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200619th International Lightning Detection Conference24-25 April • Tucson, Arizona, USA 1st International Lightning Meteorology Conference26-27 April • Tucson, Arizona, USA

THE IMPORTANCE OF RELIABLE MEASUREMENTS OF LIGHTNING CURRENTS TO ELECTRIC POWER COMPANIES

M. G. Alvim* C. Portela A. R. Nobrega FURNAS COPPE/UFRJ FURNAS

Rio de Janeiro, Brazil Rio de Janeiro, Brazil Rio de Janeiro, Brazil

*Rua Real Grandeza, 219 – Bloco A – Sala 406 Botafogo – Rio de Janeiro – RJ – Brasil – 22283-900

E-mail: alvim@furnas.com.br 1. INTRODUCTION FURNAS is an electric power company and it owns 9 hydroelectric and 2 thermoelectric power plants, with an installed capacity of 9080 MW, 41 substations, 19 000 km of overhead transmission lines with voltages from 138kV to 750 kV (AC) and ± 600 kV (DC). FURNAS’ electrical system is supervised by a telecommunication system composed of 10 optical repeater stations, 3 satellite sites and one hundred of microwave radiate stations. The electrical and telecom systems are often vulnerable to lightning effects that may be a major cause of system faults. Lightning represents a threat to people, equipment safety and quality of electricity supply. The problem is more important in Brazil than, e.g. in most of North America and Europe, due to higher lightning intensity and more unfavorable soil characteristics, by geological reasons. Adequate conception, design and operation of electrical and telecommunication systems depend on a good knowledge of pertinent lightning parameters, including aspects related to geographic distribution and to probability distribution, in design stages, and specific parameters of each relevant flash in system operation. The engineering treatment of problems related to lightning, in what concerns electrical and telecommunication systems, has the following important drawbacks: – The information related to parameters of real flashes is quite limited. For instance, reliable information covering relevant parameters for several studies covers a very limited number of measurements. For many respects, the most reliable information is still the measurements of Berger more than 30 years ago and some other measurements that also cover a very limited number of flashes.

– The information related to parameters of real flashes measured by radar or by equipment based in electromagnetic waves originated by flashes, and from which (with some “arbitrary” assumptions concerning lightning channel, soil orography and parameters) it is estimated lightning location and current amplitude, covers a large number of flashes, but is affected by a relatively high margin of error. Also, such information does not cover parameters that allow to evaluate effects of “measured” flashes for most engineering applications. As a consequence, for a specific occurrence, available information does not allow to know its relevant lightning parameters. Also, the available information of relevant parameters for most engineering applications did not increase with the newly introduced procedures to evaluate and “measure” lightning. – Due to historical and cultural reasons, most common procedures to evaluate consequences and effects of lightning consider assumptions far from reality or not adequate to deal with fast electromagnetic phenomena. Errors of several common procedures results may be of order of magnitude. For instance: a) some standards indicate procedures with important physical errors; b) soil is frequently assumed to constant conductivity and negligible permittivity, although, in frequency spectrum important for lightning effects, soil electric conductivity have order of magnitude changes, electric permittivity has influence quantitatively similar to conductivity, is also frequency dependent and has values orders of magnitude higher than assumed in some used procedures. As an example, a computational program developed by a CIGRE committee and frequently used to evaluate electrical transmission lines lightning performance, may originate order of magnitude errors in number of faults due to lightning, due to assumptions contradictory of physical behavior. This error has been practically confirmed in recent projects using such program. Although much better and robust procedures are available, e.g. [1-13], some of them with a good

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long time experience in important projects in Brazil, its use covers only a small part of engineering studies and projects. It must be emphasized that the lightning information required by electrical power companies involves several different parameters, and that relevant parameters depend on specific application, e.g.: – Probability of faults and damage in transmission and distribution lines and cables originated by lightning, considering incidence point (in several line or cable elements, or in soil in line vicinity, along the line, incidence angles, wave shape and amplitude, polarity, single or multiple strokes). Consequent effects for human safety. Consequent effects in equipment connected to lines and terminal substations, power plants and loads. Interaction with grounding systems of line towers and cables. – Probability of faults and damage in substation and power plants originated by lightning, considering incidence point (in several constructive elements, or in soil, incidence angles, wave shape and amplitude, polarity, single or multiple strokes). Consequent effects for human safety. Consequent effects in connected to lines and cables and supplied loads. Interaction with grounding systems. Interference effects in control, protection and communication systems. The engineering situation is somewhat frustrating when comparing the “expectative” that has been created of availability of “new” information related to lightning and the effective “new” information in what concern relevant parameters for many engineering studies and design. The present situation has several reasons, and involves, probably, the following aspects: – Technological difficulties, of several types, in evaluating several of such relevant parameters with some of the recently developed technologies. – Lack of interaction and “cultural differences” between people that evaluate lightning effects, in several engineering fields, and people that develop technologies and procedures to “measure” lightning parameters. – Lack of clear, reliable and objective information of limitations and error limits of recent

technologies and procedures, covering aspects related to engineering applications. – Lack of efforts to identify and develop complementary procedures and technologies to cope with aspects for which recently developed technologies have important limitations or that justify other approaches. The main objective of this paper is to call attention for this problem, in the sense of obtaining, in the future, more lightning information covering the effective needs of most engineering problems related to lightning effects. It is not pretended to cover a general discussion of the subject that would involve a large number of topics. So, we present a concrete recent example of an engineering problem, in which, by several reasons, the specific problems concerning lightning parameters has been deliberately “over simplified”, obtaining in laboratory tests results similar to lightning effects. With this approach it was possible to obtain technical solutions, although the physical mechanisms associated to field effects of lightning may be somewhat different of physical mechanisms in laboratory tests with similar effects. We hope that, with more deep analysis of lightning effects and mechanisms, a more complete interpretation of the problem will be obtained. Recently FURNAS has faced a great problem concerning to atmospheric discharges. The first optical fiber ground wires (OPGW) installed on transmission lines which crossed areas with high keraunic and lightning density levels, were often damaged by lightning discharges. In order to solve this problem, FURNAS and other Brazilian utility companies have developed their own studies looking for solutions to those specific problems in Brazil, since the technical literature on the effects of atmospheric discharges on OPGW was scarce. These studies have led to development of OPGW resistant to lightning and they were accomplished without the information of parameters of lightning flashes that originated OPGW damages. The results of the researches carried out by FURNAS on OPGW cables led to the conclusion that the impact of various lightning components involves several physical mechanisms and this not dealt with in the traditional technical literature. These physical mechanisms can be dealt with, for a specific flash, if the sensors for detection of

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discharges contain at least the records of the current waveform of discharges in real time. With the information presently available this is not the case. These studies have also showed that the integral action of the lightning currents seems not to be related with i2 dt∫ used in traditional literature to evaluate damage in a cable due to a short duration current. 2. DAMAGES ON OPGW 2.1 General The first OPGWs were installed in Brazil in the 90’s and suffered a lot of damages due to the atmospheric discharges. At that time very few countries used to have fault problems in their electrical systems due to incidence of atmospheric discharges on OPGW. In Brazil, the main damages caused by lightning occurred in the southern states of Paraná, Santa Catarina and Rio Grande do Sul and in the Central state of Tocantins. Most of the accidents caused by lightning discharges on OPGW jeopardized the electrical system. A typical damage is the breaking of the OPGW wires. Lose broken wires might hang and get too close to a conductor cable causing a short circuit. As a result, the transmission line becomes inoperative. (See Figure 1).

Figure 1

Also when the OPGW is totally broken, it can cause a short circuit by touching conductor cables consequently turning off the transmission line (see figures 2 and 3). These kinds of occurrences render the transmission lines inoperative causing great damage to the electric power companies.

Nowadays some countries such as Russia, Canada and Australia face this problem and there is a consensus, in CIGRE, of the need of studying the causes of these faults at an international level.

Figure 2

Figure 3

2.2 Representative tests of field damages By several reasons, the test parameters were chosen through the comparison of damages observed in the laboratory and those occurring in the field. The limit of these parameters was based on MIL.STD-1757A Lightning Qualification Test Techniques for Aerospace and Hardware Standard (a schematic representation of current shapes as indicated in this standard is shown in Figure 4). An additional limitation, below limit values of this standard, has been considered for component A of figure 4, due to available laboratory equipment, and practice of some entities that supply equipments that must comply with this standard. These studies were started by two separate tests intended to simulate two components of a lightning discharge current, in the laboratory. The first component was a short-duration, high–

CCOONNDDUUCCTTOORR CCAABBLLEE

OOPPGGWW

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amplitude current pulse (component A of figure 4) and the second component was a low-amplitude, long duration, DC current, intended to simulate the continuing current (component C of figure 4) of a lightning discharge.

t

Figure 4

Component Amplitude

i [kA]

Time

t∆ [ms] dtiq ∫=

[C]

dti∫2

[A2s]

A 200 ≤ 0.5 2 x 106 B 2 ≤ 5 10 C 0.2 to 0.8 250 to

1000 200

D 100 ≤ 0.5 0.25 x106

Table 1

Many combinations of current amplitudes versus time duration of components A and C were applied in the laboratory with damages similar to those obtained in the field, on OPGW cables, due to lightning discharges whose specific parameters where not known. The comparison between damages in the field and in laboratory tests, for parameter ranges used in laboratory, showed that damages caused by component A where are marginal if compared to damage caused by component C. It was decided to test the OPGW only with a simulated continuing current. It is important to emphasize that field damages in the cables may be caused by other combination current–time combinations, namely of component A type. The impulse current associated to a longer time cannot be eliminated as a cause of field damages. The earliest serious estimate of lightning current amplitude-time conditions that may originate cable damage seems to have been made by Kohlrausch (1988) [14], who found that the fusing of a metal conductor had required a current of 30 kA lasting about 2 ms [14].

Four current classes were defined through the comparison of the damages observed in the lab with those occurring in the field in according to table 2 [15]. The duration time considered was the same (500 ms) for any current class applied.

Class A B C D Electric Current (A) 100 200 300 400 Time (ms) 500 500 500 500 Electric charge (C) 50 100 150 200 Tolerance(%) ±10 ±10 ±10 ±10

Table 2

The tests reproduced in the laboratory the field damages occurred on OPGW cables. When comparing the damages caused by the lightning discharges on cable Figure (5), installed on 500 kV Serra da Mesa – Gurupi and Gurupi – Miracema transmission lines located in the central state of Tocantins (Brazil) with the damages occurred during the tests on the same cable, a similarity was observed between the number of broken wires in the field and in the laboratory for the classes of 50 C, 100 and 150 C.

Figure 5

Figure 6

i

5

Figure 6 shows tested OPGW for the class current of 50C. There were 20 similar field occurrences on OPGW.

Figure 7

Figure 7 shows tested OPGW for the class current of 100 C. There were 10 similar field occurrences on OPGW.

Figure 8

Figure 8 shows tested OPGW for the class current of 150 C. There were 4 similar field occurrences on OPGW. 2.2. Analysis of test results The analysis of test results shows that the damages are highly influenced by [16, 17]: – Diameter and material of the outer layer wires; – Coating of external layer wires; – Polarity of the test electrode. The graphs (Figures 9, 11, 12 and 13) below show, the performance of OPGW in function of the material, polarity and diameter of the wires. The

horizontal axis represents the wire diameters (mm) and the vertical axis represents the number of broken wires. Aluminum alloy and–clad steel wires present similar effect for positive and negative polarities. For this reason the damages on OPGW are represented in the same graph (Figures 12 and 13) for both polarities. Galvanized steel wires present different effects for positive and negative polarities. For this reason the damages on OPGW are represented in two different graphs. Figure 9 represents positive polarity and Figure 11 represents negative polarity. 2.2.1.Test results of OPGW with outer layer composed of galvanized steel wires.

Figure 9

Figure 9 relates to discharges of 150C with the positive polarity electrode. It can be seen that no wire was broken. During the test it was observed a removal of the zinc coating in 3 to 5 wires as can be seen in Figure 10.

Figure 10

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Figure 11

Figure 11 relates to discharges of 50 C, 100 C and 150 C with a negative polarity electrode. It can be observed that there is a tendency to occur 2 broken wires in the wire diameter range 3.00 to 3,30mm for 100 C and 150 C and 1 broken wire in the wire diameter range 3 to 3,34mm for 50C. 2.2.2. Test results of OPGW with outer layer composed of aluminum clad steel wires.

Figure 12 Figure 12 relates to discharges of 50 C, 100 C and 150 C with negative and positive polarities. It can be observed that the number of broken wires decreases when the diameter of the wire increases. Aluminum clad steel wires present a worse performance when compared to galvanized steel wires. 2.2.3. Test results of OPGW with outer layer composed of aluminum alloy wires.

Figure 13 Figure13 relates to discharges of 50 C, 100 C and 150 C with negative and positive polarities. It can be observed a higher number of broken wires for any charge applied when compared to aluminum clad steel wires. 3. PARAMETERS OF LIGHTNING FLASHES It is important to emphasize that researches presented in the world literature about damage on OPGW due to lightning: –Are mainly based in laboratory results, that try to reproduce damage similar to damage obtained in the field. – Are not validated as reasonably similar to field lightning currents that produce such damages. This type of validity limitations is in fact a practical consequence of the very limited information available about important parameters of lightning discharges that are evaluated with presently used technologies. It is important to recognize these limitations and to do efforts and take concrete actions that will eliminate them in a near future. The IEEE’s standard commission may have been prevented from conducting tests to verify the performance of atmospheric discharges on OPGW cables due to the difficulty to characterize the problems on OPGW cables stemming from atmospheric discharges. The IEC 60794-4-1 1999-01 [18] test specifications followed the schematic idea of USA Military standards for lightning qualification test technique for aerospace vehicles and hardware, but with different parameter value and ranges. At that time, the IEC Standard [19] proposed four current components as follows:

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A. Initial high peak current – with peak amplitude of 120kA, an i2 dt∫ value of 2 X 106 A2.s and a total duration not exceeding 500 µs. B. Intermediate current – with an average amplitude of 2kA flowing for a maximum duration of 5ms and a maximum i dt∫ value of 10 C. C. Continuing Current – with a long duration and low-amplitude with values to be defined by the purchaser. D. Re-strike current - with peak amplitude of 100 kA, an i2 dt∫ value of 0.25 X 106 A2.s and a total duration not exceeding 500 µs. The above IEC Standard is being revised [18] and the specified lightning test (method H2) is in accordance with the test specified in the Brazilian Standard [15]. In Brazil the lightning parameters used in the transmission line designs are mainly based on results of the researches accomplished by K. Berger [12] on the station on the Monte San Salvatore near Lugano, Switzerland, collected 30 years ago, in available meteorological information and in some other published results. In several respects, a critical analysis of available information as been done when choosing design assumptions in specific projects [1-13]. The principal parameters available on a Lightning Location System (LLS) involved on transmission line design are the magnitude of peak current, the geographic location of the incidence of lightning strokes and current polarity. The peak current of lightning strokes is indirectly measured by the detection of radiated electromagnetic fields. It is calculated by the following formula, defined by the developer of the LLS [20.].

I peak = 0.185 RNSS (kA) Where INSS is the Range Normalized Signal Strength [20]. A parameter determined concerning the magnitude for the measured electromagnetic signal. The error related to peak current measurements is in the range: 20 to 100% and the error regarding the geographic location of the incidence of lightning strokes is in the range: 0.5 to 5km [21].

These data supplied by this system are incipient. Anyhow, the type of information obtained with presently used procedures is not adequate for use as a basis to project an OPGW, transmission lines, substations and power stations in what concerns lightning effects. In what concerns studies and design, this system may only give some additional information in lightning incidence, after a reasonable number of years and a careful examination of quality of collected data. 5. CONCLUSIONS The engineering treatment of problems related to lightning, in what concerns electrical and telecommunication systems, has the following important drawbacks. The information related to parameters of real flashes is quite limited. For instance, reliable information covering relevant parameters for several studies covers a very limited number of measurements. – The information related to parameters of real flashes measured by radar or by equipment based in electromagnetic waves originated by flashes, and from which (with some “arbitrary” assumptions concerning lightning channel, soil orography and parameters) it is estimated lightning location and current amplitude, covers a large number of flashes, but is affected by a relatively high margin of error. Also, such information does not cover parameters that allow to evaluate effects of “measured” flashes for most engineering applications. As a consequence, for a specific occurrence, available information does not allow to know its relevant lightning parameters. Also, the available information of relevant parameters for most engineering applications did not increase with the newly introduced procedures to evaluate and “measure” lightning. – Due to historical and cultural reasons, most common procedures to evaluate consequences and effects of lightning consider assumptions far from reality or not adequate to deal with fast electromagnetic phenomena. Errors of several common procedures results may be of order of magnitude. Although, for a large number of applications, much better and robust procedures are available, some of them with a good long time experience in important projects in Brazil, its use covers only a small part of engineering studies and projects.

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It must be emphasized that the lightning information required by electrical power companies involves several different parameters, and that relevant parameters depend on specific application, as indicated in item 1 The engineering situation is somewhat frustrating when comparing the “expectative” that has been created of availability of “new” information related to lightning and the effective “new” information in what concern relevant parameters for many engineering studies and design The present situation has several reasons, and involves, probably, the following aspects: – Technological difficulties, of several types, in evaluating several of such relevant parameters with some of the recently developed technologies. – Lack of interaction and “cultural differences” between people that evaluate lightning effects, in several engineering fields, and people that develop technologies and procedures to “measure” lightning parameters. – Lack of clear, reliable and objective information of limitations and error limits of recent technologies and procedures, covering aspects related to engineering applications – Lack of efforts to identify and develop complementary procedures and technologies to cope with aspects for which recently developed technologies have important limitations or that justify other approaches The main objective of this paper is to call attention for this problem, in the sense of obtaining, in the future, more lightning information covering the effective needs of most engineering problems related to lightning effects. It is not pretended to cover a general discussion of the subject, as that would involve a large number of topics. We have presented a concrete recent example of an engineering problem, in which, by several reasons, the specific problems concerning lightning parameters has been deliberately “over simplified”, obtaining in laboratory tests results similar to lightning effects. With this approach it was possible to obtain technical solutions, although the physical mechanisms associated to field effects of lightning may be somewhat different of physical mechanisms in laboratory tests with similar effects. We hope that, with more deep analysis of lightning

effects and mechanisms, a more complete interpretation of the problem will be obtained. In electrical engineering studies and design, and also in operation procedures, there is a need of more lightning information covering the effective needs of most engineering problems related to lightning effects. The present situation is, at least in part, a consequence of the very limited information available about important parameters of lightning discharges that are evaluated with presently used technologies. It is important to recognize these limitations and to do efforts and take concrete actions that will eliminate them in a near future. 6. REFERENCES [1] C. Portela. – Overvoltages nd Insulation Coordination (in Portuguese), Vols. I, II, III - Vol. I, 349 p. , Vol. II, 304 p. , Vol. III, 140 p. - edition COPPE/UFRJ , Rio de Janeiro, 1982

[2] Portela, C. – Transient Regimen (in Portuguese), Vol. I, II, III, IV - Vol. I, 357 p. , Vol. II, 365 p. , Vol. III, 318 p. , Vol. IV, 280 p. - edition COPPE/UFRJ and ELETROBRÁS, Rio de Janeiro, 1983

[3] C. Portela - Frequency and Transient Behavior of Grounding Systems, I Physical and Methodological Aspects, II Practical Application Examples - IEEE 1997 International Symposium on Electromagnetic Compatibility, pp. 379-390 - Austin, United States, August 1997

[4] C. Portela - Soil Electromagnetic Behavior - Ground’98 International Conference on Grounding and Earthing, pp. 53-58 - Belo Horizonte, Brazil, April 1998

[5] C. Portela - Statistical Distribution of Parameters of Lightning Impulses in Antennas, Towers and Buildings - Methodological Aspects - Practical Application Examples - IEEE 1998 International Symposium on Eletromagnetic Compatibility, pp. 1018-1023, pp. 259-264 - Denver, United States, August 1998

[6] C. Portela - Measurement and Modeling of Soil Electromagnetic Behavior - IEEE 1999 International Symposium on Electromagnetic Compatibility, pp. 1004-1009, Seattle, United States, August 1999

[7] C. Portela - Statistical Aspects of Soil Electromagnetic Behavior in Frequency Domain - Ground’2000 International Conference on Grounding and Earthing, Proceedings, pp. 99-104 - Belo Horizonte, Brazil, June 2000

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[8] C. Portela - Grounding Requirements to Assure People and Equipment Safety Against Lightning - IEEE 2000 International Symposium on Electromagnetic Compatibility, pp. 969-974, Washington, DC, United States, August 2000

[9] C. Portela , M. Tavares - Modeling, Simulation and Optimization of Transmission Lines. Applicability and Limitations of Some Used Procedures - Transmission and Distribution 2002, IEEE - PES Society, 38 p. , Invited speech , São Paulo, Brazil, March 2002

[10] C. Portela. - Influence in Lightning Effects of Soil Electromagnetic Behavior in Frequency Domain - Proceedings International Conference on Lightning Protection 2002 , ICLP 2002 , vol. I, pp. 394-399 - Cracow, Poland, September 2002

[11] C. Portela. - Temperature Increase of Grounding Conductors due to Lightning - Proceedings International Conference on Lightning Protection 2002, ICLP 2002, vol. I, pp. 321-326 - Cracow, Poland, September 2002

[12] K. Berger, R. B. Anderson - Parameters of Lightning Flashes - Electra n0 41, pp. 23-37, July 1975

[13] M. Darveniza, F. Popolansky, E. R. Whitehead - Lightning Protection of UHV Transmission Lines - Electra n0 41, pp. 39-69, July 1975. [14] R. H. Golde – Lightning - Vol. 1. - London, 1977. [15] Brazilian Standard (in Portuguese) - 14586 (SET/2000): “Optical Ground Wires (OPGW) for overhead transmission lines – Determination of the effects of lightning discharge – Test Method”. [16] M. G. Alvim - The effects of lightning on Optical Fiber Ground Wires (OPGW): Field Measurements and Laboratory Simulations Field - International Conference on Grounding and Earthing & 3rd Brazilian Workshop on Atmospheric Electricity Rio de Janeiro - Brazil November 4-7, 2002, pp. 237-242. [17] M. G. Alvim - Improved Performance of OPGW Under Lightning Discharges in Brazilian Regions with a High Keraunic Level - CIGRE 2004, 40th Session, 2004 – August 29-September 03, Paris, France, 2004. [18] IEC 60794-4-1 1999-01: Optical fiber cables- Part 4-1 - Aerial optical cables for HV power lines.

[19] IEC 60794-1-2 – 86A/599/CD: Measurements Procedures - Method H2: Lightning test method. Protection, São Paulo, Brazil, November 21-25, pp.76-81. [20] User´s Guide 10144 REV 9707, 1998: FALLS Fault Analysis and Lightning Location System. Global Atmospherics, Inc., Appendix A, pp. A-6. [21] O. Pinto – The Art of War against Lightning – Oficina de Textos, São Paulo, Brazil, 2005, pp. 56.