optimized pit configuration for efficient grounding of the power system in high resistivity soils...

Optimized Pit Configuration for Efficient Grounding of the Power System in High Resistivity Soils

using Low Resistivity Materials A. A. Al-Arainy, Y. Khan, M. I. Qureshi, N. H. Malik and F. R. Pazheri

Saudi Aramco Chair in Electrical Power Department of Electrical Engineering, College of Engineering, King Saud University,

Riyadh 11421, Saudi Arabia Email: aarainy@ksu.edu.sa

Abstract In an electrical power system, the integrity of the grounding system is very important to maintain a reference point of potential for equipment and personnel safety, to provide a discharge point for lightning currents as well as to prevent excessive high voltages on the power system. Therefore, to maintain ground system effectiveness, proper design, installation and testing of grounding system is required. In Saudi Arabia, the weather is dry and the soil resistivity varies significantly from area to area because the geodetic terrain varies from sea shore to the arid desert and dry mountains. In most of the inland desert areas, the soil resistivity is significantly high and it is difficult to get the low earth resistance with conventional methods. Therefore to get a low value of grounding resistance, a good design of the grounding pit is necessary which can be achieved by using low resistivity materials. When such materials are used, it is important to optimize the pit design in order to have an economical and efficient grounding system. This paper presents a novel technique for finding the optimum size of grounding pits commonly used in the electrical power systems. With this technique, we can easily find out an optimized pit design that can effectively reduce the grounding resistance to an acceptable value. The obtained results can be readily used by engineers to obtain a good earth pit configuration for efficient grounding of the power system in the high resistivity soils by using low resistivity materials (LRM). Keywords Low Resistivity materials, grounding rods, grounding pits, pit optimization, ground resistance reduction.

I. INTRODUCTION This Power system is generally protected from lightning

strokes by surge arresters which are provided with a low earth resistance connection to enable the large currents encountered to be effectively discharged to the general mass of earth which offers some resistance to the flow of current. This earth resistance depends on electrode arrangements as well as the surrounding soil resistivity. Ideally, to maintain a reference potential for instrument safety, to protect against static electricity, and to limit the equipment ground voltage for operator safety, the ground resistance should be zero ohms. In reality, this value cannot be achieved. Therefore, low ground resistance values are required and typical values are usually specified by utilities for different situations.

The integrity of the grounding system is very important in an electrical power system for the following reasons: (i) To maintain a reference point of potential for equipment and personnel safety, (ii) To provide a discharge point for traveling

waves due to lightning and to provide return path for fault currents, and (iii) To prevent excessive high voltages on the power system. Therefore, to maintain ground system effectiveness, proper design, installation and testing of grounding system is required.

The earth is a poor conductor and, therefore, when it carries high magnitude current, a large potential gradient will result and the earthing system will exhibit an earth potential rise. Earth potential rise is defined as the voltage between an earthing system and the reference earth. The magnitude of earth fault currents can range from a few kA up to several tens of kA, and earth impedances of high voltage substations may lie in the range from 0.05 to over 1 [1]. Although higher fault current magnitudes are generally associated with lower magnitude earth impedances, earth potential rises can be as high as several tens of kV. Consequently, there is a potential risk of electrocution to people in the vicinity of a power network during earth faults, and damage to equipment may also occur unless measures are taken to limit the earth potential rise and/or to control the potential differences in critical places.

In the past, earthing systems were designed to achieve earth resistances below a specified value or on a particular density of buried conductor. In some standards, consideration is also given to the maximum earth potential rise of the earthing system [2]. Transferred potential levels are another important risk factor which is associated with presence of metallic objects in an electrical installation [1-2].

There are two ways to reduce the ground resistance; i.e. permanent and temporary. Many materials have been used for this purpose. These include bentonite, drilling rig mud, steel furnace slag, ground water accumulation using deep wells, and a variety of other methods and techniques [2-8]. At certain sites, it becomes impossible to achieve low ground resistance by adding more grid conductors or paralleling the grounding rods. Theoretically, it is known that the alternate solution of this problem is to effectively increase the diameter of the electrode by modifying the soil surrounding the earthing electrode [2-4]. Uses of chemicals such as NaCl, MgSO4, CuSO4 or CaCl2 have been used in the past to reduce the resistivity of the soil that surrounds the electrode. This treatment is advantageous when long rods are impractical because of rock strata or other obstructions to deep driving the rods [2, 5-8].

In Saudi Arabia, the ground resistivity varies in a large range because the geodetic terrain varies from sea shore to the arid desert and dry mountains. Therefore, to get a low value of

978-1-4577-0005-7/11/$26.00 2011 IEEE

grounding resistance a good design of the grounding pit is necessary. In some cases, LRM needs to be used in such pits. In such a case, it important to make an optimized pit design so that an efficient use of the LRM may be made [10, 11].

This paper presents a generalized novel technique for finding the optimum size of grounding pits commonly used in the electrical power systems. With this technique, we can easily find out an optimized pit design for use with LRM that can effectively reduce the grounding resistance to an acceptable value. The reported results can be readily used by engineers to obtain a good earth pit configuration for efficient grounding of the power system in high resistivity soils.

II. GROUNDING ARRANGEMENTS An In the low resistivity soils, a simple copper (Cu) or

copper clad steel rod of suitable length L(m) is inserted in the ground as shown in Fig.1(a) for the grounding purposes. The grounding resistance R1 () of such an earthing rod can be expressed as [3]:

++

++= 221 )2(1)2()2(11)

2ln(2 L

rL

rLr

rL

LR

. (1)

where is the soil resistivity (-m) and r is radius of the grounding rod (m). The effect of grounding rod ends has been neglected in the derivation of the eq.(1).

Fig.1. Grounding arrangement: (a) Rod with natural soil, (b) Rod totally covered with LRM, (c) Rod embedded in LRM placed in a circular pit

When the surrounding soil has very high resistivity, multiple parallel rods have to be used where the spacing between rods must be at least twice the rod depth. However, when the soil resistivity is either too high or the space is insufficient to construct the grounding network of required number of parallel grounding rods, one of the following two methods of employing LRM can be used for reducing the high grounding resistance, i.e. (i) augured hole method and (ii) pit methods. Both methods will provide a low impedance path for fault and lightning induced currents ensuring maximum safety from the internal system faults as well as the impact of lightning strokes.

In augured hole method, LRM is used as a reduction agent to reduce the high ground resistance. The buried electrode in the hole (as shown in Fig.1(b) is fully surrounded by the LRM having resistivity ( c) with thickness d-r (m). The LRM is surrounded by uniform high resistivity native soil of resistivity (-m). The grounding resistance R2() of the rod fully embedded in LRM arrangement in the augured hole can be expressed as [3]:

+

++

++=LL

rLr

Lr

rL

LR cc

2

)2

(1)2

()2

(11)2ln(2

222

++

++ 22 )2

(1)2

()2

(11)2ln(L

dL

dL

ddL ..(2)

where d is diameter of the hole. All other symbols have the same meanings as explained in eqn.(1).

In the second method, a pit of suitable dimensions (DL) is prepared and the vertical grounding rod of radius r(m) is embedded at the center of the horizontal layer of LRM having diameter D(m) and height H (m) as shown in Fig.1(c). Equation (3) [3], below gives the grounding resistance R3 for pit method shown in Fig.1(c).

[ ] +

++

++

+=

223 )2

(1)2

()2

(11)2ln()(2

. Lr

Lr

Lr

rL

HLHR

c

c

[ ]

++

+++

+ 22 )2

(1)2

()2

(11)2ln()(2

)(L

DL

DL

DDL

HLHLH

c

c

.. (3)

In choosing the augured hole method (methods-A) or the pit method (method-B), one has to consider the amount of resistance reduction achieved with certain volume of the LRM.

The ground resistance values were evaluated using a range of data as shown in Table 1 for all the variables mentioned in equations (1) ~ (3).

TABLE 1: INPUT DATA RANGES [10, 11]

Specifications Unit Value

From ~ To Radius of the grounding rod (r) m 0.0085 Grounding rod length (L) m 1.2, 2.4, 3.6 Soil resistivity ( ) -m 10 ~ 2000 LRM resistivity ( c) -m 1, 2, 5, 10 Diameter of the augured hole (d) m 2r ~ 50r Diameter of the pit (D) m 0.1 ~ 3 LRM layer thickness (H) m 0.001L ~ L

III. RESULTS AND DISCUSSION The grounding resistance for the three configurations as

shown in Fig.1(a)~(c) were calculated and based on the results, an optimized pit configuration that provides the acceptable earth resistance value is determined as explained next.

The grounding resistance (R1) for the grounding rod in native soil without LRM as shown in Fig.1(a) is given by eqn. (1). This equation clearly shows that R1 is directly proportional to surrounding soil resistivity () and increases with the increase in soil resistivity. In our earliar papers [10, 11], it has been shown that the most economical grounding rod length is 2.4m since in this case the percentage reduction is more as compared to 1.2m and 3.6m rods.

A. Effect of LRM volume on grounding resistance (R3) Refer to Fig.1(c), R3 is the grounding resistance for the bar

installed in a pit of some appropriate dimensions which is partially filled with LRM material of some suitable resistivity. The effect of the LRM volume (Vol3) was examined by varying pit dimensions such that 0.1m

TABLE 2:

R2, OPTIMUM R3, D AND H (WHEN C=2M)

(=500m) d 2r 5r 10r 15r 20r 25r 30r

Vol. (m3) 0.00163 0.01307 0.05393 0.12202 0.21736 0.33993 0.48973 R2 () 177.146 147.061 124.461 111.359 102.147 95.0641 89.3283 Opt. R3() 131.102 103.365 84.3511 73.6451 66.2822 60.7439 56.3487 D (cm) 11.27 26.48 48.08 67.78 86.29 103.85 120.66 H (cm) 4.12 5.94 7.43 8.45 9.29 10.03 10.71

(=1000m) Vol. (m3) 0.00163 0.01307 0.05393 0.1220245 0.21736 0.33993 0.48973 R2 () 354.201 293.91 248.618 222.363 203.901 189.707 178.212 Opt. R3() 243.893 188.006 150.386 129.6185 115.489 104.987 96.9374 D (cm) 14.75 35.47 64.47 90.62 115.02 138.01 150 H (cm) 2.4 3.31 4.14 4.73 5.23 5.68 6.93

For each calculated optimum value of R3, as shown in Fig.7, the corresponding optimum values of D and H are also calculated and the corresponding results are as shown in Table-2, for different values of the surrounding soil resistivities. Fig. 8 below summarizes the optimum values of D and H (cm) corresponding to optimum value of R3.

Fig 5. R3 vs H/L (using Optimization)

Fig 6. R3 vs D (using Optimization)

Fig 7. Grounding resistance vs LRM Volume used (using Optimization) (=500m, c=2m)

Fig 8. Optimum R3 and pit dimensions D(cm) and H(cm) vs LRM Volume

used (using Optimization) (=500m, c=500m)

IV. CONCLUSIONS The soil resistivity is significantly high in the inland desert

areas, which leads to high grounding resistance. This high resistance can be reduced by using low resistivity material

0 0.2 0.4 0.6 0.8 160

70

80

90

100

110

120

H/L (by Optimization)

R3 (

)

d=11.76rd=20r

=500 m

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.960

70

80

90

100

110

120

D in meters (by Optimization)

R3 (

)

d=11.76rd=20r

=500 m

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.540

60

80

100

120

140

160

180

Volume (m3)

Res

ista

nce

(Ohm

s)

R2Optimum R3

0 0.1 0.2 0.3 0.4 0.50

20

40

60

80

100

120

140

Volume (m3)

Opt

imum

val

ues o

f R3,

D &

H

D (cm)H (cm)R3 (Ohms)

(LRM). This paper presents different configurations of grounding pits commonly used with LRM applications. It presents a novel technique for determining the optimized pit dimensions that can effectively reduce the grounding resistance to an acceptable value. The optimum value of the grounding pit dimensions can be easily calculated for any surrounding soil resistivity by the proposed optimization method. The results also indicate that in the pit design, the use of too high volume of LRM does not reduce the earthing resistance in a corresponding manner

ACKNOWLEDGMENT The authors are highly indebted to Saudi ARAMCo Chair

in Electrical Power, King Saud University for supporting this work.

REFERENCES [1] Abdel-Salam M., Anis H., El-Morshedy A. and Radwan R., High

Voltage Engineering, Chapter 13, Marcel Dekker Inc., USA, 2000. [2] Blattner C.J.,"Prediction of Soil Resistivity and Ground Rod Resistance

for Deep Ground Electrodes", IEEE Trans. on PAS, Vol. PAS-99, No. 5, pp. 1758-1763,1980.

[3] Chen S. D., "Granulated Blast Furnace Slag used to reduce Grounding Resistance", IEE Proc. On Generation, Transmission and Distribution, Vol. 151, No.3, pp.361-366, 2004.

[4] He. J., Yuan J., Zeng R., Zhang B., Zou J. and Guan Z., "Decreasing Grounding Resistance of Substation by Deep-Ground Well Method", IEEE Trans. on PD, Vol. 20, No.2, pp. 738-744, 2005.

[5] IEEE Std. 80, "IEEE Guide for Safety in AC Sub-station Grounding", 2000.

[6] Jones W.R., "Bentonite Rods Assure Ground Rod Installation in Problem Soils", IEEE Trans. on PAS, Vol. PAS-99, No. 4, pp. 1443-1346, 1980.

[7] Kostic M.B., Radakovic Z.R., Radovanovic N.S. and Tomasevic-Canovic M.R., "Improvement of Electrical Properties of Grounding Loops by Using Bentonite and Waste Drilling Mud", IEE Proc. on Generation Transmission and Distribution, Vol. 146, No. 1, pp. 1-6, 1999.

[8] Meng Q., He J., Dawalibi F.P., and Ma J., "A New Method to Decrease Ground Resistance of Substation Grounding Systems in High Resistance Regions", IEEE Trans. on PD, Vol. 14, No. 3, pp. 911-916, 1999.

[9] Nor N.M., "Effect of Enhancement Materials when Mixed with Sand Under Impulse Conditions", Proc. of 8th ICPADM, Bali, Indonesia, pp. 916-919, June 2006,.

[10] A. A. Al-Arainy, Y. Khan, N.H. Malik, M.I.Qureshi, Grounding Pit Optimization for Uses in High Resistivity Areas, Proc. GCC-CIGRE 2010, 18-20th Oct, 2010, Qatar.

[11] A. A. Al-Arainy, N.H. Malik, M.I. Qureshi and Y. Khan, Grounding Pit Optimization Using Low Resistivity Materials for Applications in High Resistivity Soils" International Journal of Emerging Electric Power Systems. Vol. 12, Issue 1, Article 3, 2011.

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