· nie author is deeply indebted to my supervisors professor W. Janischewskyj and professor AM....
142
CUJZRENTS AND FIELDS OF CN TOWER MüLTISTROKE LIGHTNING FLASHES Mohamed Abdel-Rahman A thesis sabmitted in eonformity with the reqairements for the degree of Master of Appüed Science Graduate Department of Electricai and Cornputer Engineering University of Toronto @Copyright by Mohuned Abdef-Rahman, 1998
Transcript of · nie author is deeply indebted to my supervisors professor W. Janischewskyj and professor AM....
CUJZRENTS AND FIELDS OF CN TOWER
MüLTISTROKE LIGHTNING FLASHES
Mohamed Abdel-Rahman
A thesis sabmitted in eonformity with the reqairements
for the degree of Master of Appüed Science
Graduate Department of Electricai and Cornputer Engineering
University of Toronto
@Copyright by Mohuned Abdef-Rahman, 1998
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CURRENTS AND FIELDS OF CN TOWER MULTISTROKl3
LIGHTNING FLASHES
Mohamed Abdel-Rahman
Master of Applied Science
Graduate Department of Electrid and Cornputer Engineering
University of Toronto
January 1998
ABSTRACT
The CN Tower in Toronto provides an excellent sight for the study of the lightmng
phenornenon. Simultaneous meaSuTements of CUrrents and fields to the CN Tower have
been perfonned since 1991.
One of the most interesthg features of lightning is the multiplicity ofstrokes within
many lightning flashes, which means that the lightning flash codd contain a =ber of
successive discharges. Studying this phenornenon is of great importance as it should be
taken into account when designhg protection meam for power systems and for EMC
purposes. In this thesis both ~urrents and magnetic fields of dtistroke Iightnhg events
to the CN Tower are studied in detail in order to o b t . statistical idonnation about
subsequent strokes in relation to the fmt stroke of a flash. The thesis provides statistics
for each year for both currait and magnetic field wavefiont parameters. Besides, a subset
of the whole data is studied that contains oniy field and current files which correlate to
each other. In this case both m e n t and field measuring systems were successfbl to
ûigger simultaneously and capture the current and the acmmpanying field waves.
n i e author is deeply indebted to my supervisors professor W. Janischewskyj and
professor AM. Hussein for their continuous guidance, patience and encouragement
throughout the course of this work. 1 would like especially to express my gratitude to
professor W. Janischewskyj for his great help d u ~ g the writing of this thesis. The many
comments of Professor Hussein on the £inal version of tfüs thesis have been taken with
appreciation.
n i e financial support provideci by both professor W. JanischewskyJ and professor
AM. Hussein is greatiy appreciated.
1 would like also to take this opportunity to thank Dr. F. Rachidi for his help and
the discussions we had during criticai moments. Professor J.S. Chang's suggestions
during the preparation of this thesis have been quite helpfùi. Finally, 1 would iike to thank
Mr. S. El-Helou, Mr. G. m i e and Mr. M. Wiacek for aii their help and 1 wish them ali
the best in their endeavors.
LIST OF CONTENTS
. . ABSTRACT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . II
. . . ACKNO WLEDGMENTS ,. . . . . . . . . . . . . . .. * . . ,.. LIST OF CONTENTS .............................................................................. iv
LIST OF FIGURES.. . . . . . . . . . . . . . . . . . . . . . ,., . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . .. .. . . . .. . . . . . .ix
LIST OF TABLES ............................................................................... .... xx
CHAPTER ONE
1 .O INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 M o u s Work about CN Tower Multistroke Flashes .......... .............. 3
CHAPTERTWO
2 .O OVERVIEW.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -. . . . .... . - . . - ...... . . . -.. . - . . . . . . ... . . . . a . . . . . . . . . 8
2.1 Physical Background.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Definition o f Lightning ............................................................ . . . . 8
2.1.2 ûrigin of Charged Clouds. .. .. .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . - - - . . . . . . . . . . 9 * . . 2.1 -2.1 Preapitation theory.. ........ ...... .. .. . ... ... ... . ... . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 9
2.1.2.2 Convection theory.. .. . . . . . . . . ... . .. . . . . .. . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1 .3 Classification of Lightning . . . . . . . . . . . -. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0
2.1 -4 Muhiplicity of Strokes within a Flash.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 Practical Importance of the Study of Multistroke Flashes.. .. .. .. .......... 12
2.2.1 Lignhtning Protection of Power Lines. ... .. . .. . . . . . .. . . -. . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Testing of Surge Arresters .............................................................................. 14
2.2.3 CN Tower Multistroke Flashes .................... .... ................................................ 15
2.2.4 Other Flash Parameters .................................................................................. 16
2.2.4.1 Flash duration .............................................................................................. 16
2.2 .4.2 Interstroke interval. ............. ., ........................................................................ 17
..................... 2.2.5 Cornparison of CN Tower Observations with World Wide Da ta.. 18
........................... 2.3 Interfience in Current and Field Exprimental Data 19
2.3.1 Interference in Cwents .................................................................................... 19
2.3 -2 Inîerfierence in Fields ........................................................................................ 20
...................... 2.4 Noise Removal nom Current Derivative Waveforms 21
2.4.1 Removal of M: componmt .............................................................................. 21
2.4.2 Interference due to Reflections at the CN Tower .............. ... ................... 21
CHAPTER THREE
EQUIPMENT AND TESTING ........................................................ -29
Introduction ...................................................................................... 29
3.2 The Current Measuring System ......................................................... 30
3 .2.1 OId Coi1 ................................................ . . ..... . . ............................ 31
3.2.1.1 Technical specifications of the old coi1 ................ .... ....................................... 31
3 .2.1 -2 Connection between Old Rogowski Cod and Real Time Digitizer .................. 32
3.2.2 New Rogowski Coi1 ....................................................................................... 32
3 22.1 Technical spedications of the new coii ..................................................... 32
3.2.3 Fiber Li ........................................................................................................ 34
3.2.3.1 Technical specifïcations ................................................................................. 35
3.2.4 Recordhg Equipment ....................................................................................... 35
3 .2.4.1 Technical spdcations ................................................................................. -35
3 -3 Field Measuring System .................................................................... 36
3.3.1 Magnetic Field Sensors ......................~............................................................. 36
3.3 .2.1 Technical specincations of the magnetic field senson .................................... 36
3.3 -3 Electric Field Sensor ........................................................................................ 37
3.3 .3.1 Technical speci£iCâtions ................................................................................. -37
3.4 Equipment on h a n fiom Switzerland ............................................. -37
3.4.1 Technical Spedication of Electric and Magnetic Field Sensors ......................... 38
. 3.4.1 1 Magnetic field ............................................................................................... -38
3.4.1 -2 Electric field .................................................................................................. 38
3.4.1 -3 Main features of the opticai link ..................................................................... 38
3 . 4.3 Reco rding S y stem ........................................................................................... -38
3.4.3.1 Technical specifications ................................................................................. 38
3.5 Video Monitoring System .................................................................. 39
3 .5 . 1 The Importance of the Visual Monitoring of the CN Tower ............................. -39
3 . 5. 2 S hortcornings of the Video Camera ................................................................. -40
........................................................................... 3.6 High Speed Camera 40
3.7 Wculties with the Overall System ................................................... 41
CHAPTER FOUR
4.0 STUDY OF CURRENTS ................................................................ 45
4.1 Current Peak. ............... .. .................................... 46
4.2 Maximum Current Steepness .......................................................... 48
4.3 Risetirne ........................................................................................... 50
4.4 Cornrnents on Waveshape Parameters ............................................... 51
4.5 Effect of hterstroke Intervals .......................................................... 51
CHAPTERFIVE
5.0 STUDY OF FIELDS ......................................................................... 63
5.1 Recordhg Process ............................................................................. 64
5.2 Field Peak Ratio ............................................................................ 66
5.3 Maximum Steepness of Magnetic Field .................... .. ...................... 68
5.4 Risetirne of Magnetic Field ....................... .. ................................... 70
5 . 5 Relation Between Interstroke Intend and Field Peak ........................ 71
CHAPTER SIX
6.0 CURRENTS AND FIELDS ............................................................... 86
6.1 Currents ............................................................................................. 86
6.1.1 Cwrent Peak ..................................................................................................... 87
............................................................................. 6.1.2 Maximum Cu- Steepness 87
6.1.3 Risetirne ......................................................................................................... 3 8
6.2 Fields ................................................................................................. 88
6.2.1 Field Peak ......................... , ............................................................................. -88
6.2.2 MAxhum Field Steepness ............................................................................... ..89
6.2.3 Field Risetime ............................ .,. ....... ,. ............... -89
6.3 Comments .......................................................................................... 89
CHAPTER SEVEN
7 . O CONCLUSIONS AND RECOMMEND ATIONS .............. 97
REFERENCES ............. .. .................................................................. 100
APPENDIX A ..................................................................................... 104
APPENDIX C ....................................................................................... 108
APPENDIX D ................................................................................... 109
APPENDIX E ................... .. .................................................................. 111
APPENDIX F ....................................................................................... 113
APPENDIX G ....................................................................................... 115
APPENDIX H ....................................................................................... 117
APPENDIX 1 ......................................................................................... 119
LIST OF FIGURES
Fig . (1.1) Current wowfonns of the fw strokes in negafive p m d
flash (GO 7C5225) qizrred on Juiy7.199 1 ................... ... .................. 5
F ig. (l. 24) Linear correlùtion &pen&nce of marhum steepness
.............................................................. .............. on peak mrrent .. 6
F ig. (1.24) Lineur correlalion &pendence of measured maxtmaxtmum
................................... m e n t on measwedpeak m e n t ai Flw& -6
F i , (1.3) Regress~~on lines for the rutio I d l with fitst
.......................................................... sfroke current Il as purameter 7
Figo (2.1) Categon'zation of the jour typs of fighmmg
according !O Berger.. .............. ... ....................................................... -23
Fig . (2.2) A &uwing illustratmg some of the vananous processes co@ng
........................................ a negorive cfmd to g r o d lightning- -23
Fzg . (2.3) Disfrtsfrthtion of overvolage peuk along a tranmiss~~on fine ................. 24
F ig. (2- 4) Temperaire elevaton for a wge arrester utuier singe
.................................................................. and quintuples îüscharges -24
Fig(2.5) Cumulutive distribution of stroke ntultipficity ...... ....... .......... ....... . J
Fi g. (2.6) Frequency distribution of stroke multiplicity ........................................ 25
........................................... Fig . (2.7) Cumulative distnstnbution of flmh duration 26
Fig . (2- 8) Cumufative distrrstrrbution of interstroke tirne ......................................... -26
Fig (2.9) a-Current &rivative file fl085443.4Oj captwed
on h e 14 1996 at 11: 44: O3p . nt . and (6) its integration ............ .. .. 27
Fig. (2 10) Fieufile flO86WO. 753 qhrred on
............................................................ June 10, 1996 at 11:44:03p.m 27
Fig. (2. 11) a-Cwent &rivative file f1085443.4o-i
q t w e d on J e Iû, 1996 at II:44:03p.m. ond itr integrdon
0- Afer removing the LlC offset ................................ d
Fig. (2.12) CN Twer sirnpIe m d s for lighming m e n t propagation. ............ .28
Fig. (3.1) Schematic rlaprmn of the CN Tmer and lighmng meanring
. * .................................................................... and monztor~ng sysfems. -42
Fig. (3.2) Schernotc ciragram of the new m e n t m e m n g system.. .................. 43
Fig. (3.3) Cornrpmion of H+ record cqtured simultaneousiy by
........................................................... U of T and Swiss equipmen L... 44
Fig. (3.4) A picture oja lighming to the CN Tower q t w e d on
July 30,1996 at Ol:4Z:f 9 by the Mdeo cmnera Iocated on
the roof of Rosebrough buiküïng shows the glme accompanyrpanyrng
the Iightning discharge that obscures idenrifution of rhe
c h n e l &tails.. ............................................................................... 4 4
Fig. (4. I ) Current derivolive fle flO85443.283 captltured on
JunelO, 1996 at 23:45:03 .................................................................... 52
Fzg. (4.2) Current w a v e f m resuiting from tnteption of cument
&rivative f le f 1085443.283.. .......... .. ............................................ -53 Fig. (4.3" Cumulative distrr'bution of aiment peuk ratios for 1996.. .................... .53
Fzg. (4.4) Cumulative dktribution of m e n t peak ratios for 1992. .................... .53
.................... Fig. (4.5) CurmîhlatMe cllistribution of m e n t peoR ratios for 1994.. .54
Fig. (4.6) Cumulafive disfnsfnbution of mrrent peak ratratras for 1993.. ............-...... 54
Fis (4.7) Cumhtiw dlstnstnbution of m e n t peoA ratios for 1992. .................... .54
Fig. (4.8) Cumkùtive di*-bution of m e n t peoR ratios for 1991. .................... .54
Fig(4. 9) Cumulative dz~n?*bution of m e n t peak rcrtiosfr all
rnuiristroke frtzshesPorn 1991 to 1996. ............................................. 55
Fig. (4.10) Cumuhive di~n?~bution oft ntaxlntaxlmum cuvent
........................... ........................ &rivative ratios for 1996 ....... 55
Fig. (4. II) CumuGative distrrstrrbution of mrmrmmum m e n t
................................. derivative ratios for 19 W...
Fig. (4.12) Cumu14trive disiribution of rnmimum mrrent ciemative rafios for
......... 1994 .......................,.........................+................................ 56
Fig. (4.13) CumuIatzve distribution of rnmnmnmum m e n t
demafive ratios for 1993.. ............................... ... ................................ 56
Fig. (4.14) Cumiative distribution of ratios for mrmrmmznn
cuvent derivative 1992.. ....................... ,.., ....................................... .56
Fig. (4.15) Cumuï"Ne distnibution of maximum v e n t
derivutive ratios for 1991. ...................................
Fig(4.16) Frequency cii'stribution of nu2xirmrm mnent derivative ratios for
ail multistroke flushes from 199 1 to 1996.. ................................ . 5 7
Fig. (4. i 7) CumuIàtive distrr*bution of m e n t risetirne
................................................................................... ratr'os for 1996 57
Fig. (4.18) Cumulative distnstnbution of m e n t risetirne
................................................................................. ratios for 1995 57
Fig. (4.19) Cumulafiive distribution of current risetime
ratius for 1994. .................................... ... ....................................... 58
Fig. (4.20) CmuI(ative &stnstnbution of m e n t risetime
ratios for 1993. ..................................... .*
Fzg. (4.21) Cumulative distn-bution of m e n t risetirne
ratios for 1992. ............................................................................. .58
Fig. (4.2 1) C m W e distribution of m e n t risetirne
ratios for 1992. .............................................................. d
Fig. (4.22) Cumuiàtive distn*bution of current risetirne
ratios for 1991. ................................................................................. .58
Fig(J.23) Cumufiztive dm-bution of m e n t risetirne ratios for al1
mdtistroke j h h e s from 1991 to 1996 .............................................. -59
fig. (4.24) Ratio of m e n t peaR of subsequent io first strokes us
a funcion of time forjhh ho843165.flh.. ......................... d
fig.(4.25) Ratio of mrrent peoR of subsequent to first strokes as
a function of tirne forjlàdi fIûü5443lfUr. ...................................... -60
fig. (4.261 Ralio of m e n t peaR of subsequent to first sfrokes as
a fmrction of time forflaFh h146I916.jX ........................................... 60
fig. (4.2 7) Ratio of m e n t peak of subsequent to first strokes as
amction of time forflashhf22314iO.fl'. .......................................... -61
fig. (4.28) Ran'o of m e n t peak of subsequent tofist strokes as
................. a function of time forjhsh E2069 789.flh .................. .. -61
fig (4.29) Ratio of nrrrentpeak of absequent to first strokes a
s ahc t ion of tinte for flash E l 062 725.JJi .................. .. ............... -62
Fi g. (5 . la) A vertical lightning-h to the CN T i caplirred by the
video m e r a on Septemkr 11.1996~t 12:03:16am ........................ 72
Fig . (5.1Ib) Geometricai parameters wed in caImlating reetm stroke fie lct ...... -72
Fig . (5.2) Six strokesfield wrrwjrms captured on Mùy 20. 1996 at
......... ............ 7.31.46p.n.. the fifrh stroke shows apemiiatpeak .. 73
F ig. (5.3) Five strokes w e n t derivative w m f o m qtured on May
.................... 20. 1996 at 7:23: 09 p.m.. aif the* strokes me similm 73
Fig . (5.4) Azimuthai magnetic fie ldfie f lO86ûïO.753 q tu ted on
JunelO. 1997 at 1 1:45:03 ....................................................... 7 4
F ig. (5.5) Cumu2àtive distribution offe&i peak ratios for l 996 .......................... 74
Fig . (5.6) Cumulative &stnstnbutlion offeM peak ratios for 1995 .......................... 74
Fig . (5.7) Cumuintiw ca'strt-bution offield peaR ratios for 1994 .......................... 75
F ig. (5.8) ~tlmuiative distribution of peak peak ratios for 1993 .......................... 75
Fig . (5.9) Cumuhtive cirstributzon ofjieid peak rutios for 1992 .......................... -75
Fi, . (5.1 O) CumuZattiw distrrstrrbuiion of field peak ratios for 1991 ...................... .75
Fig . (5.11) CCumulatiw disn*butiott offield peak rutios for al1
multistroRe frccshes from 1991 IO 1996 .............................................. 76
Fig(5.12) Amplitu& spectra of the azimuthui magnetic fieu of mica2
for h e e observation points / 30 poin~sper &cade) ...................... .76
Fig. (5.13) Cumulative distribution offieki maxlmaxlrmmt
steepness ratios for 1996 ................................................................. 77
Fig. /5.14) Ctmttlhtive distribution o f f e u max~*mtnn
steepnes ratios for 1995.. ................................................................ 7 7
Fig. (5. 15) Cumulatrve dstrrstrrbution offield maximum
steepness ratios for 1994. ............................................................ 7 7
Fzg. ( X i 6 CrmtuIative distnstnbution offelid muimum
steepness ratios for 1993. ................................................................ 7 7
Fig. ('5.1 7) CumuIàtive distrhtzon offield m a - m m
................................................................. steepness ratios for 1992.. 78
Fig. (5.18) Curtlhriw dstnstnhtion offeld mmQXJmm
.................................................................. steepness ratios for 199 1. 78
Fig(5. 19) Cumuiative distributiun offieeld mmC#lmum steepness ratios for ail
multistroke flushes front 1991 to 1996. ............................................ 7 8
Fig. (5.20) adecond &rivative of first stroke of m e n t &rivative
fie fîO85443.283 q tured on Jme 14 1996 at 1l:4:O3 p.m.
&Second derivative of third strok of mrrent demative
........... filef1085443.283 capîured on Jme 10, 1996 at 11:4:03 p.m 79
Fig. ('3.21) Cumiotive distnsfnbution ofFelà risetirne
ratios for 1996 ................................................................................. 80
Fig.(5.22) Ctmtulative di~rr~bution oflieu risetirne
....................................... ratios for 1995 ....W
Fzg. 623) cumuiafive &strrstrrbution offieu risetirne
.................................................................................... ratios for 1994 80
Fzg. (3.24) Cumuhtive distribution offieu risetirne
rutias for 1993. .................... .... .................................................. -80
Fig. (5.25) CunnrJatnte ri but ion offieu rise fime
ratios for 1992. ................................................................................. 81
Fig. (5.26) C m W e dismktion offieu risetinte
ratios for 1991. .................................................................................. 81
Fig(S.2 7) Cumulative rüstnstnbution offieid risetirne rutios for all
multistroke jhshes from 1991 to 1996. ................... .... ............... 81
fig. (5.28) Ratio ofPeIdpeak of subsequent to first strokes as a
....................... function of time firflaFh h1461959.flh.. .4
fig. (5.29) W o offielpeak of subsepent to first strokes as a
fimction of tinte for@h E2070306.jX.. ........................................ -82
fig. (5.30) Rutio offielapeak of &sequent to first strokes as
......... a function of tirne forfrczsirf2232 I 06.m.. .. Fig.(5.31) Ratio offleIdpeak of subsequent tofirst strokes as
a function of time for flash fIûû60 7O.jlh.. .......... A
fig.6 32) Ratio offieIdpeak of subsequent to first stroks as
a fMction of tirne forflarh E2085719.fllr.. ........................................ 84
fg (5.33) W o of m e n t peak of subsequent to first strokes as
a jhction of tirne forjhsh Jl673157.flk.. ....... ... ......................... 84
Fig. fi 34) Regremon Iznes behveen a) Field peuks with interstroke
............................ tintes. b) Current peaks with interstroke tintes. ..... 85
Fig (6.1) Freîpency (a) and cumz~lclliiw (bl distribution of peaR
m e n i ratio of subsequent tojirst strokes (foral
........................................................... of 52 strokes in I 7jhshes}. ...Pl
Fig (6.2) Freqency (a) and MrntUI;ative (o) drstnstnibution
of maxîmum m e n t s t e e p s ratio of dsequent
............................. 10 first stroks (total of 52 strokes in I7flmhes).. -92
Fzg (6.3) Frepency (a) and CrrntzIIative (5) &cüslrbution of
m e n t risetirne ratio of dsequent tofirst stroks
. (total of 52 strokes in I 7flahes). ...................... ... ..A
Fig (6.4) Freîpency (a) and m m W e (b) &stnstnbution
of peukfield rat10 of subsequent to first strokes
( total of 52 strokes in I 7jhhes). ..................................................... .94
Fi' (6.5) Freqirency (a) und cumuJative 0) drstrrstrrbution
of rn~x1~murnfiei-i steepnes ratio of subsequnt
to 3rd strokes ( toral of 52 strokes in I 7Jashes). ........................... ..A
Fig (6.6) Frequency (a) and cumulafive 0) distribution
offiiki risetinte ratio of subsequent to first strokes
( totui of 52 strokes in 1 7 m h e s ) . . ................................................... -96
Fig. (A. 1) Experimentaï Semp ............................. .. .......................................... 1 0 4
Fig. (A.2) Output of Rogowski Coiladits integrato n. ............................... 1 0 5
F ig. (C . i) n e testhg remits supplied by the manufacturer.
the tesring puIse and the caimlations are sh m. ........................... 108
Fig 0.1) Frequency distrrstrrhtion of m e n t peok rutios for 1996 .................... ,109
Fig 0 . 2 ) Fre pency distrrstrrbution of ciment peak ratios for 1995 .................... 109
F ig. (D . 3) Frequency dissbiation of m e n t peok ratiis fur 1994 ................... -109
Fig . (ïl . 4) Frequency disfnsfnbution of cuvent peak ratios for 1993 .................... 109
Fig . (D . 5) Freqirency distnstnbution of m e n t peaA ratios for 1992 .................... 110
F ig. (D. 6) Frequency dïstrrstrr&ution of m e n t peak tananos for 199 1 .................... 110
Fig@ . 7) Frepency distribution of m e n t peak ratios fop ail
ml t i s t t oke~hes from 1991 to 1996 .................... ... ..................... II0
Fig.(E. I ) Frepency dlstristribution of marclmarclmznn m e n t &riYatjYe
ratios for 1996 ................................................................................ 111
Fig . (E . 2) Frepency dlstrr'bution of ~ t ~ # ~ ' m u m m e n t derivative
ratios for 1995 ................................................................................. I I I
Fig . (E . 3) Frequency clstnstnhtiort of mrmmrmmum m e n t &rivative
ratios for 1994 ................................................................................. 111
Fig . (E . 4) Frequency distribution of mîmQXImum cuttent derivative
ratios for 1993 ................. ........ ................................................... 111
Fig . p.5) Frepency distribution of maximum m e n t derivative
ratios for 1992 .......................................................................... 112
Fig . @ . 6) Frequency di'stribution of max~~ntum m e n t &rivative
ratratros for 1991 ................................ .. ....................................... 1 1 2
Fige . 7) CumulCrtive dishlCrtive&ution of maximum m e n t derivative
ratios for a21 muItistrokeflarhesfrom 1991 to 1996 .......................... I I 2
Fig . . 1) Frequency distribution of m e n t nsetime ratios for 1996 ................. 113
Fig . (î? 2) Frequency cii'stribution of m e n t risetinte ratios for 1995 ............... -113
Fig . (l;. . 3) Frequency distribution of m e n t risetirne ratios for 1994 ............... -113
Fig . (E 4) Frequency dishbution of m e n t risetirne ratios for 1993 ................. 113
Fig . 5) Frequency distribution of amen1 nsetime ratios for 1992 ................. 114
Fig . fi 6) Frequency &stnstnbution of cument risetirne ratios for 1991 ................. 114
Fig (F. 7) Frequency aïsiribution of m e n t risetirne rutios foi
uZI mZtistrrokeflarhs from 1991 to 1996 ................... ... ................ 114
F ig. (G . 1) Frequency disfrrsfrrbution offield peak ratios for 1996 ....................... -115
Fig . (G . 2) Frequency dstnaution of fieldpeaC ratios for 1995 .......................... 115
Fig . (G . 3) Frequency &rBbution o f f i e l peak ratios for 1994 .......................... 115
Fig . (Ga 4) Fretpency dlistrr'htion offieMpeak ratios for 1993 .......................... 115
Fig . (G . 5' Frequency mStrbution of field peak ratios for 1992 ....................... -116
Fig . (G . 6) Frequency distribution of field peak ratios for 1991 .......................... 116
Fig(G- 7) Frequency distribution of field peak ratios for all
multistroke jkshesfiom 199 1 to 1996 ........................................... 116
Fig . (H . 1) Frequency distrr*bun'on of mmcrmcrmum PeId steepness
rutios for 1996 .................................................................................. 117
Fi g. (H. 2) Frequency distrrstrrbution of rn~~~~mtmtfield steepnes~
............ ratios for f 995 ............... .... .............................................. .. 117
Fig . (If . 3) Frequency &stnstnhtion of ~ ~ ~ # l ~ m z n n fiehi steepness
ratios for 1994 ............... ....... ....................................................... 117
Fig . (H . 4) Frequency distnstnbution of maxrlaxrlmtntlfieiü sîeepness
rmios for 1993 .................................................................................. 117
Fig. (H.5) Frequency distnstnbution of maxhmfieU steepness
ratios for 1992 ................... ... ....................................................... 118
Fig . .(H. 6) Freqiency distrrstrrbution of m~l~~~mznnfiehi steepness
ratios for 1991 ...................... ... ..................................................... 118
Figw 7) Frequency distribution of mamammtnn fie Id steepness
ratios for all m21Itistroke~esfiom 1991 to 1996 .......................... 118
Fig . fl . 1) Frequency distribution ofjehi risetirne ratios for 1996 ..................... 119
F ig. ('7.2) Frequency distributton of jehi risetirne ratios for 1995 ..................... 119
Fig . (l.. 3) Frequency distribution offield risetirne ratios for 1994 ..................... 119
Fig . (/ 4) Frequency distrrtrrbu~ion of fieM risetinte ratios for 1993 .................... 119 .
Fi' (7.5) Frequenq distrr'bution offield risetirne ratios for 1992 ..................... I20
Fig . (T. 6) Frequency dstniartion offield risetime ratios for 1991 ..................... 120
Figfl . 7) Ftequemy cüstrrstrrbution offieId risetime ratios far dl
muItistrokJrmhes from 1991 to 1996 .............................................. 120
LIST OF TABLES
Tabe (A . 1) Remits of expriment on Mq 2 7. 199 7 .................... ... ....... 1 0 6
Table of resuh of testrng the new c d on Mq 23. 1997
in rhe Lighming Reserach Laboratory .......... .. ................................................ 107
CHAPTER ONE
INTRODUCTION
Lightning is one of the most interesting phenornena that has inspireci humans
throughout dl ages and around the whole worid. Legends and myths about lightning have
dominateci humanity for long tirne.
In the modem age, lightning is still an exciting phenomenon. Liberated from the
myth of the past. Lightning now is known to be a transient m e n t discharge within a
cloud, from one cloud to another, or between a cloud and ground.
In this thesis, an effort is made to statistically study the phenomenon of multiplicity
of strokes in a Iightning flash and to understand the behavior of lightning curmts and their
associated electromagnetic fields.
This effort was based on the data gathered from lightning events to the CN Tower
in Toronto since 1991. Monito~g of lightnhg strikes to the CN Tower started in 1978.
Now, with equipment to measure m t s and radiated fields, Uistalleci in 1991, the CN
Tower sight has become one of the important centers to study the characteristics of
lightning in the world.
In this thesis a study of multiple stroke flashes is carrieci out, as the CN Tower
data was rarely studied in detaii fiom this point of view. The uniqueness of this study is
that it deals with both currents and fields at the same tirne, as the nature of data associated
with CN Tower flashes provides the ability to observe both, m e n t and field,
simultaneously.
A lighbling flash may contain several strokes. Multistroke flashes to the CN
Tower constitute more than 55% of the data. During the last two decades, the number of
strokes within a CN Tower flash was found to range from one to ten Only two ten-
stroke flashes were observed, the first in 1980 and the second in 1992.
The study of multistroke flashes is of practical importance, as lightning protection
equipment should be designed to withstand several successive discharges within a short
duration of time. Transmission line overvoltages couid result fiom a direct hit or fiom
radiated fields due to a nearby l i g b g flash, and couid lead to flashovers. Performance
of sensitive electronic equipment could be dso detrimentdy affêcted by radiated
electromagnetic fields associatecl with lightning discharges.
In this thesis, lightning retum stroke current information coîiected at the tower and
the azimutha1 magnetic field data resulting fkom CN Tower strokes, recorded 2km north
of the tower since 1991, wil be analyzed. Relations between waveshape parameters
(wavefiont steepness, first peak and risetime) of the first stroke and those of the
subsequent strokes will be searched so as to establish statistics which wouid be of practical
importance for the estimation of the threat level due to multistroke flashes. A separate
study will be perfonned to establish the interrelation between the interstroke tirne and the
characteristics of t he subsequent stroke.
The analysis of the current data yields significant results, as it is found that the £irst
aroke is less steep, with longer risetirne and with slightly higher peak than subsequent
stro kes.
The andysis of field data showed that like the ment, the first stroke is generally
less steep and with longer risetmie. However, for subsequent strokes field peak has been
found to be in average larger than that of the nrst stroke.
1.1 Previous Work about CN Tower Mulîistroke Flashes
The interest in the CN Tower muitistroke flashes has arisen a long tirne ago. It
was reported in previous documentation [l] that simüarity in the shape of the fïrst stroke
and the subsequent ones was found in many flashes. That study ody considered data
captureci in the Summer of 199 1. Fig.(l -1) shows m e n t waveforms of four sîrokes in a
negative upward flash.
Regression analysis was made [ l ] to find an interdependence among the different
parameters of wavefront of the aimnt. It was found that for subsequent strokes the
strongest correlation is between the current peak and the m;urimum value of m e n t
derivative, whereas for the first stroke the strongest corretation is between the m e n t
peak and the current fiont steepness, which is defineci as the average steepness between
the 10% and the 90% points on the wavefkont. For subsequent strokes, these findings
wincide with results obtained by triggered lightning where it is reported that in general the
larger the current, the iarger is the cumnt derivative [2]. When we consider that in
triggered lightning the metaiîic conductor acts as a dart leader, then there will be no
stepped leader for the first stroke. Therefore, the first stroke of triggered lightning will
have sirniiar characteristics as the subsequent strokes of a natural flash- As a reSuIt, first
stroke cfiaracîeristics of n a t d lightning cannot be studied from tnggered lightning
obseniations. Linear correlation dependence of maximum steepness on peak curent for
triggered lightning, CN Tower and world wide data are shown in Fig.(l.2a) [l].
Triggered iightning scperiments were carried out in Florida and in Centrai France. Fig.
(1.2b) shows the regression relations as were obtained fiom the Florida tests.
Furhermore, the dependence of the ratio of subsequent stroke curent peak to that of the
first stroke on the interstroke tirne, with the nrst stroke m e n t peak as a parameter, is
developed in [3] and is shown in Fik(1.3). For the risetirne (1% and 90% of the peak,
caiied T-IO), the correlation with wrent peak was not too sisnificant, but it was found
that the sign of correlation of this parameter was different for first and subsequent strokes
[2]. These results indicate that there exists a ciifference between the nature of the first and
subsequent strokes.
It was these findings which stimulated cwiosity to investigate the ciifference in
behavior of lightning during the fïrst stroke and during subsequent ones. A previous
exploratory study showed that subsequent strokes are characterized by their shorter
risetimes, smailer peak current amplitudes and higher maximum steepnesses 131, [4].
From the previous discussion, it is clear that a study in the area of lightning
characteristics exhibited by first and subsequent mkes is needed both for its importance
fiom the practical point of view of direct strokes as well as from the standpoint of induced
voltages. This later aspect is investigated on the basis of fields and currents
simultaneously m m e d for CN Tower flashes.
The thesis has two introductory chapters to give the reader an idea about the CN
Tower project and the measuring systems involved. Chapter two is an overview on the
problem and has the purpose to supply at least minimal information about the physics
behind the lightning discharge. In chapter three the details are provided on the equipment
used to wiiect the CN Town. lightning data. The three are chapters of the thesis are
chapters four, five and six. Chapter four treats multiplicity of strokes in cwent files
captureci by the m e n t measuring equipment, whereas Chapter f i e treats field files
capnired by the field meamring equipment. Chapter six uses the data of currents and
fields that are correIated to each other and analyzes it by the same method to understand
the behavior of lighting m e n t and field for the same event. Chapter seven gives
conclusions and recommendations for M e r research in this direction.
Fig. (I. 1) Cwenr wavefoms of four strokes in a negaliw upwmdjiash (W IC5225)
cqtured on July 7.1991 [ 1 1.
II/di (kfVmi cros. 1 c i a ,
Fig. (1.2-b) Linear correlation dependeence of mearured mamammum m e n t on nteasured
peak aiment ut FIon'da [2]
CHAPTER TWO
2.0 OVERVIEW
In this chapter, a brief description of the phenomenon of lighbmng is highiighted
with more details concerning the multiplicity of strokes withui a lightning flash. To start
with, a physical background is provided to give the physicai basis behind the phenornenon.
A description of the relevant work at the CN Tower and at other places in the world is
provided. A brief discussion about noise removd techniques and rnodeling of the CN
Tower is supplied.
2.1 Physical Background
2.1.1 Definition of Lightning
Lightning is a naîurai phenomenon that has sometimes thought of as representhg
the anger of nature towards man. hie to its disastrous consequences ranging from k i h g
people to darnaging modem day equipment, lightning has attracted the attention of many
researchers with the a h of investigating the phenomenon in order to overwme havoc and
problems resulting from it. The advances in material science and the wide spread use of
plastics and cerarnics with their low shielding effectiveness accompanied by the advances
in equipment employing electronics, which is very sensitive to the sunoundhg
electromagnetic environment, have raised the question of detrimental inauences of
lightning on present-day equipment. The investigations have thus widened to hclude the
dkts of l@&nhg cunent not only on the struck object but on the neighboring ones as
well.
As for the d e t i o n of lightning, Uman [5] mes: " N d LightnUig couid be
defined as a transient, high cun-ent electric discharge in the aîmosphere whose paîh length
is in kilometers".
2.1 -2 Ongin of Charged Clouds
The first question that arises &er knowing that iightning is an electricd discharge
is what produces charged clouds in the first place. Formation of charged clou& couid be
acplained by two theones: the precipitation theory and the convection theory [SI. It
should be boni in mind that there always exists an electrostatic field between the earth and
the ionosphere. Under f'air-w&er conditions the elechic field at the s u r h of the earth
is positive, which means it is directed into the surfâce of the ground. The usual magnitude
is about 100 V/m [6] and it varies with the thne of day since ionosphere activity is affected
by sun rays.
2.1.2.1 Precipitation theory
In this theory, heavy precipitation pamcles f h g d o m under gravity effe*
interact with light particles carried up by upward drafts. hiring the interaction process the
heavy particles acquire negative charges whereas the iight particles become positively
charged. The charging process is said to be due to collisions between hail and ice crystals.
Hence, the charge is separated inside the cloud, and the cloud tums out to be a charged
dipole.
2.1.2.2 Convection theory
In this theory, ionization takes place near the ground Surface due to radioactive
dat ion, cosmic rays, and similar. These charges are accumulateci near the earth surfiice
or regions of varying air conductivity. The ionized particles are carrieci upwards by
convection associateci with thunderstorm.
It is worth saying that neither theory can f U y account for the details in the
formation of the chargeci clouds [SI.
2.1.3 Classification of Lightning
More than half of the lightning discharges take place within the thunder cloud i td f
and are called intradoud discharges, whereas cloud to cloud lightning is less common.
The more important type of lightning, from the point of view of its destructive egects, is
the cloud to ground lightning. This kind of lightning was classified by Berger [5 ] with
reference to the direction of motion of the leader and the type of charge descendhg to
ground. A leader may move either in the upward or the downward directions, and the
charge deposited fiom the cloud is either positive or negative. World wide data show that
more than 90 % of cloud to ground lightning is downward negative lightning. Fig.(2.1)
shows Iightning as classified by Berger.
2.1.4 Multiplicity of Strokes within a Fiash
Each flash is composed of at least one discharge event called "stroke ".
Occasionally, &er the disappearance of a iightning stroke, another discharge event takes
place dong the same lightning channel. In that case the flash has a mdtiplicity of strokes.
Each stroke usually lasts l e s than a millisecond and the separation between consecutive
strokes is in the order of tens or even few hundreds of &seconds.
A negative cloud-to-ground iightning event may be swmnarized with referace to
Fig.(2.2) as foliows.
First, charged clouds are fomed, as indicated in Fig.(2.2 ) at times t = O and t = 1 -00 ms.
Next, at 1.10 ms a stepped learder initiates the process leading to the first retum stroke by
propagating a small amount of charge Eum cloud to ground in discrete steps. Leader
steps are typicaliy 1 ,us in duration and tems of meters in length with a pause time between
steps in the order of 50 p. During its trip towards the ground the stepped leader may
contain branches that are forxned in a downward direction, as indicated at t = 19-00 ms.
As the stepped leader brings the charge closer to ground, the electric field near the ground
and especially at protniding objects becomes large mou& to break down the air and
initiate an upward moving discharge, as seen in the fiame t = 20.00 ms. Finally one of
these discharges grows into an upward leader and eventualiy meets the downward aepped
leader in the attachment process d e d thefinaijump.
The retum stroke travels upward over the path preionized by the leader. An
electromagnetic pulse precedes the current and renders the channel highiy conductive.
The heating &ect of the rehirn stroke cumnt is responsible for the shock waves that
produce thunder.
M e r the return stroke has finished, everythg rnay end and then we have a single
stroke flash. In some cases, &er the lightning channel becomes exfinguished, foiiowing a
pause called t~erstroke intenazi, additional charge may become available in a ceii at the
cloud end of the channel and a dmt leader propagates almg the residual ionized channef
and leads to another retum stroke. The process continues until the charged celis in the
vicinity of the lightning channel are exheusted.
For the CN Tower case, with its 553 m height, upward negative Lightning is the
most expected type to happen, since for a given charge distribution, the concentration of
the field at the tip of the CN Tower in most cases exceeds the field conceutration at the
cloud. For that teason, an upward leader starts fiom the tip of the tower. This resembles
the case of lightning to a rnountain top or to a transmission h e tower in mountaineous
areas. Most of the strikes to the CN Tower are upward initiated and bring down fiom the
cloud a negative charge.
2 -2 Practicd Importance of the Study of Multisîroke Flashes
The study of multistroke flashes has gained an increasing importance in the last
few years as recent studies show that the eect of multistroke flashes is not only more
severe than that of single stroke ones. but also more severe than originaily anticipated [7].
These considerations should be taken into acwunt when des img equipment for lightning
protection of transmission lines.
It is obvious that excessive overvoltages occu when a transmission h e is
subjected to a multistroke lighming flash as a result of refiection and of U r superposition
with overvoltages fiom subsequent strokes. For air insulation, the t h e between individual
strokes is of importancey since the withstand voltage of the air gap may be reduced by
residual voltage stresses fkom a preceding stroke. For solid insulation and for devices
such as lightning amesters* thermal stresses of subsequent strokes play a sigdicant role.
In these cases, duration of imerstroke in teds determine the opportunity for coohg of
components subjected to a tightning flash.
2.2.1 Lightning Protection of Power Lines
In the course of discussion concerning transmission hue lighmuig protection, it
should be bom in Mnd that transmission lines can be protected against direct hits by
means of ground wires. The effectiveness of groundwires depends on the nature of the
route of the transmission line (Le. ground resistivity7 surroundings, .. .etc.). Ground wires
provide to a certain extent protection against back fiashover. However, the only effective
means to protect transmission iines against induced overvoltages due to nearby tightnùig,
is to use surge arresters. At the same the, this is the most Wrely Lightning event that
transmission lines are exposai to.
Fig(2.3) shows the results obtained by Y. Murooka et al. [8] on the comparative
effiveness of overhead ground wires and surge arresters. !t was found that surge
arresters are usually more effective than overhead ground *es, as the ratio of the peak
overvoltage at the striking point with an overhead g c m d wire to that without was found
to be 0.63, whereas the ratio between the peak ovcmdtqp with the use of surge amesters
to that without was 0.20. This finding points to the superior protection that may be
achieved by the use of surge arresters. Yet, as already pointed ouf arresters are subjected
to both electricai and thermal streses when multiple strokes occur. As a result, the nature
of multistroke flashes shouid be sfudied in order to reach an optimum design for surge
mesterS.
2.2.2 Tesîing of Surge Amesters
Direct strokes can discharge signincant amounts of energy through a surge mester
[9]. Present testing procedures require that the surge mesters are tested by a 100 k&
4x10ps test impulse with an energy of 4.66 kJ per kV . The heavy duty distribution class
arresters must sluvive two of these tests, but tirne is allowed for cooling between tests.
Fomuuuely, in reality arresters on adjacmt poles share a sipificant arnount of the stroke
energy. Although these arresters do not appreciably reduce the peak m e n t through the
stnick arrester, they help to dissipate a signifiant portion of the CUrrent in the tail of the
stroke. This will effectively decrease the tail time constant of the struck arrester's
discharge currmt, which means the reduction of the energy in the tail.
As mentioned above, during standard laboratory tests of surge amesters time for
woling is dowed, and the tirne interval between successive impulses is selected to be one
minute. Darveniza et al. [9] showed both experimentdy and analyticdy that the effects of
multipulse avrents are cumulative as they can cause damage by direct effect or can lead to
thermal instability. Fig.(2.4) shows a cornparison of temperature elevation as a result of
single and quintuples (four-pulse) testing of surge amesters.
Furthemore, plasma enhancement and varistor d a c e coating were found to play
a dominant role in the surface flashover of arresters [q. hiring testhg by Darveniza et al.
[9] it was established that before reaching thermal instability the more limiting &ect than
the failure of the mester was the sudhce flashover of the varistors. This indicated that
effeas of multipulse applications may not necessarily be limited to thermal energy
absorption. The generated plasma rnay not be SUfficiently deionised in the inter-pulse time
interval of about 35 ms, whereas deionisation could take place in the standard lightning
impulse test with time intervais of 60 seconds between succasive pulses. So a new
testing procedure must be developed taking into account the statistics of naturally
occurring mdtipiicity of strokes.
2.2.3 CN Tower Multistroke Flashes
Statistics of the multiplicity of strokes in lightning flashes terminating on the CN
Tower was studied before using video data [IO]. In that study 528 flashes were
considerd among which 235 or 44.5 % were single stroke. At the same tirne the
incidence of single stroke flashes among the world wide data is also 45 %. Thus it could
be said that ta11 structures and neighbo~g objects are exposeci to the same percentage of
flashes with multiple strokes as flashes in open country. However, while the percentage of
multiple stroke flashes is almost the same, the multiplicity of strokes in a flash is found to
depend on the height of the object. A cornparison of CN Tower stroke multiplicity with
other sights in the world is presented in Fig.(2.5) [IO]. The 50 % value of stroke
multipiicity for the CN Tower is 2.4, which is l e s than most 0 t h sights except for
Berger data coliected in the Swiss Alps. Also, the largest number of strokes in a CN
Tower flash was 10. This has occurred twice in the 20 years of CN Tower observations.
While the above analysis indicates that lightning data coiiected at the CN Tower are
somewhat different fiom those in the flat country, there &sts the argument for a great
seleaive suitabiiity of CN Tower multiplestrokes statistics to certain applications.
Mountainous terrain with its tendency for field concentration at protnisions of rugged
mountah peaks, simüarly to the CN Tower, will be subjected to preferentially upward
initiated lightning flashes. In these areas, because of high soil resistivity7 grounding of
power h e structures is difEicult and for that reason lightnhg arresters are preferably
employed for lightning protection. I n f o ~ o n on CN Tower lightning characteristics is
therefore moa suitable for development of performance requirements for these devices.
2.2-4 Other Flash Parameters
There are other characteristics of lightning flashes, besides the occurrence of
multiple strokes, that affect the penormance of apparatus installeci on power lines. The
m o a important of these wu be discussed next.
2.2.4.1 Flash duration
Flash duration is the t h e elapsed fiom the beginning to the end of a flash. It
includes the time taken by individual strokes as weii as the between them, the so-calied
interstroke tirne. The flash duration of CN Tower Eghtning events was studied in [IO].
The statistical distribution of the number of strokes per flash for ali the 528 observed
flashes is shown in Fig.(2.6). The most frequently ocanring values of flash duration (69
% of the studied cases) is seen to be ranging nom 100 to 600 rns. The longest flash was
1433 ms. In g e n d flashes with long duration were noted to have a large number of
strokes. However, the relation between multiplicity of strokes and flash duration is not a
simple linear correlation. An asyrnptotic behavioc was observed as presented by equation
(2.1 } developed in refèrence [l O] and pre~nfed by equation (2.1 } .
Where T is the flash duration in m n d s m is the number of stroke in the flash, A, B and
a are parameters detennined by regrasion analysis to be 0.304 s, 0.2 s and 0.124,
respectively .
Fig. (2.7) shows the cumulative distribution of flash duration for structures of
different heights. In general the flash duration increases with the height of the structure.
It is expected that lightning flashes with long durations impose more severe conditions on
electrical installations. However, not oniy flash duration but also characteristics of the
lightning CUrtent wave and of the radiated electrornagnetic fields, as well as the tirne
elapsed between strokes, must be taken into consideration when assessing the severity of
lightning strikes.
2.2 -4.2 Interstro ke int erval
The time elapsed from the end of one stroke to the beginning of the next is defïned
as the interstroke interval. Statistics of interstroke intervais for the CN Tower were
established in [ 1 O] and are shown in Fig.(2.8). It is observed that the most freguent value
(41.3 % of all cases) for the interstroke interval lies between 67 and 133 ms, only 19 %
are less than 67 ms. It should be mentioned that these values are based on counting the
number of fiames recorded by the video camera between successive strokes. As will be
described in section 3.4, the time resolution of individual h e s is 33x11s. Thus, the
recorded interstroke time is multiples of 33 ms. This is a restriction that is being
overcome by the ment acquisition of a high speed camera as stated in section 3.5.
2.2.5 Cornparison of CN Tower Observations with World Wide Data
As seen from Fig.(2.5) CN Tower data shows a reduced multiplicity of multistroke
flashes. This could be explaineci by the following argument. Ai the moment when retum
m k e w e n t reaches the upper end of the lightnuig channe1 a junction streamer may be
formed which progresses towards an untapped charge c d . Two sequences of events rnay
o m r . The fint is that while the junction streamer is growing, the co~ection to earth is
st3.l established. Then the charge h m the new charge ce1 wiU join the discharge process
without any pause in the Iightning process. nie other case exists when the current to the
earth becornes extinguished before the junction streamer is converted into a wnducting
charnel. In that case a new leader is needed to reionize the extinguished path and initiate
a new stroke [ I l ] .
For the case of a very high structure like the CN Tower, its exceptional height will
lead to a reduced amount of coilected charge, and a reduced total number of charge cells
established within the cloud. Hence, larger interceii distances are found between cells.
This is one factor that can account for the reduced number of multiple strokes to the CN
Tower. Furthemore, since the CN Tower height reduces the lightning channe1 length
noticeably, the lightning m e n t can maintain its connection to the earth more easiiy as a
s d e r potentid ciifference is needed to keep the chamel. This also contributes to
reduction of the probability that the CN Tower is stmck by a muitistroke lightning flash.
2.3 Interference in Current and Field Experimental Data
It is important to be aware that records of the airrent derivative measured at the
CN Tower and those of the electrornagnetic field measured 2 km away are wntaminated
by noise. The m e n t records in addition contain rdections that are due to the structural
shape of the tower.
2.3.1 Interference in Currents
The noise in the Gwent derivatives files wuld be classifieci as follows:
1) DC offset.
This is a unidirectional displacement of the record that after integration redts in a
large rarnp fùnction which obscures the wave shape of the Lightning aiment and rnakes it
diflicult to derive the desired parameters (See Fig. 2.9). The DC offkt may be the result
of the power frequency voltage or might be caused by the A D converter itself.
2) High frequency noise
In case of fast rishg ~rrents with high peaks, high fiequency noise will not
contaminate the wavefront due to its fhst rise t h e . On the other hand it cari affect
waveforms h a h g lower peaks and longer risethes.
3) Low fiequency noise at about 100 lrHz
As shown in Fig.(2.9) it is noticeable at both m e n t derivative and its integration
that there exists a low Eequency oscillation. This noise may be caused by a carrier
telecommunication on the power system. As this noise is very slow wmpared to the
risethne of the m e n t derivative and the current tse& it may not noticcably &kt the
values of wavefront parameters but the problem will be in finding the startuig point of the
lightning wavefiont.
Beside these three classes of noise, reflections at various points of discontinuities
at the CN Tower cannot be ignoreci. A detailed study of reflections by modeling the CN
Tower as a series of cascaded transmission lines was carrieci out by R Rusan [l 21.
2.3 -2 Int derence in Fields
As rnay be seen h m Fig. (2.10), noise in field data does not cotlstitute such a
large problem as is the case in current derivative files. The noise in field files cm be
classified as folIows:
1) DC offset
As show in Fig. (2. IO), if the DC offset is noticed in the field at dl, it is u d l y
small and it wiIl not resuit in any detrimental effeas when the field p m e t a are
extracted from the field wave. Field files are used directly, without any htewtion or any
other operation.
2) High frequency noise
This component of noise exists in field data simiiar as in current data, brrt it only
affects the fields with low peaks. It should be noted that there is no low fkequency noise
in field data. However, since the electromagnetic field is deterrnined by the nirrent along
the whole length of the lightning path, the refiections the curent goes through while
traveling along the tower will have their &kt on the field waveshape.
2.4 Noise Removal fiom Current Derivative Waveforms
Several approaches were used to remove the noise from the current derivative.
One of the approaches was to use linear filtering technique which was succes& with high
fiequency noise, but was restncted to a limited bandwidth for the low fiequency noise
[13]. Another two approaches were used to remove the noise by Y. Chen [14]. The first
was the use of fiequency domain analysis. In this approach the file was segmentecl in a trial
to extract the spectmm of the noise. The main problem was the discontinuhies between
the several segments. The most successfùi approach was the use of wavelets technique to
get rid of the noise presented by Y. Chen [14 1.
2.4.1 Removal of DC component
In order to remove the DC offset fiom current derivative to establish the statistics
produceci in this thesis, the average of the pretrigger intend was taken and subtracted
fkom the whole data. Fig.(Z. 1 1) shows the redts of this approach. It is clear that it is
quite successful in removhg the DC.
2.4.2 Interference due to Reflections at the CN Tower
The question of the influence of the tower structure was treated by Jlaniscshewskyj
et a1.[15], [16] and in [12] by R Rusan. In fkct reflections arise from the poims of
discontinuity dong the m e n t path. The main points of discontinuity are :
1) The connection between the iightning chanael and the CN Tower structure.
2) The ground.
3) At the points where the physical dimensions of the tower change above and below the
restaurant Iwel.
4) At the point where the steel structure mets the concrete structure (the space deck).
In [IS] and [16], the CN Tower was studied using Transmission Iine theory.
Severai models were deveioped for the CN Tower. Fig. (2.12) shows the models used in
which the tower is represented by up to 3 transmission lines in cascade..
Fig. ('2.2) A &awi~ig iliustruting some of the vanvanous processes comprising a negative
c i d fo ground lighmmgmh[5]
O 200 400 600 800 LOUU Distance fmm tdt End (m)
Fig. (2.4) Temperature elevation for a surge mester under single a& quintuples
Fig. (2.6) Frequency distribution of siroke muitiplicity [1 O ]
O 0 3 0.4 0.6 0-8 1-0 1.2 1.4 FIash m o n (s)
Fig. (2.9) a-Current derivatzve file f 1085443.405 ccrpwed on June 1 O. 1996 at
Fig. (2.1 O) Field file fl0860 70.753 qtured un June 10,1996 of 1 I : M 03p.m.
l l:44:03p.m. cmd its integration b- Afer removing the DC ogsel
Fig. (2.12) CN Tmer simple m&Is for l ighing current propagation
CHAPTER THREE
3.0 EQUIPMENT AND TESTING
In order to study lightning characteristics, a measuring and monitoring system with
various kinds of equipment is needed. The measuring system is to masure lightning
current and the associated electromagnetic radiation. The monitoring system is to monitor
visuaiiy lightning so as to find out the shape of the lightning channei and to foliow the
steps of lightning development. The foilowing chapter provides a description of the whole
system associated with CN Tower lightning and its testing.
3.1 Introduction
Characteristics of lightning strikllig the CN Tower are captureci by several
rewrding systems. They allow measuement of such parameters as the the derivative of
ment , the associated electric and magnetic fields as well as the variation in the trajectory
of the lightning channel.
Characteristics and operation of individuai measuring systerns have been d d b e d
before by Peter Dzurevych [17], neana Rusan [18] and Radu Rusan [12], and recorded in
the technical literature [19]. In the following sections, ernphasis will be placed on testing
of these quipments and on modifications witnessed by the author of this thesis. Fig.O.1)
presents a schematic diagram that shows the CN Tower and the equipment used during
the S u m e r 1997 for measurement:
1) 3m Rogowski a i l . (Refierred to as the old c d )
2) Triax double shieided co~ecting cable.
3) 6rn Rogowski c d (Referred to as the new c d )
4) Fiber ünk.
5 ) Three RTDs. (Real t h e digitizers)
6) Two magnetic field sensors.
7) One electric field sensor.
8) Two vide0 cameras.
9) One high speed camera.
In addition, durhg the summer of 1997, two magnetic field sensors and an electric
field sensor were bomwed fiom the Swiss Federal Instmite of Technology. Readings
obtained with these seasors are compared with simuitaneuus records of the correspondhg
University of Toronto field sensors for the purpose of calibration and verification
3.2 The Current Measuring System
The derivative of the niment is the actuai measured parameter. The curent
denvative is meastueci by means of a Rogowski Coil and recorded by one of the three real
t h e digitizers. The obtained records are transferred to a fioppy and numerically
integrated off-he to obtain the airrent.
During the 1997 lighaiing season, there were two ails instalied at the tower. The
h, which is refmed to as the old c d , is a two-section, three-meter long Rogowski Coil
which surrounds only one fifth of the steel mast at the 474 level of the CN Tower. A
factor of five is hcluded in the sensitivity of the mil, assuming that the current is equally
divided over the pentagonal cross section of the tower, which is a fhir assumption but was
never experimentally proven bdore. A t r ia cable is used to wmect the mil to the
digitizer. One of the major problems of this comection is the noise that is superimposed
on the output of the coü. This noise, its origïn and methods to remove it was discussed in
more details in chapter 2. The wmponents of the induced noise could be raimmarized as
follows:
1) DC offset possibly rdting fiom analog to digital converter.
2) High fiequency noise generated by CN Tower transmitting anteanas.
3) Law fkequency noise component at 100 Hz.
The second mil, calied the new coi1 is a four-section, six-meter long Rogowski Coi1 which
encircles the whole tapering steel mast at the 500 m level of the CN Tower. In order to
reduce noise, the new coii is connected to the digitizer by a fiber optic system.
3 -2.1 Old Coi1
The aiment measuring c d , refmed to as the old mil, is a two-sections 1 Sm long
each Rogowski Co& located at the 474 m above ground lwel (AGL). Then the output of
this coi1 is C O M ~ C ~ ~ through a triax double shielded cable to the real t h e digitizer at the
372 meters AGL through a 50 ohms termination to avoid reflections.
3 -2.1.1 Technical specifications of the old coi1
Sensitivity : 0.3 5 1 V/(A/ns)
Band Width: 40 MHz
Length: 3 m
Impedance: 50 Q
The mil was calibrateci on May 27,1997 at the CN Tower and the sensitivity was
found to be 0.35 1 V/(A/ns). In fàct the r ed t s of this test c o ~ s what was obhined on
May 25,1992 and May 10 1993 which proves that the c d didn't deteriorate as a result of
the atmospheric conditions at the tower. The details of this test are provided in Appendix
"AW.
3.2.1.2 Connection between Old Rogowski Coil and Real Tirne Digitizer
nie output of the wü. is co~ected through a 50 ohms tennination to the
Tektronix real tirne digitizer by a 102 meter long double shielded triax cable in order to
suppress the noise. The outer cable sheath is grounded at both the coi1 location and at the
reai thne digitizer.
3.2.2 New Rogowski Coil
In order to improve on the current measuring system, and especidly to overwme
ditficulties caused by noise, a oew current derivative measurement system was instalied in
the Sumrner of 1997. The new current measuring system, shown schematically in
Fig.(3.2), wnsists of :
1) Four-part 6m Rogowski Coil.
2) Fiber iink.
3) The RTD.
3 -2.2.1 Technical specifications of the new coi1
Sensitivity: 1 -2862 V/(A/ns)
Band Wldth: 40 Mhz
-1 6 m
Impedance: 50 R
This Rogowski coi1 was installed at the 500 m AGL of the CN Tower. Like the
old mil, this coi1 was supplied by Physics International (2700 Morood Street, P.O. Box
SOlQ San Leandro, CA94577, (510) 577-7283). It encircles the whole tapering steel
mat, so as to measure the wrent derivative of the whole current flowing in the rnast.
This arrangement wiU permit, through cornparison of sirnultaneous aimnt records fkom
the new and old systems, to test the assumption of equal division of the cumnt over the
five corners of the pentagonal steel mast. The new mil was tested on May 23,1997 in the
laboratory with an overlap of 5 cm in each section of the coii as specified by Physics
International. The sensitivity was found to be 1.1985 V/(A/ns). This confirms the value
quoted by the manufacturer which was 1.19 V/(A/ns). The experimental setîing and
resuits are describeci in Appendix "Bn. The results of the manufacturer testing is provided
in Appendix "C". It is worth mentionhg that the mil was designed to measure the
derivative of wrent flowing through its center. The experimentai setup only encircles one
of the four sections of the mil. As a r d t , the four sections must be tested and the
average of th& sensitivities is taken as the sensitivity of the whole c d . But, due to the
installation and space requirernents at the CN Tower, the actual overlaps (10.8 cm.)
between the parts of the coi1 were dEerent from that specifïed by the manufacf~rer. Mer
the first thunder storxn, the output of the coi1 was analyzed. It was found that the curent
derivative wntained high fkquency oscillations. The iightning current denvative was not
recognizable and upon integrating this signal, an odd waveshape that doesn't represent the
lighallng wrent was recovered. Consequently, a visit to the tower was arrangeci and the
mil was removed fkom the CN Tower and retested in the laboratoty. Mer investigating
reasons for oscillations, it was found that some coaxial cables connechg various sections
of the coi1 were i n t e d y disconnecteci. The d e f d v e coaxial cables were replaced and
the coi1 was recaliirated with the overlap of 10.8 cm at the laboratory on August 15,
1997. The sensitivity was found to be 1.2862 VI(A/ns),which is not much different 601x1
that with the specified overlap. This is the value which should be used in ail caldations
involving the data collecteci by this mil.
3.2.3 Fiber Link
In an attempt to overcome the problem of noise associated with the old coi1
system, a fiber link (OP 2000 A optical transmission system) was obtained from Nanofast
( 416 West Erie, Chicago, Illinois 60610 - (3 12) 9434223 ) for connecting the n m coi1
with the RTD.
The fiber link consists of:
1) Transder.
2) Optical fiber.
3) Reciever.
The transrnitter is an OA-1 opticai analog transmission plug-in and a seva inch
high Rn trammitter chassis to reduce the extemal electromagmric nuise by mort thml 20
dB. Moreover, a five kV spike on the AC h e creates les than 100 rnicrovott signal
referred to the input.
The receiver is an OA-1 optical analog receiver plug-in and a twelve and a half
inch high RR receiver chassis. The length of the optical fiber between the tnuismitter and
the receiver is 1 Sûm.
3.2.3.1 Technical specifications
1- Input to transmitter: From - 1 volt to +1 volt fidi scale into 50 R
2- output of receiver : From -1 volt to +I volt fùil sale into 50 i2
3- Band width: 100 MHz ( 3dB points are 35 Hz and 1 OOMHz)
A total of 28 dB attenuation ( 20 dB attenuator + 8 dB attenuator) is used between
the coi1 and the transniitter to reduce the output of the Rogowski coi1 so as not to sahirate
the fiber link. As the output of the Rogowski coi1 is expected to reach 16 volts in extreme
cases.
3.2.4 Recording Equipment
A two-channel Sony-Tektronix RTD 710A Real time digitizer is used at the si@
of the CN Tower to record the measured data. A Tektronix PEP 301 controlIer is
connected to the degitizer-
The connedon between the old Rogowski Coil and the RTD is a tria cable of
102 m length. In fact the induced voltage in this cable is responsible for a major part of
the noise superimposed on the recorded cumnt derivative signai. The new Rogowski
Coil is connecteci to the sarne RTD by the fiber Link systern.
3 .M. 1 Technical specifications
Sample rate: 100 MHz
Nwnber of channels: 2
Mernory : 128 kB/channel
Recording mode : Advance
Trigger mode: Single, bislope
Output vertical resolution: 10 bits
3.3 Field Measuring System
Two mgnetic field saisors and an electric field saisor are used to measure the
radiated electromgnetic fields. Those senson are located on the roof of the University of
Toronto Rosebnigh Building 2.0 km away 6om the CN Tower itself.
The meas~.iêd components are :
- Azimutha1 component of the magnetic field 04).
- Radial component of the magnetic field a). - Vertical component of the electric field (EJ.
These notations are used based on the cylindrical wordinate system with the z axis
coincident with the CN Tower.
3.3.1 Magnetic Field Sensors
There are two identical sensors, one is directed towards the CN Tower to measure
the azimutha1 componmt of the magnetic field. The other is perpendidar to it, so as to
measure the radial component of the magnetic field.
3.3.1.1 Technical specifications of the magnetic field senson
Type: active single Ioop antenna.
1) Sensitivity: O -42 1 V/(A/m)
2) 3- dB Iow roilsff fi-equency : 697 Hz
3) 3- dB high roii-off frequency: 150 MHz
4) Maximum linear output: 0.62 V(m)
3.3 -3 Electric field sensor
3.3 -3.1 Technical spdcations
1) Type: Active hemispherical antema-
2) SensitiMty: 2.3 8 mV/(V/m)
3) 3-dB low roll-off frequency: 47 Hz
3) 3 4 3 high roll-off fkquency: 100 M H z
4) Maximum linear output: 0.62 V (rms)
The recordhg system is identical to that used for the measurement of current
derivative but two RTDs are used instead of one.
3.4 Equipment on Loan from SwitzerIand
In addition to these sensors, during the 1997 lightning season, another systern was
borrowed fiom the Swiss Federal Institute of Technology ( S m . This system consists of
two magnetic field sensors and one electric field sensor to masure the same panmeters
recorded by our system [21]. The sensors are connecteci through a fiber optic Wr to a Le
Croy 94 24 degitizer. The whole setup is located in Rosebrugh building .
3 -4.1 Technid S pecification of SFIT Electric and Magnetic Field Sensors
3.4.1.1 Mhgnetic field
1) Tangentid setlsitivity: -50 to +50 CLA/m
2) Accuracy: -0.5 dB to M.5 dl3
3) Band width: 4 kHz to 150 MHz
4) Band p a s ripple: -1 to +1 dB
5) Risetirne: 2.8 11s
3.4.1.2 EIectric field
A spherical sensor
1) Range : 1 V/m to 316 kV/m
2) Band width: 1 lcHz to 150 MHz
3 -4.1 -3 Main fatures of the opticai liok
1) Bandwidth: 100 Hz to 150 MHz
2) Peak to peak signal RMS noise: 53 dB
3) Range: 15.5 dB
4) Accuracy: - 0.5 to + 0.5 dB
3 -4.3 Recording System
The recording system consists of three fiber links connecting the senson to the ''Le
croy" degitizers [2 1 1.
3 -4.3.1 Technical specifications
1) SampIing hquency: 500 MHz
1) Sampihg fiequency: 500 MHz
2) Mernory: Drum rnernory associatecl with a system of pre and post
triggerhg 5000 points.
3) Built in FFT.
The results of both systems for a CN Tower flash for the H~JI rne~i~ttred by both systems is
&en in Fig(3.3). It is obvious that the Swiss equipment gives bigher magnetic field peak
than U of T equipment (about 3 times larger), but both records have the same waveshape,
which shows that there is a scaling problem.
3 -5 Video Monitoring System
The CN Tower is continuously monitored by two video carneras one of them is on
the roof of Rosberough Building 2.0 km north of the CN Tower, the other is on the roof
of Ontario Hydro Technologies building 1 1.8 km west of the tower. The two images can
be digiîized and used in a specially developed cornputer program [20] to determine the
three dimensional path of the iighming trajectory.
The cameras used are Hitachi VM-3 100A with the speed of 60 -es per second.
The video cameras work wîth 1:2 interlace which results in the 33 rnillisecond resolution
between individual pichires
3 -5.1 The Importance of the Visual Monitoring of the CN Tower
The visuai monitoring of the CN Tower flashes is very important. The resuitant
pichires from both cameras, which are taken fiom two aearfy perpendicular direaios, can
be used to obtain three dimeosiod images of the lightmng trajectory as mentioned before.
In this way it can be established whether the lightning channel was vertical or not. By
counting the fiames where lightning exists, the duration of each stroke, the number of
strokes in the recorded flash, and interstroke imenals can be daennineci. Furthemore, in
some cases, TV monitoring of a flash may help to find its initiation direction.
3.5.2 Shortcomings of the Vide0 Camera
nie main difEcuities with the use of the video cameras is the large tirne between
recorded fiames. W~ the highest resolution of 33 ms, a Lightning stroke d d be easily
missed. Besides, the intensity resolution is not very high, so that @are often obscures the
details of the lightning stroke, as iiiustrated in Fig.(3.4)
3.6 Hi& speed camera
To overcome the main disadvantage of the video system, which is the large tirne
between individual fiames, a high speed camera with capability of recording up to 1000
m e s per second was acquired. The " Phantom" camera is supplieci by VISION
RESEARCH (Wayne, New Jersey, tel. (201) 6964500). During 1997, this camera was
operated at the speed of 500 h e s per second. In this way interstroke t h e can be
estimated with a much improved accuracy in multiples of 2 ms. This level of resolution
allows observation of details in the development of a iightnllig flash. This camera is
triggered through a separate circuit which is activated by the change of light intensity.
The saving time of a cine of 500 fbmes is about 50 seconds.
3.7 Difnculties with the Overd System
L i g h h g discharge processes occur on the microsecond d e . Fof that reason,
very -te the-keeping systems are needed to recognVR simultmeous occurrence of
events captureci by the different recording systems. In view of these requirements, the
existing overail lightning measuring and observation installation is associated with three
main problems
1) As wery system has its own clock. variations of the tirne between the different systems
usually appear and a difnailty arises to correlate the output of each system An involvecl
matchhg procedure is needed at present to establish simultaneous events. Idealiy, a master
clock operating by a world-wide timing system should be used and all subsystems should
be connecteci to that clock. Udortunately, avdable fùnds do not allow this lwel of
sophistication.
2) Sioce different systems are triggered on difkent parameters, one syaern may be
ûiggered while the others are not.
3) The various instniments used require different tirnes for saving the collected data.
During these intervals, collection of data by a pmticular instrument may be intmpted.
As a consequeme, individual instruments may miss certain events that are recorded on
other systems. Hopefully, technical progress in digital recording instniments wiil lead to
reduction of saving tirne. However, it is hard to expect that it couId be reduceû to zero.
For that to happen, recordhg of events would have to be written directly into the
permanent memory, a task not achievable at this thm.
New mil and pticai link t m m d î a , at the second damper IeveI 500 m A.G.L.
c U of T Rosberugh Building: 1 - Magnetic field sensors. 2- Electric &id sensor. 3- Video camera. 4- Magnetic field semm h m S m . 5- Electric field sensor fiom SFIT. U of T. S d o r d Fleming Building 1- Fast camera.
(3.1) Schematic diaqm of the CN 7'' and iighniing mecswing d monitoring
systems.
RTD
Fig. (3.2) Schematic diagram of the new m e n t rneasuring syshm.
mm in mictcnrcoiidr
Fjg. (3.3) Comp&son of H+ record copnaed simuItarnemiy by U of T mid Swis
Fig. (3.4) A picme of a lightning to the CN Tower q t u r e d on July 30.1996 at 01:42:19
by the vide0 m e r u Iocated on the roof of Rosebraugh buildmg s h s the
grcire accompanyrhg the ïightning discharge thot obscures i&nnicafiCafion of the
channel&aik
CHAPTER FOUR
4.0 STUlDY OF CURRENTS
The interest in the study of the CN Tower multistroke flashes started with the
work of Janischewssj et al. that is descnied in the technical literature [4]. The study of
Janischewskyj et al. provided a methodology of approach but was Iimited to the CUrrents
of multistroke flashes captured during 1996. Nevertheiess, the study r d t e d in signincant
conclusions. As the set of data studied showed that ments in subsequent strokes are
characterized by a lower peak than in the fht stroke and that subsequent strokes seem to
be steepa and have less risetime than the first stroke. There has been need to co&m
these results through a detailed study of the data gathered fkom 1 99 1 till 1996, to find out
if the results obtained in [4] were p d a r to the 1996 lightning season or if these were
cornmon features among all the data of ail years.
In the foîiowing work the data is grouped by year; ail these subsets of data are
treated by the same method. Statistics are produced for each year and an over all statistics
of al1 the data tiom 1991 tiil 1996 is also produced. Another subset studied were the
m e n t nles that were correlated to field files and these will be treated separately in
Chapter 6.0.
Figs. (4.1) and (4.2) show file fl085443-283 of current derivative and its
integration of lightning aiment respectively. This is the third stroke in the flash, captured
on June 10, 1996 at 23:45 :03. Three parameters are treated in this study, which are the
peak of the current, the maximum current derivative peak and the risetirne of the cunent
wz~veform.
4.1 Current Peak
As descriied before, the current derivative is the actual quantity measured at the
CN Tower (see Fig.(4.1)). As discussed in chapter 2, the ment waveshape is obtained
fiom the digitized record of the current denvative by numerid integration (see Fig. 4.2).
The measured CUrrent denvative is contaminateci with ciiffirent kinds of noise. In addition,
the current derivative has the "signature" of the CN Tower structure through the
successive reflections at various points [22]. It is signifiant that the nrst refiection arrives
at the meamring point after 200 ns. Thus both, the m e n t denvative wave and the
m e n t wave are clear of tower reflections up to that the. In many cases the current
wave reaches its peak before the 20ûns point. However, even in these cases other kinds of
noise contamination may still be present. For slower waves, both, refledons and noise,
distort the peak.
Nevertheiess, the fht peak (see Fig.4.2) is considered to be the characteristic of
the m e n t that is least affiected by various distortions that deform the obtained resuits.
Thus the fira peak is used as the basis of the cornparison between the m e n t in the first
stroke and those in successive strokes of a flash,
To compare between the current peaks, the ratio 4 is fonned where:
where
Ratio of current peak of the k" stroke to that of the first stroke
rk Current peak of the kth stroke
11 Current peak of the first stroke.
and k > 1, as for k = 1 the ratio & wili be 1 which is not suitable to produce the required
statistics-
The total set of data used contains 33 flashes with 125 strokes. Cumulative
distriiutions of peak went ratios for ali subsequent strokes w a e obtained for each year
f?om 1991 tili 1996. The results are shown in Figs. (4.3) to (4.8). Frequency distributions
are provideci in Appendk "Dy'. From presented statistics it is clear that for al1 the years
except for data of 1995 and 1993 the 50 % value is less than 1 (about 0.8). For 1995 and
1 993 the 50% value was 1.75 and 1.25 respectively .
Cumulative distribution of peak aiment ratios for data accumulateci between 1991
and 1996, represented in Fig. 4.9, shows the 50°! value to be around 0.9. This is the
same resuit as that reached by Janishewskyj et d.[4], which means that subsequent
strokes are most likely to have d e r current peaks than the first stroke. This observation
is consistent with physics of iightning. The stepped leader requires more energy to
propagate through the un-ionized air, whereas the dart leader will foliow the partidy
ionized channel rernaining fiom the previous discharge with less effort. 44 % of
subsequent strokes had m e n t peaks ranging fiom 1 to 4 times the value of the first
stmke peak, while 16.8 % had theh current peaks less than haif of the nirrent peak of the
first stroke.
Among all the 125 current strokes obtained between 199 1 and 19%, 45.6 % had
the m e n t peak ratio greater than one, with a mean value of 1.7027. Compared to other
researchers [24], based on Berger's data the gaometrical mean was 1.2 and 15% of ail the
mdtistroke flasiies wnsidered containeci at least one absequent stroke whose peak is
greater than the first. In the case of Chi Tower 2 1 flashes, out of 3 3 flashes which is 64%,
have at least one stroke that is greatu than the tirst. This shows that our results are
biased towards a higher m e n t peak ratio as will be discussed in Chapter 6.
4.2 Maximum Current Steepness
The second parameter that is under consideration is the marr imu m e n t
steepness which always occurs on the front of the current wave. The actuai current
derivative that was capturai by the Rogowski coi1 was used to find the maximum current
steepness (see Fig.4.1). The same approach that was used in the analysis of the current
peaks was also used to analyze the current steepness as described in equation (4.2)
where
& Ratio of tnaximum curent derivative of the km stroke to thet of the
first stroke.
Sk Maximum current denvative of the k* stroke.
Si Maximum current derivative of the fint stroke.
and k > 1, as for k = 1, the ratio & wiii be 1 which is not suitable to produce the
required statistics.
The results are shown in Figs. 4.10 to 4.15 for individual years and for
accumulated data fiom 199 1 to 1996. Frequency distributions are provided in AppendDc
"E". In wntrast to the just discussed current peak ratio statistics, subsequent strokes are
found to be much steeper than the first. In each yearly set of maximum steepness the SV!%
value is always higher than one, in fact, except for 199 1 and 1992, the 50% value is more
than 2. And even for 1992 and 199 1 the 50% vdue is about 1.5.
In Fig(4.16) overd statistics is provided for d the accumuiated data fkom 1991
to 1996, the 50 % value is around 2. Only 32.8 % of the data is less than one and about
7.2 % of the data are more than 10 times higher than the first stroke. This all means that it
is highiy unkely for the subsequent stroke to be less steep than the fint one. These
results coincide with the results found in reference [4] and of course it is noticeable in this
case that the range of variation of the current steepness is much wider than that of CUiTent
peak ratios. It varies fiorn less than one to 16 times the maximum steepness of the fint
stroke. This wuld be explained by the fàct that for subsequent strokes the channe1 is fully
re-ionized by a quick progressing dart leader, but the degree of re-ionization varies over a
wide range. Yet, in general the degree of re-ionization is higher dong the whole length of
the lightning channe1 than in the case of the first stroke. The partialiy ionized channel
requires a lesser amount of energy for initiation than in the fkst stroke which explains the
lower peak ratio, while at the same tirne it provides more conductive path for the charge
to follow.
4.3 Risetime
As shown in Fig.4.2 the terrn risetime, used in this thesis, is d&ed as the M i e
needed by the lightning current wave to rise nom 10 % to 90 % of its peak value. The
10-9û?? value is used to avoid the possible misleading results that may arise due to the
&sting noise whai considering the peak itseif and the base value. In a similar approach
to that used in treating CUrrent peaks and amcimum current derivative, the foilowing
quantities are defined.
where
Rn Ratio of risetime of the km stroke to âhat of the first stroke
Risetime of the k' stroke
Tl Risetimeofthefirststroke.
and k > 1, as for k = 1, the ratio Rn will be 1 which is not suitable to produce the required
statistics.
The results for each year fiom 1991 to 1996 and for the whole 1991 to 1996
pend are show in Figs. (4.17) to (4.22). Frequency distributions are provided in
Appendix "F". 11 is clear fiom these figures that it is very rare for the risethe of
subsequent stroka to be greater than the r i d e of the nrst stroke. In fàct in al1 the
years the 50% d u e was around 0.5 except in 1992 where it was nearIy one. Fg. 4-23
shows the overd statistics of d the data fiom 1991 to 1996. The 50% value of the
overall statistics is about 0.7. 28.8 % of the accumulated data has risetirne ratio greater
than one, while 32.8 % has its risetime ratio les than 0.50- This means that for the
subsequent strokes, it is most likely that their risetimes are going to be shorter than that of
the fht stroke. This finding is consistent with obsewations made on the maximum current
steepness ratio and was explained by the more d o m chamel ionization in a subsequent
stroke. Peak of a subsequent stroke is most likely to be smaller than that of the first
stroke, while the subsequent stroke is much steeper than the f h t . In addition it was found
that subsequent strokes have shorter risetimes, a Einding consistent with the steeper fiont
of subsequent stroke waves.
4.4 Comments on Waveshape Parameters
In this chapter, the methodology introduced in [4] for cornparison of m e n t
characteristics associated with subsequent strokes to those of the first return stroke, has
been applied to study current peak values, maximum current steepness as well as risetime
of much extended set of data Perfomed analysis of the three waveshape parameters
confirmed the p r e l i i findings given in [4]. To sum up, within a multistroke flash,
subsequent strokes have smaller curent peaks than the first stroke, whereas. they are
expected to have steeper wavefkonts than the first. A h , subsequent strokes risetimes are
most likeiy to be less than the first one.
4.5 Effect of Iterstroke Itervals
In order to study the tluctuation in current peaks ofco~lsecutive strokes. each flash
was analyzed separately. Only the data from 1996 and 1995 were used as in these records
the interstroke time is in multiples of 14 ms, whereas it was in muitipies of 50 ms i0
previous years. W~th the 5ûms tesolution time, excessive uncertaiaty is introduced into
the measure of time between successive strokes. Six flashes with stroke multiplicity
ranging &om 4 to 8 strokes per flash are shidied in detaü and shown in Figs(4.24) to
(4.29). In these figures the peak ratio is piotted as a fùnction of t h e within the fiash.
Despite of the fluctuations that are noticeable in these graphs fiom large to small values. it
generally couid be not id that a large peak is preceded by a large interstroke the and
vice versa. This couid be amunted for by the time needed for a larger number of charge
cells to be wnne*ed to the lightning channel. It is suspectai that the thermodynamics of
the channe1 plays a role in this process and for that reason a more d d e d study should be
conducted to investigate this aspect.
peak ratios@ 1996. peak ratios for 1995.
Fig&-5) Cumulative tüsîri3ution of cwmt Fig.(4.6) Crmiuiatfve distn3ution of czvrent
peak ruîios for 1994. peak ratios for 1993.
peok ratios for 1992. paà d o s for 1991.
Fig(4- 9) Cumuhtive distrihtiion of m e n t p a k ratios for al2 multistroke fIcrrhes from
1991 to 1996.
Fig. (4.10) Cumulorive distribution op maximum
Ctlwent derivative ratios for 1996.
F?g.(Q. I I ) Cumulative distribution of mdx~-mtun
current demtive ratios for 1995.
Fig. (4.13) Cumu futive distribution of maximum Fig. (4.12) Cumu fative disIribut1on of maximum
m e n t derivative ratios fur 1993. m e n t derivative ratios for 1994.
Fig. (4.14) Cumulative disbbution of ratios for Fîg. (4.15' Cumulative distribution of maximum
current derivative ratios for 199 1. maximum current denvative 1992
Fig(J. 16) Frequency drstrrstrrbutzon of mrmmrmmum m e n t derivative ratios for al1 mulrisnok
Fig. (4.1 7) Cumulative disaibution of m e n t Fïg. (4.18) Cumulative distnstnbuticm of m n t
Fig.(4.19) CÙmuIutive distribution of m e n t
risetirne ratios for 1994.
Fig. (4.2 1)- Cumulative distn-6 ution of current
Fig.(4.20) Cumulative distribution of m e n t
risetime ratios for 1993.
Fig. (4.22) CumuZative disatsatbution of m e n t
risetim ratios for 1991.
Fig(4.23) Cumulative distrihtion of m e n t risetirne ratios for al/ multistroke flahes
mm rinrn hrri [ml
fg. (4.21) Ratio of m e n t peak of nrbsequent to first sttokes as O functtion of tirne for
flash ho843165.flh
jigi(4.25) h t i o of m e n t peak of subsequent to 3rs1 strokes as a functzon of time for
fig.(4.26) Raiio of current peak of mbsequent to first sbpkes as a function of time for
fig. (4.2 7) Ra~ïo of Ctlrrent peak of subsequent to first strokes as a fwctzon of rime for
fig.(4.28) Raiio of current peok of subsequent to first strokes us a function of tzme for
jig.(4.29) Ratio of m e n t peak of subsequent to first strokes aî a fincrion of t h e for
flash El O62 725.fllh
CHAPTER FlVE
5.0 STUDY OF FIELDS
In this chapter fidd fdes of CN Tower lightnhg flashes are anaiyzed in the same
way as was done for currents in Chapter 4. Ideaily we would like to deai with fields of
same events that were used for currents. Unfortunately, for reasons that will be explained
in the ne= section, this is not possible. Consequentiy we wiii deal in this chapter with aU
recorded field files, that resulted fiom iightning saikes to the CN Tower, irrespective
whether there are current mes available for these iïghtning events or not. As a result, field
statistics wiil encompass a different set of events than m e n t statistics. However, in view
of the large number of files involved in each set, a cornparison of observations made for
nirrents and for fields should be statistically valid. To test that validity, in Chapter 6 a
subset of all data is çimilarly tested. The subset comprises characteristics of currents and
of fields for only those wents where both, current files and field files have been recorded.
In the foiIowing statistics the waveshape parameters of the magnetic rather than
the electric field are tdzed, as the electric field sensor &ers f?om saturation dier a
certain level. Equations 5.1 and 5.2 show the interrelation between the components of the
lightning current and the electric and magnetic field respectively.; Fig. (5.la) shows a
schematic that shows the co~espondine quantities.
where R = Jr + ( z -
From these it can be seen that the azhmthd component E& of the magnetic field,
although la-cking the integral component of the verticai elecnic field, wili provide the
essentiai i n f o d o n about the behavior of the electromagnetic fields associated with a
iightnbg discharge. When the lightning channe1 is verticai, the 24 component is equal to
the total magnetic field. In case of CN Tower lightning this is dosely approached in most
cases as Uustrated in Fig.(S. lb) showing a record captureci on Septanber 11,1996 at
l2:O3 : 16 am. by a video camera p W on Rosebrough Building.
5.1 Recording Rocess
Let us oow turn back to the difEcuIties that are msing the set of field files to
include records of events that are not contained in the set of curreat files aed vice versa
The first difiidty arises h m the fàa that the two systems are mggered by different
signals. While recording of m e n t files is inîtiated by reaching a preset level of the
m e n t derivative, field systern is triggered on the azimuthai component H+ of the
magnetic field that depends on both, the current and its derivative, as is shown in equation
(5.2a). So the field measuring system may trigger and the m e n t measuruig system may
not. The reverse «in occur as weli. A h , since the field measuring system is mggered by
any lightning event in its vicinity, occasionally, a non CN Tower lightning discharge will
be recorded. An added problern arises f?om the fàct that the time clocks of the two
systems (one located on the CN Tower, the other at Rosebrugh Building ) run
independentiy and therefore cannot keep the subrnfisecond synchronization required for
identification of simuitaneously recorded lightning strokes. An involved process described
in reference [17l must be used to correlate airrent and field data. Furthemore during the
process of swing a file the measuring system is unable to record another went. As a
result of these mon a flash rnay be recorded by one system and not by the other. With
field records, it may even happen that one of the strokes recorded in a sequence may
belong to another flash.
Correlating field files to the appropriate m e n t files on the bais of interstroke
thes and the shape of the m e n t and field wavefom will help in excluding those £iles
that do not belong to a given CN Tower Iightning flash. Fig.(5.2) shows an example of
such a case. Flash e207306.flh is a six stroke flash. However, field stroke nurnber 5 does
not belong to this set of the CN Tower lightning flash. This can be ascertained by
reférence to Fig.(5.3) which shows the corresponding current wave shapes. A check of
Fig. (5.3) will reveal that ail current waveforms are sirnilar. At the same tirne, Fig.(5.2)
indicates that the record in question has a peailiar peak while all other field records in this
file have similar waveshapes. Thus this field record is not a CN Tower stroke. Should
this happen to be the first record in the set, the whole flash becornes unusable in the
present study. Nevertheles, 33 flashes with a totai of 125 strokes have been recorded by
the Rogowski Coi1 on the CN Tower and 27 flashes with 91 strokes by the magnetic field
measuring system at the Rosebrough Building. This last set of data will be now d y z e d .
5.2 Field Peak Ratio
Fig(5.4) shows a typical H,, waveshape for a CN Tower lightnllig flash. It is
characteriseci by a rapidly nsing eont and two consecutive peaks with about 1 ps in-
between. The second peak is found to be missing in most of first strokes within
multistroke flashes used for these statistics.
In a similar approach to that used with m e n t peak ratios of subsequent strokes
with respect to the fkst one, the corresponding ratios are formed and statistics are
established for field waves of rnultistroke lightning events.
where
p, Ratio of magnetic field peak of the k%troke to that of the first stroke.
H, First peak of the azimutha1 magnetic field wave in the k'L stroke.
H, Fust peak of the azimutid magnetic field wave in the first stroke.
and k > 1, as for k = 1, the ratio p, wiu be 1 which is not suitable to produce the
required statistics.
As mentioned, the data set used compriseci 27 flashes with 91 strokes. Cumulative
distributions were obtained for each year fiom 1991 to 1996. The results are shown in Fig.
(5.5) to Fig.(S. 10) for the yean from 199 1 to 1996 respecfively. Frequency distributions
are provided in Appendix "W. The 50% value of m e n t ratio for aii years evem for 1991
which corrtains only one flash, is lying around 1.3, 26.37% of the data lies between 2 and
10. A total of69.2 % isgreaterthan one while only 30.7 % is less than 1.
in Fig. (5.1 1) the data accumuiated fiom 199 1 to 1996 is presented in the same
manner. The 500! value of the whole data sample is about 1.3. Thus the subsequent
magnetic field peaks are Orely to be greater than the peak of the first stroke. This is in
contrast to hdings for currents of subsequent strokes. In the previous chapter t was
observed that curent peaks of subsequent strokes are more likely to be less than the
m e n t peak of the first stroke. Yet CN Tower r d t s for the magnetic field agree with
the findings of Uman and Thottappillil[21] for the radiated electric field.
The fact that the rnagnaic field peak of subsequent strokes is most likely to be
more than the peak of the first stroke could be justifieci by looking at equation (5.2) where
the magnetic field is composeci of two components; an induction wmponent and a
radiation component. The first is dependent on the m e n t itself whiie the latter is
dependent on the current derivative. In the previous chapter, while the 50% value of
current peaks was somewhat less than one the 50% value of m e n t derivative ratio was
found to be much more than one. Thus these two fkctors, whtn combined, result in the
peaks of the magnetic field for subsequent strokes to be lwger than the peak of the fbt
one. In a study by Nucci et al. [Dl], the fiequency spectra of the azimuthal wmponent of
the magnetic fieid for fust stroke and subsequent strokes were found at various points at
different distances fiom the li-g chiuuie1. As shown in Fig.(S. 12) there exists a cross
over point at about 150 kHz, where the subsequent stroke spectral components becorne
greater than those of the first stroke. This supports Our finding that it is the radiation
wmponent that is responsible for the peaks of the subsequent strokes to be higher than
that of the f b t stroke.
5 -3 Maximum Steepness of Magnetic Field
Unlike the current, the maximum aeepness of the magnetic field wavefkont was
approxhated by a straight line satisfjrhg the conditions of the least square error method.
Several fittings are tried over various parts of the wave front until the IIIZUCIlIIum dope is
reached. Numerical differentiation was not used in this part because the superimposed
high frequency noise could lead to an erroneous value of the field maximum denvative.
As far as the cornparison between steepnesses of field waveforms in subsequent strokes
with those in the first one is concemed, the approach is the same iike that used for
currents and Wre that used in the previous section when dealing with field peaks.
The approach is SUmmarized as follows:
where
pok Ratio of maximum steepness of the azimutha1 magnetic field wave of
the kU stroke to that of the first stroke.
ok Maximum steepness of the azimuthai magnetic field wave in the kh stroke-
ai Maximum steepness of the azimuthd magnetic field wave in the 1. stroke.
and k > 1, as for k = 1, the ratio pek wiii be 1 which is not suitable to produce the
requued statistics.
The results are shown in Fig.(S. 13) to Fig(5.18) for the years fiom 199 1 tiil 1996
respectively. Frequency distributions are provided in Appendix 'W. The 50% value of all
the years is ranghg fiom 2 to 6. This is tme even for 1991 which contains ody one flash
with four subsequent strokes. In Fig. (5.19) the accumuiated data fiom 199 1 to 1996 is
repre~enfed in the same manner. The 50% value of the whole data sample is about 2.3.
60.4% of the data lies between 1 and 4, 16.5% is greater tban 4, while ody 23.1% is less
than one. Hence, subsequent strokes are most likely to have steeper magnetic fields than
those associated with the first one.
This result shouid be expected as by dserentiating both sides of equation (5 . la)
with respect to tirne we will £ind that the field denvative is composed of two wmponents
the first is dependent on the current derivative, while the second is dependent on m e n t ' s
second derivative. The effect of the first term is consistent with our obsewation that the
current of subsequent strokes have a higher cunent derivative. As for the e f f i of the
second term, Fig.(5.20) indicates on the basis of differentiation of 199 1 to 1996 m e n t
derivative observations, that the second derivative of the current is aiso greater for the
subsequent strokes than for the first stroke. Hence, the field maximum steepness of
subsequent strokes are most likely to be greater than that of the first one.
5.4 Risetime of Magnetic Field
For the sake of consistency with data obtained for currents in the previous chapter,
the risetirne is d&nd as the ciiflierence in t h e between the 100h and the 90% points on
the fiont of the field wave. Again the approach used for determining the ratio is the sarne
as in the previous sections and that used in the wrents, which could be summarized as
follows:
where
P* Ratio of r i h e of the azimuthi magnetic field wave of
the km stroke to that of the 1' stroke.
+k Risetirne of the azimuthal magnetic field wave in the k' stroke.
TI Risetime of the azimuthal msrgnetic field wave in the 1' stroke.
and k > 1, as for k = 1, the ratio p d wiîi be 1 which is not suitable to produce the
required statistics.
The results are shown in Fig(5.2 1) to Fig(5.26) for the yean from 199 1 îdl 1996
respectively. Frequency distributions are provided in Appendix "I". The 50% value of ail
the years includiag 1991 which contains ody one flash (with four subsequent strokes) is
ranging fiom 0.4 to 0.9. Also, 43.9 % of the data lies between O and 0.5 while only 23.1
% is greater than one. In Fig.(5.27) cumulative probability for al1 data aanmiulated
between 1991 and 1996 is shown. The 50% value of the whole data sarnple is about 0.6.
Hence, the field waves of subsequent strokes are most Eely to have srnaller risetime than
those associated with the 6rst one.
5.5 Relation Between Interstroke Interval and Field Peak
In order to study the fluctuation of the consecutive stroke field from large to s m d
values a closer look was to be made by andyzhg each flash separately. As in the case of
aments, again only the data fkom 1996 and 1995 was studied. The interstroke time for
these mes is in multiples of 14 ms while it was in multiples of 50 ms for mes of previous
years, which does not provide sufncient resolution between successive strokes. Six
flashes with stroke multiplicity ranging fiom 4 to 8 strokes per flash are studied in detail-
In Figs.(5.28) to (5.33) the peak ratio is plotted as a bction of time within each flash.
Similar to the study of curent, in spite of the fluctuations that are noticeable in these
graphs, it generally can be noticeci that a large peak is preceded by a large interstroke t h e
and vice a versa. This could be expected as the m e n t is the factor that produces the
field, so if large currents are widely spread out fiom each others, the fields wiIl be also the
same. In order to ftrther explore the relation b-een peak values of the magnetic field
and interstroke the, a regression analysis of field peaks was made. The results are shown
in Fig(5.34). The figure shows both regression lines for field peaks and cureut prrL*
These results indicate that with long interstroke interval a large peak both for cumnt and
magnetic field should be expected.
Fig. (5. la)
"f- i'
Geom Ir.
Fig. (Wb) A wrtr-cal lighmingjhdi to the CN Tower cuptwed by the vi&o camera on
seplember 11,1996 ai 12:03:16 am.
airPiaPuiYryi sl 01 s O s- ot- SI'
l &P
-*uy SI 01 s O 0- 01-
t Ci-
% . iœ1-
Fig. (5.4) Azimuthal magnetic field file f1086070.753 capiured on June10.1997 (II
1 l:tT:O3.
Fig. (5.5) Cumulative distribution of Jield peak
ratios /or 1996.
Fig. (S. 6) ~ Y R W ~ W ~ distribution of Jeld peak
ratios for 1995.
Fig. (5.7) Cumufative distribution of jkid pak
ratios for 1994.
Fig. (5.9) Cumulative distribution of field peak
ratios for 1992.
Fig. (S. 8) Cumulative disbrbrbution of fieid peak
ratios for 1993.
Fig. (S. IO) Cumulative distribution of field peak
ratios /or 199 1.
Fig. (5- I I ) Cumulative distnstnbution of fiekt peaC &os for all muitistroke flahes from
Fig(5.12) Amplitude spectra of the ananmuthal magnetic field of typical Jrst ami
&sequent r e m strokes mlmiated at grmnd ievel for three observation
points ( 30 pintF per de&) [24].
Fig. (5.13) Cumulative distribution of field
maximum steepness ratios for 1996-
Fig. (5.15) Cumulative distribution of field
maximum steepnes ratios for 1994.
Fîg(S. 14) Cumulative dish.ibutton of field
maximum steepnes ratios for 1995.
Fig. (5- 16) Cumulative distribution of Jeld
mmtimum steepnes ratiosfor 1993.
Fïg.(5.17) Cumufotive distribution of field
mmimrrm steepness rulias for f 992
Fig. (S. 18) Cumulative distribution of Jeld
rnarcimum steepness ratios for 1991.
Fig. (5.20) a-Secmd derivative of first stroke of mrrent derivative file fI085443.283
urprltred on Jzme 10. 1996 of 1 1-4: O3 p. m.
b-Secomi derivative of third s t d e of m e n t Mvutive file f1085443.283
capnrred on J~me 1 û, 1996 at I i:4:O3 p. m.
Fig. (5.22) Cumulative distribution o/ Jeid Fîg. (5.21) Cumulative distrrburim of /ield
risetime ratios /or 1996. risetime ratios for 1995.
ai ! O O. 5 1 1 5 2 2 5 3 3.5 4
Rue r- Ratm
Fig. (5.23) Cumulative distribution of Jeid Fig. (5.24) Cumulative distribution of Jield
risetime ratios for 1994 risetime ratios for 1993.
Fig.(5.25) Cumulative distribution of fieid Fig. (5-26) Cumulaiive distribution of Jeld
risetime ratios for 1992. risetirne ratios fur 1991.
Fi@. 2 7) Cmulalive distribu fion offieid rise fime ratios for ulf multistrok flashes from
Jg. (5.28) h t i o of fieid peak of &sequent to first strokes as afunczm of t h e for flash
h1461959.jTh
fg. (5.29) Ratio offeld peak of sirbsequent fo first strokes as a funetion of time for flah
fg. (5.30) Ratio of field peuk of mbsequent ro firsî strokes m a function of time for
Fig. (5- 3 &O offield peak of dsequent tu first strokes as O function of tirne for j b h
fig. (5.32) Ratio of field peak of subsequenl tofltst strokes as a function of t h e for frmh
O 20 u) 60 a0 100 123 140 160 r i wimn mso [ m s ~
&(5.33) Ratio of ament peak of subsepent tofirst strokes us a function of lime for
j7erh Ji 673 i'i 7.frh
Fig.(s_ 34) Regresszon iines between a) Field pe& with interstroke times. bb) Current
peaks with interstroke times.
c m SIX
6.0 CURRENTS AND FIELDS
This chapter attempts at the correlation between field and m e n t resuits obtained
fiom CN Tower flashes. As explained in section 5.1, UIlfortunately not ail curent records
have wrresponding field results. In addition to the fact that field files may wntain non-
C N Tower events, a ditnculty arises fiom the hct that the two systems are tnggered by
different signals. While recording of airrent files is initiateci by reaching a pre-set level of
current derivative, the field system is tnggered on the azimutha1 wmponent (H+) of the
magnetic field. Consequently, the field measuririg system may rrigger while the current
system would not and vice versa. Furthemore, there are simply situations where one of
the systems is in the recording mode or is not operational at the time of a lightning went.
In this chapter a subset of the data, which contains the files of fields and currents that
correspond to each other, is Jaidied. This subset consists of 52 subsequent strokes in 17
flashes for ai i the years fiom 1991 to 1996. The approach used and the parameters
studied are the same as those in the previous two chapters.
This subset of current files is the subset of current strokes that correspond to field
mes. As mentioned before, a lighniing m e n t wuld be missed as result of having low
steepness which is most Likely to happa to the first stroke. As a result it should be boni
in our miads h t our resuits are biased towards lower maximum m e n t derivative ratios
because recordeci first stroke have higher steepnesses than the mis& ones.
6.1 - 1 Current Peaks
Fig. (6.1) shows both the cumulative and fiequency distribution of current peak
d o s for the subset under consideration The 50 % ratio is about 1.01 which is neady
one. Unlike our expected resuit that the 5û% value of the current peak ratios shouid be
less than one, the 500/0 value is slightiy -ter than one. This cm be accounted for by the
fàct that this subset is somewhat different fkom the whole set. Inclusions into the subset
requires that first strokes simuitaneously exceed a minimum steepness of m e n t and a
minimum level of field. These requirmients may be responsible for the bias observed
within the subset.
6.1 -2 Maximum Current Steepness
Fig(6.2) shows both the cumulative and frequency distribution of maximum m e n t
steepness ratios for the subset under consideration. The 50 % ratio is about 4.5 which is
higher than the result obtained in chapter four for the overall data where the 50% value
was about 2. This confhs the bias for the wrent denvative of this subset with respect
to the results of the whole set. The observed higher 509/0 value of this subset over the
50% vatue of the whde set may again be explainecl as the wnsequence of the statistical
chance with which the subset happened to be selected fkom the whole set. The bias here is
in the sarne direction as in the case of the m e n t peak ratio.
6.1.3 Risetirne
Fig(6.3) shows both the cumulative and fiequency distribution of risetirne ratios
for the subset under consideration. The 50 0/, ratio is about 0.4 which is les than the
around 0.7 ratio for the overall set. This result agrees with the overall trend of the
absequent strokes to have less risetirne than the nrst stroke, and is consistent with the
observations made for the peak cument ratio and the maximum m e n t derivative of the
subset. While the risaime for this subset is lower than the overall one, the ratios for the
current peak and for the maximum steepness have higher 50% values for the subset in
cornparison to those of the whole set.
6.2 Fields
This subset of field files is the subset of field strokes that correspond to current
files. As mentioned before the problem with the field sensor is that they trigger upon
which could be due to a strike to the CN Tower or to one anywhere else. During the
saving time any strike to the tower wiii not be recorded. Besides, as the field system is
triggered upon & which is a hction of both the lightnhg current and its derivative, it
may not be triggered by a given lightning event whereas the current system which depends
only on current derivative would trigger.
6.2.1 Field P eaks
Fig.(6.4) shows both the cumulative and fkpency distribution of field peak ratios
for the subset under consideration. The 50 % ratio is about 1.3 which agrees with the
result of the overall data. So, again this confirms that unlüre the m e a t s , field
subsequent strokes are going to have larger peaks than the first one. An explmation of
this was presented in Chapter Five.
6.2.2 Maximum Field Steepness
Fig.(6.5) shows both the cumulative and fiequency distribution of maximum field
steepness ratios for the subset under consideration. The 50 % ratio is about 2. This
coincides with the overall behavior of all data set where the steepness of subsequent fields
are greater than the first one.
6.2.3 Field Risetirne
Fig.(6.6) shows both the cumulative and frequency distribution of maximum field
risetirne ratios for the subset under consideration. The 50 % ratio is about O.S. This again
confirms the findings of the overall data, that the subsequent fields are going to have
smdler risetimes than the first one.
6.3 Comments
Comparing the results of this subset with the overall data indicates that this
particular subset of current data is biased towards a higher value of current peak ratio, a
lower maximum steepness ratio and a lower risetime ratio. Meanwhile, for the fields, this
subset has shown no noticeable difference in its behavior fiom the overaii data. To
improve the situation, the trigger level of the current measuring system should be set
lower. However, the is not possible with the old coi1 since this would resutt in Mse
triggering of the systern due to noise at the tower. W~th the aid of the new equipment (the
optical fiber and the new mil) much lower triggering level should be achieved and a more
compatible subset of current waveshapes shouid resuit.
F i , (6.1) Frequency (a) and mmulative (b) distnstnbution of peuk nurent ratio of
Fig (6.2) Frequency (a) und ctrmulative (5) distribution of muximm m e n t steepness
ratio of subsequent fofirsr strokes ( rota[ of 52 strokes in t 7flashe.s).
Fig (6.3) Freq<le~cy (a) and eurnz~iative (ô) Cirstribution of m e n t risetirne ratio of
-
Peak Field Rato
Peak Field Raoo
Fig (6.4) Frequerzcy (a) and cumulalive (b) disrribution of peukfieid ratio of subseipent
to Jrst stmks ( total of 52 strokes in i 7flashe.s).
'"i r> 7 - œ -L
9 Z 6 - - C (I>
8 S -
5 2 4 - O
% 3 - 7
2 -
1 -
O O
Fig (6.5) Frequency (a) and cumuialiw (b) distrzbu~ion of mminrum field steepness ratio
Risetirne Ratio
Fig (6.6) Frequency (a) utxi andmulative fi) distribution offieid risetime ratio of
subseque~~t to first snokes (tuml of 52 strokes in I7jlashes).
CHAPTER SEVEN
7.0 CONCLUSIONS AND RECOMMENDATIONS
The analysis of stroke characteristics based on lightning data coilected as part of the
CN Tower lightning project between years 1991 and 19% has lead to the following
conclusions and recomrnendations.
1) Subsequent strokes are likely to have srnalier m e n t peaks than the first moke. The
50 % value of the subsequent to first peak ratio was found to be 0.9. This observation
is consistent with the physics of a Lightning flash. The stepped leader requires more
energy to propagate through the un-ionïzed air, whereas the dart leader wiU follow the
partiaily ionized channel l& nom the previous discharge with less energy requirernent.
In the study it was determineci that 16.8 % had their current peaks less than half of the
current peak of the kst stroke while 44 % of subsequent strokes had current peaks
ranging nom 1 to 4 times the value of the first stroke peak. 45.6% of all 125 strokes
considered had the current peak ratio greater than one, with a mean vaiue of 1.7027.
2) Subsequent strokes are rnuch steeper than the iïrst stroke. Overall statistics based on
al1 the accumulated data f?om 1991 to 1996, shows that the 50 % value is around 2.
Only 32.8 % of the data are l e s than one and about 7.2 % of the data are more than 10
times higher than the aeepness of the first stroke. This all means that it is highly
unlikely for the subsequent stroke to be less steep than the first one. This could be
explained by the fiin that for subsequent strokes the channel is fiilly re-ionized by a
quiclr progressing ciart ieader. The degree of re-ionization varies over a wide range,
yet, in general it is higher and more uniform dong the whole length of the lightning
channel than in the case of the fkst stroke.
3) For the current of a subsequeat stroke, the risetime is most likely to be shorter thaa that
of the first stroke. The 50% value of the overail risetime ratio statistics is about 0.7.
Also, 28.8 % of the accumulateci data has risetime ratios greater than one, while 32.8
% has its risetime ratio less than 0.50. This finding is consistent with observations
made on the maximum m e n t steepness ratio and is explaineci by the more uniform
channel ionization in a subsequent stroke.
4) Peak values of magnetic field waves in subsequent strokes of a muitistroke lightning
flash are most likely to be greater than the first peak. The SV? value of the whole data
sample is about 1.5. 69.2% of the data was greater than one. This is in contrast to
findings for currents of subsequent strokes. The magnetic field, however, is composed
of two wmponents an induction component and a radiation component. The first is
proportional to the m e n t itseif while the latter is proportional to the m e n t
derivative. The radiation component is the one that is responsible of our finding.
5) Subsequent strokes are most iikely to have steeper magnetic field waves than those
associated with the first one. The 50% value of the whole data sample is about 2.5.
This is because the field derivative is composed of two components. The first is
dependent on the current derivative, while the second is dependent on the second
derivative of the current. The effect of the first term is consistent with Our observation
that the current of subsequent strokes has a higher m e n t derivative. The second
derivative of the current is also greater for the absequent strokes than the first stroke.
6) Field waveforms of subsequent strokes are moa Siely to have smaller risetimes than
those associated with the first one. The 50% value of the whole data sample is about
0.6.
7) The study of the statistcally biased subset of the data that bas both magnetic field files
and current derivative files, confimeci in a statistical sense the above conclusions.
8) It is recommended for future work to study the thermodynamic behavior of the
lightning charnel and the speed of retm stroke propagation in order to explain
physical rasons for the observeci Merences between characteristics of the fkst stroke
and those of subsequent strokes in a lightning flash.
9) To improve coflecfion of lightaing current data and overwme the observed bias in
recordeci waveforms, the trigger level at the CN Tower should be lowered as low as
possible. The use of the fiber link and the new coi1 should be helpful in that regard.
However, it should also be investigated whether the lightning current rather than its
derivative, as is done at this t he , should be used as the triggering signal.
REFERENCES
[l] W. Janischewslryj, AM. Hussein, P. Dziurewicz, V. Shostak and W.A Chisolm.
"Characterization of the m e n t wav&ont parameters of lighming strikes to the CN
Tower in Toronto", ISH, Yokohama, Japan, Paper 70a (pp- 1-4). August 23-27,
1993 .
[2] Chnstiane Leteinturier, Joel H-Hamelin, Andre Eybert-Berard, " Submicrosecond
characteristics of i ighhg retum-stroke currents", IEEE Transactions on
Electromagnetic Compatiblity, vol. 33, No. 4, November 199 1.
[3] W. Janischewskyj, V. Shostk AM. Hussein and P. Dziurewicz, "Line Arresters and
parameters of upward-initiated lighming flashes" . CIGRE SC 33.93(COLL), New
Delhi, 1993.
[4] W. Janischewskyj, A.M. Hussein and J.S. Chang, "Characteristics of CN Tower
multistroke flashes", The lûth International Symposium on High Voltage
E n g h e e ~ g (ISH), Montreal, Quebec, Canada, pp. 29-34, Aug. 25-29. 1997.
[SI M.A. Uman, 'The Lightning Discharge", Academic Press, Inc., 1987.
[6] R.H.Golde, "Lightning", volume 2, Academic Press, London., 1977.
[7] M. Daweniza, L.R. Turna, B. Richter, D.A Roby, "Multipulse Lightning currents and
metal oxide arresters", IEEE Transactions on Power Delivery, Vol. 12, No. 3, pp.
1 158- 1 175, July 1997.
[8] Y. Mwooka, S. Yokoyama, A. Asakawa, "Protection of power distribution lines
against lightning-induced overvoltages by means of surge arresters and overhead
ground wires", ISE& Yokohama, Japan. pp. 397400, August 23-27, 1993.
[9] M. Darveniza, D. Roby and L.R Tumma, "Laboratory and analytical studies of the
effects of multipuise lightning a>rrent on metal oxide arresters", IEEE T d o n on
Power Delivery, Vol. 9, No. 2, pp. 764-770. April 1994.
[IO] W. Janischewskyj, AM. Hussein, V. Shostak 1. Rusan, J.X. Li and J.S. Chang,
"Statistics of lightning strikes to the Toronto Canadian National Tower", IEEE
Transaction on Power Delivery, Vol. 12, No.3, pp. 12 10-1221, July 1997.
[ i l ] W. Janischewsiy, G.G. Hu, T.R McComb, H. Linclq J.S. Chang, W. Chisoim,
"Characteristics of lightning sPikllig an extremely ta11 tower", Wth International
Conference on Atmosphenc Electricity, Albany, N.Y., pp. 365-3 59, June 3-8, 1984.
[12] R Rusan, "Computation of electromagnetic fields radiated by lightning strikes to the
Toronto CN Tower", M.ASc thesis, University of Toronto, Toronto, Ontario,
Canada, 1997.
[13] A Obaid, "Improving the SNR of the lightning records captured at the CN Tower
using Linear Filtering Techniques", B. A Sc. Thesis, University of Toronto, Toronto,
Ontario. Apr. 1993.
[14] Y. Chen, "Wavelet Analysis and Statistics of (SN Tower Lightning Current
Waveforms", M.ESc. Thesis, University of Western Ontario, London, Ontario, May
1997.
[15] W. Janischewskyj, V. Shostak, J. Barratg A.M. Hussein, and J.S. Chang, "Collection
and use of lightning r e m stroke parameters taking into account characteristics of
the stnick object", 23" International Conference on Lightning Protection (ICLP),
Florence, Italy, pp. 16-23, Sept. 23-27, 1996.
[16] H. Motoyarna, W. Janischewskyj, AM. Hussein, R Rusan, W-A Chisolm and J.S.
Chang, 'ZElectromqpetic field radiation mode1 for lightning strokes to tail structuresy7,
IEEE Transactions on Power Delivery, Vol. 1 1, N0.3, Juiy 1996.
[17] P. Daire- "Measurement and Analysis of Lightnhg Strokes to the CN Tower in
Toronto", M. A Sc. Thesiq University of Toronto, Toronto, Ontario, Sept. 1993.
[18] 1. Rusan., "CN Tower Lightning Parameters", M.ESc. Thesis, University of Western
(harb, London, Ontario, May 1996.
[19] A.M. Hussein, W. Janischewslqj, J.S. Chang, V. Shostalq W.A Chisolm, P.
Dzurevych, and Z . -1. Kawasaki, "Simultaneous measurement of lightning parameters
for strokes to the Toronto Canadian National Towei', J o u d of Geophysical
Research, Vol. 100, No. D5, pp. 8853-8861, May 20, 1995.
[20] Pak Lai Wong, " The construction of three dimensional lightning stroke trajectory
fkom two dimensionai pictures and calculation of three dimensional tightning
velocity'', B. A Sc. Thesis, University of Toronto, Toronto, Ontario. Apr. 1995.
[21] M. Rubinstein, F. Rach& M.A Uman, R Thottappillil, V.A Rakov and C.A. Nucci,
"Characterization of vertical electric fields 500 m and 30 m from triggered
hghtning", Journal of Geophysical Research, vol. 100, No D5, pages 8863-8872,
May 20, 1995.
[22] W. JanischewslryJ, A.M. Hussein and V. Shostak, "Propagation of lightning m e n t
within the CN Tower", CIGRE SC 33.97(COLL), Toronto, Ontario, Canada, Sept.
2-3, 1 997.
[23] R Thottappülil, V.A Rakov, M.A. Uman, W.H. Beasley, M.J. Master and D.V.
Sheiukhin, "Lightning subsequent stroke electric field peak greater than the first
stroke peak and multipie ground terminaîions", Journal of Geophysicai Research,
vol. 97, No D7, pages 7503-7509, May 20, 1992.
[24] C.A Nucci, C. Mazzetti, P. Rachidi, M- Ianoz, " Analysis of electromagnetic field
onginated by tightning rehirn stroke in the time and fiequency domah'', Annales de
Telecommunication, no. 1 1 - 12, pp.625-63 7, 1988.
APPENDIX A
The expenmental setup used in testing the old coil on May 27,1997 is show in
Fig.(A. l), The outputs of the current transformer and the coil are shown in Fig.(A,Z). The
results are shown in table (A. 1 )
Fig. (A. 1) Eirperirnentui Set up [ 161 RTD - Real T h e Digtizer, GGPIB - General P.pose Interface Bus, PC - Personal
Compter
teskgle27 1 3552-391 0.4. 1 1 I I 1 I r 1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 - - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -
time:of the peak = 8.06- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -
0-1 - . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . curr.deriv: . = . . . ~ ~ 3 4 2 5 . -
0 B . . . . . . - - - -100 -80 -60 -40 -20 O 20 40 60
time in micro-seconds
time in micro-seconds
Fig. (A-2) Ouput of Rogowski Coii and its integrution.
Table (A. 1) R e d i s of experimenf on Mq 2 7.199 7
File name [CurrentpeakinA lCurrentinteg~ionpeakv.usec ~Sensiüvityv/(Nns) e27131 90,171 I 584 1 0.2007 1 0.3437 é27 1 3552-39 1 I 576 1 O -2064 1 0.3583
The following table shows the results of testing the new coi1 on May 23, 1997 in
the Lightning Research Laboratory.
- file name jCumnt peak in A e2350652.761
i
! 482.4 e2350710.091 ! 480.5
Cumnt integration peak v.u.sec 0.798
.0.7953 O. 5466 0.526
0.501 6 0.5484,
e2351 126.151
Sensitivity v/(A(ns) 1.6542 1,655 1 1 ,0601 1,0438 1 .O398 1 -0361
S i 5.6
0.~51 81 1.0388
e2351164.681 1 503.9 e2351191.351 e2351823.581 e2351867.321
482-4 529.3 531.2
APPENDIX C
The result of testing of the new coi1 supplied by the manufacturer (Physics
International ) is shown in Fig.(C. 1 )
Fig.(C. 1 ) The testing results supplied by the manufacturer, the testing pulse and the
ddat ions are showri
APPENDIX D
F&(D- l j Frepency .disaibution of current
peak rarios for 1996.
Fig.(D.31 Frequency disbibution of m e n t
peak raîïos&r 1994.
Fig.(D.?) Frepency distribution of cumnt
peak r a s for 1995.
Fig(D.4) Frequcncy distribution of cumnt
peak ratios /or t 993.
Fig.(D.5) Freqirenq distribution of mmnr
peak ratios /or f 9 92.
F m . 7) Frequency dï~ar~bution of Ctlnenf peat ratios for all mdtistroke jI'4she.s from
1991 to 1996.
APPENDIX E
Maximum Current ûematm Ram Fig.(E. I ) Frequenqv distrlburion of mmsirnurn
MaxlmumCun%rrtOsnvaaMRaaio
Fig,(E.Z) Frequency distribution of ltuaclltUaClrnum
current derivative ratios for 1996. m e n t derivative ratios for 1995.
l L
6 1
8 10 12 14 16 18 20 Maximum Current Oemative Ratio
Fig. (E31 Frequency distribution of maximum Fig.64) Frequenqv disrriburion of maximum
current derivative ratios /or 1994. current derivative ratios for 1993.
Fig.(ES) Frequency disniburion of maximum
current dcrivcrtive ratios for 1993.
Fig. (E. 6) Frequency disrriburion of maximum
APPENDIX F
f Riitime 2 Raio 3 4 5 w -1 O I L 1 . - 2 Ratio 3 4 I
Fïg. (;E I } Frequency distribution of current Fig..(F.2) Ftequency dim&ution of -nt
risetime ratios fur 1996. risetime ratios for 1995.
Fig.F.4) Frequency d i . s b l b ~ ~ z ~ n of cwrent Fig. (E 3) Frequency distribution of current
nmttme 1993. risetime ratios for 1994.
Risetirne Ratio
Fig. (;E 5) Frequency distribution of current Fig- p. 6) Frequency distribution of current
risetirne ratios for 1992. rrserirne ratios for 1991.
Risetirne Ratio Fig(F. 7) Freqnency aïs~ibutzm of Rcrrei>r rlsetrme ratmsjor ai[ multistroke flashes from
1991 to 1996.
APPENDIX G 12 'r
Peak Ratio -
Peak Ratm klg- (G. 1) Frequency d~sfr~burton oj jreld peuk Fig. (G. 2) Frequency d~sirtbutton of field peak
ratios for 1996. ratiosfor 1995.
Peak Raia
Fig. (G.3) Frequency distribution o//ield peak
ratios /or 1994.
Peak Ratio
Fîg-(G. 4) Frequency di~ln~button of jeld peak
ratios for 1993.
Peak Ratio Peak Ratio
Fig. G.5) Frequency distribution of fleld peak Fig. (G. 6) Frequency distrib ution of jîeld peok
ratios for 1992. ratios /or 1 99 1. 301 1 I l
Frequency diszribution Peak Üatio
of-field peak raiios for muitrstroke flashes from
Fig. fi. 1 j Frequency distributr on of maxrmum Fig. (H. 2) Frequenq d~strrbution of maximum
Jeld steepness ratiosfor 1995. Jeid steepness ratios for 1996.
Fig.fl.3) Frequency distrroution of maximum Fig. (;H.4) Frequency distribution of muximum
field steepness ru rios for 1994. Jeld steepnes ratios for 1993.
Fig.fl.5) Frequency distribution of mmimum m..(n.6) Frequency distribution of marirntrm
field steepness ratios /or 1991.
h l a x i m m S ~ R a t i 0 Fig(H. 7) Frequerzcy distribtition cf rnmzmum field sfeepness ratlos ]or all mrdtzstroke
APPENDIX 1
I
fig. fl. 1) Frequenqv distnbutzon O fjield risetirne Fig. (7-2) Frequency dislnbution of field rise frme
ratios for 1 996. s
ratios for 1995.
1 0- 1.5 2 2 5 3 3.5 L O
RUe Time Ratio 2 2 5 3 3.5
Rise Tme Ratio Fig. fl. 4) Frequency distribution o f w d risetime
Fîg. (7.3) Frequency distribution ol/ied risetime
ratios for 1993. ratioslor 1994.
j ! 1 1.5 2 3 3s 25
Rise Time Ratio
F J ~ . @-5) Frequency distribution o ffield risetirne Fig. (l. 6) Frequency distribution m e l d riserime
rafins for 1992 ratios /or 1991.
O 0.5 1 1 .S 2 2 5 3 3.5 4 R i s e fima Aatio
Figti. 7) Freipency &tribution of j i e d risefime ratzos for aii mu~tzsnoke flashes jrom
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