An Investigation into Substation Grounding and Its Implementation on Gaza Substation

doi:10.4236/epe.2011.35074

Paper Menu >>

Journal Menu >>

Energy and Power En gi neering, 2011, 3, 593-599

doi:10.4236/epe.2011.35074 Published Online November 2011 (http://www.SciRP.org/journal/epe)

An Investigation into Substation Grounding and Its

Implementation on Gaza Substation

Ahmed Hammuda1, Hassan Nouri1, Mohammed Saleh Al-Ayoubi2

1Department of Engineering Design and Mathematics, University of the West of England, Bristol, United Kingdom

2Faculty of Mechanical & Electrical Engineering, University of Damascus, Damascus, Syria

E-mail: [email protected], [email protected], [email protected]

Recieved October 2, 2011; revised November 10, 2011; accepted November 18, 2011

Abstract

An investigation into the optimal design of a substation grounding system for the transmission substation in

Gaza City, Palestine has been carried out. A research into the most influential parameters on the effective-

ness of the substation grid system has been performed and its results have been incorporated into the Gaza

case study. Through modelling and simulating the power station in Gaza while considering some field data,

an optimal substation grounding grid has been designed and has shown complete conformance to safety. It is

thus considered that such a design will protect personnel in any area of the substation in addition to the in-

stalled machinery if the largest possible fault current was to traverse the earth.

Keywords: Gaza, Grounding Grid, Ground Resistance, Substation, Step Voltage, Touch Voltage

1. Introduction

Grounding is by far one of the most imperative aspects

of electrical systems design the significance of which has

attained modest mention. The design of the substation is

complex and constitutes a copious number of interlinked

factors that all need taking into account. A substation

grounding system is an underground, regular mesh con-

ductor network that serves the purpose of providing the

path of least resistance to the traversing current so that in

the case of a fault it is distributed in all directions of the

underlying earth. If efficient, the resulting ground poten-

tial due to a fault and the ensuing touch and step voltages

will be low enough to guarantee the safety of personnel

working on the substation in addition to safety of the

installed machinery.

This paper research investigates the effects of altering

certain parameters on the effectiveness of the grounding

system, focusing on the most relevant parameters before

applying the findings of the investigation on the substa-

tion of the Gaza Power Generating Company in Gaza

City, Palestine. The selection of this case study is due to

the intrinsic characteristics of the substation earth being

sandy and in close proximity to the sea, and for this rea-

son, at least the accessible top layers will be of consid-

erably high resistivity. Additionally, the power station

being the only one locally generating power exhibits a

substantial fault current in such a case. The substantial

necessity to protect the personnel and the dependable

machinery stipulates the design of a completely trust-

worthy and effective grounding system exhibiting touch

and step voltages within tolerable margins.

Seeing as the ground resistance

R is a major deter-

minant of the system safety, it becomes of interest to

study parameters that aid in reducing this quantity. It

should be noted that a low ground resistance does not

necessitate an even distribution of surface potentials

across the grid thus it becomes necessary to study some

parameters that help to regulate surface voltages. For this

parametric analysis and for the corresponding design

pertaining to Gaza substation, the grounding grid analy-

sis module in ETAP is utilised. While to determine the

fault current that can potentially be available at Gaza

power station, PSCAD is used.

2. Ground Potential Rise, Touch Voltage and

Step Voltage

2.1. Ground Potential Rise

The ground potential rise (GPR) is the product of the

ground resistance

R which is a function of the number

of grid conductors, its area, its depth and the resistivity

of the surrounding soil multiplied by the current G

entering the grid during a fault [1].

A. HAMMUDA ET AL.

594

2.2. Touch Voltage and Step Voltage

At the instant of a fault, the potentials that occur at the

surface of the earth are such that voltage “spikes” appear

above the grid conductors while depressions occur above

the mesh areas. At typical operational frequencies, this

potential distribution is relatively equal regardless of the

point of current injection [2].

The touch voltage results from a person making con-

tact with a grounded piece of equipment which resem-

bles the GPR while standing on any point on the substa-

tions surface. Thus the touch voltage becomes

VGPRV (1)

where e is the voltage at the point where the person is

standing. Clearly, where is lowest the touch voltage

is greatest [3].

The step voltage is then simply the difference of po-

tential occurring between two points on the surface of the

earth, 1m apart. If “p” and “q” are the locations where a

mans feet touch the earth surface, the step voltage be-

comes

VVV (2)

Both phenomena can be diagrammatically as in Fig-

ure 1.

In the general case, the human can tolerate a greater

step voltage than the touch voltage seeing as in the for-

mer case, a given tolerable current level b

will traverse

from one foot to the other each with resistance

R, en-

countering the body resistance, all in series (Figure 2)

while in the touch voltage case the current will traverse a

body resistance in series with two parallel foot resis-

tances (Figure 3) [4].

Earth surface

Grid section

Earth

surface

voltage

Figure 1. Diagrammatic representation of the step voltage

and the touch voltage appearing on the substation earth

during a fault.

Figure 2. Touch voltage and body resistance, adapted from

[4].

Figure 3. Step voltage and body resistance, adapted from

[4].

3. Comparing Simulation with Calculation

For the purpose of adding validity to the results of the

parametric analysis in the forthcoming section, where

possible, the curves obtained through ETAP simulation

will be shown alongside those calculated utilising the

accepted expression for

R derived by Sverak [5],

11 1

() 1

20 20

Rh LAhA





















 





















(3)

where



is the soil resistivity in Ωm, T is the total

length of all of the conductors combined, h is the depth

of the grid and A is its area. Simulations not involving

ground rods will be carried out using the Finite Element

(FEM) functionality in ETAP as oppose to the IEEE

method.

A. HAMMUDA ET AL.595

4. Grounding Grid Performance Results and

Analysis

4.1. The Ground Resistance against the Area

Bounded by the Grid

The first consideration after conducting any thorough

field study is the area of the substation in which the

grounding system is to be installed. It can be seen in

Figure 4 that increasing grid size is one of the most fun-

damental and effective factors in reducing the ground

resistance.

Since most substations are well above 2000 m2, it is

clear that the entire area of the substation should be cov-

ered by the grid to ensure the lowest possible resistance

(1 Ω or less). The results were obtained for a grid at a

depth of 0.5 m in soil of resistivity 100 Ωm with a con-

stant mesh size of 25 m2.

The other advantage of such design is to ensure that

the substation work area is not built over the grid pe-

rimeter where the step voltage and touch voltages are

greatest due to the abrupt change in surface potential.

4.2. The Ground Resistance against the

Conductor Length

The typical relationship between the two variables, c

and

R is most appropriately shown on a logarithmic

scale to demonstrate the “saturation” effect that occurs

when the length of the conductor is minimal. This effect

is the result of interaction between the neighbouring grid

conductors, such that as the conductors tend towards one

another, the mutual interaction begins to limit the amount

of current that can be ejected thus increasing saturation

(Figure 5). The results were obtained for a grid of size

3600 m2 at a depth of 0.5 m in soil of resistivity 100 Ωm.

It can be said that the reduction in the conductor

length or the increase in the number of meshes along one

05000 10000

Grid size (metres squared)

Calculated

Simulated

Ground resistance (Ohms)

Figure 4. The variation of the ground resistance with re-

spect to the area occupied the grid.

Ground resistance

(

Ohms

)

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

110 100

Conductor length (m)

Calculated

Simula te d

Figure 5. The variation of the ground resistance with re-

spect to the grid conductor length (mesh size).

side of the grid has limited effect in reducing the ground

resistance beyond a certain number of meshes. For this

reason, this particular enhancement can be optimized.

4.3. The Ground Resistance against the Soil

Resistivity

Quite understandably, the relationship between the soil

resistivity and the ground resistance is linear for the

simulated curve using the FEM method and likewise

through calculation as



is the external multiplying

term in Sverak’s ground resistance calculation formula as

can be seen from Figure 6.

It can be seen from Figure that for low resistivities, an

increase from 100 Ωm to 200 Ωm will result in a two

fold increase in

R. The results pertain to a 2500 m2

grid with meshes of 25 m2 at a depth of 0.5 m modelled

in single layered soil.

4.4. The Ground Resistance against the Inclusion

of Rods

An enhancement that shows remarkable decrease in

05001000 1500

Soil resistivity (ohm-m)

Ground resistance (ohms

)

Calculated

Simulated

Ground resistance (Ohms)

Soil resistivity (Ohm-m)

Figure 6. The variation of the ground resistance with re-

spect to the soil resistivity surrounding the grid.

A. HAMMUDA ET AL.

596

grounding resistance when a two soil model is used, yet

small decrease in

R when a homogenous soil model is

used is the inclusion of ground rods bound to the grid.

The two layer soil model is a more accurate model that

more closely resembles the practical situation of the

earth beneath the substation. This assumes that the soil is

split into two layers, one above the other, each with its

own resistivity value. This model is characterized by the

reflection factor K defined by









(4)

where 1



is the upper soil resistivity and 2



is the

lower soil resistivity [6]. The more the reflective factor

tends towards –1 or 1 the greater the respective differ-

ence in resistivity between the two layers. A reflective

factor of 0 denotes that the soil is uniform as was the

assumption for the previous analysis. If the lower soil

resistivity is much lower than the upper soil resistivity

the value of K will tend towards –1 and vice versa.

Figure 7 shows the effect of adding 8 m rods to a grid

of size 2500 m2 when the reflection factor is 0 (1000 Ωm

homogeneous soil) and –0.8 where the 1000 Ωm upper

soil extends to a depth of 5 m before the presence of the

lower soil of resistivity 100 Ωm. The mesh size in either

case is 25 m2 and the rod diameter is 2 cm.

It can be seen from Figure 7 that rods are only greatly

effective if they penetrate the lower resistivity soil layers,

reducing

R by a factor of 0.5 following the inclusion

of 10 rods in the above situation. Therefore the feasibil-

ity of their addition is determined by the studied soil

model in concern and is constrained by their length.

4.5. Touch and Step Potentials against the

Length of the Grid Conductor

Increasing the number of meshes, or equally reducing the

length of the grid conductors will significantly draw nea-

rer the difference of surface potential between any two

points on the grid (Figure 8).

This has the effect of reducing the difference between

two points on the earth’s surface, thus the step potential

and will draw nearer the value of the surface potential to

the GPR thus reduce the touch voltage as shown graphi-

cally in Figure 8.

The grid occupies an area of 3600 m2 and resides in

the soil of resistivity 100 Ωm at a depth of 0.5 m. The

meshes are square thus 60 m is the extreme 1 mesh sce-

nario showing the greatest touch and step voltages as

anticipated. Seemingly the effect of mesh size reduction

on the touch voltage is greater. This is confirmed by the

numerical results where it is found that the drop in the

touch voltage between the greatest mesh size and the

010203040

Number of rods in grid area

Ground resistance (ohms

)

K = 0

K = -0.8

Ground resistance (Ohms)

Figure 7. A graph showing the relationship between the

grounding resistance against the number of ground rods in

two layer soil.

200

400

600

800

1000

1200

1400

020406

Conductor length (m)

Voltage (v

)

To u ch

Step

Voltage (V)

Figure 8. The touch and step voltages against the conductor

length (number of meshes).

smallest is 82.5% while that of the step voltage is 57.1%.

4.6. Surface Layer Incorporation against the

Tolerable Touch (T-tol) and Step Voltages

(S-tol)

The significant effect of adding a surface layer for the

purpose of increasing the series resistance of the person-

nel hence raising tolerable voltage levels [7] is not em-

phasised enough. Figure 9 shows the results obtained

through incorporating a 1000 Ωm and a 5000 Ωm (typi-

cal for pea gravel) surface layer. The grid is 2500 m2

with 25 m2 meshes and the soil is of 100 Ωm resistivity.

It can be seen that the incorporation of a 2000 Ω sur-

face layer as thick as just 5 cm can improve the tolerable

touch voltage significantly, precisely by 130.5% while

increasing the tolerable step voltage by 374.9%. It is

A. HAMMUDA ET AL.597

500

1000

1500

2000

2500

00.05 0.10.15

Resistive layer thickness (m)

1000 ohm TTouch

1000 ohm TStep

2000 ohm TTouch

2000 ohm TStep

Tolerable voltage (V)

1000 Ohm T-tol

1000 Ohm S-tol

2000 Ohm T-tol

2000 Ohm S-tol

Figure 9. The resistive layer thickness against tolerable vol-

tages.

noted that surface layers of much higher resistivity are

widely available. This cost effective approach can con-

siderably increase the safety of the system while avoid-

ing excessive structural improvements. This technique

was employed for improving the Gaza grounding system

after reaching the “saturation” of structural improve-

ments as will be shown in the succeeding section.

5. Modeling the Gaza Study

5.1. Substation Field Study

Following consultation with the management of the Gaza

Power Generating Company (GPGC) [8] it was found

that the substation occupies an area of 24267 m2 (204 m

× 119 m). The substation is approximately 7.5 km from

the Mediterranean shoreline and built above a region of

dry sandstone [9] almost 7 m above sea level [10]. Dry

sandstone can resemble a resistivity as high as 1000 Ωm.

It is suggested that the earth at depths below 7 m will be

both rich in moisture and will possess a high salt content

due to sea water intrusion. According to the curves shown

in [6] this can reduce the resistivity up to 100 fold due to

the electrolytic nature of water and salt thus the resistive-

ity for the second sandstone layer below a 7 m depth is

modelled at 10 Ωm.

5.2. Generating Station Model for Fault

Determination

Further consultation with the management of the (GPGC)

[8], it was found that the power station constitutes 4 ma-

jor generators. Generators 1 and 2 can potentially pro-

duce a power output of 30.470 MW and have a MVA

rating of 35.847 MVA. They develop a voltage of 11 kV

and are connected to separate transformers. The trans-

formers step up the generating voltage at the substation

to 66 kV and are both rated at 31 MVA. The reactance of

generators 1 and 2 are arbitrarily assumed to be 0.7 Ω.

The generators 3 and 4 can potentially produce a power

output of 59.3 MW, are rated at 67.764 MVA and de-

velop a voltage of 11 kV. Generators 3 and 4 are arbi-

trarily assumed to have a reactance of 0.5 Ω. They are

connected to a bus coupled with two parallel connected

transformers rated at 62.5 MVA each stepping up the

voltage to 66 kV. It is assumed that a fault occurs at the

transmission bus coupled with the aforementioned trans-

formers and that the fault is of three phase to ground na-

ture as shown in the PSCAD model of Figure 10.

5.3. Simulating the Three Phase Fault

The phases are faulted after 0.5 s of normal operation

assumingly lasting a period of 0.5 s. The resultant wave-

form is shown in Figure 11, attaining a peak of 15.46 kA

before settling at a magnitude of 6.05 kA. Closer evalua-

tion of the resultant waveform yielded a DC component

attenuation time constant of 2 s.

This time constant





T and the fault period





can be placed into the formulation for the “decrement

factor”

D [7] and multiplied by the symmetrical cur-

rent

I to obtain the symmetrical fault current equiva-

lent over the 0.5 s period where







 









(5)

yielding a decrement factor of 1.6. The symmetrical fault

current “IF”, over the initial period of 0.5 s, has a value of

ABC > G

Figure 10. Figure 8 PSCAD model of Gaza power station

with a connected 3 phase to ground fault logic component.

A. HAMMUDA ET AL.

598

Figure 11. Three phase fault current waveform produced

by Gaza power station.

6.05 kA × 1.6 = 9.68 kA. Finally, and assuming the cur-

rent that traverses the grid is 80% of the fault, guided by

the curves deduced by Garett et al. [11] and allowing for

the worst case scenario, the “split factor”, and

the grid current .

0.8

S

6.05 kA1.60.8

I

5.4. Initial Design and Tolerable Voltages

The conductor length is commonly in the region of 5 - 10

m depending on the area of the substation. If the con-

ductor length is 8.5 m for a square mesh, 24 and 14

meshes can be placed in the x and y direction respec-

tively.

The grid contains 710 copper conductors and resides

in a soil of resistivity 1000 Ωm. The tolerable touch

@Ttol

and tolerable step @Stol

voltages for this par-

ticular situation are 1554.2 V and 555.1 V respectively

for an average 70 kg individual. The objective of the

system is to develop voltages below these limits.

V V

5.5. Initial Simulation

Following a 7.7 kA current injection, the resultant touch

and step voltages greatly exceed the permitted limits at

3360.4 V and 2319.8 V correspondingly. ETAP valued

R at 3.06 Ω. The imperative task is thus to reduce the

touch voltage, and on regulating this, it is expected that

the step voltage will also adhere to safety.

5.6. Improvement of Design by Rod

Incorporation

As aforementioned, a two layer soil structure with a

negative K factor can be harnessed to the advantage of

the engineer by incorporating resistance reducing rods.

Figure 12 shows this effect and demonstrates how the

inclusion of 8 m penetrating rods has caused the ground

resistance to fall to a value well below 0.5 Ω a very sat-

isfactory decrease of 83.7%.

Adding more than 150 rods is unfeasible in terms of

reducing the grounding resistance. The curve reaches

almost a horizontal gradient and any further addition of

0.5

1.5

2.5

050100 150

Number of rods in grid area

Ground resistance (Ohms)

Figure 12. Adding resistance reducing rods to the initial

design.

rods will only affect the potential gradient rather than the

grounding resistance and the overall GPR of the system.

The new touch voltage after re-simulation has fallen

35.2% to 2179 V while the step voltage has dropped

18.4% to 1893.2 V. More enhancements are required to

meet the tolerable criteria.

5.7. Improvement of Design by Mesh Size

Reduction

Revisiting the previous section, it is seen than a reduc-

tion in mesh size promotes a respectable reduction in the

touch voltage. Decreasing the mesh size to 5.2 m5.2 m



resulting in 39 and 23 meshes in the x and y direction

correspondingly, the system can be re-simulated to yield

a further drop in the touch voltage of 30.2% (1520 V). It

is interesting to note that the new step voltage is 11.1%

greater at 2105.2 V. The increase in the step voltage is an

exceptional case and has occurred due to the fact that the

grid potential has been raised with respect to the area at

the immediate vicinity of the grid. It is suggested that

this exceptional case occurs when the grounding resis-

tance reaches a “saturated” low value that no longer falls

significantly when physical structural enhancements are

made to the grid. The result of this is that the GPR essen-

tially remains at its previous value. The decrease in touch

voltage then occurs due to raised surface potentials.

5.8. Incorporation of a High Resistivity Surface

Layer

It is forecasted that further physical improvement to the

metallic grounding structure is uneconomical, thus the

system’s solid structure is ready to incorporate a layer of

high resistive surface material to raise tolerable voltages.

Adding a surface layer of pea gravel of depth 0.15 m

and resistivity 5000 Ωm according to [7] and as valued

by ETAP produces new results showing full confor-

mance to safety for both the touch and step voltages.

This adjustment shows a remarkable increase in the

A. HAMMUDA ET AL.

599

7. Acknowledgements touch and step tolerable voltages, 184.6% and 263.7%

correspondingly. The results are summarised in Table 1.

Dr Rafiq Maliha, the plant manager at the main power

station in Gaza, Palestine is acknowledged for supplying

useful information and for his correspondence in the

formation of the research.

6. Conclusions

A grounding system for the transmission substation in

Gaza City, Palestine has been designed and simulated

and is believed to safely dissipate the largest possible

fault current at the plant. The final design shows that the

resultant touch and step voltages are within tolerable

regions and no more enhancements are necessary. It was

found that in the study, the incorporation of ground rods

long enough to penetrate the moist soil layers deemed

reachable at depths beyond 7 m decreases the overall

grounding resistance by above 80%. The touch and step

voltages are reduced by 35% and 18% correspondingly.

8. References

[1] IEEE, “IEEE Recommended Practice for Determining the

Electric Power Station Ground Potential Rise and In-

duced Voltage From a Power Fault,” IEEE Std 367-1996,

1996, pp. 1-125.

[2] J. Ma and F. P. Dawalibi, “Modern Computational Meth-

ods for the Design and Analysis of Power System Ground-

ing,” Proceedings of International Conference on Power

System Technology, Beijing, Vol. 1, 18-21 August 1998,

pp. 122-126.

Further structural improvements for the purpose of

reducing the touch voltage included the reduction of the

mesh size before the incorporation of a pea gravel sur-

face layer of depth 0.15 m and of resistivity 5000 Ωm.

This adjustment remarkably increased the touch and step

tolerable voltages to 184.6% and 263.7% respectively

and formed the adequate grounding system for the Gaza

study. It also establishes the importance of including a

high resistivity surface layer if it is found that the touch

and step voltages are beyond acceptable while further

structural improvements in the grid are unfeasible. In this

particular case its inclusion is indispensable.

[3] S. Ghoneim, H. Hirsch, A. Elmorshey and R. Amer, “Sur-

face Potential Calculation for Grounding Grids,” IEEE

Power and Energy Conference, Jaya, 28-29 November

2006, pp. 501-505. doi:10.1109/PECON.2006.346703

[4] IEEE, “IEEE Recommended Practice for Industrial and

Commercial Power System Analysis,” IEEE Std 399-1997,

1997, pp. 1-483.

[5] J. G. Sverak, “Simplified Analysis of Electrical Gradients

above a Ground Grid, Part I: How Good Is the Present

IEEE Method?” IEEE Power Engineering Review, Vol.

PER-4, No.1, 1984, pp. 26-27.

doi:10.1109/MPER.1984.5525426

The performed parametric analysis and research estab-

lishes that the most effectual grid improvement factor is

its area being inversely proportional to the grounding re-

sistance. This is followed by the soil resistivity being

directly proportional. Other design modifications are use-

ful in obtaining specific results. Rods are only effective

in two layer soils of negative K coefficient when they are

long enough to penetrate the lower soil. Reducing the

mesh size is an admirable touch voltage reducing factor

and when accompanied by a reduction in the overall

ground resistance assists in reducing the step voltage.

[6] F. Dawalibi and D. Mukhedkar, “Parametric Analysis of

Grounding Grids,” IEEE Transactions on Power Appa-

ratus and Systems, Vol. PAS-98, No. 5, 1979, pp. 1659-

1668. doi:10.1109/TPAS.1979.319484

[7] IEEE, “IEEE Guide for Safety in AC Substation Ground-

ing,” IEEE Std 80-2000, 2000, pp. 1-192.

[8] R. Maliha, “Gaza Power Station Management,”

([email protected]). Part of Gaza power station grounding

project at UWE, 2010. (a[email protected])

[9] H. Baalousha, “Analysis of Nitrate Occurrence and Dis-

tribution in Groundwater in the Gaza Strip Using Major

Ion Chemistry,” Global NEST Journal, Vol. 10, No. 3,

2008, pp. 337-349.

Table 1. A summary of the results produced in the iterative

grid enhancement process. [10] Google Earth, “Gaza Aerial View,” 2010.

http://www.google.co.uk/intl/en_uk/earth/index.html

Design

Stage

(Ω) T

V(V) @Ttol

V(V) S

V(V) @Stol

V(V)

5.5 3.06 3360.4 555.1 2319.8 1554.2

5.6 0.17 2179 555.1 1893.2 1554.2

5.7 0.17 1520.1 555.1 2105.2 1554.2

5.8 0.17 1520.1 1579.8 2105.2 5653.3

[11] D. L. Garrett, J. G. Myers and S. G. Patel, “Determina-

tion of Maximum Substation Grounding System Fault

Current Using Graphical Analysis,” IEEE Power Engi-

neering Revie w, Vol. PER-7, No. 7, 1987, pp. 49-50.

doi:10.1109/MPER.1987.5526974