1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF TU The protection of telecommunications lines and equipment against lightning discharges (Chapters 9 and 10) Geneva 1995 O 1995 All rights reserved. No part of this publication may be reproduced or utilized in an
2、y form or by any means, electronic or mechanical, including photocopying and microfilm. without permission in writing from the . 4862591 Ob7911l 626 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU The protection of telecommunications lines and equipment ag
3、ainst lightning discharges (Chapters 9 and IO) Geneva 1995 ISBN 92-61 -05501 -X CONTENTS Page Chapter 9 . Fibre optic cable lightning damage assessment 1 General . 2 Introduction 3 Direct buried cables Lightning damage mechanisms in buried cable Annual damage rate for buried cable (first approach) A
4、nnual damage rate for buried cable (second approach) Improvement due to shield wires Improvement due to route redundancy . 4 Aerial cables . 3.1 3.2 3.3 3.4 3.5 4.1, 4.2 4.3 Annual lightning damage rate assessment for aerial cable . Improvement due to joint use with a power line (aerial construction
5、) . Improvement due to route redundancy . References . Chapter 10 . Overvoltages and overcurrents measured on telecommunication subscriber lines 1 General . 2 Measuring equipment . 3 Classification of lines . 4 Parameters of the overvoltages and overcurrents . 4.1 Classification 4.2 Peak value x, 4.
6、3 Front time or rise time Ti 4.4 Steepness of the front or rate of rise S 4.5 Equivalent decay time to half-value T2 . 4.6 Specific energy . Statistical evaluation of lightning parameters . 4.7 4.7.1 Introduction . 4.7.2 Logarithmic normal distribution . 4.7.3 One-side truncated logarithmic normal d
7、istribution . 4.7.4 Heterogeneous population 5 Results 5.1 General 5.2 Rural area 5.3 Urbadsuburban area . 6 Conclusions References . Table of Contents 1 1 2 3 4 4 8 8 9 9 10 11 11 12 12 12 13 13 13 13 14 14 14 14 15 18 18 18 19 22 23 24 1 Page Appendices to Chapter 10 . Classification and statistic
8、al evaluation of parameters Appendix I . Measurement results in Canada . 25 1.1 Introduction 25 .2 General survey description . 25 1.3 Monitoring techniques 1.3.1 Level monitors 1.3.2 Digital waveform monitors . 1.3.3 Carbon arrester analysis Comments on the carbon analysis method 1.3.4 1.3.4.1 Trai
9、ning . 1.3.4.2 Limitations 1.3.4.3 Accuracy of analysis . 1.3.4.4 Discharge mark characteristics . 25 25 25 26 26 26 26 26 27 1.4 Results 27 Appendix II . Measurement results in France . 31 II.1 Introduction 31 11.2 Measuring equipment and experimental line characteristics 31 II.3 Analysis of induce
10、d overvoltages . 33 II.4 Statistical analysis of induced overvoltages . 33 11.4.1 Analysis of common-mode voltages . 33 11.4.2 Analysis of the differential-mode voltages . 37 11.5 Conclusions and outlook . 39 References . 39 Appendix III - Measurement results in Germany . 40 m.1 Introduction 40 III.
11、2 Measuring equipment . 40 III.3 Application of surge voltage counters 40 III.3.1 Sites . 40 II.3.2 Connection of counters . 41 ITI.4 Results 41 JII.4.1 Direct readings 41 Normalization to 20 thunderstorm days and per line III.4.3 Influence of the length of a line 42 Influence of the aerial section
12、of a line . lII.5 Conclusions 42 III.4.2 41 III.4.4 42 Appendix IV - Measurement results in Italy . 43 IV.1 Introduction 43 IV.2 Measuring equipment, location and route characteristics . 43 IV.3 Parameters of measured overvoltages and overcurrents . 46 IV.3.1 Classification of surge events . 46 IV.3
13、.2 Parameter distributions . 46 IV.3.3 Relationship between voltage and current 48 ii Table of Contents m 4b2593 Ob79LL4 335 Page Appendix V . Measurement results in Japan V.l Introduction V.2 Lightning surge voltage V.2.1 Measurements . V.2.2 Cable conditions V.2.2.1 Cable types V.2.2.2 Cable lengt
14、h V.2.2.3 Buried and aerial cables V.2.2.4 Terminal conditions . V.2.3 Geographic factors V.2.3.1 Soil conductivities . V.2.3.2 Lightning surges in winter and summer seasons . V.2.4 Lightning surge-voltage distributions . V.3 Lightning surge current . V.3.1 Measuring method V.3.2 Measurement results
15、 . References . Appendix VI . Measurement results in United States of America VI.1 Introduction VI.2 VI.3 Measuring equipment, location and route characteristics . Parameters of measured overvoltages and overcurrents . VI.3.1 Peak voltage VI.3.2 Peak current VI.3.3 Relationship between voltage and c
16、urrent VI.3.4 Voltage rate of rise VI.4 Conclusions References . Table of Contents 49 49 49 49 52 52 52 52 52 53 53 53 53 58 58 58 60 61 61 61 61 61 63 64 65 65 66 . 111 M 4862573 0679115 271 CHAPTER 9 FIBRE OPTIC CABLE LIGHTNING DAMAGE ASSESSMENT 1 General An optical fiber cable containing metallic
17、 components is susceptible to lightning damage for both aerial and buried constructions. Two theoretical methods to calculate the annual frequency of fiber damage due to lightning effects, are presented. Ail the graphs, etc., needed for one of the methods, are given; a numerical integration is neces
18、sary for the other. The annual damage rate estimates thus obtained can be used to quantitatively compare route system design and optical fiber cable design alternatives. The algorithms are given for improvement gained by route redundancy, shield wires and joint use with power lines. 2 Introduction A
19、n optical fiber cable which has metallic components, either in the sheath or in the core, is susceptible to lightning damage. The reasons for having metallic components may be to provide tensile strength, moisture barriers, rodent protection andor communication (“talk-pair”) facilities. Cable locati
20、ng for maintenance and repair activities is also facilitated by metal in the cable. The fact that lightning can damage fibers in such cables has been evidenced in the field and in lightning simulation experiments I. This methodology provides two numerical approaches to estimating the primary damage
21、rate. Primary damage refers to instances where the fiber is out of service. The result yielded by the first assessment technique is useful for comparison purposes whether various route designs or cable designs are being evaluated. It may also be used when an absolute value is needed. The result yiel
22、ded by the second assessment technique may be used when a conservative value for Annual Damage Rate is needed, or for comparative studies. Work is currently under way to study the actual field damage rates and compare them to the calculated rates. Secondary damage, for instance pinholing which may i
23、ncrease the corrosion rate, can be evaluated by means of the formulas given in Appendix 5 of Chapter 7, or by the algorithms presented in 5. Any lightning damage mitigation technique, e.g. route redundancy, shield wires, more rugged cable, etc. should only be considered in cases where the lightning
24、damage rate is significant in comparison to other sources of cable damage, e.g. cable cuts. Improvement in the overall system availability can be calculated using the methods given. Direct buried cables are first considered, then aerial cable. 3 Direct buried cables 3.1 Lightning damage mechanisms i
25、n buried cable When lightning strikes the ground in the vicinity of a buried cable, arcing can occur between the stroke point and the buried cable if the stroke current is large enough to cause soil ionisation 2, 31. The stroke can cause three possible problems resulting in primary damage. Dielectri
26、c breakdown between metallic members of an optical fiber cable (e.g. metallic sheath and strength member or copper pair) can cause an arc to occur. Fiber damage is probable in cases where the fibers are near or in the path of the arc i. Cable crushing can occur due to compressive forces set up aroun
27、d the cable by the passage of lightning current through the soil. Field observations indicate that a lightning stroke causing a hole in the sheath metallic layer adjacent to the fibers, can cause a shockwave to enter the core. Damaged fibers have been found in the vicinity of such holes. Chap. 9 1 _
28、 = 4862591 06771116 1OB 3.2 Annual damage rate for buried cable (first approach) To estimate the damage rate per year, the annual lightning stroke incidence (Nb) is first determined as follows: strokedyear (9-1) where: Ng is the number of strokes to earth per square km per year and is given by Kt x
29、F, x Td; Kt is the terrain factor which is equal to unity for flat terrain. For a more exposed area (e.g. on a hill, or close to tall trees). Kt would increase to 2 or 3 3,4; F, is the stroke factor which indicates the number of lightning strokes to earth per km2 per thunderstorm day. F, is dependen
30、t on the storm type and can vary between extreme values of 0.05 and 0.25 per km2 per thunderstorm day, with the lower and higher values generally being representative of convectional and frontal storms, respectively. A typical value used in a North American convectional storm area is 0.1 l/km2/Td 3;
31、 Td is the number of days during which thunder is heard at a specific observation point. Td can be estimated using the isokeraunic maps given in Figure 9-1. More detailed isokeraunic maps of limited areas also exist and can be obtained from national government agencies; 2DL in equation (9-1) constit
32、utes the area which is susceptible to direct lightning strike or arcing from a stroke point. L is the route length in km. 20 is the surface width affected by lightning and D is calculated using the equivalent arcing distance 2: D = 0.365 .Ip m; forp I 1OSZ.m (9-2a) D = 0.22 .Ip m; forp 2 1000Q.m (9-
33、2b) where p is the soil resistivity in a.m. and is the reciprocal of soil conductivity. The value of p can be found from soil resistivity maps or it can be measured. An algebraic or graphical interpolation method which assumes a straight line equation may be used to calculate D for values of p betwe
34、en 100 S2. m and loo0 Cl. m. Figure 9-2 illustrates the graphical technique. As an example, using Figure 9-2 the equivalent arcing distance, D = 5.15 for p = 500 m. Note that maps of Ng may be available in some countries. To now estimate the Annual Damage Rate (denoted by ADR), the stroke incidence
35、must be multiplied by the probability of the damaging current occurring on buried cable. Figure 9-3 provides the probability distribution of lightning stroke current 3 for buried structures. ADR = Nb x p (2 I) damageslyear (9-3) where: I is the current in kA which causes damage to the fiber. It can
36、be determined experimentally l; p indicates the probability of I being exceeded. The inverse of the ADR due to lightning will yield the mean time between failures in years. 2 chap. 9 4 hence, the term 21in equation (9-18). References 11 2 3 4 5 ITU-T Recommendation K.25, Lightning Protection of Opti
37、cal Fiber Cables. SUNDE, (E. D.): “Earth Conduction Effects in Transmission Systems” D. Van Nostrand Company Inc., 1949. BODLE (D. W.), GHAZI (A. J.), SYED (M.), WOODSIDE . L.): Characterization of the Electrical Environ- ment, University of Toronto Press, 1976. AT - the parameters peak value, front
38、 time or rise time, steepness of the front or rate of rise, equivalent decay time to half value and specific energy. These data are necessary in standardizing the surge voltage generator for test purposes. To record voltage and current waveforms and then evaluate the above parameters, Transient Moni
39、toring Systems (TMS) were used. A short description of the different TMS used can be found in the Appendices II, IV, V and VI. In many cases relatively inexpensive surge counters - to measure only one or two parameters such as peak voltage or current, duration and specific energy - were developed (s
40、ee Appendices I, III, IV and V). Chap. 10 11 _ - _ m lfBb259L ObiL2b 057 m Lightning current data were also obtained by measuring the discharge vestige areas on carbon-block arresters installed at the subscriber line ends. Lightning current parameters were estimated by comparing the vestiges with re
41、ference vestiges on carbon blocks specially prepared in the laboratory using known current source (see Appendices I and V). Measuring equipment - TMS, counters and new carbon block arresters - were instailed in switching centers and at subscriber premises between one conductor of a pair and earth. A
42、rresters were removed from the lines monitored using TMS and counters. Voltages were measured across a 200-2000 ohm termination resistance at the exchange end and across a 200 ohm or high (-100 megaohm) termination resistance at the subscriber end. The results indicate that surge voltages are not se
43、nsitive to variation in resistance above 200 !2 (see Appendix V). Currents were measured on the conductor to ground (short-circuit currents of the line) at the exchange or subscriber end. 3 Classification of lines For telecommunication equipment the ITU-T Recommendations 1, 21 suggest two levels of
44、resistibility: a lower level suitable for “unexposed” environment and a higher level for “exposed” environment. In fact the environment is the main factor which has an impact on the values of the overvoltage and overcurrent parameters; therefore it is expedient to classify the collected data as func
45、tion of the area where the line is installed. One possible classification is the following: - - area outside buildings: urban, suburban and rural zone; area inside buildings: normal, tall and large building. For the surveys, it was normally considered expedient to select only lines on which a suffic
46、ient number of surges was likely to occur. In city centers (urban zone) and inside buildings, in particular normal buildings, this condition usually is not met. Therefore, most of subscriber lines were chosen in rural zones and only few surveys considered lines in urbanhuburban zones (see Appendices
47、 I and III). Therefore, in the following, the summarized data are divided in two groups as function of the area where the monitored line was located: - rural area; - urbadsuburban area. The monitored lines were formed by a mixture of both underground and aerial cables and both unshielded and shielde
48、d cables. So it is not possible a classification of the lines as function of the cable type or cable installation. 4 Parameters of the overvoltages and overcurrents 4.1 Classification The lightning records are categorized into flashes and strokes. A lightning flash typically consists of several well
49、 defined strokes and is normally less than one second in duration. Accordingly, lightning events separated by less than one second are considered to have been caused by strokes belonging to the same flash. This classification proves operationally useful, despite its physical ambiguity, since closely spaced pulses may produce thermal effects in terminal equipment which are different from those produced by the same pulses widely spaced in time. Typical waveforms of strokes, showing voltage as function of time, are given in th
