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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.

As a thought-provoking pastime for readers, this chapter offers three simple questions in technology or physics. No answers to these questions are given in this book.

Recently, the number of outages of wind turbine generator systems has been increasing. For rational lightning protection design, the concept of lightning risk management has been proposed. In this paper, lightning risk assessment of the wind turbine generator systems is carried out and it is compared with field experiences. Furthermore, lightning risk management scheme is discussed.

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.

After reorganization of power industry in 1951, power system of Japan was remarkably expanded. To cope with this, R&D in vast area was conducted in the academy, MITI, electric power companies, manufactures, and research institutes. This paper generally describes the technical contents that the three research institutes, namely Electrotechnical Laboratory (ETL), High Voltage Power Laboratory (HVPL), and Central Research Institute of Electric Power Industry (CRIEPI), have contributed to the development of power transmission technology after the war.

Upward lightning is often observed on the coast of the Sea of Japan in winter season. Space charges by corona discharges at the grounded structures and on the ground or from the sea surface greatly affect the upward leader initiation, however, no studies of the effect of space charges have been carried out yet. In this paper, we calculate the electric fields around a conductor considering the effect of corona discharge from the conductor itself and on the ground using the DC ion flow calculation program that was developed by CRIEPI. Calculation results show that the upward leader initiation criteria are difficult to be satisfied for small conductors than large conductors because space charge reduces the electric field for the small conductors, and that space charge from conductors gives remarkable effects including polarity change on the ground level electric field in the downwind area.

As a research aiming at obtaining basic information useful for the development of maintenance-free insulation, the status quo and trends of insulation monitoring and diagnosis methods for electric power apparatus that have been done in Japan are analyzed by reviewing about 8800 reports published for the last 50 years. It is found that the accuracy of monitoring and diagnosis has been improved well and unmanned monitoring and diagnosis have been developed significantly with the introduction of measurement systems using sophisticated computer-aided analyzing and fiber-optic monitoring techniques. Further implementation of such sophisticated methods in the whole area of insulation monitoring and diagnosis should even increase the total reliability of power apparatus, which will undoubtedly reduce the necessary personnel and costs to maintain the apparatus.

Lightning has caused lot of damages to not only human beings, but structures, power transmission and distribution systems and transportation systems. Furthermore, recent information-oriented society becomes more vulnerable to lightning than that of old days. To understand lightning, several kinds of lightning observations have been carried out and novel observation equipments using opt-electronics technology have been developed. Based on the observations and theoretical analysis, lightning protection design has been established. In this article, lightning hazards, lightning observation techniques and lightning protection design are reviewed. Problems for future studies are also demonstrated.

Side flashes often occur when lightning strikes a tree and cause injury or death of human beings staying nearby the tree. Necessary clearance to prevent the side flash has been said to be 2 m, but the physical meaning of the value is unclear. In this paper, we have proposed a model of side flashes based on the physics of discharges and necessary clearance has been calculated. Furthermore, we have carried out model experiments of the side flash using real trees of a heights of 3.5 m and a naturally growing tree of a height of more than 10 m. The results of the model experiments have verified the proposed theory and it is concluded that clearance of more than 3 m from a tree is necessary to prevent side flashes from the tree.

To clarify the characteristics of lightning on the coast of the Sea of Japan in the winter season, the current waveform and the progression of lightning discharge to a high stack were observed. Observation results from 1989 to 1994 show that: (1) there are two types of lightning discharges-one with large currents and strong luminosity in the lightning path, and the other with small currents of long duration and weak luminosity of lightning path; (2) the parameters of lightning currents of the two types of discharges are quite different; and (3) there is a quantitative correlation between the change of luminosity in a lightning path and the lightning current waveform.

Guiding and triggering of the electrical discharges in lm air gaps by excimer lasers (KrF (λ=248nm), ArF (λ=193nm)) have been performed. A laser pulse (KrF: 750mJ, ArF: 550mJ) produced a plasma channel over a distance of more than 0.5m by focused with a lens. Characteristics of guided discharges along the plasma channel were investigated in detail. From these results, it is found that an excimer laser pulse can guide the positive discharges and propagate in the plasma channel produced by itself without loss. However the excimer lasers have little ability to trigger the discharges. Further the guiding ability is not maintained for a long time compared with that of infrared lasers. On the basis of these experimental results, we discussed the basic concept of the laser triggered lightning with an excimer laser.

Experiments to guide electric discharges with a chain of air breakdown plasmas (plasma channel) produced by a laser were carried out for use in power system protection applications. Electric discharges were guided up to 4.5m with a high power CO/sub 2/ 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 are reported. The electric discharge was guided up to 2 m with a high-power CO/sub 2/ laser focused by a 10 m focal length mirror. Voltage was applied at selected delay times tau following laser radiation. The relationships between 50% flashover voltage of a gap filled with laser-produced plasmas and delay times, and between guided length and the peak of the applied voltage, were obtained experimentally. The effects of the polarity of the applied voltage and the position of the plasma channel on flashover voltage are described. The development of the guided discharge is discussed. >

We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets. The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

Third International Symposium on Winter Lightning; Sapporo, Japan, 15–16 June 2011 Japan's meteorological setting in winter is unusual: It is an island in a relatively warm sea frequently overswept by colder air from Siberia. This sets up appreciable atmospheric instability in the fringe of the land adjacent to the Sea of Japan. Heavy snowstorms overlap the edge of the island and produce extraordinarily energetic lightning flashes that initiate from points on the ground (known as ground-to-cloud (GC) strokes) and wreak havoc on power lines and, more recently, wind turbines. These troublesome and costly conditions set the stage for the third in a series of conferences on winter lightning.