Hideo Akagami

Fundamental Characteristics of AC Flashover on Contaminated Insulators Covered with Ice

The fundamental characteristics of ac contaminated flashover on iced insulators were studied for the three icing states, i.e., glaze, hard rime and soft rime, which are observed most frequently in service conditions. The results showed that thawed snow with a relatively low salt concentration can produce a salt deposit density (SDD) high enough for flashover on an iced insulator. For the same concentration of contamination liquid, average values of SDD for icing states are 5 to 9 times higher than those for icing-free conditions. The results also showed that the minimum flashover stress of the soft rime condition is 20% lower than that of icing-free conditions. For this reason, we deduced that the number density of Na atoms in the discharge space at flashover tends to become highest under the soft rime conditions.

AC Flashover Characteristics on Trapezoidal Contamination Models of Actual Insulators

The fundamental characteristics of an ac contaminated flashover were investigated by using a simplified model which consists of a trapezoidal unit and a series of units in order to simulate the current density distribution on actual insulators. It is found that the lowest flashover voltage decreases with an increase in the voltage build-up rate, and after passing a minimum, tends to increase. Experiments also show that, if the leakage length is equal, a disc insulator string model having a metal plate on the connected area of units tends to flashover at a lower voltage than a long rod insulator model having no metal plate. The results are discussed according to the characteristics of the ac reignition voltage on contaminated surfaces in previous reports. It is deduced that the accumulation of Na ions in the discharge space at flashover dominates the reduction of contaminated flashover voltage.

Flashover Characteristics of Impulse Voltages on Solid Dielectric Surfaces under High Humidity

For the design of electrical insulating devices, it is very important to observe discharge phenomena on the needle-to-plate electrode gap under high humidity. In this study, impulse flashover phenomena in a needle-to-circular-electrode gap on solid dielectric surfaces under a high humidity condition have been discussed. As a result, it was found that impulse flashover voltages are greatly influenced by both horizontal and vertical gap lengths. When the horizontal gap length is shorter than the vertical one, the flashover characteristics approach those in air. On the other hand, if the horizontal gap length is longer than the vertical one, characteristics more closely approach those on the dielectric surface. The flashover characteristics also depend upon the polarity of the needle electrode. The characteristics for positive polarity seems to be similar to those in air, and those for negative polarity approach the creepage characteristics on the dielectric surface.

Reignition Voltage and Arc Voltage on Contaminated Insulator Surfaces

This paper reports investigations of flashover phenomena on insulator surfaces contaminated by NaCl. It discusses the relations between the reignition and the arc voltages, a high alternating voltage with a 50 Hz power frequency is applied to a contaminated insulator surface. The discharge characteristics in air on insulator surfaces are also discussed. These characteristics were obtained between non-contaminated electrodes, and the results are compared with those obtained with contaminated electrodes. The flashover characteristics in air were found to be very different from those on a contaminated surface. This means that under NaCl contamination, the reignition and the arc voltages are greatly influenced by the behavior of Na ions in discharge space and arc column.

Study of Characteristics of AC Arc Discharges on Polluted Insulator Surfaces

To elucidate the mechanism of pollution flashovers, the discharge characteristics of an arc were investigated by the authors in the following two cases. In the first case, the arc voltages were measured with flat plate insulators. It was found that if the length of the polluted portion is about 50% of the total gap length, the arc voltage when there is a portion free from the pollution between the electrodes is nearly equal to the voltage when the gap between the electrodes is polluted uniformly. In the second case, the arc discharge characteristics on the skirt surfaces of outdoor insulators were investigated, and it was found that the arc route at flashovers is such that the length of the arc discharges along the insulator surfaces is nearly equal to the length between skirt spacings bridged-over by the arcs.

Suppression and utilization of spurious pulse occurrence in organic GM-counters

The authors have made a study of suppression and utilization of spurious pulse occurrence in organic GM-counters. Almost all spurious pulses in the organic GM-counter are the delayed pulses which occur being dependent upon the radiation intensity. The occurrence rate of the delayed pulses against the radiation intensity is affected by the intensity of the electric field in the vicinity of the cathode of the GM-counter. The occurrence of the delayed pulses can be suppressed when the electric field in the vicinity of the cathode is kept at high value. On the contrary, the occurrence of the delayed pulses can be utilized for the dosimetry of the pulsed radiation by means of increasing the space of the weak electric field in the GM-counter.

AC Discharge Characteristics of a Needle-to-Semicircular-Electrode Gap with Solid Dielectric Surface under High Humidity

In this study, the discharge phenomena combined with the air and the creepage discharges have been discussed experimentally with a simplified model electrode gap. A conventional frosted glass plate was used as the solid dielectric. As a result, some noticeable phenomena have been found. The flashover voltages on the solid surface depend upon the relative humidity in air and decrease with increasing humidity. Especially, when the vertical gap length (or the height) is given to the needle electrode to increase the flashover voltages, the flashover voltages in high humidity seem to be free from the vertical gap length of the needle electrode, in which the inclined angle formed with the horizontal gap length and the vertical gap length is less than 30°.

AC Reignition Voltage on Partially Contaminated Insulator Surfaces

In an attempt to clarify flashover phenomena on contaminated insulator surfaces, the authors have investigated the relations between the ac reignition voltage and the arc voltage characteristics at 50 Hz power frequency. The present paper discusses some of the characteristics of partially contaminated surfaces. As reported before for contaminated surfaces, the reignition voltage in each half cycle decreases with the number of reignitions, reaches a minimum, and then increases again. This characteristic is due to the thermal ionization of Na atoms diffused and accumulated in the discharge space by alternate reignitions. A similar tendency has been observed on partially contaminated surfaces. On the partially contaminated surface, however, the value of the reignition voltage depends upon whether the gap ends are contaminated or not. As a result, the critical flashover current on a contaminated insulator surface could be estimated.

Ac minimum flashover voltages and wave number of occurrence of local discharges prior to flashover on polluted insulators

The relation between ac minimum FO voltages V<inf>ℓ</inf> and voltage build-up rates B has been investigated with respect to the wave number of occurrence N of local discharges prior to FO. The results show that V<inf>ℓ</inf> occurs when N equals N<inf>ℓ</inf> (5∼11); i.e. N<inf>ℓ</inf> increase from 5 to 11 as the pollution levels increase, and is independent of lengths for a given width. Additionally, similar informations are obtained for the cyclic number of ac minimum reignition voltages in contaminated conditions. It is also shown that the relation between the minimum FO voltages and the wave number of occurrence of local discharges is able to explain qualitatively with respect to the accumulation of Na atoms in discharge space at FO.

Ac reigniting discharge characteristics on contaminated insulator surface

The characteristics on the reignition and the arc voltages which were measured under both the contaminated and the non-contaminated (in air) conditions have been discussed. The results obtained are as follows; [1] With increase of the cycle number of reignitions, the reignition voltage v<inf>r</inf> and the arc voltage v<inf>a</inf> decrease to a minimum value and then increase again. The trend is independent of whether the insulator surface is contaminated or not. [2] The minimum value v<inf>R</inf> of reignition voltages v<inf>r</inf> occurs in larger cycle number of reignition than the minimum V<inf>A</inf> of the arc voltages v<inf>a</inf>. This means, more reignitioncycles are required for the voltage v<inf>R</inf> than for the voltage V<inf>A</inf>. [3] When the arc current i<inf>A</inf> is larger, the voltage v<inf>R</inf> occurs in smaller cycle number of reignition. [4] The voltage v<inf>R</inf> in air occurs in smaller cycle number of reignition than that under the contaminated condition. When the insulator surface is not contaminated, voltages v<inf>r</inf>, after the minimum v<inf>R</inf> is attained, cease to rise rapidly and remain constant approximately. [5] Under the partially contaminated condition, the voltage v<inf>R</inf> seems to occur in slightly smaller cycle number of reignitions than that under the whole gap contaminated condition. [6] The cycle numbers for the voltages v<inf>R</inf> and V<inf>A</inf> seem to approximate to that obtained by the other contaminating test method. [7] Under the contaminated condition, when the arc current is about 0.6 A or below, the reignition voltage v<inf>R</inf> is proportional to the −1 power of the arc current. The arc voltage V<inf>A</inf> also shows the same characteristic as the voltage v<inf>R</inf>, when the arc current is about 0.35 A or below. [8] In air, both the voltages v<inf>R</inf> and V<inf>A</inf> are expressed in proportion to the −0.7 power of the arc current, and the values become extremely higher than that under the contaminated conditions.