Design optimization of high voltage bushing using electric field computations

This paper illustrates the use of electric field computation to optimize the design of gas filled high voltage composite bushings. The United States Navy employs these bushings in high power very low frequency/low frequency transmitting stations. Commercially available 2D and 3D computational packages based on the boundary element method were employed to analyze the electric fields. The optimized design uses both internal and external elements for electric stress grading at critical parts of the bushing. It has been shown that the location and magnitude of the maximum electric field have been optimized which should result in a substantially higher corona free operating voltage     

Aging of distribution cables in service and its simulation in the laboratory

This study focuses on efforts to characterize aging occurring in 15 kV distribution cables in a dry climate. It has been shown that similar changes can be produced by a suitably planned accelerated thermal aging testing in the laboratory. The Arrhenius equation is used for establishing the accelerated aging test parameters. Different modes of statistical analysis namely analysis of variance (ANOVA), Andersen-Darling test for normality, F-test, t-test are performed to validate results from accelerated aging tests with field aging     

Field and laboratory aging of polymeric distribution cableterminations. I. Field aging

This, two-part paper, describes the results of a research project aimed at understanding the magnitude of aging of polymeric cable terminations used in distribution. The termination types evaluated are currently used; hence, they have satisfied IEEE Standard 48. This standard does not address the issue of aging. Terminations made from 3 different polymer families and porcelain were evaluated. Presented in Part I, are the results of field aging from 5 outdoor sites in the USA over a 3 year period; and in Part II, results from accelerated aging fog chamber laboratory tests. The results from field aging show that only minor changes were produced. Some differences were observed in the electrical performance of formulations within the same polymer family, and among different polymer families. However, it was concluded that the performance of the terminations evaluated would not be compromised by aging produced changes in the weathershed housing     

Computation of ac Wet Flashover Voltage of Ceramic and Composite Insulators

This paper presents a theoretical method for calculating the flashover voltage of ceramic and composite insulators under wet conditions. The insulator surface is assumed to be clean. A 3-Dimensional computer package (Coulomb) was used to calculate the electric field of wet surfaces. The surface wettability has been varied from hydrophobic to hydrophilic using the Swedish Transmission Research Institute (STRI) method of classification. The calculated results are in good agreement with published data.     

AC and DC performance of polymeric housing materials for HV outdoorinsulators

An experimental study was conducted to investigate the performance of several polymer outdoor insulator formulations under AC and DC electrical stresses. The performance of the polymers was evaluated in terms of their charge accumulation and decay characteristics as a function of humidity, and their tracking and erosion resistance. The results obtained from this study indicate that significant differences exist in surface charging phenomena between AC and DC. A likely contribution of DC polarization processes leading to hydrophobicity loss is believed to influence additionally the performance of materials subjected to pollution and DC electrical stresses. A substantial reduction in the tracking and erosion resistance of the polymeric materials is observed with DC stresses, in comparison to AC. The poorer electrical performance of the materials stressed with DC voltage was found to be due to higher magnitudes and longer duration times of the discharge current     

Field and laboratory aging of polymeric cable terminations. II.Laboratory aging

For pt.I see ibid., vol.13, no.2, p.316-22 (1998). This paper presents the results of laboratory aging tests on polymeric cable terminations. The goals of the laboratory aging were two-fold: (1) develop an accelerated aging test that can simulate the changes produced in the field, and (2) provide an assessment on the magnitude of aging induced changes on the electrical performance of terminations. Monitoring leakage current and material changes during laboratory tests were useful for assessing the variations possible in aging due to experimental parameters used. It has been shown that a laboratory aging test that is based on evaluating the termination's leakage current suppression capability and the ability to withstand a moderate level of discharge activity is suitable not only for assessing the long term performance, but also for ranking the electrical performance in accordance with field exposure     

Contamination performance of silicone rubber cable terminations

The authors describe the results of a systematic study of the performance of silicone rubber terminations under contaminated conditions. The terminations evaluated were of the 15 kV class cold shrink type for concentric neutral cables. They were evaluated in a fog chamber where the electric stress, water conductivity, water flow rate, and the deenergization time between subsequent fog chamber exposures were varied in order to alter the test severity. Changes in the surface hydrophobicity were monitored periodically by analyzing the samples using ESCA (electron spectroscopy for chemical analysis), SEM (scanning electron microscopy), and contact angle measuring techniques. The results indicate that the experimental conditions play a significant role in the outcome of the tests. It was demonstrated that the recovery of hydrophobicity can be studied by using several techniques     

Sn diffusion coefficient and activation energy determined by way of XRD measurement and evaluation of micromechanical properties of Sn diffused YBa2Cu3O7−x superconducting ceramics

This study manifests not only the effect of Sn diffusion on physical, electrical, mechanical, structural and superconducting properties of the bulk YBa₂Cu₃O_7?x (Y123) superconductors prepared by the conventional solid-state reaction route by use of electrical resistance, X-ray diffraction analysis (XRD), electron dispersive X-ray, scanning electron microscopy, transport critical current density (J _c ) and Vickers microhardness (H _v ) measurements but also the diffusion coefficient and the activation energy of tin (Sn) in the Y123 material for the first time. The diffusion coefficient and the activation energy of Sn are investigated in the temperature range 500–945 °C using the change of the lattice parameters extracted from the XRD patterns. The resistance (at room temperature), critical (onset and offset) temperature, variation of critical temperature, hole-carrier concentration, crystallinity, lattice parameter, texturing, surface morphology, lotgering index, element distribution, critical current density, oxygen content, load dependent microhardness, elastic modulus and yield strength values are obtained for the pure and Sn diffused samples and compared with each other. One can see that all superconducting parameters given above depend sensitively on the Sn diffusion on Y123 system. The obtained results exhibit that the room temperature resistance enhances with the Sn diffusion because of the hole filling when the onset $ (T_{c}^{onset} ) $ ( T c onset ) and offset $ (T_{c}^{offset} ) $ ( T c offset ) critical temperatures are obtained to be about 93.4 and 89.6 K for the pure sample as against 92.2 and 88.1 K for the Sn diffused sample, respectively. This may be attributed to the fact that the decrement in the critical temperatures is due to the deterioration of crystallinity and descend in the grain size. As for the critical current density measurements, J _c values are obtained to be about 125.4–65.3 A/cm², respectively, for the undiffused and Sn diffused materials. This may be led to the decrease of the flux pinning mechanism stemming from the stacking faults, planar and micro-defects. At the same time, XRD measurements display that the samples produced in this work exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with Sn diffusion in the system; however, new peaks belonging to the minor (BaCuO₂) phases start to appear for Sn diffused sample confirming both the reduction of the grain size and degradation of the critical temperature. Moreover, the pure sample is confirmed by both enhancement of a and b lattice constants and the decrement of the cell parameter c of the sample in comparison with that of the Sn diffused sample. According to SEM examination, the crystallinity and grain connectivity suppress with the Sn diffusion. EDX measurements illustrate that not only do the elements used for the preparation of the Y123 superconductors with and without Sn diffusion distribute homogeneously but also the level of Cu element reduces with the Sn diffusion, presenting that the Cu²⁺ ions may partly be diffused by tetravalent tin (Sn⁴⁺) ions. Further, surprising results of the Vickers microhardness values demonstrate that the pure sample visualizes Indentation Size Effect (ISE) feature; however, the Sn diffused sample reports Reverse Indentation Size Effect (RISE) nature. Additionally, the diffusion coefficient is observed to increase from 1.11 × 10^?9 to 2.82 × 10^?8 cm² s^?1 as the diffusion-annealing temperature increases, verifying that the Sn diffusion at lower temperatures is much less significant as compared to the higher ones. Temperature dependence of the Sn diffusion coefficient and activation energy in the range of 500–945 °C is defined with the aid of the following equation: $$ D = 7.78 \times 10^{ - 6} { \exp }\left[ {\left( {( - 0.590 \pm 0.005){\raise0.7ex\hbox{${\text{eV}}$} \!\mathord{\left/ {\vphantom {{\text{eV}} {k_{B} T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{B} T}$}}} \right)} \right] $$ D = 7.78 × 10 ? 6 exp [ ( ( ? 0.590 ± 0.005 ) eV / eV k B T k B T ) ] .     

Computer-aided design of porcelain insulators under pollutedconditions

The development of a user-friendly, interactive personal computer package for designing insulators to be used in polluted conditions is presented. This is accomplished by integrating a dynamic arc model that computes the pollution flashover voltage within the Microsoft Windows application program. Suspension, station post and pin type insulators of numerous shapes have been incorporated into the package. The model is capable of handling AC and DC voltages, in addition to uniform and nonuniform pollution distributions on the insulator surface. The use of the model as a design tool to predict the insulation requirement for a given system voltage, and as a maintenance tool to determine the critical contamination level (in terms of ESDD) that will cause flashover of the insulator at the operating voltage, has been demonstrated. Wherever possible, comparisons of the model predictions with experimental data have been made, and good agreement has been obtained     

Role of diffusion-annealing time on the superconducting, microstructural and mechanical properties of Cu-diffused bulk MgB2 superconductor

In this study, the effect of various annealing time (0.5, 1, 1.5 and 2 h) on microstructural, mechanical and superconducting properties of the Cu-diffused bulk MgB₂ superconducting samples is investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers microhardness (H _v ) and dc resistivity measurements for the first time. The critical transition temperature, grain size, phase purity, lattice parameter, surface morphology, crystallinity and room temperature resistivity values of the bulk samples prepared are compared with each other. Electrical-resistivity measurements show that the sample (annealed at 850 °C for 1 h), exhibiting the highest room temperature resistivity, obtains the maximum zero resistivity transition temperature (T _c ). From the XRD results, all the samples contain MgB₂ as the main phase with a very small amount of Mg₂Cu phase. Moreover, SEM investigations conducted for the microstructural characterization illustrate that not only does the grain size of the samples studied enhance gradually, but the surface morphology and grain connectivity also improve with the increase in the diffusion-annealing time up to 1 h beyond which all the properties obtained start to degrade. Indeed, the worst surface morphology is observed for the Cu-diffused bulk MgB₂ superconductor exposed to 2 h annealing duration. At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values are calculated separately for the pure and Cu-diffused samples. It is found that the microhardness values depend strongly on the diffusion-annealing time. Furthermore, the diffusion coefficient of the Cu ion in the bulk MgB₂ superconductor is obtained to change from 1.63 × 10^?7 to 2.58 × 10^?7 cm² s^?1. The maximum diffusion coefficient is observed for the sample prepared at 850 °C for 1 h whereas the minimum one is noted for the sample annealed at 850 °C for 2 h, confirming that the annealing-time of 1 h is the best ambient to improve the mechanical, microstructural and superconducting properties of the samples produced.