- Email: [email protected]
Lightning effects in the vicinity of elevated structures
ARTICLE IN PRESS
Journal of Electrostatics 65 (2007) 342–349 www.elsevier.com/locate/elstat
Lightning effects in the vicinity of elevated structures F.H. Silveira, S. Visacro LRC—Lightning Research Center, UFMG—Federal University of Minas Gerais, Av. Antoˆnio Carlos 6627, Pampulha 31.270-901, Belo Horizonte, MG, Brazil Received 13 September 2004; accepted 13 September 2006 Available online 26 October 2006
Abstract The overvoltage generated on electrical systems due to lightning strikes to the structure of nearby radio-base telecommunication station was evaluated by computational simulation. Representative configurations were adopted for the low voltage network, for the structure and for the grounding electrodes of the station. The relevance of the contribution of both the potential rise in the soil and the voltage induced by the flow of return stroke current was analyzed. Severe overvoltages were found for all analyzed cases. It exceeded 1 MV for specific critical conditions. r 2006 Elsevier B.V. All rights reserved. Keywords: Lightning-induced voltages; Radio-base station; Low voltage networks; Grounding; Soil resistivity
1. Introduction Lightning is known as the major cause of faults in power systems. Its effects are derived from direct strikes to structures and from the induced voltage caused by the electromagnetic field associated to the return stroke current. Nowadays, a large dissemination of elevated structures has been observed, mainly in urban areas. Such structures are generally associated with the installation of radio-base stations for mobile telecommunication, which adopts structures like towers and masts with heights around 50 m. Due to their height, these structures constitute preferential points for lightning incidence. Lightning strikes to elevated structure may cause several effects in the station vicinities, including the soil potential rise, current and voltage transference through nearby grounded electrical systems, induced voltages [1] on overhead distribution lines. Such effects might be extremely severe to electrical system due to the proximity of the telecommunication station. Besides that, these effects can be transferred to consumer service entrances that are connected to the system. During storms, consumer Corresponding author. Tel.:/fax: +55 31 34995455.
E-mail addresses: [email protected], [email protected] (F.H. Silveira). 0304-3886/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2006.09.003
complaints regarding the occurrence of damages on their electronic equipments (computer, television, vcr, etc.) are quite common. This scenario justifies the development of evaluations concerning this kind of occurrence that seems to be lacking in literature to the best of authors’ knowledge. This paper aims to investigate the overvoltage levels developed at low voltage electrical network due to lightning strike to nearby elevated structures. The performed evaluations were developed by means of systematic simulations adopting an electromagnetic model. 2. Developments 2.1. Comments about the phenomenon The overvoltage developed along the electrical system due to lightning incidence on a nearby telecommunication elevated structure comprises two different effects. The first one corresponds to the induced voltage generated by the current distribution along the lightning channel and the elevated structure. Several factors affect the intensity and shape of induced voltages. They can be classified into discharge parameters (current peak, current rise rate, current wave velocity, corona sheath, channel core losses [2]), environment parameters (mainly, soil
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
343
resistivity and orography), geometric parameters concerning the relative position of line conductors and lightning channel (stroke location, attachment point height) and line configuration (presence and position of ground wire or neutral conductor, number of down-conductors, grounding impedance value etc.). Sensitivity analyses considering the influence of the mentioned factors on the amplitude of lightning induced voltages are found in [3,4]. The second effect is related with the overvoltage generated by the injection of current into neutral conductor due to potential rise in the soil. Such overvoltage is dependent on the value of soil resistivity as well as the grounding configurations of the electrical system and of the radio-base station. Moreover, the presence of a direct connection between these two grounding systems affects the resultant overvoltage. The evaluation of such overvoltages comprises complex matters, such as the transient behaviour of grounding systems [5,6], and of the tower struck by lightning [7–9], the current distribution along lightning channel [10] and the electromagnetic coupling between overhead line and channel [11,12].
potential, it is possible to calculate connective relations that determine vectors of currents and potentials for a specific condition of external current injection. Once all currents are found, the model also allows determining fields in the system vicinity and their integration to obtain voltages such as lightning induced voltage. The solution is performed in frequency domain and the results in time domain are obtained applying the Inverse Fourier Transform. This model was originally developed for determining the transient behavior of grounding electrodes [13]. Later, it was improved to allow its application to lightning related problems. Details about its formulation are found in two recent publications [14], [2], being the second specially dedicated to evaluations of current distribution along lightning channel. The literature shows another complementary applications of this model to evaluate overvoltages in overhead lines due to direct strikes [15], lightning induced voltages [16] and transient behaviour of towers due to lightning strikes [17].
2.2. Employed methodology
Typical configurations used in Brazil for the low voltage network and for the grounding system of radio-base stations were considered in simulations. Fig. 1 illustrates the elevated structure inside a station, represented as a 50 m height circular mast with 0.5 m radius, positioned 20 m away from the distribution line. The configuration of grounding electrodes connected to the mast is depicted in Fig. 2. Electrodes buried 0.5 m deep compose two equalization rings around the mast and the container where station electronic equipments are installed. As indicated, vertical rods (3 m long) are derived from these rings. An additional, rod is derived from the bottom of the mast structure (buried 7 m from soil level). All electrode radii were assumed 0.5 cm. The simulations considered different soil resistivity values (100, 500, 1000 and 2500 O m). Soil ionization was disregarded due to the adopted electrode length. Moreover, the electromagnetic
The results of this work were obtained by computational simulation. The adopted model is capable to simultaneously compute all the above mentioned aspects. It was developed for evaluation of lightning associated problems. Based on field equations, this model (HEM—hybrid electromagnetic model) is capable to estimate the electromagnetic fields and the induced voltage nearby the lightning striking point as well as the voltage and current transferred through grounding electrodes due to the soil potential rise. Basically, this model represents the system under analysis by a set of finite elements composed from the partition of all metallic components involved in the current path. A set of variables is attributed to each element: longitudinal current, transversal (divergent) current, voltage drop and average potential (in relation to remote earth). Following, the coupling relations between each pair of elements in frequency domain is developed, according to the relative position of the elements and computing the media characteristics in which they are placed (air and soil). Such relations are calculated adopting electric scalar potential and magnetic vector potential formulations. The transversal current that crosses the element surface and is spread through the surrounding medium is responsible to determine the average potential of the element. On the other hand, the longitudinal current that flows along the element establishes the voltage drop along the element itself and contributes to the voltage drop along the other elements of the system. Propagation effects are also included in the formulations. Then, based on the current continuity principle and assuming the voltage drop between adjacent elements as the difference of their average
2.3. Simulated system and assumptions
External 2.i current source
i
40 m
i
35 m
35 m
40 m Neutral Phase
Elevated Structure (H = 50 m) 20 m
Fig. 1. Scheme of the investigated problem.
soil
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
344
The lightning channel was represented as a vertical conductor (conductivity of copper) assuming no corona sheath around it. These assumptions lead to low core losses and high propagation velocity (around 3 108 m/s). The expected effect of such unusual high velocity is a moderate decrease on the induced voltage level [3]. The attachment point was assumed 100 m above the mast top. At this point, a ramp-type current waveshape (2 kA peak value, 1/50 ms) was supposed to be injected, as shown in Fig. 1. As the equivalent surge impedances seen by the source above and below attachment point are very similar, at the beginning of the phenomenon, this leads to a 1 kA current wave travelling upwards and downwards. 3. Results and analyses Fig. 2. Radio-base station grounding configuration.
Neutral Conductor 7.2 m
0.2 m Phase Conductor
40 m 35 m
soil
35 m 40 m
Fig. 3. Simplified design of a Brazilian low voltage network.
Neutral conductor
Z1 Z1 = 694 Ω Z2
Z2 = 485 Ω Z3 = 688 Ω
Phase conductor
Z3
Fig. 4. Parameters of matching resistors at the extremity of the line.
coupling between aerial and buried elements was not considered. On the other hand, the behaviour of grounding electrodes subjected to lightning current was accurately computed, considering the interaction between the grid that involves the elevated structure and the electrodes of the distribution line. Fig. 3 illustrates the adopted 150 m long low voltage network. Its neutral conductor is placed 7.2 m above soil. The highest phase conductor is positioned 20 cm below it. In order to avoid undesirable reflection effects, both conductors were matched in their ends, as shown in Fig. 4. Along the line there are five earth terminations (3 m long vertical rod), connected to the neutral conductor by down-conductors.
In this work, the results were organized in two parts. First, there is an analysis about the influence of soil resistivity on the potential rise in the station grounding system. The second part is dedicated to evaluate the amplitude of the overvoltage developed along the low voltage electrical network. For such evaluation, two conditions are analyzed: no load connecting neutral and phase conductors in the low voltage network (Case 1) and a 30 O load connecting both conductors (Case 2). Besides that, specifically in case 2, the influence of a direct connection between the grounding systems of the station and of the low voltage network on the overvoltage developed at the electrical system was evaluated. The results are presented in the text by graphs showing the phase-to-ground and phase-to-neutral overvoltages at the centre and at the extremities (75 m from phase centre). The voltages are expressed in kV/kA. Therefore, it is necessary to multiply the instantaneous values of the overvoltage shown in the graphs by the lightning current peak in order to find the voltages due to real current. This procedure is allowed due to the linear relation between induced voltage and lightning current amplitudes, according to evaluations presented in [18]. 3.1. Radio base station grounding behaviour Fig. 5 shows the response of grounding system in the radio-base station to the injection of a 1 kA ramp-type current waveshape (1/50 ms), in terms of the developed ground potential rise (GPR). The current injection point is illustrated in Fig. 6. For low resistivity soils, the conductive nature of soil prevails and the grounding works practically as a resistor [6]. This becomes evident for the 100 O m curve, since the potential wave presents the same ramp shape and the same front-time of the injected current wave. Both peaks occur around 1 ms. The increase on soil resistivity value promotes a distortion of the potential waveshape. For high resistivity soils, the capacitive behaviour of the grounding becomes more relevant. It promotes the delay of the potential wave in relation to the injected current wave.
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
345
3.2. Overvoltage developed along the low voltage network
Fig. 5. Potential waves at the current injection point of the radio-base station grounding system.
3.2.1. Case 1: no loads connecting neutral and phase conductors The configuration adopted for such evaluation is the same one presented in Fig. 3. The overvoltage is analyzed for the highest phase conductor, which is positioned 20 cm below the neutral conductor. In this case, the presence of loads connecting neutral and phase conductor was disregarded. Fig. 7 shows the resultant overvoltage developed at phase centre (a) and 75 m from phase centre (b) for different values of soil resistivity. As it is shown in Fig. 7, the resultant phase-to-ground overvoltage is extremely high, for all considered values of soil resistivity. The most intense stress level is observed at the line centre. This is the region that receives the direct illumination of the lightning electromagnetic fields. For a 2500 O m soil, the peak value of such overvoltage is around 6.4 kV/kA. Thus, the amplitude of phase-to-ground overvoltage at the line centre can be higher than 250 kV if 40 kA is assumed for the peak current (typical value for first negative strokes, according to the measurements performed at Morro do Cachimbo station, Brazil [19]).
Fig. 6. Current injection point.
Table 1 Impulsive impedance values of the radio-base station grounding system Soil resistivity (O m) r
Impulsive impedance ZPr (O)
Ratio of impedances ðZPr =ZP100 Þ
Ratio of soil resistivity (r/100)
100 500 1000 2500
3.2 16.9 34.5 82.2
1 5.3 10.8 25.7
1 5 10 25
The values of the impulsive grounding impedance ZP for the analyzed configuration are summarized in Table 1. This impedance is here defined as the ratio between the peak values of voltage and current waves [6]. It is observed an almost linear relation between the values of this impedance and of the soil resistivity.
Fig. 7. Phase-to-ground overvoltage developed at phase conductor (Case 1). (a) phase centre; (b) 75 m from phase centre.
ARTICLE IN PRESS 346
F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
The overvoltage developed at the phase conductor for this kind of occurrence is composed by the opposite effects of the induced voltage and of the potential rise in the soil compose. This explains the bipolar overvoltage profile shown in Fig. 7. The induced voltage phenomenon is closely related to the rate of current rise (di/dt). As long as the instantaneous value of current increases, a similar raise is experienced on induced voltage wave. When the current stops increasing, the induced voltage amplitude presents a fast and significant decay. Thus, the induced voltage phenomenon is responsible for the first microseconds of the overvoltage wave represented in Fig. 7. After that, the effect of the overvoltage due to the soil potential rise prevails and the voltage changes polarity. It is also observed in Fig. 7 that the increase of soil resistivity makes the bipolar feature of the overvoltage wave more remarkable. The potential developed at soil surface explains such behaviour. According to the result presented in Fig. 5, an increase on soil resistivity value enhances the amplitude of potential wave. For soil resistivity values above 500 O m, the potential waves are still increasing after 1 ms (time to crest of the injected current). Such continuous increase contributes to raise the absolute value of the overvoltage at phase conductor until a level referred to the potential rise at soil surface. Fig. 8 illustrates the phase-to-neutral overvoltage observed at the centre of the low voltage network. The obtained values are extremely intense. The maximum amplitude for the 2500 O m soil is approximately 5 kV/kA. Assuming a discharge with 40 kA current peak, the phase-to-neutral overvoltage can exceed 200 kV, causing serious damages to the load. 3.2.2. Case 2: 30 O load connecting neutral and phase conductors For this kind of analysis, 30 O loads were assumed to connect neutral and phase conductors like depicted in Fig. 9.
Fig. 8. Phase-to-neutral overvoltage at line centre (Case 1).
Neutral Conductor 7.2 m
Load connecting Neutral and Phase conductors
0.2 m Phase Conductor
40 m 35 m
soil
35 m 40 m Fig. 9. Simulated configuration—Case 2.
Two kinds of simulations were implemented. First, it was evaluated a condition in which it was not considered the presence of a direct connection between the grounding systems of the low voltage network and of the station. After this, the effect of such connection on the resultant overvoltage developed at the network was evaluated. 3.2.2.1. No connection between station and low voltage network grounding systems. The phase-to-ground overvoltage developed at phase centre and 75 m from phase centre for such condition is presented in Fig. 10. According to Fig. 10, the presence of loads connecting neutral and phase conductors acts to increase the effect of soil potential rise on the resultant overvoltage developed at phase conductor. This behaviour can be seen after the first microseconds of the overvoltage wave that reaches absolute values around 8 kV/kA at phase centre for a 2500 O m soil. Besides that, the contribution of the lightning induced voltage on the total overvoltage diminishes in comparison with the case in which there are no loads connecting neutral and phase conductors (Case 1). In spite of such behaviour, the overvoltage values are still extremely high, constituting a serious risk for the consumer’s safety and protection, as well. The phase-to-neutral overvoltage developed at line centre for case 2 is remarked in Fig. 11. It can be seen that such value is very low if compared with the phase-toneutral overvoltage presented in Fig. 8 (Case 1). It happens because the presence of loads connecting neutral and phase conductors approximates the overvoltage value and waveform on both conductors. Consequently, the difference between them will be small. Nonetheless, the phase-toneutral overvoltage is also high and can reach values around 0.5 kV/kA (2500 O m soil). Such value is very severe for the supportability level of the load. 3.2.2.2. Direct connection between station and low voltage network grounding systems. In Brazil, the grounding systems of the radio-base station and of the low voltage network are usually connected. Due to the potential rise in the soil such connection may contribute to the overvoltage developed at the electrical system by the injection of current into neutral conductor. As indicated in Fig. 12, the presence of a buried conductor connecting one rod of the
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
347
Fig. 12. Schematic representation of the direct connection between radiobase station and low voltage network grounding systems.
Fig. 10. Phase-to-ground overvoltage developed at phase conductor (Case 2). (a) phase centre; (b) 75 m from phase centre.
The direct connection between the two grounding systems promotes a significant increase on phase-to-ground overvoltage amplitude. The maximum values at the phase centre are approximately 3.5 kV/kA for 100 O m, 20.3 kV/kA for 500 O m, 38 kV/kA for 1000 O m and 80 kV/kA for 2500 O m. Such values will surely cause huge damages to the consumers and the electrical system. Evaluations performed for other observation points along the phase conductor demonstrated that the above overvoltage values are approximately the same for each soil resistivity, independently of the location of the observation point along the line. Another point to be remarked is that the phenomenon of voltage transference through grounding systems due to the soil potential rise became much more strong than the effect of induced voltage on the total overvoltage developed at the phase conductor for the values of soil resistivity adopted in this work. Fig. 14 presents the phase-to-neutral overvoltage at the centre of the low voltage network for this case. It is observed the huge increase on such overvoltage with the increase of soil resistivity value. Such raise can result in phase-to-neutral overvoltages around 6 kV/kA for 2500 O m soils. 4. Conclusions
Fig. 11. Phase-to-neutral overvoltage at line centre (Case 2) no connection between grounding systems.
container equalization ring to the rod of the downconductor placed at the centre of the low voltage network was simulated in order to investigate such condition. This 20 m long horizontal electrode was buried 0.5 m deep and had a 0.5 cm radius. The obtained results for the overvoltage developed at phase centre are depicted in Fig. 13.
This paper reports an investigation above the levels of overvoltages developed along low voltage networks due to lightning strikes to elevated structures in nearby telecommunication stations. The results shown that very severe overvoltages are established at phase conductors due to strikes to the station mast for typical configurations of Brazilian low voltage network and telecommunication station. These voltages are able to promote serious damages to loads connected to these conductors. The voltage level is critical in the closest portion of the line.
ARTICLE IN PRESS 348
F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
Fig. 13. Phase-to-ground overvoltage at line centre (CASE 2)—Influence of a direct connection between station and low voltage network groundings. (a) soil resistivity: 100 O m; (b) soil resistivity: 500 O m; (c) soil resistivity: 1000 O m; (d) soil resistivity: 2500 O m.
Fig. 14. Phase-to-neutral overvoltage at line centre (CASE 2)—Influence of a direct connection between station and low voltage network groundings.
The overvoltage wave developed at phase conductor has a bipolar profile. The induced voltage is responsible for the positive parcel of the wave at the first microseconds. After that, the overvoltage due to the soil potential rise prevails,
changing the wave polarity. As higher the soil resistivity value is, more relevant is the contribution of potential rise on the total overvoltage developed at the phase conductor. The total overvoltage at the consumer service entrance (phase-to-ground) is extremely intense when loads connect phase and neutral conductors. This may constitute a serious risk to the consumer’s safety and requires special attention regarding the protection of consumer’s installations. Though the phase-to-neutral overvoltage for such condition is comparatively low, it can be much higher than the supportability level of typical loads. The connection of the grounding systems of the radio base station and of the low voltage network promotes a huge increase on the overvoltage at phase conductor. This voltage is more intense as soil resistivity increases. The resultant overvoltage is surely able to cause severe damages to the electrical system. References [1] C.A. Nucci, F. Rachidi, M. Ianoz, C. Mazzetti, Lightning-induced voltages on overhead lines, IEEE Trans Electromagn. Compat. 35 (1) (1993) 75–86.
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349 [2] S. Visacro, F.H. Silveira, Evaluation of current distribution along the lightning discharge channel by a hybrid electromagnetic model, J. Electrostatics 60 (2–4) (2004) 111–120. [3] C.A. Nucci, Lightning-Induced Voltages on Overhead Power Lines— Part III: Sensitivity Analysis, CIGRE WG 33.01, Bologna, Italy 2000. [4] F.H. Silveira, S. Visacro, Lightning induced overvoltages: the influence of lightning and line parameters, GROUND2002—International Conference on Grounding and Earthing, Rio de Janeiro, Brazil, November, 2002, pp. 105–110. [5] L. Grcev, F. Dawalibi, An electromagnetic model for transients in grounding systems, IEEE Trans. Power Delivery 5 (4) (1990) 1773–1781. [6] S. Visacro, A didactic general approach of grounding behavior for lightning currents, Proceedings of the 27th ICLP—International Conference on Lightning Protection, Avignon, France, Vol. 1, September 2004, pp. 508–513. [7] S. Guerrieri, C.A. Nucci, M. Ianoz, F. Rachidi, M. Rubinstein, On the influence of elevated strike objects on directly measured and indirectly estimated lightning currents, IEEE Trans. Power Delivery 13 (4) (1988) 1543–1555. [8] V. Rakov, Transient response of tall object to lightning, IEEE Trans. Electromagn. Compat. 43 (4) (2001) 654–661. [9] F. Rachidi, V. Rakov, C.A. Nucci, J.L. Bermudez, Effect of vertically extended strike object on the distribution of current along the lightning channel, J. Geophys. Res. 107 (D23) (2002) doi: 10.11029/ 2002JD002119. [10] C.A. Nucci, G. Diendorfer, M.A. Uman, F. Rachidi, M. Ianoz, C. Mazzetti, Lightning return stroke current models with specified channel-base current: a review and comparison, J. Geophys. Res. 95 (D12) (1990) 20,395–20,408. [11] S. Rusck, Induced lightning overvoltages on power transmission lines with special reference to the overvoltage protection of low voltage networks, Trans. RO Inst. Technol. Stockolm 120, 1958.
349
[12] A.K. Agrawal, H.J. Price, S.H. Gurbaxani, Transient response of a multiconductor transmission line excited by a nonuniform eletromagnetic field, IEEE Trans. Electromagn. Compat. EMC-22 (2) (1980) 119–129. [13] S. Visacro, C.M. Portela, Modelling of earthing systems for lightning protection applications, including propagation effects, 21st ICLP— International Conference on Lightning Protection, Berlin, Germany, September 1992, pp. 129–132. [14] S. Visacro, A. Soares Jr., HEM: A model for simulation of lightning related engineering problems, IEEE Trans. Power Delivery 20 (2) (2005) 1206–1208. [15] A. Soares Jr., M.A.O. Schroeder, S. Visacro, Transient voltages in transmission lines caused by direct lightning strikes, IEEE Trans. Power Delivery 20 (2) (2005) 1447–1452. [16] S. Visacro, F.H. Silveira, A return stroke model based on a hybrid field-circuit approach, Proceedings of the 27th ICLP—International Conference on Lightning Protection, Avignon, France, Vol. 1, September 2004, pp. 95–100. [17] S. Visacro, F.H. Silveira, Lightning current waves measured at short instrumented towers: the influence of sensor position, Geophys. Res. Lett. 32 (L18804) (2005). [18] P.P. Barker, T.A. Short, A.R. Eybert-Berard, J.P. Berlandis, Induced voltage measurements on an experimental distribution line during nearby rocket triggered lightning flashes, IEEE Trans. Power Delivery 11 (2) (1996) 980–995. [19] S. Visacro, M.A. O Schroeder, A. Soares Jr., L.C.L. Cherchiglia, V.J. Sousa, Statistical analysis of lightning current parameters: measurements at Morro do Cachimbo Station, J. Geophys. Res. 109 (D01105) (2004) 1–11.
Journal of Electrostatics 65 (2007) 342–349 www.elsevier.com/locate/elstat
Lightning effects in the vicinity of elevated structures F.H. Silveira, S. Visacro LRC—Lightning Research Center, UFMG—Federal University of Minas Gerais, Av. Antoˆnio Carlos 6627, Pampulha 31.270-901, Belo Horizonte, MG, Brazil Received 13 September 2004; accepted 13 September 2006 Available online 26 October 2006
Abstract The overvoltage generated on electrical systems due to lightning strikes to the structure of nearby radio-base telecommunication station was evaluated by computational simulation. Representative configurations were adopted for the low voltage network, for the structure and for the grounding electrodes of the station. The relevance of the contribution of both the potential rise in the soil and the voltage induced by the flow of return stroke current was analyzed. Severe overvoltages were found for all analyzed cases. It exceeded 1 MV for specific critical conditions. r 2006 Elsevier B.V. All rights reserved. Keywords: Lightning-induced voltages; Radio-base station; Low voltage networks; Grounding; Soil resistivity
1. Introduction Lightning is known as the major cause of faults in power systems. Its effects are derived from direct strikes to structures and from the induced voltage caused by the electromagnetic field associated to the return stroke current. Nowadays, a large dissemination of elevated structures has been observed, mainly in urban areas. Such structures are generally associated with the installation of radio-base stations for mobile telecommunication, which adopts structures like towers and masts with heights around 50 m. Due to their height, these structures constitute preferential points for lightning incidence. Lightning strikes to elevated structure may cause several effects in the station vicinities, including the soil potential rise, current and voltage transference through nearby grounded electrical systems, induced voltages [1] on overhead distribution lines. Such effects might be extremely severe to electrical system due to the proximity of the telecommunication station. Besides that, these effects can be transferred to consumer service entrances that are connected to the system. During storms, consumer Corresponding author. Tel.:/fax: +55 31 34995455.
E-mail addresses: [email protected], [email protected] (F.H. Silveira). 0304-3886/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2006.09.003
complaints regarding the occurrence of damages on their electronic equipments (computer, television, vcr, etc.) are quite common. This scenario justifies the development of evaluations concerning this kind of occurrence that seems to be lacking in literature to the best of authors’ knowledge. This paper aims to investigate the overvoltage levels developed at low voltage electrical network due to lightning strike to nearby elevated structures. The performed evaluations were developed by means of systematic simulations adopting an electromagnetic model. 2. Developments 2.1. Comments about the phenomenon The overvoltage developed along the electrical system due to lightning incidence on a nearby telecommunication elevated structure comprises two different effects. The first one corresponds to the induced voltage generated by the current distribution along the lightning channel and the elevated structure. Several factors affect the intensity and shape of induced voltages. They can be classified into discharge parameters (current peak, current rise rate, current wave velocity, corona sheath, channel core losses [2]), environment parameters (mainly, soil
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
343
resistivity and orography), geometric parameters concerning the relative position of line conductors and lightning channel (stroke location, attachment point height) and line configuration (presence and position of ground wire or neutral conductor, number of down-conductors, grounding impedance value etc.). Sensitivity analyses considering the influence of the mentioned factors on the amplitude of lightning induced voltages are found in [3,4]. The second effect is related with the overvoltage generated by the injection of current into neutral conductor due to potential rise in the soil. Such overvoltage is dependent on the value of soil resistivity as well as the grounding configurations of the electrical system and of the radio-base station. Moreover, the presence of a direct connection between these two grounding systems affects the resultant overvoltage. The evaluation of such overvoltages comprises complex matters, such as the transient behaviour of grounding systems [5,6], and of the tower struck by lightning [7–9], the current distribution along lightning channel [10] and the electromagnetic coupling between overhead line and channel [11,12].
potential, it is possible to calculate connective relations that determine vectors of currents and potentials for a specific condition of external current injection. Once all currents are found, the model also allows determining fields in the system vicinity and their integration to obtain voltages such as lightning induced voltage. The solution is performed in frequency domain and the results in time domain are obtained applying the Inverse Fourier Transform. This model was originally developed for determining the transient behavior of grounding electrodes [13]. Later, it was improved to allow its application to lightning related problems. Details about its formulation are found in two recent publications [14], [2], being the second specially dedicated to evaluations of current distribution along lightning channel. The literature shows another complementary applications of this model to evaluate overvoltages in overhead lines due to direct strikes [15], lightning induced voltages [16] and transient behaviour of towers due to lightning strikes [17].
2.2. Employed methodology
Typical configurations used in Brazil for the low voltage network and for the grounding system of radio-base stations were considered in simulations. Fig. 1 illustrates the elevated structure inside a station, represented as a 50 m height circular mast with 0.5 m radius, positioned 20 m away from the distribution line. The configuration of grounding electrodes connected to the mast is depicted in Fig. 2. Electrodes buried 0.5 m deep compose two equalization rings around the mast and the container where station electronic equipments are installed. As indicated, vertical rods (3 m long) are derived from these rings. An additional, rod is derived from the bottom of the mast structure (buried 7 m from soil level). All electrode radii were assumed 0.5 cm. The simulations considered different soil resistivity values (100, 500, 1000 and 2500 O m). Soil ionization was disregarded due to the adopted electrode length. Moreover, the electromagnetic
The results of this work were obtained by computational simulation. The adopted model is capable to simultaneously compute all the above mentioned aspects. It was developed for evaluation of lightning associated problems. Based on field equations, this model (HEM—hybrid electromagnetic model) is capable to estimate the electromagnetic fields and the induced voltage nearby the lightning striking point as well as the voltage and current transferred through grounding electrodes due to the soil potential rise. Basically, this model represents the system under analysis by a set of finite elements composed from the partition of all metallic components involved in the current path. A set of variables is attributed to each element: longitudinal current, transversal (divergent) current, voltage drop and average potential (in relation to remote earth). Following, the coupling relations between each pair of elements in frequency domain is developed, according to the relative position of the elements and computing the media characteristics in which they are placed (air and soil). Such relations are calculated adopting electric scalar potential and magnetic vector potential formulations. The transversal current that crosses the element surface and is spread through the surrounding medium is responsible to determine the average potential of the element. On the other hand, the longitudinal current that flows along the element establishes the voltage drop along the element itself and contributes to the voltage drop along the other elements of the system. Propagation effects are also included in the formulations. Then, based on the current continuity principle and assuming the voltage drop between adjacent elements as the difference of their average
2.3. Simulated system and assumptions
External 2.i current source
i
40 m
i
35 m
35 m
40 m Neutral Phase
Elevated Structure (H = 50 m) 20 m
Fig. 1. Scheme of the investigated problem.
soil
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
344
The lightning channel was represented as a vertical conductor (conductivity of copper) assuming no corona sheath around it. These assumptions lead to low core losses and high propagation velocity (around 3 108 m/s). The expected effect of such unusual high velocity is a moderate decrease on the induced voltage level [3]. The attachment point was assumed 100 m above the mast top. At this point, a ramp-type current waveshape (2 kA peak value, 1/50 ms) was supposed to be injected, as shown in Fig. 1. As the equivalent surge impedances seen by the source above and below attachment point are very similar, at the beginning of the phenomenon, this leads to a 1 kA current wave travelling upwards and downwards. 3. Results and analyses Fig. 2. Radio-base station grounding configuration.
Neutral Conductor 7.2 m
0.2 m Phase Conductor
40 m 35 m
soil
35 m 40 m
Fig. 3. Simplified design of a Brazilian low voltage network.
Neutral conductor
Z1 Z1 = 694 Ω Z2
Z2 = 485 Ω Z3 = 688 Ω
Phase conductor
Z3
Fig. 4. Parameters of matching resistors at the extremity of the line.
coupling between aerial and buried elements was not considered. On the other hand, the behaviour of grounding electrodes subjected to lightning current was accurately computed, considering the interaction between the grid that involves the elevated structure and the electrodes of the distribution line. Fig. 3 illustrates the adopted 150 m long low voltage network. Its neutral conductor is placed 7.2 m above soil. The highest phase conductor is positioned 20 cm below it. In order to avoid undesirable reflection effects, both conductors were matched in their ends, as shown in Fig. 4. Along the line there are five earth terminations (3 m long vertical rod), connected to the neutral conductor by down-conductors.
In this work, the results were organized in two parts. First, there is an analysis about the influence of soil resistivity on the potential rise in the station grounding system. The second part is dedicated to evaluate the amplitude of the overvoltage developed along the low voltage electrical network. For such evaluation, two conditions are analyzed: no load connecting neutral and phase conductors in the low voltage network (Case 1) and a 30 O load connecting both conductors (Case 2). Besides that, specifically in case 2, the influence of a direct connection between the grounding systems of the station and of the low voltage network on the overvoltage developed at the electrical system was evaluated. The results are presented in the text by graphs showing the phase-to-ground and phase-to-neutral overvoltages at the centre and at the extremities (75 m from phase centre). The voltages are expressed in kV/kA. Therefore, it is necessary to multiply the instantaneous values of the overvoltage shown in the graphs by the lightning current peak in order to find the voltages due to real current. This procedure is allowed due to the linear relation between induced voltage and lightning current amplitudes, according to evaluations presented in [18]. 3.1. Radio base station grounding behaviour Fig. 5 shows the response of grounding system in the radio-base station to the injection of a 1 kA ramp-type current waveshape (1/50 ms), in terms of the developed ground potential rise (GPR). The current injection point is illustrated in Fig. 6. For low resistivity soils, the conductive nature of soil prevails and the grounding works practically as a resistor [6]. This becomes evident for the 100 O m curve, since the potential wave presents the same ramp shape and the same front-time of the injected current wave. Both peaks occur around 1 ms. The increase on soil resistivity value promotes a distortion of the potential waveshape. For high resistivity soils, the capacitive behaviour of the grounding becomes more relevant. It promotes the delay of the potential wave in relation to the injected current wave.
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
345
3.2. Overvoltage developed along the low voltage network
Fig. 5. Potential waves at the current injection point of the radio-base station grounding system.
3.2.1. Case 1: no loads connecting neutral and phase conductors The configuration adopted for such evaluation is the same one presented in Fig. 3. The overvoltage is analyzed for the highest phase conductor, which is positioned 20 cm below the neutral conductor. In this case, the presence of loads connecting neutral and phase conductor was disregarded. Fig. 7 shows the resultant overvoltage developed at phase centre (a) and 75 m from phase centre (b) for different values of soil resistivity. As it is shown in Fig. 7, the resultant phase-to-ground overvoltage is extremely high, for all considered values of soil resistivity. The most intense stress level is observed at the line centre. This is the region that receives the direct illumination of the lightning electromagnetic fields. For a 2500 O m soil, the peak value of such overvoltage is around 6.4 kV/kA. Thus, the amplitude of phase-to-ground overvoltage at the line centre can be higher than 250 kV if 40 kA is assumed for the peak current (typical value for first negative strokes, according to the measurements performed at Morro do Cachimbo station, Brazil [19]).
Fig. 6. Current injection point.
Table 1 Impulsive impedance values of the radio-base station grounding system Soil resistivity (O m) r
Impulsive impedance ZPr (O)
Ratio of impedances ðZPr =ZP100 Þ
Ratio of soil resistivity (r/100)
100 500 1000 2500
3.2 16.9 34.5 82.2
1 5.3 10.8 25.7
1 5 10 25
The values of the impulsive grounding impedance ZP for the analyzed configuration are summarized in Table 1. This impedance is here defined as the ratio between the peak values of voltage and current waves [6]. It is observed an almost linear relation between the values of this impedance and of the soil resistivity.
Fig. 7. Phase-to-ground overvoltage developed at phase conductor (Case 1). (a) phase centre; (b) 75 m from phase centre.
ARTICLE IN PRESS 346
F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
The overvoltage developed at the phase conductor for this kind of occurrence is composed by the opposite effects of the induced voltage and of the potential rise in the soil compose. This explains the bipolar overvoltage profile shown in Fig. 7. The induced voltage phenomenon is closely related to the rate of current rise (di/dt). As long as the instantaneous value of current increases, a similar raise is experienced on induced voltage wave. When the current stops increasing, the induced voltage amplitude presents a fast and significant decay. Thus, the induced voltage phenomenon is responsible for the first microseconds of the overvoltage wave represented in Fig. 7. After that, the effect of the overvoltage due to the soil potential rise prevails and the voltage changes polarity. It is also observed in Fig. 7 that the increase of soil resistivity makes the bipolar feature of the overvoltage wave more remarkable. The potential developed at soil surface explains such behaviour. According to the result presented in Fig. 5, an increase on soil resistivity value enhances the amplitude of potential wave. For soil resistivity values above 500 O m, the potential waves are still increasing after 1 ms (time to crest of the injected current). Such continuous increase contributes to raise the absolute value of the overvoltage at phase conductor until a level referred to the potential rise at soil surface. Fig. 8 illustrates the phase-to-neutral overvoltage observed at the centre of the low voltage network. The obtained values are extremely intense. The maximum amplitude for the 2500 O m soil is approximately 5 kV/kA. Assuming a discharge with 40 kA current peak, the phase-to-neutral overvoltage can exceed 200 kV, causing serious damages to the load. 3.2.2. Case 2: 30 O load connecting neutral and phase conductors For this kind of analysis, 30 O loads were assumed to connect neutral and phase conductors like depicted in Fig. 9.
Fig. 8. Phase-to-neutral overvoltage at line centre (Case 1).
Neutral Conductor 7.2 m
Load connecting Neutral and Phase conductors
0.2 m Phase Conductor
40 m 35 m
soil
35 m 40 m Fig. 9. Simulated configuration—Case 2.
Two kinds of simulations were implemented. First, it was evaluated a condition in which it was not considered the presence of a direct connection between the grounding systems of the low voltage network and of the station. After this, the effect of such connection on the resultant overvoltage developed at the network was evaluated. 3.2.2.1. No connection between station and low voltage network grounding systems. The phase-to-ground overvoltage developed at phase centre and 75 m from phase centre for such condition is presented in Fig. 10. According to Fig. 10, the presence of loads connecting neutral and phase conductors acts to increase the effect of soil potential rise on the resultant overvoltage developed at phase conductor. This behaviour can be seen after the first microseconds of the overvoltage wave that reaches absolute values around 8 kV/kA at phase centre for a 2500 O m soil. Besides that, the contribution of the lightning induced voltage on the total overvoltage diminishes in comparison with the case in which there are no loads connecting neutral and phase conductors (Case 1). In spite of such behaviour, the overvoltage values are still extremely high, constituting a serious risk for the consumer’s safety and protection, as well. The phase-to-neutral overvoltage developed at line centre for case 2 is remarked in Fig. 11. It can be seen that such value is very low if compared with the phase-toneutral overvoltage presented in Fig. 8 (Case 1). It happens because the presence of loads connecting neutral and phase conductors approximates the overvoltage value and waveform on both conductors. Consequently, the difference between them will be small. Nonetheless, the phase-toneutral overvoltage is also high and can reach values around 0.5 kV/kA (2500 O m soil). Such value is very severe for the supportability level of the load. 3.2.2.2. Direct connection between station and low voltage network grounding systems. In Brazil, the grounding systems of the radio-base station and of the low voltage network are usually connected. Due to the potential rise in the soil such connection may contribute to the overvoltage developed at the electrical system by the injection of current into neutral conductor. As indicated in Fig. 12, the presence of a buried conductor connecting one rod of the
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
347
Fig. 12. Schematic representation of the direct connection between radiobase station and low voltage network grounding systems.
Fig. 10. Phase-to-ground overvoltage developed at phase conductor (Case 2). (a) phase centre; (b) 75 m from phase centre.
The direct connection between the two grounding systems promotes a significant increase on phase-to-ground overvoltage amplitude. The maximum values at the phase centre are approximately 3.5 kV/kA for 100 O m, 20.3 kV/kA for 500 O m, 38 kV/kA for 1000 O m and 80 kV/kA for 2500 O m. Such values will surely cause huge damages to the consumers and the electrical system. Evaluations performed for other observation points along the phase conductor demonstrated that the above overvoltage values are approximately the same for each soil resistivity, independently of the location of the observation point along the line. Another point to be remarked is that the phenomenon of voltage transference through grounding systems due to the soil potential rise became much more strong than the effect of induced voltage on the total overvoltage developed at the phase conductor for the values of soil resistivity adopted in this work. Fig. 14 presents the phase-to-neutral overvoltage at the centre of the low voltage network for this case. It is observed the huge increase on such overvoltage with the increase of soil resistivity value. Such raise can result in phase-to-neutral overvoltages around 6 kV/kA for 2500 O m soils. 4. Conclusions
Fig. 11. Phase-to-neutral overvoltage at line centre (Case 2) no connection between grounding systems.
container equalization ring to the rod of the downconductor placed at the centre of the low voltage network was simulated in order to investigate such condition. This 20 m long horizontal electrode was buried 0.5 m deep and had a 0.5 cm radius. The obtained results for the overvoltage developed at phase centre are depicted in Fig. 13.
This paper reports an investigation above the levels of overvoltages developed along low voltage networks due to lightning strikes to elevated structures in nearby telecommunication stations. The results shown that very severe overvoltages are established at phase conductors due to strikes to the station mast for typical configurations of Brazilian low voltage network and telecommunication station. These voltages are able to promote serious damages to loads connected to these conductors. The voltage level is critical in the closest portion of the line.
ARTICLE IN PRESS 348
F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349
Fig. 13. Phase-to-ground overvoltage at line centre (CASE 2)—Influence of a direct connection between station and low voltage network groundings. (a) soil resistivity: 100 O m; (b) soil resistivity: 500 O m; (c) soil resistivity: 1000 O m; (d) soil resistivity: 2500 O m.
Fig. 14. Phase-to-neutral overvoltage at line centre (CASE 2)—Influence of a direct connection between station and low voltage network groundings.
The overvoltage wave developed at phase conductor has a bipolar profile. The induced voltage is responsible for the positive parcel of the wave at the first microseconds. After that, the overvoltage due to the soil potential rise prevails,
changing the wave polarity. As higher the soil resistivity value is, more relevant is the contribution of potential rise on the total overvoltage developed at the phase conductor. The total overvoltage at the consumer service entrance (phase-to-ground) is extremely intense when loads connect phase and neutral conductors. This may constitute a serious risk to the consumer’s safety and requires special attention regarding the protection of consumer’s installations. Though the phase-to-neutral overvoltage for such condition is comparatively low, it can be much higher than the supportability level of typical loads. The connection of the grounding systems of the radio base station and of the low voltage network promotes a huge increase on the overvoltage at phase conductor. This voltage is more intense as soil resistivity increases. The resultant overvoltage is surely able to cause severe damages to the electrical system. References [1] C.A. Nucci, F. Rachidi, M. Ianoz, C. Mazzetti, Lightning-induced voltages on overhead lines, IEEE Trans Electromagn. Compat. 35 (1) (1993) 75–86.
ARTICLE IN PRESS F.H. Silveira, S. Visacro / Journal of Electrostatics 65 (2007) 342–349 [2] S. Visacro, F.H. Silveira, Evaluation of current distribution along the lightning discharge channel by a hybrid electromagnetic model, J. Electrostatics 60 (2–4) (2004) 111–120. [3] C.A. Nucci, Lightning-Induced Voltages on Overhead Power Lines— Part III: Sensitivity Analysis, CIGRE WG 33.01, Bologna, Italy 2000. [4] F.H. Silveira, S. Visacro, Lightning induced overvoltages: the influence of lightning and line parameters, GROUND2002—International Conference on Grounding and Earthing, Rio de Janeiro, Brazil, November, 2002, pp. 105–110. [5] L. Grcev, F. Dawalibi, An electromagnetic model for transients in grounding systems, IEEE Trans. Power Delivery 5 (4) (1990) 1773–1781. [6] S. Visacro, A didactic general approach of grounding behavior for lightning currents, Proceedings of the 27th ICLP—International Conference on Lightning Protection, Avignon, France, Vol. 1, September 2004, pp. 508–513. [7] S. Guerrieri, C.A. Nucci, M. Ianoz, F. Rachidi, M. Rubinstein, On the influence of elevated strike objects on directly measured and indirectly estimated lightning currents, IEEE Trans. Power Delivery 13 (4) (1988) 1543–1555. [8] V. Rakov, Transient response of tall object to lightning, IEEE Trans. Electromagn. Compat. 43 (4) (2001) 654–661. [9] F. Rachidi, V. Rakov, C.A. Nucci, J.L. Bermudez, Effect of vertically extended strike object on the distribution of current along the lightning channel, J. Geophys. Res. 107 (D23) (2002) doi: 10.11029/ 2002JD002119. [10] C.A. Nucci, G. Diendorfer, M.A. Uman, F. Rachidi, M. Ianoz, C. Mazzetti, Lightning return stroke current models with specified channel-base current: a review and comparison, J. Geophys. Res. 95 (D12) (1990) 20,395–20,408. [11] S. Rusck, Induced lightning overvoltages on power transmission lines with special reference to the overvoltage protection of low voltage networks, Trans. RO Inst. Technol. Stockolm 120, 1958.
349
[12] A.K. Agrawal, H.J. Price, S.H. Gurbaxani, Transient response of a multiconductor transmission line excited by a nonuniform eletromagnetic field, IEEE Trans. Electromagn. Compat. EMC-22 (2) (1980) 119–129. [13] S. Visacro, C.M. Portela, Modelling of earthing systems for lightning protection applications, including propagation effects, 21st ICLP— International Conference on Lightning Protection, Berlin, Germany, September 1992, pp. 129–132. [14] S. Visacro, A. Soares Jr., HEM: A model for simulation of lightning related engineering problems, IEEE Trans. Power Delivery 20 (2) (2005) 1206–1208. [15] A. Soares Jr., M.A.O. Schroeder, S. Visacro, Transient voltages in transmission lines caused by direct lightning strikes, IEEE Trans. Power Delivery 20 (2) (2005) 1447–1452. [16] S. Visacro, F.H. Silveira, A return stroke model based on a hybrid field-circuit approach, Proceedings of the 27th ICLP—International Conference on Lightning Protection, Avignon, France, Vol. 1, September 2004, pp. 95–100. [17] S. Visacro, F.H. Silveira, Lightning current waves measured at short instrumented towers: the influence of sensor position, Geophys. Res. Lett. 32 (L18804) (2005). [18] P.P. Barker, T.A. Short, A.R. Eybert-Berard, J.P. Berlandis, Induced voltage measurements on an experimental distribution line during nearby rocket triggered lightning flashes, IEEE Trans. Power Delivery 11 (2) (1996) 980–995. [19] S. Visacro, M.A. O Schroeder, A. Soares Jr., L.C.L. Cherchiglia, V.J. Sousa, Statistical analysis of lightning current parameters: measurements at Morro do Cachimbo Station, J. Geophys. Res. 109 (D01105) (2004) 1–11.