untitled 114 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL 53, NO 1, FEBRUARY 2011 Field Analysis of the Occurrence of Distribution Line Faults Caused by Lightning Effects Teru Miyazaki, Member, IEEE, and Shigemitsu Okabe, Member, IEEE Abstract—Electric supply reliability is an issue of wide impor tance to both an information oriented society and electric power companies This paper focuses on lightning effects on distribution lines Field research is now underway in the northern part of.
114 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL 53, NO 1, FEBRUARY 2011 Field Analysis of the Occurrence of Distribution-Line Faults Caused by Lightning Effects Teru Miyazaki, Member, IEEE, and Shigemitsu Okabe, Member, IEEE Abstract—Electric supply reliability is an issue of wide importance to both an information-oriented society and electric power companies This paper focuses on lightning effects on distribution lines Field research is now underway in the northern part of the Kanto Plain in Japan, and voltage and current waveforms in distribution lines due to lightning hits have been observed since 1996 There are now 284 datasets; these include data on 62 direct flashes to lines This research reveals that a distribution line can be protected from direct lightning strokes In some cases, no power follow current was confirmed after a multiple phase flashover, and a statistical analysis was conducted to investigate factors affecting the generation of power follow current These results can serve as a valuable resource to help clarify the mechanisms underlying the production of distribution-line faults caused by lightning effects Index Terms—Direct lightning stroke, distribution line, flashover, indirect lightning stroke, lightning protection, power follow current I INTRODUCTION ISTRIBUTION-line faults can be caused by indirect lightning strokes near distribution lines as well as direct hits to the line because of their low-insulation level [1] Electric power companies have installed lightning-protection devices, such as shielding wires and surge arresters, to reduce the rate of distribution-line faults, and these devices can contribute to higher reliability of the lines than those without the devices [2], [3] Experimental studies have pointed out that distribution lines can be protected from even direct strokes by appropriately arranging surge arresters and shielding wires [4]–[6], due to the function of those devices to prevent forming the lightning impulse flashover and power-frequency arc [3] The line fault in Japanese distribution lines, however, is still mainly caused by lightning strokes, and how lightning affects lines still remains a matter for debate Field studies of lightning-stroke effects, particularly the mechanisms that produce distribution faults, is essential for designing rational lightning protection for distribution lines Given the aforementioned background, the Tokyo Electric Power Company has conducted field research of phenomena accompanying lightning strokes to 6.6 kV distribution systems since 1996; voltages and currents in the distribution lines were D Fig Construction of distribution lines and observation apparatus (a) Configuration of current sensors (b) Measurement points of voltage sensors measured directly, and observed lightning effects were classified as direct or indirect using lightning-activated camera systems [7]–[9] Combining lightning surge waveforms with photographs, provides a comprehensive observation of each lightning flash and its effects In previous study, each observational data was introduced in detail [9] But this paper focuses on the mechanisms that produce the distribution-line fault, and a statistical analysis of observed data was conducted aiming to quantify the factors affecting the production of line faults due to lightning strokes The results clarify the effect of lightningprotection devices, such as ZnO elements on the formation of flashovers or subsequent power follow currents in distribution lines, and can serve as a valuable resource for designing their effective placement on distribution lines II OUTLINE OF OBSERVATION SYSTEM A Observation Sites and Construction of Distribution Lines Manuscript received December 17, 2009; revised June 8, 2010; accepted August 9, 2010 Date of publication December 10, 2010; date of current version February 16, 2011 The authors are with the High Voltage and Insulation Group, R&D Center, Tokyo Electric Power Company, Yokohama 230-8510, Japan (e-mail: miyazaki.teru@tepco.co.jp; okabe.s@tepco.co.jp) Digital Object Identifier 10.1109/TEMC.2010.2068301 Regions experiencing a high ground-flash density were selected as observation sites in the Kanto Plain [7]–[9] Field research has been conducted since 1996 The constructions of the distribution lines are shown in Fig Usually in such regions, shielding wires are installed on the top of most concrete poles But in some areas, they are not installed 0018-9375/$26.00 © 2010 IEEE MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS 115 for observations Shielding wires are connected to grounding electrode via common earthing wire by metal wires Concerning insulation coordination, insulators supporting voltage wires have the highest insulation level in distribution lines because their faults affect a power failure of large area On the other hand, transformers have the lowest level But they include ZnO elements This means transformers are protected from lightning strokes B System Configuration The observation apparatus can simultaneously provide photographs and waveforms for a single lightning event [7]–[9] In both the camera and the waveform measurement instrument, the time is recorded using global positioning system (GPS), which can then be matched with lightning-position and trackingsystem (LPATS) data [10], [11] Camera systems monitor the field sites to capture lightning flashes The cameras are installed on 63 poles in observation sites and designed to take photos automatically by controlling the shutter according to external lightning intensity Two types of sensors have been used to observe lightning surge waveforms: a current-measuring sensor and a voltage-measuring sensor The voltage and current sensors measure insulator voltage and grounding lead current waveforms The measuring sensors are installed on 103 poles in same sites Fig shows an overview of the measurement points of the sensors Frequency bandwidth of the sensors is 250 Hz–250 kHz Fig Relation of the number between observed lightning flashes and subsequent faults (1996–2006) TABLE I LIGHTNING ATTACHMENT POINT TO DISTRIBUTION SYSTEM (A) DISTRIBUTION LINE WITH SHIELDING WIRES (B) DISTRIBUTION LINE WITHOUT SHIELDING WIRES III OBSERVATIONS During 11 years from 1996 to 2006, 284 flashes were photographed (62 direct and 222 indirect), with all the observed events being in summer times Thus, the discussion in this paper is limited to summer lightning The percentage of positive discharges during that period was about 1% A Line Faults by Direct and Indirect Flashes All of the observed events were classified as direct or indirect based on the photographs of the flashes It was noted that direct denotes a lightning hit to any distribution equipment including shielding wires, power lines, reinforced concrete poles, and so on, while a lightning hit to other than the distribution system was regarded as indirect Fig relates the number of direct and indirect flashes and the subsequent line faults during that period, causing overcurrent relays in substations to operate Investigating whether overcurrent relays were operated or not at the time of the observational photographs GPS time, Fig data were collected These results confirm that direct flashes caused 83% of the faults, which suggests that direct hits were the major cause of line faults, while the data also show that 48% of all the direct hits caused faults, whereas only 3% of all the indirect events provided faults In other words, about half of direct flashes did not cause line faults, which means that the lines can be protected from direct hits that were thought would inevitably damage lines The authors focused on this issue, and analyzed observed data from direct hits B Lightning Attachment Point Due to Direct Hits Based on the photographs of direct flashes, the lightning attachment points were estimated and classified into five patterns All results derived are summarized in Table I Lightning to the pole head recorded the highest ratio whether shielding wires were installed or not (see Table (a) and (b)) and the lightning hits to the power line without shielding wires accounted for only 5% A report experimentally showed that lightning is likely to strike bare wires compared to covered wires because the generation of upward leader is suppressed due to insulator [12] This 116 Fig IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL 53, NO 1, FEBRUARY 2011 Direct lightning flash to a pole head without shielding wires result suggests that the lightning striking distance of insulated wires is smaller than that of bare wires In some cases without shielding wires, as shown in Table I(b), metallic caps were equipped at the top of reinforced concrete poles, which might affect the ratio of direct lightning to the power line In these observational sites, the length of the line equipped with shielding wires is 10 times as long as that without shielding wires There is room for further investigation Fig shows an example of photographs of a lightning flash to a pole head without shielding wires C Analysis of Observed Direct Lightning Event Observed surge waveforms due to direct lightning hits, which are considered to be the primary cause of line faults, were validated using the electromagnetic transients program (EMTP) in order to estimate whether or not flashover were generated In our previous paper [8], one example of direct lightning hit to a pole head was selected, and a comparison of the observed and calculated waveforms was conducted Fig shows an example of the observed and calculated insulator voltages, and the details of the analysis model can be seen in the papers [8], [13] The distance between the measurement point and the strike location is 57 m The calculated voltage using the flashover model shows opposite polarity excursions of the waveform starting around μs later when a flashover occurred [see Fig (b)], which can also be confirmed in the observation, as shown in Fig 4(a) [8] In this paper, a waveform was also calculated without simulating flashover generation As shown in Fig (b), the calculated waveform simulated without flashover generation does not shows the opposite polarity excursions, which is consistent with the observed surge waveform, but shows an inversion of polarity in the wavefront when a flashover was generated [7] Fig Comparison between observed and calculated insulator voltage waveforms (a) Observed waveform (b) Calculated waveform 1) Production of Flashover: The production of flashovers was estimated based on the insulator voltage waveform characteristics described previously While a waveform with a rapid reverse of polarity in the wave front suggests that a flashover must have been produced, a waveform with the same polarity means no flashover generation If the transient is measured far away, e.g., at the end of the feeder, the numerous reflections of the traveling wave at the various connections of laterals and transformers are expected to alter the waveform and make its analysis more difficult 2) Production of Power Follow Current: The production of power follow current was also estimated based on the observed insulator voltage waveforms over a time range up to 20 ms, in which one cycle of power-frequency (50 Hz) voltages can be confirmed Voltages remaining zero over that period at two or three phases indicate the production of a power follow current, which will cause an overcurrent relay to operate in a substation B Rate of Flashover IV FIELD EXAMINATION OF LINE FAULTS The production of flashover and power follow current was examined to clarify the mechanisms that cause distribution-line faults based on the observed direct-lightning data A Patterns of Lightning Surge Waveforms Surge voltage waveform characteristics can help estimate whether flashovers and power follow currents are produced Of the observed 62 direct data with cameras, 56 flashes accompanying voltage waveforms were selected as data for the analysis to estimate whether or not a flashover was produced The results are shown in Table II; lightning strokes caused a flashover to occur in 42 (75%) among 56 cases, and the flashover in three phases shows the highest percentage (55%) In the case of flashovers at two phases, flashovers were confirmed at only R- and T-phases, which is probably because voltage of S-phase is suppressed compared with the other phases due to coupling MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS 117 TABLE II NUMBER OF OBSERVED FLASHOVERS DUE TO DIRECT LIGHTNING HITS TABLE III NUMBER OF CONFIRMED POWER FOLLOW CURRENT AFTER FLASHOVERS DUE TO DIRECT LIGHTNING HITS Fig Cumulative frequency distribution of peak current of lightning strokes based on LPATS data for the observed patterns of flashovers (cumulative probability was calculated for each category) lower value than after three phases flashover (67%) Even when power follow currents are generated after three phases flashover (20 cases), in 13 cases current was generated only at two phases In 77% (= 23/30) of the cases that caused flashover to generate at three phases, the arc of the flashover was extinguished at one phase or more Taking all the cases into account, the rate of the generation of power follow currents is a total of 53% The fact that only 53%of the data with flashovers caused the power follow current can provide insight regarding line-fault mechanism and lead to improving the method of calculation of distribution outage rate due to lightning, since multiple phase flashovers are regarded as the fault under present studies, including calculations for line faults due to lightning The operation of overcurrent relays is caused by multiple attachment flashovers and subsequent power follow currents So “multiple attachment flashovers” is defined as “faults” practically effect It is noted that 25% of the direct lightning strokes cause no flashover generation, a remarkable fact given that direct lightning strokes had been thought to inevitably cause flashovers because of the low-insulation level C Rate of Power Follow Currents A power follow current after multiple phase flashovers causes a relay in a substation to operate Productions of a current on 40 cases were estimated, with associated surge waveforms measured up to 20 ms, in which power follow current can be confirmed, out of 42, the set of direct data with flashover is given in Table II Guessing the reason for the different results obtained for the different conductors, S-phase seems to be the most protected of the three phases by electromagnetic coupling with shielding wire because of the shortest length to that The result is shown in Table III No power follow current occurred after flashovers in single phase The reason is probably as follows: most distribution lines in Japan use isolated neutral systems If flashover occurs only in single phase, the power follow current will be naturally extinguished due to a small ground-fault current Neither was a power follow current confirmed after the production of flashovers in two or three phases, and the rate of power follow current after two phases flashover (20%), in particular, is a V FACTORS THAT AFFECT ON LINE FAULTS In Section IV, the frequencies of the production of flashover and subsequent power follow current were estimated using associated surge waveforms In some cases, multiple phase flashovers caused no power follow current to generate It is important to note that the production of the flashover and power follow current could be affected by several factors such as shielding wires, surge arresters, impedance of a line, phase angles of power-frequency voltages, and so on Further detailed studies were conducted to evaluate which factors can affect the flashover phenomena and power follow current due to direct strokes to the distribution system A Effect of Lightning Current The amplitude of lightning currents associated with lightning overvoltages can affect the probability of occurrence of line faults Fig indicates the cumulative frequency of currents amplitude estimated by LPATS data of each flashover types Fig indicates the cumulative frequency of power follow currents due to the observed direct lightning hits Types of flashovers were estimated based on associated voltage waveforms In these figures, each set of distributions was plotted by selecting lightning currents associated with particular events in terms of types of 118 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL 53, NO 1, FEBRUARY 2011 Fig Cumulative frequency distribution of peak current of lightning strokes based on LPATS data for the observed patterns of power follow current after flashover at three phases (cumulative probability was calculated for each category) flashover or subsequent power follow current There were no data exceeding 60 kA, and these distributions show the median 25 kA, showing almost no characteristic difference from previous studies [14], [15] In Fig 5, the cases of single-phase flashovers are only six So they should be collected and analyzed to find the correlation lightning currents, peak of transient voltage clearly in future A few remarks should be made concerning uncertainness of LPATS data An experimental study suggests that negative first stroke currents estimated by LPATS tend to be smaller than first peaks of measured values especially for large currents exceeding 40 kA [10], [11] When looking at Figs and 5, the lightning currents amplitudes inferred by LPATS system could have underestimate the actual values A study reported that median variability of lightning currents inferred by the lightning location system (LLS) was estimated to be from 20% to 30% [16] The “20%–30%” uncertainty in peak-current estimates in [16] is for negative subsequent strokes in existing channels The uncertainty can only be larger for first strokes The accuracy of lightning currents estimated by this LLS is estimated to be equivalent to that by the LPATS in our research Thus, it is considered that Figs and can be evaluated with the aforementioned accuracy Fig Relation of the number between observed direct lightning strokes and multiple phase flashovers and line faults C Effect of Phase Angle of Power-Frequency Voltage Phase angles of the power-frequency voltage when the flashover occurred was estimated based on the observed surge waveforms due to direct lightning hits The phase angles were confirmed in the waveforms of 52 events of direct lightning, and Fig shows the phase angles of the line voltage when two or three phase flashovers were generated, showing almost no characteristic having effects on the production of flashovers Fig also shows the phase angles of line voltage when the subsequent power follow current was generated after flashovers at three phases When the power follow currents were generated at two phases, most angles concentrate around the maximum value of the power-frequency voltage [see Fig 9(a)] These facts suggest that the phase angle of power-frequency voltage can have an effect on the production of the power follow current, which results in distribution-line faults In the case of the power follow current at three phases, the phase angles are dispersed and no relation between the angle and the power follow current was confirmed, as shown in Fig 9(b) B Effect of Shielding Wires D Effect of Impedance Between a Substation and Strike Location Fig relates the number of direct lightning strokes, subsequent multiple phase flashovers and line faults to the presence or absence of the shielding wire on the struck points When the shielding wire was installed above the lines, the proportion of the events of the multiple phase flashovers is 56% (= 22/39), which is low compared to the value of 82% (= 14/17) in the case with no shielding wire on the struck point On the other hand, the proportion of line faults after multiple phase flashovers in the case with shielding wire is 59% (= 13/22), which is almost the same as the value of 57% (= 8/14) in the case with no shielding wire These suggest that the presence of shielding wires on the struck point plays a more important role in the production of the multiple phase flashovers than that of the subsequent power follow current The impedance of the lines between a substation and attachment point was investigated because that can affect the shortcircuit current during the duration of the power follow current The impedance was calculated by cable length from a substation to struck point using impedance value per m shown in specifications Fig 10 shows the cumulative frequency distribution of the impedance in the cases with or without flashover, and the impedance was calculated on the basis of 10 MVA The shape of both distributions was roughly the same Fig 11 shows the cumulative frequency distribution of impedance to the patterns of the power follow current after flashovers at three phases, and each shows the same distribution The correlation between the impedance and the production of the flashover or power follow current is estimated to be weak in the lines MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS Fig Phase angles of line voltage when flashover was generated (data were plotted for each category) (a) Flashover at two phases (b) Flashover at three phases 119 Fig Phase angles of line voltage when power follow current was generated after three phase flashovers (data were plotted for each category) (a) Power follow current at two phases (b) Power follow current at three phases E Effect of the Distance Between ZnO Elements and the Strike Location Next analysis was the distance between a pole with ZnO elements, such as a surge arrestor and a struck point Fig 12 shows the cumulative frequency distribution of the distance between the ZnO elements and the struck point The cases without flashovers represent lower distances than those with flashovers If looking at the 50% value, it is 64 m in the case of flashover while 30 m without flashover Fig 13 shows the cumulative frequency distribution of the distance to the patterns of the subsequent power follow current after the production of flashover at three phases The distances show lower values in case of no power follow current compared to cases with the production of the power follow current, and the distances in the occurrence of the current at three phases show higher value than those at two phases These results reveal that the distance between the ZnO elements and the strike point plays a decisive role in protecting the lines against direct lightning strokes, since it affects both the formation of flashover and power follow current But the measured distance in Figs 11 and 12 seems to include expected errors, 10 m at the most, by measurement from two or more directions F Evaluation of the Results It may be desirable to briefly review the results In Section V, several factors, such as the effects of ZnO elements were evalu- Fig 10 Cumulative frequency distribution of the impedance of the lines between a substation and a struck point in the cases with or without flashover (cumulative probability was calculated for each category) ated for their potential impact on the production of the flashover and power follow current due to direct lightning strokes The results are summarized in Table IV 1) Generation of Flashover: The observation confirms two factors that effect the production of flashovers: the presence or absence of shielding wires on the struck pole, and the distance between the ZnO elements and the struck point 2) Generation of Power Follow Current: The distance between the ZnO elements and the strike point also affect the formation of the power follow current as well as flashover, while the effect of the phase angle of power-frequency voltage was 120 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL 53, NO 1, FEBRUARY 2011 TABLE IV ESTIMATION OF FACTORS THAT AFFECT THE PRODUTION OF FLASHOVER AND POWER FOLLOW CURRENT Fig 11 Cumulative frequency distribution of the impedance of the lines between a substation and a struck point to the patterns of the power follow current after flashovers at three phases (cumulative probability was calculated for each category) Fig 12 Cumulative frequency distribution of the distance between ZnO elements and a struck point in the cases with or without flashover (cumulative probability was calculated for each category) Fig 13 Cumulative frequency distribution of the distance between ZnO elements and a struck point to the patterns of the power follow current after flashovers at three phases (cumulative probability was calculated for each category) confirmed in the production of power follow current only at two phases after three phases flashovers VI LIGHTNING-PROTECTION DEVICES IN DISTRIBUTION SYSTEMS The analysis of the observation indicates that ZnO elements have a function to prevent forming flashovers and subsequent power follow currents at poles even without ZnO elements Let us devote a little more space to consider it from viewpoint of the traveling of surge waveforms A lightning surge travels through common earth wire (or shielding wire) from the injection point to the closest ZnO elements, and the current flows into grounding electrode Then, the insulator voltages of a struck pole will be suppressed after the reflection of the surge voltage travels back to the struck pole Thus, the effects of ZnO elements of another pole reaches the struck pole with time delay, and this might be one of the causes why the distance between ZnO elements and a struck point affects the formation of the power follow current as well as the flashover Schoene et al [17] obtained experimental data from test with rocket-triggered lightning currents, and estimated that arresters reduce surge voltages against a direct lightning stroke with time delay On the other hand, shielding wires have an effect on the generation of only flashover The explanation for this is probably that the effect of shielding wires on the reduction of insulator voltages can be obtained immediately after lightning hit to a pole because of the diversion into the wires Let us, for the moment, consider other factors The results also confirmed no effect of the lightning current or the impedance between a substation and a struck point With regard to the production of the power follow current, the correlation of phase angles of power-frequency voltage was confirmed only after the case of the production of the power follow current at two phases after the three phases flashovers The facts indicate that the lightning-protection devices, such as shielding wires and ZnO elements play a more decisive role against lightning stokes compared to the effect of other factors in the lines when considering distribution systems with a number of lightningprotection devices But excessive arresters (e g., on every pole) not always protect distribution lines A consideration has to be given to the arrester failures, which will cause more serious outage rate of lines than ordinary flashovers [18] Regarding the effect of arrestors in the observed areas, the authors also confirmed that most insulator voltages tend to concentrate less than 36 kV, which is the limit voltage of ZnO elements in a transformer [8] These results indicate that surge voltages not grow in proportion to the lightning-stroke current, and which can be one of the causes why lightning currents seem to have no effect on the formation of line faults as explained in this MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS section It is noted that a correlation between insulator voltages and lightning-stroke currents estimated by LPATS was investigated in our previous study [9], and the analysis also confirmed that the distance between a ZnO elements and a strike location affects surge voltages due to lightning strokes VII CONCLUSION This paper mainly focuses on the observed direct lightning events to the lines, and analyses were conducted to clarify which factors can affect the production of the flashover and power follow current due to direct strokes The results mainly confirm two factors that affect the production of flashovers: the presence or absence of shielding wires on the struck point, and the distance between ZnO elements and a struck point The distance between the ZnO elements and the strike location is estimated to have affected the generation of the power follow current as well as flashover, and the effect of the phase angle of power-frequency voltage was confirmed in the generation of power follow current only at two phases after three phases flashovers, while no relation between the phase angle and the power follow current was confirmed in the other cases It should be concluded, from what has been said previously, that several factors can affect the generation of line faults and lightning-protection devices, such as surge arrestors and shielding wires are effective for the protection of distribution lines against direct lightning strokes In future, the authors will work on designing rational lightning protection for distribution lines by analyzing more observation data and improving a method for the calculation of distributionline faults VIII ACKNOWLEDGMENT The authors would like to thank S Amemiya of Tokyo Electric Corporation, who extended support and cooperation to this study They are also grateful for the support and cooperation of related departments, such as the distribution department of their company REFERENCES [1] P D Kannu and M J Thomas, “Lightning induced voltages on multiconductor power distribution line,” Inst Elect Eng (IEE) Proc Gener Transm Distrib., vol 152, no 6, 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distribution linesdischarge characteristics of open wire and insulated wire,” Trans Inst Elect Eng., Jpn, vol 115-B, no 12, pp 1508–1514, Dec 1996 [13] S Sekioka, “Lightning-surge analysis model of reinforced concrete pole and grounding lead conductor in distribution line,” presented at the Int Workshop High Voltage Eng., Sapporo, Japan, 2004 [14] R B Anderson and A J Eriksson, “Lightning parameters for engineering Application,” Int Conf on Large Electric High-Tension Systems (CIGRE) Electra, no 69, pp 65–102, Mar 1980 [15] K Berger, R B Anderson, and H Kroeninger, “Parameters of Lightning flashes,” Int Conf on Large Electric High-Tension Systems (CIGRE) Electra, vol 41, pp 23–27, Jul., 1975 [16] K L Cummins, M J Murphy, E A Bardo, W L Hiscox, R B Pyle, and A E Piper, “A combined TOA/MDF technology upgrade of the U S national lightning detection network,” J Geophysical Research, vol 103, no D8, pp 9035–9044 [17] J Schoene, M Uman, V Rakov, A Mata, C Mata, K Rambo, J Jerauld, D Jordan, and G Schnetzer, “Direct lightning strikes to test power distribution lines—Part I: Experiment and overall results,” IEEE Trans Power Del., vol 22, no 4, pp 2236–2244, Oct 2007 [18] T E McDermot, T A Short, and J G Anderson, “Lightning protection of distribution lines,” IEEE Trans Power Del., vol 9, no 1, pp 138–152, Jan 1994 Teru Miyazaki (M’07) received the B.Eng and M.Eng degrees in electrical engineering from the University of Electro-Communications, Tokyo, Japan, in 1995 and 1997, respectively He received the Doctorate degree from the Shonan Institute of Technology, Kanagawa, Japan, in 2008 In 1997, he joined Tokyo Electric Power Company, Tokyo, Japan, where he is currently a member of the High Voltage & Insulation Group, the R & D center His main research interest includes the lightning protection design of a power distribution line Shigemitsu Okabe (M’98) received the B.Eng., M.Eng., and Dr degrees in electrical engineering from the University of Tokyo, Tokyo, Japan, in 1981, 1983, and 1986, respectively He has been with Tokyo Electric Power Company since 1986, where he is currently the Group Manager of the High Voltage & Insulation Group, the R & D Center In 1992, he was a visiting scientist at the Technical University of Munich He has also been a Guest Professor at the Doshisha University since 2005, at the Nagoya University since 2006, and a Visiting Lecturer at the Tokyo University He is a Secretary/Member at several WG/MT in CIGRE and IEC Dr Okabe is an Associate Editor of the IEEE Dielectrics and Electrical Insulation ... OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS 117 TABLE II NUMBER OF OBSERVED FLASHOVERS DUE TO DIRECT LIGHTNING HITS TABLE III NUMBER OF CONFIRMED... why lightning currents seem to have no effect on the formation of line faults as explained in this MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING. .. is estimated to be weak in the lines MIYAZAKI AND OKABE: FIELD ANALYSIS OF THE OCCURRENCE OF DISTRIBUTION-LINE FAULTS CAUSED BY LIGHTNING EFFECTS Fig Phase angles of line voltage when flashover