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2009 Second International Conference on Computer and Electrical Engineering

Designing a lightning protection system using the rolling sphere method

M. Nassereddine

A. Hellany

University of Western Sydney School of Electrical Engineering Sydney, Australia [email protected]

University of Western Sydney School of Electrical Engineering Sydney, Australia [email protected]

Abstract—lightning is a natural occurring phenomenon that can cause damage harm and fatality. This paper discusses the lightning protection of a property using the rolling sphere method. In addition, it introduces an easy approach on how to design an effective lightning protection and finally it provides a case study

2

(3)

S = 8I 0.65

(4)

S = 3.3I 0.78

(5)

IEEE

Keywords; lightning, soil resistivity, protection mast

I. INTRODUCTION Lightning stroke can cause fatality structural damage, and could lead to malfunction of the electric equipment. The lightning stroke will vary by characteristics from area to area. The lightning itself is an emission or discharge of electricity from cloud to ground, from ground to cloud and from cloud to cloud. When the lightning strikes the ground, it chooses a path with low resistance. According to the IEEE standard 998-1996 “the stroke occurs in two steps, the first is ionization of the air surrounding the centre and the development of stepped leaders, which propagate charge from the cloud into the air”. The second step is return stroke, according to the same standard, “the return stroke is the extremely bright streamer that propagates upward from the earth to the cloud following the same path as the main channel of the downward stepped leader”. II.

S = 9.4 I 3

Suzuki

Where

I is the return stroke current in kA S is the strike distance in meters Many leading lightning investigators such as J. G Anderson and Mousa [2] support the usage of IEEE equation. This study will use the IEEE equation. There are two common methods to approach the lightning design: • the fixed angle • the rolling sphere, This paper will discuss the rolling sphere method design. In this method the value of the lightning direct strike current will determine the radius of the circle. Many countries including Australia set in their standards the level of protection based on the level the stroke current, table I shows the four level of protection in Australia and its relevant sphere radius and stroke current.

THEORETICAL STUDY

During the first part, the last step of leader will determine the striking distance (S). Many scientists studied this striking distance and came up with different equations to determine the distance, below is the most commonly used equations: Darveniza

TABLE I.

TABLE I LIGHTNING CURRENT CAPACITY WITH RESPECT TO THE STRIKE DISTANCE

−I ⎞ ⎛ S = 2I + 30⎜1 − e 6.8 ⎟ ⎟ ⎜ ⎠ ⎝

(1)

Love

S = 10I 0.65

Sphere radius (m)

Interception Current (kA)

1 2 3 4

20 30 45 60

2.9 5.4 10.1 15.7

To determine what level of protection is needed it is recommended to liaise with the local Meteorology Bureau to determine the probability level of lighting in the desired area. If this information is not available it is recommend to use protection level one in the design.

(2)

Whitehead

978-0-7695-3925-6/09 $26.00 © 2009 IEEE DOI 10.1109/ICCEE.2009.140

Protection Level

502

The idea behind mast is to find a low resistive path for the lightning to discharge into the ground. The ground resistivity should be less than 10 ohms for the lightning system according to many standards such as IEEE and AS/NZS. This resistivity will by the soil resistivity value and the type of grid used.

ρ=

πL2 R 2l

(6)

Where: L is the distance the centre from the outer probe l distance to the centre from the inner probe

III. EASE OF USE One of the main elements in the study of the induced voltage as a result of HV lines is the determination of soil resistivity of the surrounding area for the proposed pipeline. There are many ways to measure the soil resistivity, below is the most commonly used methods: A. Wenner Method Wenner method consist of four electrodes, two are used for current injection and two for potential measurement, figure 1 shows the Wenner method [1].

Figure 2. Figure (2) Schlumberger Array layout

C. Driven Rod Method This method is also called the three probe method or three pin method. This method is suitable the most for an area where the physical layout makes the usage of the previous two method difficult, the soil resistivity under this method can be calculated using equation 3:[1].

ρ= Figure 1. Figure (1) Wenner four probe arrangement.

The soil resistivity formula associated with Wenner method is shown in equation 1:

ρ = 2πaR

2πlR ⎛ 8l ⎞ ln⎜ ⎟ ⎝d ⎠

(7)

Where: l is the length of driven rod in contact with earth in meters d driven rod diameter in meters

(5)

Where: a is the probe spacing in meters R is the resistance measured in Ohms Wenner array is the least efficient from labour perspective. It requires four people to perform the task in a short time. On the other hand it is the best method when it comes to ration of received voltage per unit of transmitted current. B.

Schlumberger Array This method is more economical than the Wenner array when it comes to the man power required to perform the task. The outer electrode can be moved four or five time for each movement of the inner electrode. Figure 2 shows the arrangement for the Schlumberger Array. When contact resistance is a problem the reciprocity theorem can be applied to the Schlumberger array, this method is known as the Inverse Schlumberger Array. This method provides safer working environment for the tester under high current supply and also reduces the heavier cable that can be needed during the test. The soil resistivity can be calculated using equation 2 [1]:

Figure 3. Figure (3) Driven Rod test layout

IV. ROLLING SPHERE Rolling sphere is one of the most used methods of lightning protection. The rolling sphere method can use one or multiple mast to protect the house.

503

A. Single mast protection Figure 4 shows the proposed method of using single mast to protect an object; the circle shows the rolling sphere of the lightning strike.

Figure 5. Figure (5) double mast protections Figure 4. Figure (4) single mast protection

2 2 2 H = a − a − ⎛⎜ a − (a − d ) + T ⎟⎞ ⎠ ⎝

⎛M⎞ H = a + d − a2 − ⎜ ⎟ ⎝ 2 ⎠

2

(8)

2

(9)

C. Three masts protection Using three masts to protect the house will lead to further decrease in the height of masts. Figure 6 shows the three masts protections, this will be ideal to protect the house and it doesn’t required high masts to complete the design.

Where a: the radius of the sphere d: the heights of the protected object T: the distance between the mast and the far corner of the protected object Knowing the dimension of the house and the location of the mast, equation 8 is used to determine the heights of the required mast. B.

Double masts protection Sometimes using one mast to protect the house required a very high mast. Reduction of the height is possible by using two masts to protect the house. Figure 5 shows the method of protection using 2 masts: Where: a: the radius of the sphere d: the heights of the protected object M: the distance between the two masts Note that this formula (9) will only protect a thin object like a Bus-Bar and will not provide protection for a cubical object like house. More information will be shown in the case study section.

(a)

504

Figure 8. Figure (8) house layout. (b) Figure 6. Figure (6) three masts protection

R=

M 2Cos(30)

(10)

M should not be greater than 1.7 × a D. Four masts protection Using four masts as shown in figure 7 to protect the house is possible and the height can be calculated using equation 11:

(

H = a + d − a 2 − 0.25 L2 + G 2

)

(11) Figure 9. Figure(9) the location of the mast.

T can is calculated using the dimension of the house plus the separation distance between the house and the mast. 2

T = 5 2 + (5 +1) = 7.81m Using equation 8 gives: 2 2 2 H = 20 − 20 − ⎛⎜ 20 − ( 20 − 3) + 7.81⎟⎞ ⎠ ⎝

Figure 7. Figure (7): four mast layout

V. CASE STUDY A small house in the mountain need to be protected from lightening, the dimension of the house is shown in figure 8. The design will be carried out using the three proposed methods; level 1 protection will be used during the design. First step is to determine the location of the mast and then measuring the distance from the mast to the far corner of the house. Figure 9 shows the proposed location of the mast.

2

H = 12.03m

A mast of 12.03 meters heights should be used in the proposed location to protect the house from lightning strike. Installing two masts as shown in figure 9 and using equation (9) will gives the following: H = 3.9m

505

These two masts will only protect the line between the masts and won’t protect the edges of the house; point A in figure 10 won’t be protected. Therefore in order to use two masts, equation 12 must be used with T as the distance between the mast and point A. The mast in figure 10 should be 9.52m.

Figure 12. Figure (12): four masts protection

VI. EARTH GRID Earth grid is essential when it comes to lightning design. Using the soil resistivity of the ground leads to the determination of the earth grid resistance for the lightning mast. According to IEEE 998 and to Australian Standards 1768, the lightning grid must have a value less than 10 ohms. This grid will aid in dissipation of the lightning in the ground. Usually a single electrode is sufficient for the lightning grid, equation 16 can be used to determine the grid resistance for the tested soil resistivity results: Rg = Figure 10. Figure (10) two mast protection

ρ ⎛ ⎛8 L ⎞ ⎞ ⎜ln ⎜ ⎟ −1 ⎟ 2π L ⎜⎝ ⎝ d ⎠ ⎟⎠

(12)

Where L is the buried length of the electrode in meters D the diameter of the electrode in meters

Figure 11 shows the layout of the three masts to protect the house.

VII. CONCLUSION This paper shows an easy effective way to design a lightning system to protect a house or any valuable equipment from lightning strike. This paper also shows that single mast will be the optimum design to protect a typical house from lighting. REFERENCES [1] [2]

Figure 11. Figure (11) three mast top view

Using equation (10) to calculate the height of the mast

[3] [4]

AS/NZS 1768:2007 Lightning protection E. Koncel “potential of a transmission line tower top when stuck by lighting” AIEE transmission and distribution, New York 1956.

[5]

R. Hans, ‘A practical approach for computation of grid current’, IEEE transactions on power delivery, Vol. 14, No. 3, July 1999.

[6]

A. Vladimir “review and evaluation of lightning return stroke models including some aspects of their application” IEEE transactions on electromagnetic compatibility Nov. 1998. R. Markowska “Step and Touch Volatage Distributions at GSM Base Station during Direct Lightning Stroke”. International Conference on high voltage and application, China. November 9-13, 2008.

H = 7.57m [7]

Using a four mast as shown in figure 12 to protect the house will required a mast of height:

[8]

H = 4.24m

506

‘IEEE guide to safety in AC substation grounding, 2000’ (IEEE, New York, 2000). ‘IEEE Guide for direct lightning stroke shielding of substations” (IEEE, New York, 1996).

B. Grupta, “Impulse impedance of grounding grids” IEEE trans. Power Appar. And Syst, 1980...