Joachim Holbøll

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.

Partial discharge tests using CIGRE method II

In this paper, the results of an experimental project on insulating material aging, performed in both Denmark and Italy, are reported. This study was concerned with partial discharge (PD) behavior at temperatures between 30 and 80/spl deg/C using CIGRE method II. The material tested was a commercial polymethylmethacrylate (PMMA) which was chosen not for its good dielectric properties but rather because much of its discharge resistance data at ambient temperature is already well documented. A description is given of the theoretical and experimental methodology followed in this work. Mixed Weibull analysis techniques in terms of the PD amplitude and phase distribution characteristics were employed to distinguish the presence of different aging mechanisms. Such a difference was observed at 30 and at 80/spl deg/C. At 30/spl deg/C the analysis inferred a single discharge aging process acting until breakdown, while at 80/spl deg/C the results suggested the predominance of a single PD aging mechanism for the first half of the insulation lifetime; however, for the remainder of the insulation life time, an additional degradation mechanism was evinced.

Space charge profiles in low density polyethylene samples containing a permittivity/conductivity gradient

Laser induced pressure pulse space charge measurements were made on 1.5 mm thick plaques of high purity low density polyethylene equipped with vacuum-evaporated aluminium electrodes. Temperature differences up to 20 °C were maintained across the samples, which were subjected to dc fields up to 1.5×107 V m-1. Current density was also measured as a function of temperature and field. Space charge due exclusively to the temperature gradient was detected, with density of order 0.01 C m-3. The activation energy associated with the transport of electrons through the bulk was calculated as 0.09 eV. However, measurements of the temperature dependence of the current in isothermal samples yielded activation energies in the range 0.9-1.3 eV. It is deduced that most of this larger activation energy is associated with charge injection at the electrodes. Electron mobilities not less than 2×10-12 m2 V-1 s-1 were inferred by combining the space charge and current density measurements.

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.

Partial discharge patterns and surface deterioration in voids in filled and unfilled epoxy

Results are presented from analyses of pulses from partial discharges (PDs) in single spherical voids in filled and unfilled epoxy plastic and related to the observed surface deterioration. The filler types used were dolomite, alumina, and silica. Long-time aging tests including pulse phase/height analyses were performed over a period of 2400 h and showed very characteristic discharge patterns for each material combination. A unique behavior with regard to changes of pulse repetition rate and maximum apparent charge was observed for PD in alumina- and silica-filled epoxy. The void surfaces were investigated by scanning electron microscopy for voids exposed to PD as well as voids without discharges. Different kinds of surface deterioration were found. An attempt was made to establish a relationship between aging phenomena as exhibited by void surface changes and discharge patterns.<<ETX>>

Partial discharge patterns related to surface deterioration in voids in epoxy

Results are presented from an investigation of the relationship between changes in partial discharge patterns and the surface deterioration process taking place in small naturally formed spherical voids in epoxy plastic. The voids were exposed to a moderate electric stress above inception level, where partial discharges were present for more than 1500 h. Two types of electrical tree growth were found, the bush like tree and a single channel-like tree, which led to very different partial discharge patterns. It is concluded that the formation of crystals on a void surface leads to an immediate and easy-to-detect increase in the partial discharge activity with subsequent severe surface deterioration (deep pit formations) in the vicinity of the crystal. However, the partial discharge signal from a specimen with a black channel-like tree structure did not give any indication of channel growth.<<ETX>>

Design Study of Fully Superconducting Wind Turbine Generators

In this paper, two fully superconducting generators employing MgB2 armature winding, with YBCO and MgB2 field winding respectively, are presented and analyzed. The ac loss in armature winding is estimated, and a simple comparative study is carried out. The results show that both electromagnetic designs for fully superconducting generators are promising with respect to the power density. However, the cost of removing ac loss in armature winding is as high as

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

900 000. It is also noted that with the current price of YBCO tape, the generator employing MgB 2 field winding would have lower cost.

Estimation of medium voltage cable parameters for PD-detection

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.

Temporal and spatial development of partial discharges in spherical voids in epoxy related to the detected electrical signal.

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.