Mogens Henriksen

Effect of Insulation Properties on the Field Grading of Solid Dielectric DC Cable

The development of solid dielectric dc transmission class cable is a priority throughout much of the world, to avoid risks associated with placing hydrocarbon fluids in underwater environments. The conductivity of polymeric solid dielectrics tends to be a strong function of temperature and electric field, however. Based on measured material properties, we demonstrate the effect of such dependencies on the field grading of dc cable for the range of measured material properties and provide an analytical approximation for computing the field of resistively graded dielectrics, including the effect of temperature and field-dependent conductivity.

The influence of electrodes and conditioning on space charge accumulation in XLPE

The accumulation of space charge in planar crosslinked polyethylene (XLPE) samples under dc electric fields at room temperature was investigated using the pulsed electroacoustic (PEA) method. Three different organic semiconductor (semicon) materials containing carbon black at concentrations /spl sim/30 %wt, and vacuum-evaporated gold, were used as electrodes. Three different conditioning procedures were investigated. Unconditioned samples developed heterocharge with density increasing from zero at the electrodes, to a maximum at roughly one-third of the thickness, and then falling to zero around the center. They also developed homocharge close to the electrodes. The choice of electrode material had little effect on the heterocharge profile in unconditioned samples. Conditioning by holding at 80/spl deg/C for four days, at rotary pump pressure or at atmospheric pressure, suppressed the accumulation of heterocharge. Homocharge accumulation close to the electrodes in samples with semicon electrodes was affected little by this conditioning, but was reduced considerably in samples with gold electrodes. Conditioning by holding at room temperature for seven days at rotary pump pressure had little effect. The heterocharge originates in inhomogeneous polarization due to a spatially inhomogeneous distribution of permanent dipole molecules, probably acetophenone. The dipole concentration decreases in going from the electrodes towards the center of the samples, as a result of diffusion of the molecules in the opposite direction. The homocharge originates in electronic charge injection/extraction across the electrode/XLPE interfaces.

The influence of titanium dioxide additive on the short-term DC breakdown strength of polyethylene

The effect of the addition of 1% by weight of titanium dioxide fine particles to low-density polyethylene (LDPE) on the short-term DC breakdown strength of the LDPE was investigated using direct and reverse polarity voltages. The samples used were cylinders of both plain and doped materials, with hemispherically tipped cylindrical electrodes completely embedded in the material, with a minimum gap length between the electrode tips of 0.25 mm. All tests were conducted at room temperature. Results indicate that, although the addition of TiO/sub 2/ reduces the DC breakdown strength of the doped material if compared to the plain material, it significantly improves its DC reverse polarity characteristics. The doped material seems to be insensitive to the DC polarity reversals. The observed beneficial effect of the addition of TiO/sub 2/ on the DC reverse polarity characteristics is attributed to the role of this additive in modifying the trapping levels in the polymer, and the consequent change in the space charge pattern in the doped material.<<ETX>>

Effect of semicon-dielectric interface on conductivity and electric field distribution

Development of solid dielectric dc transmission class cable has become a priority throughout much of the world. Interdiffusion between the semiconducting electrode materials and the dielectric inevitably causes variations in conductivity of the dielectric near the semicon which results in distortion of the electric field and space charge formation under dc conditions. Analytical approximations and numerical computations provide a basis for analyzing space charge measurements, and based on such space charge measurements and the analysis, we estimate the field distortion for several material systems.

Estimation of medium voltage cable parameters for PD-detection

Medium voltage cable characteristics have been determined with respect to the parameters having influence on the evaluation of results from PD-measurements on paper/oil and XLPE-cables. In particular, parameters essential for discharge quantification and location are measured. In order to relate a measured signal at the cable terminations to a specific PD-amplitude and location on the cable, the attenuation and the transmission speed of PD-pulses on the cable have to be known. Consequently, the main parameter to be determined is the complex propagation constant which consists of the attenuation and phase constants. A method to estimate this propagation constant, based on high frequency measurements, is presented and is applied to different cable types under different conditions. The influence of temperature and test voltage is investigated. The relevance of the results for cable diagnostics is discussed.

Space charge build-up in XLPE-cable with temperature gradient

Space charge build-up in standard XLPE insulated AC cables has been studied under varying temperature and field conditions. The cables were triple-extruded with the inner semicon on a solid aluminum conductor, 5.5mm XLPE-insulation and an outer semicon. The cables were stressed up to 15kV/mm DC and temperatures were applied in the 20 - 80/spl deg/C range with gradients across the insulation of up to 15/spl deg/C. In this paper, the observed charge phenomena in the bulk and at the interfaces are related to the external conditions, in particular to the temperature gradient. The measured space charge distributions are compared to calculated values, based on available conduction models and the deviations are discussed thoroughly.

Superconducting generators for wind turbines: Design considerations

The harmonic content of high temperature superconductors (HTS) field winding in air-core high temperature superconducting synchronous machine (HTS SM) has been addressed in order to investigate tendency of HTS SM towards mechanical oscillation and additional loss caused by higher flux harmonic. Both analytical expressions for flux distribution and current sheet distribution have been derived and analyzed. The two main contributors to the AC loss of HTS rotor winding are also identified and their influence addressed on general level.

Laboratory results from dielectric spectroscopy of field aged XLPE cables with respect to water trees

This paper deals with the application of frequency domain dielectric spectroscopy for the detection of water trees in XLPE cables. During the research, dielectric spectroscopy measurements have been carried out on cable samples with and without water trees while it was attempted to preserve many of the uncertainties present in the field. The experiments support the applicability of a high voltage test developed primarily at KTH in Sweden, which utilizes the nonlinearity of the dissipation factor. Subsequently it was investigated whether this high voltage method also seems reliable at low voltages. It turned out that the fundamentals of the high voltage method were not directly applicable at low voltages as the nonlinearity of the permittivity is not noticeable at low voltages, at least compared to the deviations. Instead a novel method for interpreting the low frequency measurements is proposed though it needs further verification. Finally a theoretically possible modification of the measurement system is proposed, which deals with the problem of a limited current capability of the amplifier

High temperature superconductor machine prototype

A versatile testing platform for a High Temperature Superconductor (HTS) machine has been constructed. The stationary HTS field winding can carry up to 10 coils and it is operated at a temperature of 77K. The rotating armature is at room temperature. Test results and performance for the HTS field winding comprising four coils wound with two types of HTS tapes are shown and discussed. The field winding produces up to 0.62T in the 10mm air gap which constitutes 78% of the armature design value. Recommendations for the field winding operation are proposed and verified, which resulted in an increase of available magneto-motive force of the field winding up to 25%.

Space charge and electrical conductivity in LDPE doped with titanium dioxide

DC conductivity and space charge density were measured in 1mm thick samples of polyethylene with density 0.885 g/cm/sup 3/ and approximately 17% crystallinity. 2 wt % of the rutile form of titanium dioxide was added to some samples, and 2 wt % of the anatase form to others. Current measurements using semiconductor electrodes were made in the temperature range 40-80 /spl deg/C, and at applied dc voltages of 20 and 40 kV. The effect on the conductivity of holding the samples at 70 /spl deg/C and 3-4 /spl times/ 10/sup -2/ mbar for several days before commencing measurements was also investigated. Space charge density measurements were made at 20 and 30 /spl deg/C, and at 20 kV, using the LIPP technique and semiconductor electrodes. The effect of a temperature gradient on the space charge profiles was also investigated.