simulations of lightning effects on the metallic structure of large radio communication ..... the signal conductors from transmission to switch- ing equipment at the ...
LIGHTNING CURRENTS AND ELECTROMAGNETIC FIELDS IN LARGE RADIO COMMUNICATION CENTERS WITH TOWERS Renata MARKOWSKA Bialystok Technical University
Abstract. The paper presents some results of numerical simulations of lightning effects on the metallic structure of large radio communication center consisted of a high communication tower located in the vicinity of central office building. The electromagnetic fields inside the building, currents’ flows in the metallic structure, overvoltages and overcurrents induced in internal and external cabling have been analyzed. Different earthing of particular blocks inside the building and the presence of lightning protection system have been also considered.
The aim of this study was the evaluation of: - the electromagnetic fields inside the central office building; - the current flowing into the building thorough the antenna cable; - the overvoltages and overcurrents in internal d.c. power line and signal cabling. These values have been evaluated for two different arrangements taking into account some protective means. 2. Analyzed configurations
1. Introduction As it is well known from many field experiences, the radio communication towers are one of the preferential points for direct lightning strikes. These towers are always located in the vicinity of either the central telecommunication office building or container. In both cases the direct lightning stroke to the tower is considered as possible and very serious source of damages of the electronic equipment inside the building or container. Another point is that continuous reduction of dimensions and signal levels in modern electronic equipment makes the equipment more and more susceptible to damage. Lightning current flowing through the tower structure, the existing grounding system and the conducted structure of the building or container creates a strong electromagnetic pulse as well as large voltage differences between conductors. As a result of both the galvanic and the electromagnetic coupling, the overvoltages and overcurrents in external and internal cabling of the equipment inside the building are generated. This paper presents a study of the electromagnetic fields inside the central telecommunication office building as well as currents’ flows and overvoltages in the internal and external cabling in case of direct lightning strike to the telecommunication tower. In these considerations it has been taken into account a 40 m high telecommunication tower located in the vicinity (about 4 m) of the central office building as it is shown on figures 1 and 2.
Figures 1 and 2 present the two analyzed configurations. Basically, both configurations are the same structure of the 40 m high communication tower located about 4 m away from the central office building. The dimensions of the building are: length – 14.8m, width – 10.8m and height – 12m. Both configurations are composed with the following common elements: - the steel structure of the tower, which consists of 4 slanted legs, some horizontal elements linking these legs every 4 m along with the tower height and the air termination connected to the tower and its grounding conductor; - the simplified steel structure of the building; - the simplified models of conducted parts of the tower’s foundation block; - the grounding system, consisted of two ring earth electrodes around the building and the tower buried at a depth of 80cm with some vertical rods and horizontal earth electrodes; - the tower grounding conductor linking the air termination with the grounding system; - the simplified models of some chests containing the electronic equipment: switching equipment on the 2-nd floor, transmission equipment on the 1-st floor, main distribution frame (where the subscriber lines are connected) and d.c. power plant on the ground floor; - some d.c. power conductors from d.c. power plant to the transmission and switching equipment;
Current injection point
Transmission equipment
Switching equipment
d.c. power plant Distribution frame
Fig. 1. Analyzed configuration – case 1.
Fig. 2. Analyzed configuration – case 2.
-
some signal conductors from the transmission to the switching equipment; - some signal conductors from the switching to the distribution frame; - the model of the antenna cable’s shield from the top of the tower to the transmission equipment (this shield is connected to the tower grounding conductor as well as to the grounding system at the entry to the building). The differences between configurations from fig.1 and 2 relate to the following: - In case 1 (shown on fig. 1) the earthing configuration of the system blocks is a star configuration, where all the system blocks are isolated and connected to the grounding system by separate conductors with a common single point [1]. In case 2 (shown on fig. 2) this star configuration was replaced by the lightning protection and earthing system that consists of ring conductors at each floor along the inside perimeter of the building and the network of vertical conductors and horizontal grounding conductors forming the external lightning protection system. Both the internal and external lightning protection systems are connected together and to the building’s grounding system approximating a Faraday cage; - In case 1 (shown on fig. 1) the antenna cable’s shield is connected to the tower’s grounding conductor at two points: on the top of the tower and bottom, at the exit from the tower. In case 2 (shown on fig. 2) the antenna cable’s shield is connected to the tower’s grounding conductor every 4 m along with the conductor. In both configurations the uniform soil model with a 100Ωm resistivity, a relative permittivity of 1 and relative permeability of 1 has been assumed. 3. Lightning current In all cases, the calculations have been performed with one, double exponential waveform of the lightning current: 1/50 µs and with amplitude of 100 kA. The equation for the lightning current [2] used for the computations is as follows:
I (t ) =
I
η
(e
−αt
)
− e − βt ……….….(1)
where: t – time α = 14×103 s-1 – reciprocal of time constant – reciprocal of time constant β = 6×106 s-1 I = 100 kA – peak current η ≅ 0.98 – correcting factor The current surge was injected into the top of the tower in all cases. 4. Numerical modeling The computer simulations have been performed by the MultiFields [2], [3] software package, which is a part of CDEGS package.
By means of Fourier Transform, the scalar potential and electromagnetic field in the time domain are given by:
1 V (t ) = 2π E (t ) = H (t ) = where:
1 2π 1 2π
+∞
∫ V (ω )e
jωt
∂ω ……...….(2)
∫ E (ω )e
jωt
∂ω …………(3)
∫ H (ω )e
jωt
−∞ +∞
−∞ +∞
∂ω ………...(4)
−∞
V (ω ) = Vo (ω )I (ω ) ………….….(5) E (ω ) = E o (ω )I (ω ) ………….….(6) H (ω ) = H o (ω )I (ω ) ………...…..(7) I (ω ) =
+∞
∫ I (t )e
− jωt
∂t ………….....(8)
−∞
I(ω) is the frequency spectrum of the lightning current surge and Vo(ω), Eo(ω), Ho(ω) are the unmodulated scalar potential, electric field and magnetic field in frequency domain, respectively [5]. These unmodulated quantities are computed by a unit current energization of the conductor network. The computation methodology is as follows: 1. Frequency decomposition of the time domain signal: Forward Fast Fourier Transform of the time domain signal using FFTSES [2]; 2. Computation of the frequency domain electromagnetic field response: compute the unmodulated frequency domain system response using HIFREQ [3]; 3. Computation of the time domain electromagnetic field response: Inverse Fast Fourier Transform using FFTSES [2]. The frequency domain computations are assumed to be a simple harmonic. More complicated shapes can be expressed as a superposition of such simple harmonics. For the purpose of the computation, each conductor in the network is partitioned in small segments (large enough to meet the thin wire approximation). Each such segment is represented by an electric dipole located at its center. The electromagnetic field at an observation point is obtained as the sum of the contributions from all of the dipoles. The field of the dipole is expressed as the sum of the source term, the image term and the Sommerfeld integral. The Sommerfeld integrals have been computed by the Double-Integration method i.e. numerically, without any approximation [3]. Partitioning of conductors in small segments has been done so that the current is assumed to vary linearly along with the segment for all analyzed frequencies. For the current determination and to impose the boundary conditions at the conductors’ surface, the twopotential (scalar and vector) moment method is used. For the considered current surge shape, all computations have been made in frequency range from 0 to 20 MHz.
5. Simulation results 5.1. Electromagnetic fields inside the building Fig. 3 presents the total electric and magnetic fields in the middle of the building on the 2-nd floor (about 9.4m above ground) for the two analyzed configurations: from fig. 1 and fig. 2 (case 1 and case 2 respectively). a) case 1
b) case 2
Table 1. Calculated results of the peak values of electromagnetic fields in various points inside the building. Point inside the building Ground floor (2m above ground), in the middle of the building
Case 1
Case 2
210 kV/m 960 A/m
72.8 kV/m 616 A/m
1-st floor (5m above ground), in the middle of the building
92 kV/m 828 A/m
70 kV/m 736 A/m
2-nd floor (9.4m above ground), in the middle of the building
123 kV/m 980 A/m
72.8 kV/m 800 A/m
172 kV/m 1680 A/m
116 kV/m 1720 A/m
2-nd floor (9.4m above ground), about 1m away from the corner of the building
5.2. Currents flowing into the building through antenna cable The aim of the next step of the analysis was to determine the currents flowing into the building through the antennas cable, precisely through the shield of the cable, and whether they can be reduced or not by equipotential bonding of the shield and the tower structure. The two configurations from figures 1 and 2 with 2-point and multipoint bonding respectively have been analyzed. a) case 1
b) case 2
Fig. 3. Electromagnetic fields on the 2-nd floor (9,4 m above ground) in the middle of the building for the analyzed configurations: a) from fig. 1; b) from fig. 2.
The shape of the time domain magnetic field is similar to the shape of the lightning current 1/50 µs. The observed oscillations are caused by the reflection of the main current wave in the tower. The time domain electric field has much faster front time and contains much more higher harmonics. Tab. 1 presents some results of electromagnetic fields computations in various points inside the building. The table shows that the electromagnetic fields inside the building can arise to quite high levels, especially the magnetic field. It is clear also that in the case 2 the fields’ levels are lower, but the reduction is not significant. About 2-9 dB and 1-4 dB for the electric and magnetic field respectively in comparison with case 1. In the vicinity of the building’s corner the magnetic field in case 2 is even a little higher. This is due to close proximity of the building’s lightning protection conductors.
Fig. 4. Currents in the antenna cable’s shield inside the building for the two analyzed configurations: a) from fig. 1; b) from fig. 2.
Fig. 4 presents the currents flowing into the building through antenna cable for the two analyzed configurations. The diagrams show the currents in the cable’s shield inside, just after entering the building. The current in case 2 is almost two times lower than that in case 1. However, this is not the result of bonding the cable’s shield to the tower grounding conductor, because the current in the cable’s shield outside, just before entering the building in case 2 was even larger
(23.6 kA) than that in case 1 (22.5 kA). So the reduction of the current in antenna cable’s shield inside the building is caused by the earthing and lightning protection system of the building (better equalization of potentials and currents’ draining off).
case 2). Fig. 5 represents circuit with the return conductor connected to ground at both ends and fig. 6 circuit with the return conductor connected to ground at one end only (at transmission equipment). a) case 1
b) case 2
5.3 Overvoltages and overcurrents in internal d.c. power line and signal cabling For determining overvoltages and overcurrents induced in d.c. power and signal cabling inside the building, two circuit configurations have been assumed: - configuration 1 – return conductor connected to ground at both ends; - configuration 2 – return conductor connected to ground at one end only (at the proper signal source). The proper signal sources have been assumed to be the sources with zero internal impedance. The open circuit voltages and short circuit currents with the following conductors have been computed: - the d.c. power conductors at the interfaces of the transmission and switching equipment; - the signal conductors from transmission to switching equipment at the interfaces of the switching equipment; - the signal conductors from the switching equipment to the distribution frame at the interfaces of the distribution frame. The computed short circuit currents in the d.c power cabling at the interfaces of the transmission and switching equipment for circuit configuration 1 are summarized in tab. 2 for both analyzed arrangements (figures 1 and 2). The values shown in the table are the amplitudes of the main impulse caused by the applied current surge i.e. that of the shape similar to the surge current. The maximal values are even larger due to the fast oscillations appearing at the front of the main impulse that are caused by the reflections. The currents at the interfaces of the transmission equipment have the negative values because of the current flowing through the antenna cable. Some part of this current is drained off to the grounding system through the d.c. power conductors. In case 2 this current has much shorter path to the grounding system, so the current flowing through the d.c. power conductors is much less.
Fig. 5. Computed short circuit currents in signal cabling at the switching equipment interfaces, where the return conductor is connected to ground at both ends. a) case 1
b) case 2
Table 2. Short circuit currents in the d.c. power conductors at the interfaces of the transmission and switching equipment for the analyzed system configurations. Analyzed interface Transmission equipment Switching equipment
Case 1 [A] -780 275
Case 2 [A] -60 560
The higher current at the switching equipment interface in case 2 was caused probably by the large voltage difference between grounding conductors of the power plant and switching equipment. In case 1 they were grounded at one single point. Figures 5 and 6 present the computed short circuit currents in signal cabling at the switching equipment interfaces for different system configurations (case 1 and
Fig. 6. Computed short circuit currents in signal cabling at the switching equipment interfaces, where the return conductor is connected to ground at one end only.
Tab. 3 summarizes the computed short circuit currents in signal cabling for both system configurations (case 1 and case 2) and at both the switching equipment and the distribution frame interfaces. The first values in the table are the maximal values including the fast oscillations at the front of the main current surge, and the second ones are the amplitudes of this main current surge i.e. that arises directly from the surge current.
Table 3. The computed short circuit currents in signal cabling for both system configurations at the switching equipment and the distribution frame interfaces. Description Short circuit current at switching equipment port, return conductor grounded at both ends Short circuit current at switching equipment port, return conductor grounded at one end Short circuit current at distribution frame port, return conductor grounded at both ends Short circuit current at distribution frame port, return conductor grounded at one end
Case 1
Case 2
610 A 376 A
472 A 375 A
45 A 11.5 A
39 A 10.6 A
504 A 304 A
310 A 224 A
26.5 A 4.8 A
6. Conclusions
30.2 A 2.2 A
The comparison between values shown in the table for different system configurations (case 1 and case 2) show that there is no evident benefits due to the shielding effectiveness of the earthing and lightning protection system applied in the configuration from fig. 2 (case 2). In comparison with the configuration from fig. 1 (case 1) the reduction oscillates between 0 and 7 dB. Further, it is clear that the circuit configuration in which the return conductor is grounded at one end only, provides a very strong reduction of short circuit currents flowing into the equipment through the signal lines. The benefits are from 20 to 40 dB. a) case 1
b) case 2
100
80
80
60
60
40
40 20 kV 20
kV 0
0
-20
-20
-40
-40 -60
0
0.5
1
1.5
2 µs
2.5
3
3.5
4
-60
0
0.5
1
1.5
2 µs
2.5
3
3.5
4
Fig. 7. Computed open circuit voltages in signal cabling at the switching equipment interfaces, where the return conductor is grounded at both ends. a) case 1
10
20
8
7. References
6 4
10
2
5
kV 0
0
-2
-5
-4
-10
-6
-15
-8
-20
-10
0
0.5
The computations of the currents flows in the external and internal cabling of the building associated with the radio communication tower (especially the currents flowing through the shields and return conductors) show that potential differences between grounded conductors are the main sources of overvoltages and overcurrents. Thus, voltages and currents in circuits where the return conductor is grounded at both ends are several times (from a few to even a hundred times) greater than that with the return conductors grounded at one end only. The amplitudes of both the short circuit currents and open circuit voltages in power line and signal cabling at the equipment interfaces may arise to very high levels. Also the shielding effectiveness of the building’s earthing and lightning protection system do not provide evident benefits. It is not enough to protect the equipment even for the circuit configuration with the return conductor grounded at one end. So, some further precautions have to be undertaken and some protective means should be applied to prevent the electronic equipment inside the buildings from damage. Basically, better equalization of potentials, which are the main sources of overvoltages and overcurrents in cabling, is needed, if possible. Particularly, the better draining off the current flowing through the antenna cable’s shield at the entry to the building is very important for potentials’ equalization on the area occupied by the building. Also, the cables rerouting should be considered. And finally, the shielding of cables and lightning protection devices should be applied.
b) case 2
25
15
kV
configurations (case 1 and case 2) are presented on figures 7 and 8. Fig. 7 represents circuit configuration with the return conductor connected to ground at both ends and fig. 8 configuration in which the return conductor is grounded at one end only (at transmission equipment). In this case, the shielding effectiveness of the earthing and lightning protection system from fig. 3 (case 2) in comparison with the configuration from fig. 2 (case 1) is about 2.5 – 8 dB. The reduction of overvoltages in circuit with the return conductor grounded at one end in comparison with circuit with the return conductor grounded at both ends is from 12 to 17 dB.
1
1.5
2 µs
2.5
3
3.5
4
0
0.5
1
1.5
2 µs
2.5
3
3.5
4
Fig. 8. Computed open circuit voltages in signal cabling at the switching equipment interfaces, where the return conductor is grounded at one end only.
The computations of open circuit voltages in signal cabling at the equipment ports show a similar dependence. The computed open circuit voltages in signal conductors at the switching equipment ports for the two system
1. CCITT Recommendation K 27, “Bonding configuration and earthing inside a telecommunication building”, ITU 1992. 2. “FFTSES User’s Manual: Fast Fourier Transform” Safe Engineering Services & Technologies Ltd., Montreal Canada 3. “HIFREQ User’s Manual: Frequency Domain Analysis of Buried Conductor Networks” Safe Engineering Services & Technologies Ltd., Montreal Canada 4. “How to… Engineering guide: Lightning transient study of a communication tower” Safe Engineering Services & Technologies Ltd., Montreal Canada
