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Abstract—Winter lightning in Japan is known for such characteristics as frequent occurrence of upward lightning and of positive ground flashes. On the engineering side, higher frequencies of troubles at transmission lines or wind turbines in winter due to lightning than those in summer have been experienced in the winter thunderstorm area of Japan, despite the much smaller number of lightning strokes in winter observed by lightning location systems (LLS). Such frequent troubles by lightning in the cold season are unique in Japan, which have promoted intensive research on winter lightning in Japan since 1980s. I started research on winter lightning from 1980s, and discovered a peculiar upward flash which caused multiple-line faults of power transmission lines. Our research group named the high-current pulse, included in the flash, GC (ground-to-cloud) strokes.

Index Terms—lightning, winter lightning, upward lightning,

transmission line, GC stroke

I. INTRODUCTION

inter lightning in Japan is now world-widely known, which occurs in the coastal area of the Sea of Japan on the Honshu Island. The same area had been better

known for the heavy snowfall in winter, and lightning used to be regarded as a harmless precursor of the season of snow. Recent development of infrastructures has turned winter lightning into serious threat to them. The thunderstorm days in this area are comparable in winter to those of summer; however, the number of lightning strokes observed by lightning location systems (LLS) in winter is much smaller. Nevertheless, transmission lines or wind turbines in the coastal area of the Sea of Japan have suffered from higher frequencies of serious troubles by lightning in winter than those in summer [1]-[4].

The first serious incident was a crash of a jet fighter of JASDF (Japan Air Self-Defense Force) in Kanazawa on February 8, 1969, which was believed to be due to a lightning strike. In the 1970s, lightning in the cold season attracted attention of scientists [5][6], and systematic researches of winter lightning started [7]. Also from this decade, 500 kV power transmission lines were put into operation in Japan, and those constructed in the coastal area of the Sea of Japan suffered from outages in winter much more frequent than prediction. Lightning was suspected as the cause of this unusually high outage rates in winter, and winter lightning attracted attention of electrical engineers. Since then, winter lightning has been a principal research subject of lightning research in Japan. __________________________________

Masaru Ishii is with APET, Graduate School of Engineering, The

University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan (e-mail: ishii@iis.u-tokyo.ac.jp).

Research of lightning has two aspects, that is, lightning physics and lightning protection. Lightning protection is the urgent subject in case there is necessity to protect important objects from lightning hazards. In designing lightning protection, assessment of lightning parameters is indispensable. Research on winter lightning in Japan has been strongly motivated from the needs for lightning protection; however, observation of lightning for engineering purposes has often facilitated improvement in understanding lightning physics.

II. CHARACTERISTICS OF WINTER LIGHTNING IN JAPAN In the early report on winter lightning in 1974, it was

already written that Hokuriku winter storms generally lower positive charge to earth, and upward flash started from an object lower than 15m in height [3]. These are prominent characteristics of winter lightning, though, negative c-g (cloud-to-ground) flashes actually outnumber positive c-gs.

Nagoya University and New Mexico Institute of Mining and Technology reported that Hokuriku storms also appeared to have a normal dipole structure, that is, positive charge uppermost, and they hypothesized that the dipole was severely tilted in the direction of wind shear to explain the high fraction of positive c-g flashes [6][7]. This hypothesis, so-called tilted dipole model, had been believed for more than 20 years, however, was based on only a few observations.

In the 1970s, a lightning location system (LLS) based on wideband observation of electromagnetic field in the frequency range from VLF to LF was invented in the U.S., and two systems were introduced into Japan in the beginning of 1980s. One system was installed on the coast of the Sea of Japan, which suffered from low detection efficiency of lightning strokes in winter. This also is a characteristic of winter lightning.

My research group discovered through observation of electromagnetic fields that there is seasonal variation in field waveforms associated with lightning return strokes [8]. LLS then was tuned based on observation of summer lightning, and did not detect field waveforms associated with lightning return strokes in winter, which are characterized by shorter zero-cross time. Later its detection efficiency of winter lightning was improved by tuning the waveform discrimination criteria.

Research on meteorological aspects of winter thunderstorms by using meteorological radar and VHF direction finders of JASDF had been carried out by Defence Adademy from 1980s [9]. They discovered that winter thunderstorm was active even when the altitude of the −10°C temperature level was as low as only 1.8 km, whereas in summer, its typical height is around 6 km.

Fig. 1 shows statistics of lightning outages of 500 kV lines in Japan in the late 1970s [10]. Today, the outage rate in winter has been reduced due to the improvement of footing resistance of transmission towers, but the salient

Research on Winter Lightning Masaru Ishii

The University of Tokyo

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characteristic of winter lightning that the number of double-circuit outages is comparable to that of single-line outages remains. The high outage rate of newly constructed 500 kV double-circuit transmission lines in winter motivated electric power companies and Central Research Institute of Electric Power Industry (CRIEPI) to carry out intensive field study of winter lightning by direct measurement of lightning current at tall structures and test transmission lines, and by automatic film-type cameras.

In Feb. 1995, a three-phase back flashover of a 500 kV double-circuit line was photographed as shown in Fig. 2, and it was confirmed that double-circuit outages in winter were not caused by a mysterious lightning phenomenon but by ordinary back flashover [10]; however, the photo did not clue whether the flash was upward or downward then, because the shutter of a camera was operated after detecting light pulses, so branches of lightning channels were not imaged. LLS did not detect this lightning flash. Lightning flash densities observed by LLS in the winter lightning area during winter months were less than 1/5 of those during summer months. From observation of lightning at tall structures, it was known that lightning striking such structures was mostly upward lightning [11].

Fig. 1. Seasonal lightning outage rate of 500 kV double circuit balanced insulation lines in Japan from 1974 to 1980 [10].

Fig. 2. Three-phase back flashover of double-circuit transmission line in the coastal area of the Sea of Japan in winter (Feb. 1995) [10].

III. OBSERVATION OF WINTER LIGHTNING

A. Location of positive charge related to positive c-g flashes

Electric field waveforms from lightning discharges over Fukui Plain on the coast of the Sea of Japan were observed by networks of 8 slow antennas, 8 VHF receivers and 5 fast antennas from the end of 1990s. Field waveforms recorded by each observation station are time-tagged by GPS-clocks [12]. The network of VHF receivers acted as a Lightning Mapping Array (LMA), enabled 3-D location of VHF sources by the time-of-arrival (TOA) method. Fig. 3 shows configuration of the networks.

Distributions of altitudes of gravity centers of VHF sources before the occurrence of positive return strokes of ordinary downward flashes in winter are shown in Fig. 4. Concentration of the gravity centers around –10°C level indicates that the frequent +CG strokes in winter are related to positive charge layers around –10°C level. This observation agrees to the heights of located positive charges by using the SA network [13]. In only one case out of 20, the most active region of VHF radiation prior to a positive return stroke was located at –30°C level, which was claimed by the so-called tilted dipole model [7].

Fig. 3. Seasonal lightning outage rate of 500 kV double circuit balanced insulation lines in Japan from 1974 to 1980 [12].

Fig. 4. Distribution of temperature altitudes of gravity centers of VHF sources prior to occurrence of +CG strokes in winter [12].

B. Discovery of GC strokes

Extensive networks of transmission lines can contribute to observation of rare lightning phenomena if they are properly monitored. The enigma of high double-circuit outage rate under low lightning flash density in winter was finally resolved by collating transmission-line fault records with GPS-timed records of electromagnetic field waveforms radiated from lightning discharges.

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Lightning discharges, observed by the Fukui network, which seemed to be associated with notable transmission-line faults were examined in detail. The total number of such events within 90 km of the network was 53 during 1998-2007．All the investigated faults on 77 kV and 154 kV lines were multiple-line faults. In the data of 275 kV and 500 kV lines, single-line faults were included. All the transmission lines under investigation were double circuit lines.

Some of the transmission lines are equipped with a relay system timed by GPS clocks, and 6 data among the 53 faults were from such lines. In all these 6 data, intense LEMP were found within 15 ms from the recorded relay operation times. The range-normalized amplitudes of these LEMP were several tens of V/m at 100 km, which are much more intense than that of the average of negative first return strokes, 5 to 6 V/m. In these 6 data, no LEMP having the range-normalized amplitude of 5 V/m level were found within several tens of ms from the relay operation times except the intense LEMP.

At the other 47 transmission-line fault events in winter used for the analysis, the resolution of the time of operation of relays was 1 second. In winter, the lightning flash rate is so low that it is quite easy to identify the lighting flash which may be associated with a transmission-line fault. The length of fast antenna records of the electric field changes was about 0.8 s without interruption, and the most intense LEMP during the associated lightning flash was subject to analysis. Except maximum 3 possible positive return-stroke waveforms, no return-stroke waveforms were found in the 53 field-change records. LLS located about 80% of them within few km from the transmission towers where traces of faults were confirmed, though not all the signal strengths observed by LLS matched those observed by fast antennas.

Electric field waveforms of the 21 negative discharges associated with the transmission-line faults did not involve those from negative return strokes, and they were inferred to be associated with upward flashes of an unknown type, initiated by positive upward leaders from transmission towers. They were named –GC (Ground-to-Cloud) strokes [14], as opposed to CG (Cloud-to-Ground) strokes, which are preceded by downward leaders from the cloud followed by return strokes. Fig. 5 shows an example of the electric field waveform of –GC simultaneously recorded with a transmission-line fault. It was also detected by LLS, and its peak field strength was equivalent to a negative return stroke of about -300 kA. The characteristics of the waveforms like Fig. 5 are so distinct that it is easy to find similar LEMP among field waveform data observed in winter.

Fig. 5. Electric field waveform of –GC stroke associated with a transmission-line fault in winter.

Fig. 6. Example of a set of simultaneously recorded electric field waveforms at different stations associated with a possible negative GC flash [14].

Evidence that this type of lightning discharge substantially

transferred negative charge from cloud to ground is shown in Fig. 6. This discharge occurred within the Fukui field observation network and all the electric field change records obtained at stations in three different directions viewed from the discharge showed positive shifts after the bipolar pulse, indicating that negative charge moved from cloud to ground.

The rest 29 LEMP waveforms except the three possible positive return-stroke waveforms, among the 53 field waveforms recorded in coincidence with transmission-line faults, were inferred also from upward flashes of positive polarity. Actually, different from –GC waveforms, it is hard to find clear distinction between +GC and positive return stroke waveforms except they were recorded from the close range. These newly discovered lightning phenomenon, named GC (ground-to-cloud) strokes, associated with upward lightning and characterized by lightning electromagnetic pulses (LEMP) of large amplitudes, were concluded to be causes of more than 90% of double-circuit outages in winter. Combination of frequent occurrence of upward lightning from elevated structures in winter, and existence of upward lightning associated with high current pulses, though probability of its occurrence is low, clearly explains the frequent occurrence of back-flashovers at transmission towers.

Later, a different research group reported on the same phenomenon, -GC stroke, by a different name, LBE (large bipolar event), but they only observed electromagnetic radiation [15].

To reproduce the field waveform of –GC stroke, a model in which junction between the upward positive leader and a short negative leader in the altitude occurs was proposed [16]. It predicted a narrow single pulse of little dip in the opposite polarity to be observed at the ground end of the lightning channel.

In March 2014, first simultaneous recordings of images by a high-speed camera, current and field waveforms for –GC were obtained at Tokyo Skytree, a 634-m instrumented tower in Tokyo [17]. Fig. 7 shows images of upward leader developing from the tower top, and Fig. 8 shows the directly

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observed current waveform, a prominent narrow single pulse superposing initial continuous current, as was predicted. Remote electric field, not shown here, shows a characteristic narrow bipolar waveform like Fig. 5.

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Fig. 7. Upward lightning channel hitting Tokyo Skytree observed by high speed camera (March 2013) [16].

Fig. 8. Current waveform corresponding to Fig. 7, directly observed by Rogowski coil [16].

IV. LIGHTNING RESEARCH ISSUES TO BE PAID ATTENTION Attachment of lightning channel to grounded objects at

both downward and upward is an interesting subject in both physical and engineering aspects. Out of necessity in engineering, non-physical models of attachment have been employed in designing lightning protection of structures and electrical systems. These models are practical and useful, but they are empirical, so they sometimes produce inaccurate results at structures of novel design.

By the recent development of observation instruments such as digital high-speed cameras and LMA, combined with the knowledge on the frequent occurrence of lightning hits on tall structures, there will be more chances than before to observe detailed physical parameters related to lightning attachment, leading to development of practical semi-physical models that are useful in engineering purposes.

V. ACKNOWLEDGEMENT I would deeply acknowledge Dr. J. Hojo and Dr. M. Saito

of my research group and personnel of Hokuriku Electric Power Co., for their cooperation in the observation of winter lightning, which has brought about fruitful research results.

VI. REFERENCES [1] M. Ishii, and M. Saito, “Lightning electric field characteristics

associated with transmission-line faults in winter,” IEEE Trans. Electromagnetic Compatibility, vol. 51, pp.459-465, 2009.

[2] H. Sugimoto, “Lightning protection against winter lightning,” Proc. of the 28th Int. Conf. on Lightning Protection, Kanazawa, Japan, Vol. 1, No. INV-5, pp. 26-32, 2006.

[3] D. Natsuno, S. Yokoyama, T. Shindo, M. Ishii, and H. Shiraishi, “Guideline for lightning protection of wind turbines in Japan,” 30th International Conference on Lightning Protection, Cagliari, Italy, SSA-1259, 2010.

[4] D. Wang, and N. Takagi, “Characteristics of winter lightning that occurred on a windmill and its lightning protection tower in Japan,” Proc. 3rd International Symposium on Winter Lightning, Sapporo, Japan, pp. 87-91, 2011.

[5] T. Takeuti, M. Nakano, M. Nagatani, and H. Nakada, “On lightning discharges in winter thunderstorms,” J. Meteorological Soc. Japan, Vol.51, pp.494–496, 1974.

[6] T. Takeuti, M. Nakano, M. Brook, D.J. Raymond, and P. Krehbiel, “The anomalous winter thunderstorms of the Hokuriku coast,” J. Geophys. Res., Vol.83, pp.2385–2394, 1978.

[7] M. Brook, M. Nakano, P. Krehbiel, and T. Takeuti, “The electrical structure of the Hokuriku Winter Thunderstorms,” J. Geophys. Res., Vol.87, pp.1207–1215, 1982.

[8] J. Hojo, M. Ishii, T. Kawamura, F. Suzuki, H. Komuro, and M. Shiogama, “Seasonal variation of cloud-to-ground lightning flash characteristics in the coastal area of the Sea of Japan,” J. Geophys. Res., Vol.94, No.D11, pp.13207–13212, 1989.

[9] K. Michimoto, “Meteorological aspects of winter thunderstorms along the Hokuriku coast of Japan,” 28th Int. Conf. Lightning Protection, Kanazawa, Japan, INV-1, 2006.

[10] H. Sugimoto, “Lightning protection against winter lightning,” 28th Int. Conf. Lightning Protection, Kanazawa, Japan, INV-5, 2006.

[11] M. Miki, “Observation of current and leader development characteristics of winter lightning,” 28th Int. Conf. Lightning Protection, Kanazawa, Japan, INV-3, 2006.

[12] M. Saito, M. Ishii, and N. Itamoto, “Observation of VHF sources of lightning discharges in winter,” 14th Int. Conf. on Atmospheric Electricity, Rio de Janeiro, Brazil, 2011.

[13] M. Ishii, M. Saito, J. Hojo, and K. Kami, “Location of charges associated with positive C-G flashes in winter,” Proc. 12th Int. Conf. Atmospheric Electricity, Versailles, France, pp.151-154, 2003.

[14] M. Ishii, and M. Saito, “Lightning electric field characteristics associated with transmission-line faults in winter,” IEEE Trans. Electromagnetic Compatibility, Vol.51, No.3, pp.459–465, 2009.

[15] T. Wu, S. Yoshida, T. Ushio, Z. Kawasaki,Y. Takayanagi,and D. Wang, “Large bipolar lightning discharge events in winter thunderstorms in Japan,” J. Geophys. Res., Vol. 119, Issue 2, pp.555-566, 2014.

[16] M. Saito, M. Ishii, and N. Itamoto, “Influence of geometry of –GC strokes on associated electromagnetic waveforms,” XI SIPDA, Fortaleza, Brazil, Session III-4, 2011.

[17] M. Saito, T. Shindo, T. Miki, M. Ishii, and T. Sonehara, “High current upward lightning hitting Tokyo Skytree observed by high-Speed camera,” 2014 Convention of IEEJ P&E Society, Kyoto, Japan, 2014 (in Japanese).

Masaru Ishii received the B.S.,M.S., and Ph.D. degrees in electrical engineering from the University of Tokyo, Tokyo, Japan, in 1971, 1973, and 1976, respectively. In 1976, he joined the Institute of Industrial Science, the University of Tokyo, where he was a professor during 1992-2013. He became an emeritus professor of the University of Tokyo in 2013 and is now a Senior Executive Researcher at the School of Engineering of the University of Tokyo. He

was a Vice President of IEE of Japan from 2007 to 2008 and is the President of Institute of Electrical Installation Engineers of Japan from 2015. Prof. Ishii is a Fellow of IEEE, Distinguished Member of CIGRE and a Senior Member of IEE of Japan.

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