Investigation on the Performance of Different Lightning Protection System Designs

Nicholaos Kokkinos, ELEMKO SA, Ian Cotton, University of Manchester

Abstract-- In this paper different lightning protection system (LPS) are modeled so that the scalar potentials on the LPS components and the magnetic fields that are generated due to the flow of the lightning current on the LPS conductors can be recorded. Furthermore an investigation regarding the induced voltage and currents due to the magnetic fields on cables inside the LPS will also be presented.

Index Terms-- Lightning, lightning protection system, magnetic fields, overvoltages.

I. INTRODUCTION In the event of a lightning strike on an earthed structure, the external lightning protection system (LPS) should safely conduct the lightning current through the LPS conductors into the earth. Although a direct contact between the structure and the lightning flash might be avoided (due to the presence of the LPS), the current flow on the LPS conductors will generate high potentials on the conductors themselves, which might lead to a flashover [1] between the LPS conductors and other metallic objects (e.g. Air Condition units, Doors, Windows and Fences) that are in the vicinity of the LPS conductors. But this is not the only problem that might arise due to a poor LPS design. The flow of the lightning current on a conductor will generate strong magnetic fields, which will induce overvoltages and surge currents on cables and electrical installations inside the structure. The magnetic field is proportional to the current, therefore by distributing the current on more LPS conductors (act as a Faraday cage) the magnetic fields can be reduced. Therefore it is the aim of this paper to analyse different LPS designs and to present the current distribution and

the potential on the LPS conductors and also the magnetic fields near and inside the structure. The results can be calculated either by using mathematical formulas and methods that have been invented and analyzed by previous researchers [2], [3], [4] or by using one more powerful method, a computer software simulation package. The software that was used for all the following simulation results is called CDEGS [5] (Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis).

II. CDEGS SIMULATION MODELS

For a simple rectangular building with the following dimensions: Length 40 meters width 20 meters and height 10 meters, four different LPS designs were examined and the resultant current distribution, potential on the conductors and the magnetic fields were recorded. The four different LPS to be examined are the following:

• Simple type lightning protection with just a rod to intercept with the direct lightning current and one down conductor connected to an earth ring 45x20m, see figure 1.

• LPS1 with four meshes 20x10m for air termination, four down conductors near each edge of the structure, connected to the same earth ring as in the previous case, see figure 3.

• LPS2 with four meshes 20x10m for air termination, twelve down conductors ten meter apart from each other all connected to the same earth ring as in the previous two cases, see figure 5.

• LPS3 (figure 8) with sixteen meshes 10x5m for air termination, twelve down conductors, ten meter apart from each other similar to LPS2 and all connected to an earth ring similar to the previous cases.

Contact Address: Dr. Nicholaos Kokkinos The LPS designs were based on European (EN) and

International (IEC) standards, although that the first two ELEMKO SA, Tatoiou 90 str, 144 52 GR,Metamorphosis, Attiki,, Hellas e-mail: nkokkinos@elemko.com

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28th International Conference on Lightning Protection

cases did not fulfil standards [6], [7] there are real cases where such installations occur.

The LPS conductors will be in much higher potential than any metallic object (e.g. Doors, Windows, and Fences) in the vicinity of them (unless effective bonding is provided). Therefore a potential difference between the two parts might lead to a flashover (the generation of an arc between the LPS and the metallic objects). This event is known as a sideflash.

The earthing system (which is a part of the external LPS) was installed at 0.5 meter depth in a uniform soil with a 100 Ωm resistivity, a relative permittivity of 1 and a relative permeability of 1 are also assumed.

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The lightning surge considered throughout this paper is defined as a double exponential type function

( ) ( )ttm eeItI βα −− −= where Im=30 kA, α=1.4x104

sec-1 and β=6x106 sec-1. The waveform is characterized by a rise time of 1.2 µsec and a tail time of 50 µsec.

III. CURRENT DISTRIBUTION AND POTENTIALS ON LPS CONDUCTORS

Fig. 2: Potential at the top of the down conductor of Simple type lightning protection A. Simple type lightning protection

Even nowadays where national and international regulations clearly specify on how an efficient lightning protection system should be designed there are cases were LPS installations do not fulfill these specifications. It is a fact that LPS installations with just a single rod and only one down conductor are currently used to protect a structure against lightning. Figure 1 represents the schematic configuration of this particular design. Even if such an LPS installation can provide protection against a direct lightning strike there are many disadvantages, which prove that it should not be used. These disadvantages will be discussed in this chapter.

B. Lightning protection system 1 – LPS1

LPS1 (see figure 4) design consists of four 20 x 10 m meshes on the air termination system, four down conductors at each edge of the structure and a buried earth electrode at 0.5 m into the soil, which connect all the down conductors. The lightning current is equally subdivided in the down conductors. The equal distance and geometry between the four down conductors and the energisation point and the fact that all the down conductors are bonded together with the earth ring in a uniform soil explains the equal subdivision of the lightning current.

The energisation point in figure 1 is at the top of the lightning interception conductor. The lightning current will flow through the only down conductor and discharge through the earthing ring into the soil. The potential at the top of the down conductor can be seen in figure 2.

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Fig. 3: LPS1 design and current distribution on the down conductors according to Kirchhoff's voltage law based on the resistive part of the conductors (DC-Resistance) only Figure 4 shows the voltage variation between the top and the bottom of the down conductors. The voltage difference between the two points can be easily observed especially at the very initial stages (i.e.0-0.5 µsec) due to the voltage drop across the surge impedance of the LPS conductors.

Fig. 1: Simple type lightning protection system design

The peak voltage was measured approximately 7.4MV with respect to earth. The oscillations and the negative peaks can be explained by traveling wave theory. (If a surge propagates down a conductor and finds a discontinuity point, i.e. a change in impedance, e.g. where the down conductor joints to the earthing system, some of it will be reflected back to the energisation point. For more information on traveling wave theory see [8] )

The important information that was extracted from these results was that due to the high potential at the top of the LPS conductors, a flashover is more likely to happen at the top than at the bottom of a structure. Therefore care must be taken when designing the air termination and the down conductor part of the LPS in order to avoid flashover. Unnecessary loops within the LPS conductors

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must be avoided and bonding requirements must be considered.

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Bottom Top Fig. 4: Voltage variation between the top and the bottom of LPS1 down conductors

C. Lightning protection system 2 – LPS2 Figure 6 shows the second lightning protection design LPS2, which will be under consideration. In the previous case of LPS1, although the principles of the design were based on IEC 62305 -3 and EN 62305-3 it did not fulfil all of the necessary requirements exactly as they are mentioned in the standards. Regardless of the lightning protection level, distances between the down conductors should be a maximum of 20m or 10m according to both standards.

Therefore in this case the same design as before will be examined but with twelve down conductors placed 10m apart from each other. The energisation point will remain in the middle of the air termination system. It should be noted that sets of the conductors, whose distance and symmetry are equal and identical respectively from the energisation point, will have an equal current flow through them. This is also due to the fact that they are all bonded together in a uniform soil.

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Face middledown conductor

Fig 5: LPS2 design and current distribution on the down conductors according to Kirchhoff's voltage law based on the resistive part of the conductors (DC-Resistance) only Conductors near the energisation point, carry more current. Comparing the side conductors with half face middle conductors, shows that the distance from the energisation point is the same however, the current that flow through them is not the same. That is because the side conductors are directly connected to the energisation point (without any intermediate node) while the half face middle conductors are connected through one node. In other words the current will flow through the easiest path, which will drive it to earth. Note that the current values on the down conductors in figure 6 is the percentage of

the actual lightning current that will flow though and not the real current, which also includes the electromagnetically induced current. Figure 6 and 7 show the variation of current and voltage for each set the down conductors respectively. Table 1 contains the peak values from the voltage and current graphs.

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rent

(A)

Edge conductors Side conductors Half face middle conductors Face middle conductors Fig. 6: Current distribution on the down conductors of LPS2

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Edge Conductors Side conductors Half face middle conductors Face middle conductors Fig. 7: Voltage variation on LPS2 down conduct

TABLE 1 : PEAK CURRENT AND VOLTAGE VALUES FROM FIGURE 7 AND FIGURE 8

Conductor Edge Side Half face

middle Face

middle Current

peak 1,916 A 2,803 A 2,173 A 4,155 A

Voltage peak 649 kV 801 kV 593 kV 653 kV

D. Lightning protection system design 3 – LPS3 Figure 8 represents LPS3, which is similar to LPS2 but with an improved air termination system. The air termination system was designed according to the mesh method. The mesh size is 10 x 5 meters.

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IV. ELECTROMAGNETIC FIELD ANALYSIS WITHIN A STRUCTURE DUE TO THE FLOW OF LIGHTNING CURRENT

ON THE EXTERNAL LIGHTNING PROTECTION SYSTEM

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conductor

Fig. 8: LPS3 design and current distribution on the down conductors according to Kirchhoff's voltage law based on the resistive part of the conductors DC-Resistance) only

CDEGS can compute the magnetic fields at single point and at a surface, which consists of many observation points. For the purpose of the following simulations three observation surfaces were selected, one at the top one at the middle and one at the bottom of the structure. Each surface consists of a set of profiles; each individual profile consists of a number of observation points. Figure 12 describes the schematic configuration of the computation procedure.

Figure 9 and 10 show the current and voltage variation on LPS3 down conductors respectively. Table 2 contains the peak values from the voltage and current graphs.

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Top surface

Middle surface

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Number ofprofiles per surface

Number of points per profile

Fig. 11: Three-dimension view of the simulation structure with the three observation surfaces

The lightning current has been more evenly distributed between the down conductors. The edge, side and half face middle conductors carry approximately the same amount of current. The potential difference at the top of the LPS conductors is not very high. Therefore by using more meshes on the air termination system, flashover probability can be reduced. TABLE 2: PEAK CURRENT AND VOLTAGE VALUES FROM FIGURE 10 AND

FIGURE 11 The top observation surface was 0.5 meters from the top of the air termination system. The magnetic field is measured in amperes per meter. Every color represents a range of magnetic field magnitude. All the magnetic field results are extracted at 1.2 microseconds, which is written on the bottom of the graph. The 3D graph presentation was used in order to represent the magnitude of the magnetic fields at this particular time instant (1.2 µsec). The maximum field was recorded at the time of the peak current, which is represented in table 3.

Conductor Edge Side Half face middle Face middle

Current peak 2,026 A 2,429 A 2,560 A 3,300 A

Voltage peak 556 kV 612 kV 560 kV 543 kV

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The magnetic field is proportional to the current and therefore LPS designs whose conductors carry more lightning current are expected to generate stronger magnetic fields. The benefit of having a more detailed air termination system can be understood by comparing LPS3 (figure 15), which has sixteen meshes, with the simple LPS design (figure 12), LPS1 (figure 13) and LPS2 (figure 14) whose air termination system consists of one and four meshes only. The lightning current has to flow from the energisation point to the earth. Therefore by creating more conductive paths a better distribution can be succeeded and a better distribution means lower currents through the LPS conductors and therefore lower magnetic fields near the LPS conductors. By using more LPS conductors the structure acts as a faraday cage, which results to the reduction of the electromagnetic fields inside the structure.

Fig. 9: Current distribution on the down conductors of LPS3

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Edge conductors Side conductors Half face middle conductors Face middle conductors

For the purpose of this paper only the magnetic fields that were recorded on the top of the structure will be graphically presented. The results on tope middle and bottom are discussed further in the paper and are also numerically presented in table 3.

Fig. 10: Voltage variation on LPS3 down conductors

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A. Discussion of magnetic field results

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From the simulation results some basic factors that influence the efficiency of each LPS design in terms of minimizing peak magnetic field can be stated. It is now evident that a more detailed air termination system can minimize the electromagnetic fields at the top of the structure. Comparing the observation results for LPS2 and LPS3, a significant reduction regarding both the peak values and the area that is covered by the high magnetic field can be achieved by using more meshes on the air termination system.

Fig. 12: Resultant magnetic field at the top of simple LPS at 1.2 usec (Peak value at that time was 14500 A/m) The number of the down conductors is another very

important parameter, which influences the efficiency of the external LPS. From the simple LPS type result it is obvious that just one down conductor can not provide effective protection against electromagnetic pulses. The use of four down conductors (LPS1) provides a significant reduction in both the peak values and the area that is covered by strong magnetic fields. LPS2 and LPS3 prove that by using even more down conductors a further reduction to the magnetic fields can be achieved.

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The results at the bottom of the structure prove that buried conductors can also influence the generation of magnetic fields. Regarding the middle and bottom results, it is possible to see that in the center of the structure the magnetic fields are much lower than close to the down conductors. Another important factor that should be mentioned is the fact that there is no other element apart from air between the inside of the structure and the LPS conductors. So in a real situation all the magnetic fields should be lower because concrete and especially reinforced concrete is more conductive than air. Reinforced concrete structures act as a Faraday cage, which has as a result the reduction in the magnetic fields inside it.

Fig. 13: Resultant magnetic field at the top of LPS1 at 1.2 usec (Peak value at that time was 3000 A/m)

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As a total overview of the results it is possible to state that the most efficient design was LPS3. The generated magnetic fields due to the lightning current on the LPS3 conductors present lower peak values than in all of the other simulations and also the total area inside the structure that is under a strong magnetic field can be reduced to a minimum by using the LPS3 design. Table 3 summarizes the maximum-recorded magnetic field values inside each lightning protection design for the top, middle and bottom observation surfaces.

Fig. 14: Resultant magnetic field at the top of LPS2 at 1.2 usec (Peak value at that time was 2400 A/m)

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TABLE 3: SUMMARY OF MAXIMUM MAGNETIC FIELD VALUES FOR DIFFERENT LPS DESIGNS

Maximum magnetic field A/m Simple LPS1 LPS2 LPS3

Top 55,000 18,400 7,500 6,050 Middle 49,000 17,000 6,500 5,200 Bottom 24,000 9,150 3,200 2,500

Fig. 15: Resultant magnetic field at the top of LPS3 at 1.2 usec (Peak value at that time was 1400 A/m)

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V. INVESTIGATION OF INDUCED VOLTAGE AND CURRENT SURGES ON CABLES INSIDE THE LPS DESIGNS

A metallic loop was simulated near the down conductors inside of each LPS. The induced voltage and current surges on the metallic loop were depending on the position of the loop inside the LPS design. Therefore two different positions of the metallic loop inside each LPS were examined and the maximum-recorded surges will be presented. The loop dimensions and positions can be seen in figure 16. Inside LPS1, LPS2 and LPS3, the position of the metallic loops are the same but for the simple type the loops were installed near the only down conductor. The energzation point for all the simulations was the center of the LPS. The cable type that was used in the loop was a co-axial type cable with an external 0.01 mm foil screen and 0.16 mm of PVC insulation. Each loop was individually simulated as an open circuit and as a short circuit. The voltage across the open circuit point is the value of the inductive and capacitive induced voltage across the cable. The current that will flow through the short circuit loop is the generated current due to the induced voltages. Table 4 summarizes the recorded overvoltages and surge currents values for the previously described metallic loops inside the four different LPS designs. Loops inside LPS2 and LPS3 present the lower overvoltage and surge current values, which proves that a higher efficiency LPS design, not only provides higher protection against direct flashes on the structure, but can also minimize the induced voltages and currents on the cables inside the structure.

TABLE 4: INDUCED VOLTAGE AND CURRENT ON A METALLIC LOOP

INSIDE DIFFERENT LPS DESIGNS

Efficiency and Level

Voltage surge (kV)

Current surge (A)

Simple LPS - (*) 1,550 1,175 LPS1 - (*) 380 530 LPS2 80%, IV 145 220 LPS3 95%, II 110 140

(*) Simple type LPS does not satisfy any of the international standards therefore its efficiency could not be evaluated. (**) LPS1does not fulfil all of the necessary requirements for a level IV LPS, it only has four instead of six down conductors.

18 m

4 mArea = 72 m2

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1 m

20 m

1 mShort Circuit Open Circuit

Horizontal arrangement Fig.16: Dimensions and position of the metallic loop inside the LPS designs The recorded voltage and current values on the cables are the inductive and capacitive coupled surges and are not due to resistive coupling. Although resistive coupling surges may be more severe than inductive and capacitive coupling, the protection against them is easier that the last two. By installing surge protection devices [4-2] at the main distribution boards the resistive coupled surges can be minimized. But the inductive and capacitive coupling surges may appear across installations even if they are not

connected to any part of the structure, which carry lightning current. The magnitude of the induced voltage surges varies between some tens of kV up to some hundreds of kV and the current magnitudes may reach values of some hundreds of amperes. Electrical installations connected at these cables will experience these overvoltages and surge currents. The damage that may be caused to sensitive electronic equipment, which operate with only a few tens of volts and few mA could be severe.

VI. CONCLUSIONS Designing an efficient external lightning protection system (LPS) the risk of having high induced overvoltages on cables inside the structure can be reduced. International standards provide guidance on how an efficient LPS should be designed where necessary. Simulations investigating the performance of different LPS designs have proved that just a single down conductor can not provide efficient protection regarding the generation of induced overvoltages on metallic loops in the vicinity of the down conductors. By increasing the number of down conductors the potential across the LPS can be reduced and the probability of sideflashes can also be minimised. The air termination system is also very important and it has been proved that by using a meshed air termination system a better distribution of the lightning current on the down conductors can result, which then reduces the potential differences across the LPS components. By using more down conductors and a meshed air termination system, the generated magnetic fields can also be reduced and therefore, induced overvoltages and overcurrents on cables inside the LPS can be partially controlled.

VII. REFERENCES [1] Allen, Fundamental aspects of air breakdown, High voltage engineering and testing, IEE Power, 1996 [2] Flisowski, Stanczak, Kuca, Mazzeti, Orlandi, Yarmarkin, Induced currents and voltages inside LPS models due to lightning currenr, 23rd ICLP, Firenze, 1996 [3] Noda, Takami, Nagaoka, Amantani, Basic investigation of lightning induced voltages to an electronic appliance, 23rd ICLP, Firence, 1996 [4] Sowar, Gosling, Lightning radial electric fields and their contribution to induced voltages, IEEE Symposium on EMC, 1999 [5] CDEGS, Safe Engineering Services (SES) and Technologies limited, Montreal, Canada. [6] EN 62305 -3, Protection against lightning – Part 3: Physical damage to structures and life hazard [7] IEC 62305 -3, Protection against lightning – Part 3: Physical damage to structures and life hazard [8] Pritindra Chowdhuri, Electromagnetic transients on power systems, John Wiley & Sons, 1996 [9] IEC 61643 – 12, Low voltage surge protective devices – Part 12: Surge protective devices connected to low voltage power distribution systems – Selection and application principles

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