and out of superconducting thin YBa 2 Cu 3 O 7− δ films. After field cooling below T c , the external magnetic field B ext perpendicular to the film is changed which gives rise to shielding currents in the sample. The current distribution is disturbed momentarily by heating with a focused laser pulse near the sample edge. This triggers a magnetic instability, in which a magnetic flux avalanche starts to propagate.

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A joint experimental and theoretical work to explain the electronic and geometrical structure of an in situ prepared film of iron phthalocyanine (FePc) on silicon (100) is presented. FePc molecular films have been characterized by core and valence photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS), and the results have been interpreted and simulated by density functional theory (DFT) calculations. C1s and N1s PE spectra have been analyzed by taking into account all chemically nonequivalent C and N atoms in the molecule. In the Fe2p(32) spectra it has been possible to resolve two components that can be related to the open shell structure of the molecule. By valence PES and N1s XAS data, the geometrical orientation of the FePc molecules in the film could be determined. Our results indicate that for the FePc on Si(100), the molecules within the film are mainly standing on the surface. The experimental N1s XAS spectra are very well reproduced by the theoretical calculations, which are both angle and atomic resolved, giving a detailed description of the electronic and geometric structure of the FePc film. Furthermore, the asymmetry and the intensity angle variation of the first N1s XAS threshold feature could be explained by the presented DFT calculations as due to the chemical nonequivalence of the N atoms and the symmetry character of the lowest unoccupied molecular orbital.