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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.

This chapter explains the fundamental characteristics of the electrostatic and quasi-electrostatic fields that the book covers. It deals with basic equations, boundary conditions, and the effects of conduction, among others. The “uniqueness theorem” in electric fields is also explained.

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.

Lightning observation has been carried out at Tokyo Skytree, which is a 634-m high freestanding broadcasting tower located on flat terrain at 1 m ASL. 25 downward and 40 upward flashes were observed at the tower from 2012 to 2017. This paper focuses on upward negative flashes initiated from the top of the tower observed in April 2017. Of the 3 confirmed upward flashes, all current waveforms were recorded by Rogowski coils installed on the tower at a height of 497 m. One out of 3 upward flashes was a positive flash initiated by a negative upward leader, and the rest were negative flashes initiated by positive upward leaders. For each event, we conducted simultaneous observations of the lightning current waveform, EM waveform, and optical images. Here, we report our recent observation results of lightning events

For understanding multiphase flashover phenomena of transmission lines, concurrent flashover phenomena have been studied experimentally using a parallel gap configuration. Considering the statistical characteristics of time to flashover of air gaps, the occurrence of concurrent flashover is theoretically analyzed. Experiments and the results of the theoretical analysis agree with fairly well.

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.

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.

We summarize the lightning data for 17 years from 1992 to 2008 obtained with lightning location systems of nine electric power utilities in Japan and analyze them. The observed annual number of lightning flashes of which the current is more than 10 kA is 400 000–800 000 and the 50% value of the cumulative distribution of lightning peak currents is about 23 kA in recent times. The variation of lightning occurrence characteristics by areas, seasons, and so on, is clarified. Comparison of the obtained data with those obtained by another lightning location system is also made. The relationship between lightning occurrence and climate is discussed. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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.

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 (wileyonlinelibrary.com). DOI 10.1002/eej.21143

Lightning risk is one of the important problems in the modern ICT society. Based on the lightning hazard evaluation, lightning risk assessment should be carried out for establishment of lightning risk management. An example of lightning hazard map in Japan is shown and present status of lightning risk assessment methods is presented.

Negative upward lightning flashes from a high structure in winter were observed by a current shunt and a network of VHF receivers, slow antennas and fast antennas on the coast of the Sea of Japan. VHF radiation sources associated with the lightning discharges were located three-dimensionally by the TOA method. Continuing currents of 6 upward lightning flashes were observed, and the position and the amount of associated charges in clouds were estimated from the data of the observation network. The measured charge amounts by the current shunt and the estimated neutralized charge amounts from electric field observation show good agreement.

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.

Lightning striking characteristics to wind turbine blades have been investigated by the model experiments with actual turbine blades. The effects of various types of receptors, polarities of applied voltages, pollution on the blade surface have been clarified. Based on these experimental results, lightning protection design of actual wind turbine blades by receptors has been 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.

It is important to establish a lightning risk management scheme for rational insulation design. In this paper, lightning risk assessment is carried out for a customer facility and a wind turbine blade. The calculated values of lightning risk agree with field experiences. © 2008 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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.