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The features of lightning shielding of transmission lines have been calculated using the electrogeometric model (EGM) proposed by Armstrong and Whitehead. Discrepancies, however, have been found in the case of ultra-high voltage (UHV)-designed transmission lines. UHV-designed transmission lines often pass through high-altitude mountain areas, where foggy or rainy conditions are common when there is lightning. While the electrogeometric model determines the striking distance based on the lightning stroke current, environmental conditions, and how they might affect lightning discharge paths, they are not considered. It is possible, therefore, that foggy conditions may cause differences in the performance of transmission-line lightning shielding. This study used scale models of transmission lines for discharge tests with a 1-m air gap and analyzed differences in discharge paths and locations in dry and foggy conditions. The results showed that the polarity of the discharge had a significant influence, but differences in dry and foggy tests were negligible. Index Terms--Discharge characteristics, foggy conditions, lightning shielding, ultra-high voltage (UHV)-designed transmission lines.

The electric geometry model suggested by Armstrong and Whitehead was used to calculate the performance of lightning-shielded transmission lines. However, since the ultra high voltage (UHV) designed transmission lines were brought into 500 kV operation, it has become evident that their actual performance is different from the calculated predictions in terms of their actual faults, lightning observation results, etc. It is thought that the UHV designed transmission lines are subjected to foggy or rainy conditions when lightning strikes, because they often pass through mountainous areas at higher altitudes. However, according to the electric geometry model, the striking distance is determined by the lightning-stroke current value only, and the model does not consider the environment of discharge path for the relevant lightning strokes. These considerations suggest that foggy conditions may affect transmission line lightning shielding performance. In this study, discharge tests were conducted using a scaled-down transmission line with a 5 m air gap. The discharge point to the simulated conductor was analyzed in both dry and foggy conditions. Tests were also conducted with a dc bias applied to the simulated power lines, to take the operating voltage of the lines into consideration. These experiments have shown that the effect of fog on the discharge ratio to the conductors is negligible. Index Terms--Lightning, shielding, discharge characteristics, fog, long air gap, switching impulse.

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rodrod and rod-conductor gaps. Index Terms--Switching impulse, negative polarity, flashover characteristics, long air gap.

For rational insulation design of a UHV substation, full-scale switching impulse tests of phase-to-ground (shielding ring-to-tower) and phase-to-phase (shielding ring-to-shielding ring) insulating clearance have been carried out in dry and wet conditions. In the tests, shielding rings that will be installed on bushings at actual UHV substations were used. The sparkover voltages depend on not only gap configurations but also the waveform of applied switching impulses; in other words, they show what we refer to as U-characteristics. Critical sparkover voltages are higher than those of rod-rod gaps by 20%-25% in dry conditions and by 7%-10% in wet conditions. Index Terms--Air gaps, air insulation, flashover, surges, UHV substations.

A new approach to characterizing 'anomalous' flashover phenomena is presented and a method of calculating the probability of anomalous flashovers occurring is proposed. The calculated probabilities of anomalous flashovers are compared with experimental results, and the new method is used to estimate the safety clearance for UHV substations. Key Words: Anomalous flashover, Flashover probability, Parallel gap, Insulation coordination, Safety clearance

Experiments to guide electric discharges with a chain of air-breakdown plasmas (plasma channel) produced by a laser were carried out. Electric discharges were guided up to 4.5m with a high power CO2 laser focused by a 10m focal length mirror. Unusual 50% flashover voltage characteristics were obtained and the mechanism on the laser-guided discharge is shown. Experiments of laser-guided discharge in fog or rain were also carried out to clarify the effects of precipitation on the guiding characteristics, and the results are analysed quantitatively.

Experiments to guide electric discharges with a chain of apparently discrete air-breakdown plasmas (plasma channel) produced by a laser were carried out. The electric discharge was guided up to 2m with a high power CO2 laser focused by a 10-m focal length mirror. Voltage was applied at selected delay times tau following laser radiation. The relations between 50% flashover voltage of a gap filled with laser-produced plasmas and delay times tau, between the guided length and the peak of the applied voltage, were obtained experimentally. The effects of the polarity of an applied voltage and the position of the plasma channel on flashover voltage are described. Development characteristics of the guided discharge were also investigated.

A new shielding theory is proposed based on the assumption that the occurrence of lightning strokes on the Japan Sea coast in winter is due to the inception of upward leaders from tall structures. Ratios of the numbers of lightning strokes to high structures observed there in winter show reasonable agreement with values calculated by this theory. Shielding characteristics of a high structure in various conditions are predicted.

Direct observation of lightning current is a fundamental approach to evaluating lightning parameters, which are among the most basic factors for the lightning protection design of various grounded objects. However, for a tall grounded structure, upward lightning flashes initiated from the top of the structure have usually been observed, and the direct observation results of downward flashes have been limited. We developed and installed a pair of Rogowski coils at Tokyo Skytree, which is a 634-m-high freestanding broadcasting tower in Japan (35.71°N, 139.81°E), and have conducted direct observation of lightning flashes striking the tower. As of November 2018, 70 lightning flashes striking the tower, 26 downward and 44 upward, have been observed since the end of February 2012. In this paper, we focus on the downward negative flashes that occurred at the tower for seven years from 2012 to 2018 and report on the observed results of typical waveform parameters of lightning stroke currents in comparison with other observed results on downward negative flashes. [ABSTRACT FROM AUTHOR]

Observation of lightning current by using Rogowski coils started in February 2012 at Tokyo Skytree, which is a 634-m-high freestanding broadcasting tower. Electromagnetic field waveforms have been also observed by calibrated capacitive antennas at several tens of kilometers from the tower. Simultaneously observed lightning current and electric field waveforms are reproduced by using an electromagnetic model of a return stroke over lossy ground with the help of NEC-4. Lightning channel models with inclination are reconstructed from observed images. Consequently, it turns out that the electromagnetic field waveforms are affected by the geometrical shape of a lightning channel, current refractions in the tower, ground conductivity, and current wave velocity in the lightning channel. NEC-4 is quite useful for such analysis of lightning strokes hitting high structures. [ABSTRACT FROM AUTHOR]

For the analysis of back‐flashover phenomena of transmission lines, accurate evaluation of flashover occurrence at arcing horns is necessary. In this paper, parallel gap flashover phenomena are studied theoretically in the following conditions: (i) the gap lengths are different, and (ii) after one of the gaps flashes overs, there remains some residual voltage. The formulae for the occurrence probabilities of concurrent flashover on these conditions are introduced. Based on the analysis, application to the analysis of multiphase flashover is discussed. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Recently, very tall structures with heights more than several hundred meters have been constructed all over the world and observation of lightning has been made. The striking characteristics obtained from these observations are reviewed, and conventional and new shielding models are discussed. In addition, future themes of studies to be carried out are proposed. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

SUMMARY Recently, concept of the lightning risk management has been proposed for the minimization of damages caused by lightning. In distribution systems (power distribution lines, low-voltage distribution lines, telecommunication lines, and customer appliances), lightning protection measures based on the lightning risk management are effective since the number of facilities is enormous. In this paper, for the establishment of lightning risk management method of distribution systems, we expressed the lightning risk assessment method of distribution systems and examined the influence of arrangement of customer house on the lightning risk values of power distribution lines and customer appliances. [ABSTRACT FROM AUTHOR]

Nine electric power companies in Japan have been collecting lightning data with lightning location systems. Five years of the lightning data (2009-2013) are summarized and analyzed in this paper. The recent annual number of lightning flashes of which the current is more than 10 kA in Japan is around 1 million. The variations of lightning occurrence characteristics by areas, seasons, and so on, are clarified. Meteorological conditions that may affect lightning occurrence are also discussed. Furthermore, we show that there is a clear correlation between the number of lightning strikes and the outage rate of transmission lines. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Application of Bayes' theorem for the evaluation of diagnostic results of electric facilities is described. A concept of risk assessment method based on the diagnosis results is proposed and the effects of various parameters on the risk are clarified. Based on these results, a rational renewal scheme using diagnostic results is proposed. In addition, the prior probability based on the data of actual facilities is investigated and it is found that the data available so far is insufficient and more data are required for the detailed analysis. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

The characteristics of lightning strikes on wind turbine blades have been investigated by model experiments with real turbine blades. The effects of various types of receptors, the polarities of the applied voltages, and pollution on the blade surface are clarified. Based on these experimental results, lightning protection design of actual wind turbine blades by receptors is discussed. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 176(3): 8-18, 2011; Published online in Wiley Online Library (). DOI 10.1002/eej.21143 [ABSTRACT FROM AUTHOR]

The lightning occurrence characteristics around Tokyo Skytree are investigated with Japan Lightning Detection Network, and the results are evaluated with observation data by the Rogowski coils installed at a height of 497 m and by cameras set around Tokyo Skytree. After the construction of Tokyo Skytree, the apparent density of detected strokes near Tokyo Skytree increases, but without lightning strokes to Tokyo Skytree, the density of detected strokes around Tokyo Skytree seems to be roughly constant. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Laser-guided discharge is promising for protection of power transmission systems against lightning. Several experiments concerning the laser-induced electrical discharge have been performed up to now. However, it is necessary to understand the mechanism of the laser- induced electrical discharge to achieve the laser-triggered lightning. When a high-power laser beam is focused in air, a high degree of ionization is produced as brilliant beams near the focus. In this paper. the electrical discharge induced up to 2-meters with a high-power CO² laser focused by a 10-meter focal length mirror is described. Experiments of the laser-guided discharge were carried out by application of an impulse voltage to a rod- rod gap after laser radiation. This paper deals with the effects of delay times and polarity of the impulse voltage on 50 percent flashover voltage characteristics and the observation results of discharge phenomena taken by a still camera, a special camera named ALPS (Automatic Lightning Discharge Progressing Feature Observation System) and an image converter camera. [ABSTRACT FROM AUTHOR]

Concurrent flashover characteristics with three gaps in parallel are analyzed theoretically considering the statistical characteristics of time to flashover of a gap. The analysis shows that the concurrent flashover phenomena are likely to occur if the overvoltage ratio is large even if there are three gaps. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]