An analysis of spark paths in air using 3-dimensional imageprocessing

A video camera has been used to record the path of spark breakdowns across a positive point/plane air gap. By using a prism as a mirror, two images were obtained from which the path could be determined in three dimensions by subsequent computer processing and analysis. Some 80 images were analyzed for each of five angular positions of the point electrode's axis, the gap remaining constant. The fractal dimension of each path was determined and found to increase slightly with the inclination of the pointed electrode. Each spark appeared to have propagated semi-randomly in a direction which included a `memory' factor, i.e. it tended to follow the previous direction while moving monotonically towards the opposite electrode. This was confirmed by considering a projection of the spark path onto the plane electrode which was found to form a semi-random walk in which each step was influenced by the previous step. These, and other analysis, suggest that the development of spark paths in air by successive steps is in a direction suggested by an angular probability distribution related to the field at the tip of the propagating leader     

污秽绝缘子表面干区形成的影响因素

干区的形成是绝缘子污闪发生的一个必要条件,干区的形成受电压、污秽种类和含量、天气等诸多因素的影响,研究干区形成的影响因素问题,不仅有助于更加深刻地认识干区的形成,而且可以完善外绝缘污闪理论。为此,以矩形污秽玻璃片模拟实际绝缘子,在人工雾室中试验研究了污秽和雾对干区形成的影响。试验结果表明:随着等值附盐密度(ESDD)的增加,干区的形成时间逐渐缩短,干区宽度逐渐减小;随着等值附灰密度(NSDD)的增加,干区的形成时间逐渐变长,干区宽度逐渐增大;随着雾浓度的增大,干区的形成时间不变,干区宽度逐渐减小。此外,基于热量平衡理论对这些影响趋势给出了解释。研究结果表明ESDD、NSDD和雾浓度对于干区的形成是有影响的。     

糠醛浓度判断变压器绝缘纸寿命的现场测量

详细分析了 310台线路变压器的油样。结果表明 :尽管运行时间较长的变压器糠醛浓度较高 ,但随运行时间而加大的趋势不明显。糠醛浓度发生偶然性和阶跃性增加的原因一般为变压器内部故障引起的局部过热或变压器过负荷引起的整体过热。计算机仿真结果和实测的糠醛浓度与运行时间的关系曲线相当吻合。     

Prediction of spark paths in a point/two-rod gap in air

Three-dimensional images had been obtained of some 400 positive point/plane sparks in air during previous work and analysis showed that on average each section of the spark was related to the previous one. However there was a large random perturbation (within a probability distribution) which accounted for the tortuous shapes of the spark paths. Simulations based on these findings gave satisfactory agreement in terms of, for example, appearance and fractal dimension. A more fundamental test of this analysis and simulation method has been tried by attempting to simulate a significantly different electrode system: the grounded plane electrode was replaced by two parallel grounded rods. The simulation is then used, firstly to predict the general shape of the spark paths; and secondly to predict how many of the sparks will be between the point electrode and one of the two rods, and how many between it and the other rod. By moving the point electrode in a direction at right angles to the axes of the rods, but at the same height above them, this ratio could be varied. In order to get reasonable consistency it was necessary to run 1000 trials in the simulation for each point electrode position. Different ratios were obtained for different assumptions as to the magnitude of the random element in the section-by-section prediction. A series of experiments with an analogous electrode system gave good agreement in terms of the percentage strikes to a given rod versus the position of the point electrode.     

Negative corona in air using a point/cup electrode system

Calculations and simulations of potential and charged-particle distributions have been made by various workers for single pulses along the axis of point/plane electrode systems but not for the overall current. The difficult geometry makes this inevitable. In this paper the authors describe measurements of corona currents in air using a high-symmetry `point/cup' electrode system comprising a hemispherical-tipped point which is concentric with a concave hemispherical electrode; and simulations based on a spherically symmetric geometry. The simulations assumed avalanches to originate from the random release of electrons in the inter-electrode gap. The predicted onset voltages and the shape of the experimental curves for corona current vs. Voltage were in good agreement with the experimental results over a range of point-tip radii (0.5 to 3 mm) and pressures (0.02 to 0.10 MPa). Space charge effects were not included in the simulation so it follows that the accumulation of space charge is negligible at the onset voltage in air, although it is likely that it will have an increasing effect as the voltage increases     

Positive corona in air using a point/cup electrode system

The authors have previously published some measurements of negative corona currents in air using a spherically-symmetric point/cup electrode system, and also the related simulations. In this paper the corresponding current/voltage measurements and simulations for positive-point corona, again using the highly symmetric electrode system, are described. Although the corona onset voltages are again predicted with reasonable accuracy for different point radii and air pressures, the currents are very much lower, presumably due to the effect of positive space charge accumulating close to the point electrode (since the simulations assume the absence of space charges). In agreement with this assumption, the streamer pulses are only seen at or just above the onset voltage. Only avalanches occur from about 1 kV above the onset voltage.     

The high mobility group protein Abf2p influences the level of yeast mitochondrial DNA recombination intermediates in vivo

Abf2p is a high mobility group (HMG) protein found in yeast mitochondria that is required for the maintenance of wild-type (rho+) mtDNA in cells grown on fermentable carbon sources, and for efficient recombination of mtDNA markers in crosses. Here, we show by two-dimensional gel electrophoresis that Abf2p promotes or stabilizes Holliday recombination junction intermediates in rho+ mtDNA in vivo but does not influence the high levels of recombination intermediates readily detected in the mtDNA of petite mutants (rho-). mtDNA recombination junctions are not observed in rho+ mtDNA of wild-type cells but are elevated to detectable levels in cells with a null allele of the MGT1 gene (Deltamgt1), which codes for a mitochondrial cruciform-cutting endonuclease. The level of recombination intermediates in rho+ mtDNA of Deltamgt1 cells is decreased about 10-fold if those cells contain a null allele of the ABF2 gene. Overproduction of Abf2p by >/= 10-fold in wild-type rho+ cells, which leads to mtDNA instability, results in a dramatic increase in mtDNA recombination intermediates. Specific mutations in the two Abf2p HMG boxes required for DNA binding diminishes these responses. We conclude that Abf2p functions in the recombination of rho+ mtDNA.     

Improved demodulation scheme for fiber optic interferometers usingan asymmetric 3×3 coupler

Problems associated with the demodulation of the signal produced by a fiber optic interferometer which employs a 3×3 fiber coupler are investigated both theoretically and experimentally. It is shown that existing demodulation methods do not work well, especially when the 3×3 coupler is asymmetric, as is often the case. Modifications to improve the performance of a previously proposed method are suggested. Furthermore, an alternative demodulation circuit is presented which is simple and which works well even when the 3×3 coupler is asymmetric     

提高线路绝缘子防污闪及抗泄漏性能的新方法

章介绍了通过在传统的瓷质或玻璃线路绝缘子的铁帽底加装一表面覆盖硅橡胶的金属环片的新方法,可以使绝缘子表面电场分布更均匀,以提高起晕和闪络电压;使电场方向更垂直于绝缘子上表面,以抑制放电的发展;同时金属环片外表现覆盖的具有良好防污性能的硅像胶材料可以极大地增加爬电距离,降低泄漏电流和提高闪络电压。实验结果和理论分析相吻合,证实采用该方法改进后的新型绝缘子的优良性能。经10克／升的氯化钠溶液充分浸泡后,新型绝缘子的干污闪电压比普通绝缘子至少提高了170％,起晕电压提高了近70％,泄漏电流减少了近30％。