THE IMPACT OF ELEVATED MASTS ON LIGHTNING INCIDENCE AT THE RADIO-BASE-STATION VICINITIES

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1 X International Symosium on Lihtnin Protection 9 th -13 th November, 9 Curitiba, Brazil THE IMPACT OF ELEVATED MASTS ON LIGHTNING INCIDENCE AT THE RADIO-BASE-STATION VICINITIES Rosilene Nietzsch Dias 1, Fernando Henrique Silveira, Silvério Visacro Lihtnin Research Center - UFMG - Federal University of Minas Gerais, Brazil rosilene@cdee.ufm.br, silveira@cdee.ufm.br, LRC@cdee.ufm.br Abstract This aer analyzes how a radio-base structure installed in urban areas may affect the lihtnin incidence on the vicinities as well in the owner site. Also, the rofile of the eak-current distribution on the nearby reion is evaluated. These analyses are erformed considerin different structure heihts installed in a determined lot, and some asects concerns to the lihtnin incidence on the other lots at the vicinities are estimated. 1 INTRODUCTION Nowadays, the number of radio-base-telecommunication stations (RBS) installed in urban areas is increasin fast. These stations comrise elevated structures such as tower or masts with heihts around 5 m. Due to their elevated heiht, such structures constitute referential oints for lihtnin strikes, affectin the lihtnin incidence and eak current distributions in the nearby reion. Accordin to [1-], lihtnin strikes to elevated structure cause severe effects in the station vicinities, includin the round otential rise, current and voltae transference throuh nearby rounded electrical systems and induced voltaes on overhead distribution lines. Such effects miht be extremely severe to consumers as well as the electrical system due to the roximity of the radio-base station. As shown in [1], for some secific critical conditions overvoltaes around 1 MV can be develoed. This context has motivated the develoment of this work that is dedicated to investiate the imact of the resence of the RBS elevated structure on the lihtnin incidence and eak current distributions in the vicinities of the station. Basically, the oal of this aer is to quantify the rotection level aainst direct lihtnin strikes in the vicinities of a RBS structure. Also the chanes on the eak-current-distribution rofile due to the resence of the structure are analyzed. BASIC ASPECTS It is well-known that elevated structures are referential oints for lihtnin incidence. When a RBS is installed in urban areas, the mast becomes a referential oint to lihtnin strikes in relation to the RBS vicinities. As a consequence, the lihtnin incidence and lihtnin eak current distributions are modified. On this section, some basic asects relatin to the lihtnin incidence and eak current distributions will be discussed..1 Peak current distribution Lihtnin database are obtained from different sources, as instrumented towers, lihtnin location system and triered lihtnin. Amon them, the more confident manner to obtain lihtnin current measurements is by means of instrumented towers. However, these statistical distributions are affected by the resence of the tower, which attracts more intense events that would hit the secific local if the tower not were installed in. Consequently, the statistical arameters of the eakcurrent distribution are affected. In order to eliminate the attractive effects of the instrumented tower on the eak current data, it is necessary to estimate the lonormal lihtnin eak current distribution at the round level. Borhetti et al. [3] roosed a rocedure based on Monte Carlo method that can be alied to estimate the statistical distribution at the round level from data recorded by instrumented towers. Also, Pettersson [4] derived an analytical formula that allows calculatin the lonormal arameters, as showin in (1). 1 Suorted by CNPq - National Council for Scientific and Technoloical Develoment Suorted by FAPEMIG - Fundação de Amaro à Pesquisa do Estado de Minas Gerais 91

2 e µ +,5σ = e µ +,5σ [ ex( bσ )] ; σ = σ where: σ is the scale-factor of lonormal distribution (obtained by lihtnin current measurements on instrumented tower) µ is the shae-factor of lonormal distribution obtained by instrumented tower measurements. σ and µ are the lonormal arameters of eak current distribution at the round level. b is the exonent of I on the attractive radius exression (on this case, b=.64, see next toic). The robability of occurrence of a stroke with eak current I at round level ((I )) is obtained from (): ( I P ) = I P ( ln I µ ) P 1 σ πσ. Lihtnin incidence distribution Alon the years, the electroeometric model (EGM) has been emloyed to characterize the number of lihtnin that strikes a structure. There are several studies that roose analytical equations to calculate the attractive radius (R a ) in function of the stroke eak current. Amon them, Eriksson had introduced the effect of structure heiht in the calculation of the attractive radius [5]. Equation (3) is the eneral Eriksson s formula. It was obtained considerin structures u to 5 m in heiht (H) and lihtnin eak current (I P ) u to ka in different reions. a 4 (.66+ I 1 ) R = H I e.64 P This attractive radius determines a sherical reion centered on the to of the structure. On this area, all lihtnin events that would strike the round if the mast were not bein installed there have a reat robability to hit the mast. The stroke collection area (S ) reresents the reion where an event with amlitude I is attracted to the structure, considerin the relationshi into stroke intensity and structure heiht. = π (4) S R a In order to calculate the number of strokes for each eak current (I P ) that hit the mast (N I ), some arameters have to be takin into account, as follows: (3) (1) () the stroke density in the reion, in strokes/km²/year (N s ); stroke collection area (S P ) for each eak current; robability of lihtnin eak current I at the round level ((I )) N = N S (5) I s ( I P ) The total number of strokes N that hit the structure considerin the whole eak-current distribution is calculated from (6). N I N s S (6) ( I = Max Imin P ) 3 DEVELOPMENTS AND RESULTS The analyses resented in this work are based on the alication of the EGM and intend to determine the imact of RBS masts on the number of strokes and on the lihtnin eak-current distribution at the vicinities of the radio-base station. Next sections describe all rocess erformed and their results. 3.1 Methodoloy The methodoloy adoted in this work considers an urban area with tyical square and lot dimensions. A standard lot with 36 m²-area (1 m x 3 m), situated on a flat reion with lihtnin density equals 6 strokes/km²/year, is considered. The strokes incidence as well as the eak current rofile in the lot where the elevated structure is situated and at its vicinity are calculated by the alication of the electroeometric model [5] and the eak current distribution at the soil level rovided by statistics of direct measurements [6]. The effect of the structure is evaluated comarin the results in terms of lihtnin incidence and eak-current distribution in their vicinities, considerin both the resence and the absence of the structure. 3. The effect of the resence of the radio-base-station structure (RBS) Firstly, the effects of the resence of the structure are analyzed. The lihtnin eak-current distribution obtained at Morro do Cachimbo station [6] is adoted on these simulations. The lonormal arameters at the round level are calculated from equation (1). The obtained values are σ =.597 and µ =

3 The eak-current value ranes from 1 ka u to ka (1 ka ste) in all analyses resented on this work. The number of strokes N I for each I (5) and total number of strokes N that hit the lot (6) are calculated. After that, a standard RBS structure with 5 m in heiht is added in the lot and new calculations are done. Table 1 shows the total number of strokes for the incident strokes on the lot for each analyzed case. In order to erform a sensitivity analysis, masts with 3 m and 8 m in heiht are also considered. Fi 1 resents the stroke number for each I (N I ) considerin the standard lot without the structure (dashed line on axis y on the riht side) or with RBS structure installed in (axis y on left side). Table 1: Total stroke number in the lot. Analyzed case N Lot without RBS structure.16 x 1-3 Lot with a 3 m structure.158 Lot with a 5 m structure.313 Lot with a 8 m structure.588 As exected, the number of strokes that strikes in the lot increases when a RBS structure is installed in. This number is larer for hiher structures. It is observed on increase around 73 times for a 3m-heiht structure whereas for a 8m heiht structure, such increase on the number of strokes that strikes the lot is about 7 times larer. It is imortant to remark that ractically all strokes will hit the mast for this condition. 7, (x1-5 ) 6, Stroke Number in lot 5, 4, 3,, 1,, 7, (x1-3 ) 6, Stroke Number 5 m mast 5, 4, 3,, 1,, (b) Fi. 1 Stroke incidence on RBS site without RBS mast (a) and (b) with 5 m-mast installed in. (a) The mode eak-current value increases 4% in the resence of the 5 m-mast in the lot (from 18 to 3 ka). This fact is due to the hihest attraction radii for hih eak-current values, which roitiate an increase on the more intense strokes that may hit the mast. 3.3 The RBS vicinities In order to evaluate the imact of the resence of the RBS structure on the number of strokes and the median eakcurrent values at the nearby reion, a standard square constituted by 3 lots is considered as deicted in Fi. The area of each lot is 36 m², and the mast is installed in lot A 4 m distant from their ede. The urose of this analysis is to evaluate the lihtnin incidence rofile in each lot when the RBS structure is installed in lot A. Due to the symmetry of the lihtnin incidence effects on lots A 1 until P 1, the followin analyses are erformed considerin only lots A until P. In order to erform a sensitivity analysis, masts with 3 m, 5 m and 8 m in heiht are considered. 3m A B C D E F G H I J K L A1 B 1 C1 1m M N O P D1 E1 F1 G1 H1 I1 J1 K1 L1 M1 N1 O1 P1 Fi. Standard square: 3 lots with 36 m² in area each other. The RBS structure is situated in lot A. Fi. 3 characterizes the lihtnin incidence alon the square considerin the structure installed in lot A. The number of strokes that strikes on a secific lot is related to the left Y axis and those that hit the mast installed in lot A are associated with the riht Y axis. Lot incidence,e-3 1,8E-3 1,5E-3 1,3E-3 1,E-3 7,5E-4 5,E-4,5E-4,E+ Lot incidence (3m mast) Lot incidence (5m mast) Lot incidence (8m mast) Mast incidence (h=3m) Mast incidence (h=5m) Mast incidence (h=8m),6,5,4,3,,1 Mast incidence Fi. 3 Stroke incidence alon the square considerin different mast heihts installed on lot A. A decrease of the number of stroke incidence on the mast is observed with the increase on the distance from the structure. Consequently, the number of lihtnin incidence on the lot increases. This behavior is due to the attractive Street 93

4 radius associated to the eak current values. For loner distances from the mast, only the most intense lihtnin current would be attract to the structure. Fi. 4 illustrates the rotection level (in ercentae) that the RBS structure offers to each lot on the square. For examle, considerin a 5m-heiht structure installed in lot A, only 4% of the lihtnin that would hit lot L will strike on it, namely, 6% of strokes that would strike in lot L will hit the mast. As the distance of the RBS structure and the mast heiht increases, the median eak current on each lot becomes hiher, that is, more strokes will strike the analyzed lots. The median eak current of the strokes that hit the mast also increases due to the hiher attractive radius, since only the most intense lihtnin will be attracted to the structures. 1% 9% 8% Considerin the elevated mast heihts, it is observed a hih rotection level for those lots close to the structure. When the distance from the elevated structure is increased, the robability of lihtnin incidence on the lot under analysis is hiher. Mast rotection 7% 6% 5% 4% 3% % Fi. 5 shows the median eak-current values for two distinct situations: (i) the median eak-current relative to those strokes that hit the lot under analysis; (ii) the median eak current of those events that would strike the lot if the structure were not installed in lot A but will hit the mast due to the attractive effect of the elevated structure. In Fi. 6, the lihtnin eak-current distribution for structures of 3, 5 and 8 m is resented. It is ossible to note the eak-current values of the strokes that hit the mast (the remarked ones) and those that hit the lot. Fi. 7 shows the threshold values of eak current and attractive radii that determine the lihtnin that would hit the structure. The left Y axis refers to the attractive radius and the riht Y axis refers to the eak-current values. For examle, considerin lot J and a 3m-heiht mast installed in lot A, all strokes that have eak-current values reater than 45 ka (corresondin to the attractive radius of almost 11 m) will strike the mast. 1% % Lot 3m Mast 5m Mast 8m Mast Fi. 4 Mast rotection level for each lot considerin the resence of RBS structure on lot A Lot (3m mast) Lot (5m mast) Lot (5m mast) Mast (h=3m) Mast (h=5m) Mast (h=8m) Fi. 5 Median eak-current value of the strokes that hit the mast and the lot under analysis. (h =3 m) (h=5 m) (h=8 m) Fi 6 Peak-current distribution for each lot considerin masts with 3, 5 and 8 m located in lot A. Each column means a lot (from lot A to P ). Each line means the eak current value from 1 to 1 ka. Remarked values reresent the eak current values of the strokes that hit the structure. 94

5 Attraction radii (m) Threshold arameters Attraction Radii (3m mast) Attraction Radii (5m mast) Attraction Radii (8m mast) Peak current (3m mast) Peak Current (5m mast) Peak Current (8m mast) Fi. 7 Threshold eak current and attractive radii for each lot Considerin Fis. 5, 6 and 7, those lots nearest the RBS structure will be hih rotected aainst lihtnin incidence. Only few strokes lower than ka (until lot F, H and K, resectively for structure heiht of 3, 5 and 8 m) would strike the lot under analysis. 4 CONCLUSIONS This work resents evaluations of the imact of elevated masts on lihtnin incidence rates at the radiobase-station vicinities. Their resence rotects the vicinities of the radio-base station aainst direct strikes. On the other hand, there is a concentration of lihtnin incidence on the structure, makin the nearby reion vulnerable to lihtnin-induced voltae and overvoltaes due to the round otential rise (GPR). Fi. 1 in session 3. denoted the increase of eak current with the resence of the 5 m mast in the site (4% hiher). In session 3.3, the behavior of eak-current and stroke incidence on the mast and in the RBS site accordin to the increase on the distance from the structure is quantified. Considerin each lot, a better rotection was reachin on the lots nearest to the mast site. As the distance of the structure increases, the robability of lihtnin incidence into lots is reater, mainly those of lower eak-current values. 5 REFERENCES [1] F. H. Silveira and S. Visacro, Lihtnin effects in the vicinity of elevated structures, J. Electrostatics, vol.65, issues 5-6, May 7, [] F. H. Silveira and S. Visacro, Electromanetic Interferences on Electrical Systems Due to Lihtnin Incidence on Nearby Telecommunication Elevated Structures Proceedins of VIII SIPDA - International Symosium on Lihtnin Protection, 5, São Paulo. - v [3] A. Borhetti, C. A. Nucci and M. Paolone, "Estimation of the statistical distributions of lihtnin current arameters at round level from the data recorded by instrumented towers", IEEE Trans. Power Delivery, Vol. 19, No 3, 4, [4] P. Pettersson; A unified robabilistic theory of the incidence of direct and indirect lihtnin strikes - IEEE Trans. Power Delivery, Vol. 6, No 3, 1991, [5] A. J. Eriksson: An imroved electroeometric model for transmission line shieldin analysis, IEEE Trans. Power Delivery, Vol. PWRD-, No 3, 1997, [6] S. Visacro, A. Soares Jr, M. A. O Schroeder, L. C. L. Cherchilia, and V. J. de Souza: Statistical analysis of lihtnin current arameters: measurements at Morro do Cachimbo station, Journal of Geohysical Research, Vol. 19, D115, 4. 95