Electrical characteristics of winter lightning

0021-9169(94)00072-7

Electrical characteristics of winter lightning Yukihiro *Department

Goto*

and

Ken’Ichi

Narita?-

of Electrical Engineering, Tohoku Gakuin tMiyagi Polytechnic College, Tsukidate ( Rrceiwd

in.final

form

19 Ma?

University, Tagajo 987-22. Japan

1994 ; accepterl27

985, Japan

June 1994)

Abstract-To make clear the characteristics of winter lightning flashes. the long term observation of winter lightning on the west coast of Japan has been continued using magnetic links, digital recording systems for the current oscillograms, field mills, still cameras and video camera systems for the images of lightning channels. Of the 66 magnetic links records exceeding 2 kA, 71% were negative in polarity, and 27% were positive. Median peak values for winter flashes were 17 kA for negative flashes and 45 kA for positive flashes. respectively. 145 current waveforms for the winter lightning flashes, which have current amplitudes exceeding 1 kA, have been obtained by the shunt systems and/or the coils system. They show that winter lightning flashes often have a very long duration or continuing current, and sometimes have a very large amplitude exceeding 200 kA in positive flashes. As one example flash on 9 January 1987, the maximum current amplitude was +280 kA, the maximum current derivative 1.O x IO”’As- ‘. the total charge +400 C and the action integral 1.5 x IO’ A%. The winter lightning current waveforms are classified into three types : single stroke flashes, monopolar multiple stroke flashes and bipolar flashes. Moreover, each flash is categorized as positive or negative, single peak or multiple peak. and with or without a contmuing current.

1. INTRODUCTION

ies have been conducted jointly with the Tohoku Electric Power Company (Goto et al., 1983; Goto and Narita 1989, 1992; Narita ef a/., 1989). The observation of lightning currents in winter as in this research has been made in the Hokuriku area by Miyake ef al. (I 992). This report describes aspects of these measuring systems and the electrical characteristics of winter lightning.

In the winter season, many thunderstorms occur on the northwest coast of Honshu Island facing the Sea of Japan. Many observations of winter lightning have been carried out mainly on the Hokuriku coast (Takeuti and Nakano 1983; Brook et al., 1982). It has been reported that the nature of winter thunderstorms is rather anomalous. For example, most lightning strokes to ground a lower positive charge (Takeuti et al., 1978), and winter lightning frequently causes multi-line grounding and strand fusing in the 500 kV overhead grounding wire of power transmission lines (Nakahori et al., 1982). The currents for the definition of lightning threat are based on the tower current measurements of lightning strokes performed primarily by Berger et al. (1975), Garbagnati and Piparo (1982) and Eriksson (1978). However, these lightning current data are mostly for summer lightning. Thus, it is very important to make clear the characteristics of winter lightning for designing protective measures in electric power facilities and equipment. Since the winter of 1976, the long term observation of winter lightning strikes to an isolated tower have been carried out on the Niigata coast located about 240 km northeast of the Hokuriku coast. These stud-

2. OBSERVATION

STATION

AND TOWER

Winter lightning striking an isolated meteorological tower has been observed at Maki which is located about 30 km southwest of Niigata City, near the shore of the Sea of Japan, since the winter of 1976. The tower, about 150 m high, was constructed on a ridge 125 m above sea level and about 400 m from the sea. It is supported by four steel wires in three directions as shown in Fig. 1. Two lightning rods are installed on the tower top, and both rods are connected to the tower structure through resistive current shunts. However, lightning did not strike the lightning rods only; it sometimes struck other exposed portions of the tower top. Moreover, the tower top has a complicated structure as shown in Fig. 2. Thus, lightning 440

450

Y.

Goto and K. Narita

I

1

2

I I I I1111

5

I

I 1I

Ill11

I

I

I I

lo 2P(kzo IO0.200 500

Fig. 3. Per cent cumulative frequency distribution of lightning peak currents obtained by magnetic links.

Fig. 1. Schematic configuration of the meteorological observation tower. Circled crosses show the positions where magnetic links were installed.

Fig. 2. Schematic three-dimensional configuration of the tower top.

currents to the tower can be led to Earth via multiple paths. 3. LIGHTNING PEAK CURRENT BY MAGNETIC LINK It is difficult to measure accurately the total current that flows to ground through the tower for the reason

that the complex tower configuration causes a nonuniform current distribution in the tower and the current flows through many paths to ground. In order to estimate the total current flowing to ground, 50 magnetic links were installed at 23 positions on the tower (as shown in Fig. 1). Two or three links were installed in major positions such as 12 guy wires, three main poles, two lightning rods and one lightning conductor. The peak current was estimated by summing the crest values of the currents flowing through the 14 individual paths of the tower to ground. The current in each path was determined by considering the current distribution on the tower from the records of magnetic links at various positions on the tower. Lightning peak currents exceeding 2 kA have been obtained in 66 records using magnetic links from August 1976 to December 1992. Of the 66 records, 73% were negative in polarity, 27% were positive and the median peak values of winter lightning flashes were 17 kA for negative flashes and 45 kA for positive flashes, respectively. Of course, lightning strikes to the tower were confirmed by other observations, such as digital recording systems, automatic video camera recording systems, and field mill records. The data obtained from magnetic links were infrequent over the 16 year period of this research because winter lightning strikes are infrequent and the exchange of the many links installed in the various positions on the tower is a very difficult task during the winter. This causes multiple records on magnetic links. On the other hand, we have found that lightning of both polarities struck the tower several times in one thunderstorm. Consequently, magnetic links are also recorded with many strokes. As a result, the data obtained by magnetic links may show a maximum peak current among many strokes. Figure 3 is a cumu-

Characteristics of winter lightning lative frequency distribution rents for both polarities.

4. MEASURING SYSTEMS

of lightning

FOR

LIGHTNING

peak cur-

CURRENT

WAVEFORMS

I( Two measuring systems have been prepared to record the waveforms of the lightning current. The positions of two resistive shunts and five Rogowsky type coils are shown in Fig. 2. The first system consists of two co-axial resistive shunts (2 mQ and 4 mn), E/O and O/E converters. optical fiber cables and transient recorders controlled by a computer. One shunt of 2 mQ is prepared for the main lightning rod which is installed on the tower top. Another shunt of 4 mR is prepared for the sub lightning rod installed on the shield ring for protecting the anemometer against a direct strike. The block diagram of this system is shown in Fig. 4. In this system, three transient recorders, which have memory capacities of 32 k, 32 k and 64 k words, respectively, 8 bits per word and 100 ns minimum sampling times, have been used. The first and second recorders are provided for recording each individual stroke in one flash which strikes the main lightning rod and the sub lightning rod, respectively. The memories in the first and second recorders are divided into five parts operated at a sampling time of 100 ns with a minimum period of 0.5 ms ; therefore, five strokes can be stored separately in one flash. The third recorder is prepared for a multiple stroke flash, or when continuing current struck the main lightning

t

Lightmng Rod 1

Trigger Generator

I

I

12.bit & 16KW

lMBX2

Ii

FIN. 4. Block diagram of digital ning current waveform

recording system for hghtusing two shunts.

12-bit & IhKW

I”o~~~~~~~~-~““‘,“;‘~~~“l~1 Fig. 5. Block diagram of digital recording sets of Rogowsky coils.

system using live

rod. This recorder is operated mainly at a sampling time of 2 vs. Captured digital data are transferred to dual 8” floppy disks by a micro-computer. This system has been used in place of the analogue recording system since the winter of 198 I The other system consisted of five Rogowsky type coils as sensors and has been used since the winter of 1982. It has coils. passive R-C integrators (time constant is 75 ms), optical converters. optical fiber cables and transient recorders controlled by a computer. The pick-up coils are installed at five positions on three main poles and two braces beneath the tower top as shown in Fig. 2. These coils are connected in series to obtain the total lightning current to the tower. The block diagram is shown in Fig. 5. In this system, two kinds of transient recorders were used. They had memory capacities of 4 k and 8 k word>. each 8 bits per word, and their minimum sampling times are 50 ns and 1 ps. respectively. In 1986, both recorders were replaced with units having memot-y capacities of 16 k words. 12 bits per word and a minmum sampling times of 1 ps. These new recorder\ were operated with sampling times of I 11sand IO I/S. respectively. In addition to these recorders. the ne\\ recorder having 64 k words is operated with sampling times of IO ~LSfor obtaining a multiple stroke fla\h or continuous current.

5. RESULTS Floppy Dmk Onir

tc=75ms)j

OF

LIGHTNING

CURRENT

MEASC‘REMEYI

To date. I21 current records of winter lightning flashes by the system of resistive shunts and 71 records by the system of Rogowsky coils have been obtained. 145 current waveforms, for which the maximum cur-

Y. Goto and K. Narita

452

Table 1,Polarity and stroke multiplicity of winter lightning flashes Polarity Monopolar flash Bipolar flash

+ _ +---+ Total Percentage (%)

2

14 52 * *

7 6 2 6

3 9 5 4

0 10 0 4

1 9 0 5

66 45.5

21 14.5

21 14.5

14 9.7

15 10.3

exceeded 1 kA, were recorded by the shunts system and/or the coils system and evaluated statistically. It might be that the systems recorded the greater part of winter lightning flashes striking the tower. However, all waveforms could not be obtained, owing to various problems with the measuring instruments, to program errors in the computer, to mistakes in setting the trigger level and to a limitation on recording time. The polarity and stroke multiplicity of winter lightrent amplitude

Nov. 12, 1983

Feb. 9, 1993

21:31:08

22:40:58

Oct. 15, 1986 6:34:18 h t50 % o __ 0.2 0.4 0.6 t (ms) c( -50

.-

-100

1

Number of multiple strokes 3 4 5

1

Fig. 6. Examples of current oscillograms for negative flashes with single stroke obtained by the shunt system.

6

7s

‘I otal

0 2 0 1

0 3 0 2

2s 91 7 22

3 2.1

5 3.4

145 100

ning flashes are shown in Table 1. Typical waveforms are shown in Figs &8 obtained by the shunts system and in Figs 9 and 10 obtained by the coils system. As shown in Table 1 and Figs 610, winter lightning flashes are classified into the following five types with their relative frequency of occurrence (in per cent) negative flash with single stroke (Fig. 6) positive flash with single stroke (Fig. 10) (c) negative flash with multiple strokes (Fig. 9) positive flash with multiple strokes (Fig. 9) (4 (e) bipolar flash with multiple strokes (Fig. 7)

[“d

36% 10% 27% 7% 20%

Although the percentage of negative flashes is about 63% for winter lightning, it is much lower than that for summer lightning (over 90%). On the other hand, the rates of positive and bipolar flashes are 17% and 20%, respectively. These ratios are much higher than those for the summer case. Figure 8 shows the typical current waveforms in the lightning rod installed on the shield ring. Figure 9 shows some examples of current oscillograms for multiple stroke flashes obtained by the coils system. As known from Figs 610, the current waveforms for each stroke are divided into three types; single peak stroke, double peak stroke and multiple peak stroke. The percentage of winter lightning flashes with multiple pulses in the first stroke is 56%. These multiple peak strokes are like those of cloud discharges which have been measured by airplanes (Moreau et al., 1992). The current begins with a small current pulse (some hundreds of Amperes) whose amplitude gradually increases. These pulses are superimposed on a continuing current. Figure 10 shows the current waveforms for three very large amplitude lightning flashes obtained by the coils system. These events could be superbolt lightning

Characteristics

of winter lightning

Y. Goto and K. Narita

454

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F4

Q

Characteristics

of winter lightning

Jan. 17, 1987 11:42:23 Current scale : 5 kA/div Time scale : 5 ms/div

Current scale Time scale

: :

5 kA/div 5 ms/div

O&31:37 Current scale : 5 kA/div Time scale : 5 ms/div

..

1.

Fig. 9. Examples

.’

.,,

of current

,;

oscillograms

..I..

for multiple

.:

stroke flashes obtained

by the coils system.

Y. Goto and K. Narita

456

Oct. 30, 1986

19:38: 5 Current scale : 100 kA/div Time scale : 1 msjdiv .,

..,

.,.,. ,.............. ~...+I&

I. ..I ,..,i j :. ... . . . ...... . . . ... . .....

. . ..

i

i

I

j

i i ,.

i .....

:..

...___~........

.’--..-~:.. Dec. ,4,, 1986 16:40:35 : ..,...............,..................... Y)‘..

.{

‘y.. Jar+ 9, 1987

_. .

7T.T

_

.

:

‘.C&r&t scale

: 100 kA/div

Time scale :

1 msldiv

04:17:28

.

.

Fig. 10. Examples of current oscillograms for superbolt lightning obtained by the coils system.

Characteristics of winter lightning Table 2. The current parameters of superbolt lightning Date 30 October 1986 4 December 1986 9 January 1987

1,

Action Integral [A’s]

Lk.41 e [(‘I

+ 340 + 320 + 280

t330 +I80 +400

3.6 x 10’ 1.5 x 10: 1.5x 10’

(Turmun, 1977). The current parameters for these superbolt lightning events are shown in Table 2. For the observed winter lightning flashes, the maximum peaks of current amplitude appear among successive strokes in multiple stroke flashes. Fortythree flashes are categorized into these types among 9’ multiple stroke flashes (54%). This is a special feature of winter lightning. Occasionally, strokes with very long duration have been recorded. The current flow was sustained for an interval ranging from several milliseconds to several tens of milliseconds.

6.

(3)

(4) (5)

CONCLUSIOi?

(6)

Our measuring system has recorded many current haveforms of winter lightning flashes striking the tower. From the results obtained for lightning currents, the following characteristics of winter lightning currents are emphasized :

(7)

(1) Of the 66 records from magnetic links exceeding 2 kA. 73% were negative in polarity, 27% were positive and the median peak values for winter lightning flashes were 17 kA for negative and 45 kA for positive, respectively. (‘1 Winter lightning flashes are classified into five types : both negative and positive flashes with single stroke. both negative and positive flash

dl/d/ [A s ‘] 3 x 10” 1 x IO’” 1 x 10’”

with multiple strokes, and a bipolar Rashes with multiple strokes. Of 145 flash records, 91 flashes (63%) were negative, 25 flashes (I 7%) were positive and 29 flashes (20%) were bipolar. The relating occurrences of positive and bipolar flashes were much higher than those of summer lightning. Three flashes having an amplitude exceeding 200 kA were recorded. In winter lightning flashes, the maximum peaks oflightningcurrent appeared in successive strokes in about half of the measured multiple stroke flashes (54%). Sometimes. the strokes had kery long duration. The current flow persisted for intervals ranging from several milliseconds to several tens ot milliseconds. The percentage of winter lightning Hashes ha\ ing multiple pulses in the first stroke is 56”G. These multiple peak strokes are like those ot’ cloud discharges measured by airplanes

A~knol,,/ec!yrn7c,r1r.c The present work is part14 supported by a Grant-mAId for general scientific research from the Ministry of Education, Science and Culture of Japan (I 9x7. 1988). The authors wish to express their hearty thanks for support to the associates of the Electricity ‘I‘echnologb Research and Development Center and of the Nuclear Power Department. Tohoku Electric Power Co lnc

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197x

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1983

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Take&i T., Nakano M., Brook M., Raymond D. J. and Krebiel P. Turn-tan B. N.

1992

V

H.

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