Electrical breakdown of small gaps in vacuum

The comparison of the characteristics of an electric arc breakdown in vacuum between two silver or silver-nickel alloy convex electrodes, enabled us to show experimentally that the process initiating discharge in vacuum is the electronic emission at the micrometer scale. Increased content of nickel in the alloy improves the insulation and achieves high resistance to erosion for low power arcs. Heating in vacuum demonstrates the multiple layer structure of studied alloys. Secondary electron microscopy and energy dispersing spectroscopy of the electrode surfaces after breakdown, as well as the measurements of fall and delay times, enabled us to evidence the origin of the electric arc     

Impulse partial discharge and breakdown characteristics of rod–plane gaps in air and N2 gases

In response to growing environmental concerns, we attempted to develop switchgear without using SF₆ gas. In our research, we used compressed air and pure N₂ as an electrical insulation gas, because of their low global warming potential. In this paper, we examined the impulse breakdown and impulse partial discharge characteristics under various conditions related to nonuniformity of the electric field. The experimental results show that the breakdown voltage (BDV) of air is higher than that of pure N₂ gas under highly nonuniform field conditions in the rod–plane gap. On the other hand, the discharge inception voltage of air and N₂ were almost the same. Furthermore, first partial discharge (PD), leader discharge, and its transition to the breakdown were successfully observed through the measurement of discharge current and light emissions under impulse voltage application. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(3): 36–43, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10277     

Electrical breakdown and space-charge effect in polyphenylene sulfide films

Electrical breakdown of polyphenylene sulfide (PPS) films, one of the high-temperature engineering plastics, has been investigated. The results show that the dc breakdown strength is lower than the impulse one, and that the difference in electric strength between dc and impulse becomes wider as the sample thickness decreases. The result suggests space-charge accumulation when dc voltage is applied. Further, the impulse breakdown strength in the presence of a dc prestressing voltage is measured in order to elucidate the space-charge effect. The data on the contribution of the prestress voltage to electrical breakdown are used to estimate the amount of space charge accumulated in the specimen during the dc prestressing.     

Electrical breakdown characteristics of nitrogen and air at cryogenic temperature in a quasi‐uniform electric field

The breakdown voltages of the longer‐gap configurations in gaseous nitrogen and air that are necessary in designing superconducting electrical power apparatuses are measured at temperatures of 293 and 93 K. The quasi‐uniform electric field made by a sphere‐to‐sphere electrode with a diameter of 150 mm and a gap length of about 10 to 100 mm is used in the measurement of the breakdown voltages. When 50‐Hz ac and dc voltages are applied to the sphere‐to‐sphere gap, the breakdown voltages in nitrogen and air obey Paschen's law even at cryogenic temperatures (93 K). When a 1.4/50‐μs lightning impulse voltage is applied to the gap, the 50% breakdown voltage of nitrogen also obeys Paschen's law under UV irradiation of the cathode electrode. However, the breakdown voltage in air at 93 K is higher in the case of lightning impulse voltage applications, and the delay from impulse voltage application to breakdown occurrence is apparently longer at 93 K than at 293 K. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 28–33, 2000     

Electrical breakdown in high-voltage winding insulations of different manufacturing qualities

Due to an increasing demand for new, or the refurbishment of power stations, high-voltage rotating machines continue to play a significant role in generating electrical energy. An important component of these machines is their electrical insulation. Although much research has been done to improve the material, about a quarter of all failures still are related to insulation problems. Research to improve winding insulation is very complex because a large number of factors influence insulation life, among them the raw materials, size of the mica tapes, geometrical characteristics of the bar and taping- or manufacturing techniques. The results show that electrical tree propagation is the main electrical degradation mechanism that leads to breakdown of the main wall insulation and that poor impregnation of the insulation and type of taping can reduce time to breakdown significantly.     

Puncture breakdown characteristics of protected rubber insulatinggloves

The results of puncture breakdown tests on rubber gloves are presented. A positive polarity switching surge voltage (275/2500 μs) or a 60 H alternating voltage was applied to the glove to cause breakdown. The effect of mechanical pressure on glove breakdown behavior and the influence of using a leather protector were investigated. It was found that protected gloves are more likely to break down at a position other than where the needle electrode has been placed. In both AC and switching surge breakdown tests, the standard deviation in breakdown voltage for protected samples was greater than that of the nonprotected samples by 25%. Under AC conditions, with mechanical stress applied, a bare glove will break down at a voltage which is approximately 22% below the nonstressed condition. A protected glove which is mechanically stressed will break down at a value 12.75% below the nonstressed condition. It is suggested that the results obtained will help to determine the operating limits and reliability of the glove insulation     

AC breakdown characteristics of in-service aged XLPE distributioncables

AC breakdown tests were performed on field-aged XLPE distribution cables. For these measurements, a special test assembly consisting of air/distilled water terminations was developed. The tested cable specimens were examined for the occurrence of halos and water trees. The results suggest a correlation between the AC breakdown strength and operating stress. The incidence of halos is also found to be related to the AC breakdown strength, but no such relation appears to exist for the water tree length or density     

Laser-induced breakdown characteristics of nonuniform dc coronadischarge fields

Laser-induced breakdown of nonuniform field gaps in atmospheric air was caused by an XeCl excimer laser. This laser beam was focused on the center of a rod-to-plane, needle-to-plane or hemisphere-to-plane gap with dc voltage. The laser energy dependency of breakdown voltage was investigated for positive and negative applied voltages and gap lengths of 15 to 25 mm. It was found that the positive breakdown voltage was higher than the negative one in the hemisphere-to-plane gaps or the rod-to-plane gaps with low voltages, where there is little or no space charge. However, the effect of polarity is reversed in the rod-to-plane gaps with large spacings and the needle-to-plane gaps. On the basis of the dependence of the discharge mode on this polarity effect, it can be seen that it was caused by the difference in development of positive and negative streamers in the corona discharge     

Parameters influencing breakdown characteristics of vacuum gapsduring spark conditioning

Experiments have been made to identify the key parameters in the increase of the breakdown strength of a vacuum gap through repetitive breakdowns. The current-voltage waveforms were observed, the residual stress in electrodes was measured, and the chemical composition of the electrode surfaces and the energy of the valence-band electrons were analyzed. These experiments revealed that prebreakdown currents (field emission currents) were sometimes observed and sometimes not. The residual stress before and after 500 breakdowns changed significantly. X-ray photoelectron spectroscopy analysis before and after 500 breakdowns showed that the spectral lines of oxygen and carbon due to contaminants on the electrode surface were completely removed and the copper ones appeared clearly after 500 breakdowns. The valence-band electron energy spectrum of the anode after 500 breakdowns shifted towards the lower binding energy to almost coincide with the theoretical spectrum, while the energy spectrum for the cathode shifted towards sometimes higher and sometimes lower binding energy