Earthing Desing v42-255

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AbstractElectrical railway system relies on the tractionsubstation to enhance its voltage operation. Performing High Voltage

(HV) maintenance with a multicraft work force creates a special set

of safety Challenges. The safety aspect will be discussed in this paper

from an earthing point of view. An effective approach to design the

earthing system of the traction substation will be proposed and

explained. Finally, this paper provides an analytical example to the

proposed method.

KeywordsSoil Resistivity, Earthing Design, OHEW.

I. INTRODUCTIONLECTRICAL Railway employ traction substations to

advance the operational voltage of their trains, the traction

substation play an imperative role in ornamental the power

capability of the catenary line. Many of those traction

substations are located in the vicinity of resident, some are

few meters away. The safety of the people as well as the

workers must be carefully addressed to guarantee the safe

operation of those traction substations. Earthing design is one

of the main areas of concerns when it comes to the control andthe safety of the traction substation. Earthing system provides

a safe working environment for workers and people passing

by during a fault or malfunction of the power system.

Some of these traction substations are fed through a feeder

without the existing of the over head earth wire (OHEW),

meaning that the entire fault current will reach the earth grid

and create a danger earth potential rise (EPR). Some utility

design and install a continuous OHEW between the traction

substation and the supplier, others choose to install OHEW for

few hundred meters on both side of the traction substation to

expand the substation grid. These OHEW with its associated

electrode assist the substation earth grid in the debauchery of

the fault current and enhance the touch, step and EPR voltage.

This paper will converse the case where the substation is be

fed by a feeder without the OHEW, the study will include the

OHEW for few hundred meters both side of the substation.

Also this paper shows significant between different

approaches when it comes to simulation the earth grid.

M. Nassereddine, Research student at the University of Western Sydney,

Australia (e-mail: [email protected])

A. Hellany, Senior lecture at the University of Western Sydney, Australia

(e-mail: [email protected])

II. SOIL RESISTIVITYBefore departing into the detail design of the earth grid, it is

imperative to study the ground around the site. Soil resistivity

plays a fundamental part in determining the earthing grid

details, soil resistivity can be carried out using different

method, it is essential for an efficient earthing design to have

more than a test carried out onsite, it is suggested to carry at

least two tests if possible as shown in figure 1, also it is to thebenefit of the design if a longer test can be carried out, test

around 100 meters if possible to give more accurate reading

for the bottom layer.

Fig. 1 Soil Resistivity test layout for a proposed site

Below is the most three popular methods to perform soil

resistivity test:

Wenner method

Wenner method consists of four electrodes; two are for

current injection and two for potential measurement [1], as

shown in figure 2.

Fig. 2 Wenner four probe arrangement

The soil resistivity formula related to Wenner method is

shown in equation 1.

aR 2= (1)

OHEW Earthing Design Methodology of

Traction SubstationA. Hellany M., Nassereddine

E

World Academy of Science, Engineering and Technology 42 2010

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Where

R is the resistance measured by the machine,

a is the spacing of the probe

Wenner array is considered to be the least efficient from

labour perspective as it requires four people to perform the

task in a short time. On the other hand it is considered to bethe most competent method when it comes to ration of

received voltage per unit of transmitted current. [2]

Schlumberger Array

This method is more economical when it comes to the man

power when compared to Wenner method. The outer electrode

can be moved four or five times for each move of the inner

electrode [2]. As shown in figure 3 Schlumberger method can

be performed. This method can incur an error is the top layer

is of high resistivity. Every machine has maximum loop

impedance, in some cases where the soil resistivity of the top

layer is very high, that will lead to loop impedance higher that

the maximum loop impedance of the machine. In this case

the reciprocity theorem can be applied to the Schlumbergerarray, this method is known as the Inverse Schlumberger

Array (ISA), this method provides a safer working

environment for the tester under high current supply also

reduce the heavier cable may be needed during the test. The

soil resistivity can be calculated using equation 2: [2-4]

l

RL

2

2

= (2)

Where:

L is the distance the centre from the outer probe

l distance to the centre from the inner probe

Fig. 3 Schlumberger Array layout

Driven Rod Method

This method is also known as the three probe method or

three pin method [5]. This method is mainly suitable for an

area where the physical layout makes the usage of the Wenner

and Schlumberger methods are difficult; equation 3 can be

used to compute the soil resistivity under this method:

=

d

l

lR

8ln

2 (3)

Where:

l is the length of driven rod in contact with earthd driven rod diameter

Fig. 4 Driven Rod test layout

After completion the soil Resistivity test, using software to

determine the soil structure or can be done using handcalculation relaying on IEEE standards. After the agreement

on the soil structure it is possible to compute the grid

resistance or the electrode resistance using one the following

formulas.

Equation 4 can be used to compute the earth grid of a mesh

that buried at a depth of 0.5 meter:

= 1

)(

4ln

5.0dh

L

LR

(4)

Where

h is the buried depthL length of the electrode

d diameter of the electrode

Equation 5 can be used to compute the resistance of a grid

consisted of multiple electrode in parallel.

= 1

2ln

b

L

LR

(5)

Where

L is the buried length of the electrode

b equivalent radius off the electrode at the surface

( )5.02

2

25.0

)4( shS

dhsSb

+=

=(6)

Where:

d is the diameter of the electrode

h buried depth

s distance between 2 parallel electrode

S distance from one electrode to the image of the other in

meters

Equation 7 can be used to calculate the resistance of the

electrode at each pole of the OHEW:


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= 1

8ln

2 d

L

LRg

(7)

Where

L is the buried length of the electrode in meters

D the diameter of the electrode in meters

III. SOIL RESISTIVITY TEST VERIFICATIONIt is recommended by IEEE and Australian standards, to

bear a method to verify the soil Resistivity test results; one of

these methods could be the fall of potential. For this method to

convene its purpose it is vital to swipe for the area metal

detector to ensure that there is no intrusion from any metal

objects in the area. It is recommended to drive an electrode or

cupper stick to a depth of more than 0.5 meters in the ground,

the deeper the electrode the more accurate are the results.

This method can be carried out using the same machine for

the soil Resistivity test such as SYSCAL, figure 5 shows the

layout of the fall of potential test. Figure 6 shows the layout ofthe result of the test:

Fig. 5 fall of potential test.

Fig. 6 Result of the fall of potential test.

A similar electrode characteristic will be used in a

simulation process to determine the best soil structure for the

site. This can be done by matching the fall of potential test

with the simulated one. This method will assist in

determining the most accurate soil profile for the designed

substation.

IV. SAFETY IN EARTHINGThe main reason behind the design of an earth grid is to

achieve a safe working and living environment. According to

many standards, such as IEEE and Australian standards, the

hazard can jeopardise two categories of people:

The public that can be affected by the step and touchvoltage. (50kg person)

Workers who can be affected by the step and touchvoltage as well as the earth potential rise (EPR) zone.

(70kg person)

It is a common practice, as per IEEE and AustralianStandard, to compute the step and touch voltages using the

following equations.

t

CV sstouchkg

174.011650

+= (8)

t

CV ssstepkg

696.011650

+= (9)

t

CV sstouchkg

236.015770

+= (10)

t

CV ssstepkg

942.015770

+= (11)

09.02

109.0

1+

=s

s

sh

C

(12)

Where

Cs is the de-rating factor relating to surface layer thickness

and resistivity

s is the top surface layer

t is the primary clearance time

V. EARTHING DESIGNAs mentioned before, this paper will discuss the earthing

study of a traction substation fed by a 33kV line and its

associated feeder with few hundred meters of OHEW at both

side of the substation. Figure 7 shows the proposed layout of

the design; it shows the substation grid, the feeder and the

OHEW with its associated electrode. The OHEW is extended

for a length of 1000 meters at each end of the site with a 100

meters separation between poles.

Some designers break the system into two sections, theOHEW of the feeder and the substation earth grid, this could

lead to an over engineered design or to unsafe environment

They use the split study to determine the current into the grid,

and then using this split current to compute the EPR in the

substation using the stand alone earth grid resistance

They consider this EPR to be the maximum EPR for the

system which lead to the assumption that is the maximum

EPR that can be transferred to each electrode at each pole of

the feeder. The design will be safe but way over engineered as

will be shown further in the study. The substation grid and the

OHEW on both sides must be part of the simulation design at

all time, including the substation grid with the OHEW. During

the simulation of the fault this will give more accurate reading


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for the touch voltage, step voltage and EPR in and around the

nominated site.

Fig. 7 the proposed layout for the traction substation

When the traction substation is being fed from one side, the

split current will determine the return in the OHEW of the

feeding side. The mutual impedance between the faulted line

and the OHEW of the feeding side will have an influence on

the return current. Also the split study will not determine theground current into the substation earth grid.

For example if 40% of the current return in the OHEW, that

doesnt mean that 60% goes into the substation earth grid as

some will be dissipated in the OHEW on the other side of the

substation. This side does not have any mutual impedance

from the fault current as there is no parallel section between

this side and the faulted line. The OHEW in the opposite side

of the fault will act as an extended grid to the substation. This

shows that a one simulation that tables the entire system

(substation grid and OHEW) will have a more efficient

earthing study.

Figure 8 shows the electrical circuit of the designed system.

It does not show the electrode for the poles, but shows thefaulted line impedance, the substation earth grid and the

OHEW impedance at both end of the substation. The mutual

impedance exist between the Z-OHEW and Z-Transmission-

line and can be determined using equation 13, the split current

can be found using equation 15

Fig. 8 The single line diagram of the OHEW and earth grid

++=

GM

e

gwgw

R

Dfj

fRZ

10

7

7

log10938.28

1088.9

(13)

fDe

4.658= (14)

gwZ self impedance of the OHEW in Ohms/m

GMR is the geometric mean radius of the OHEW in m

f is the frequency

efg III = (15)

Where;

gI Is the ground current

eI Is the current that return in the OHEW

eI is the current that return in the OHEW Figure 5 shows

Ie and Ig can be determined using the following

equation:

f

gw

gm

ge

gm

fe IZ

Z

RZ

RZII

= (16)

Where

mZ mutual impedance between phase conductors and

OHEW in Ohms

eZ input impedance of the OHEW in Ohms

gR resistance of the earth grid in Ohms

gmZ mutual impedance per meter between OHEW and

phase conductors in Ohms/m

The two facts below can lead to an EPR at the last electrode

of the 1000 meters of OHEW much higher than the one at the

substation:

The electrode resistance of the poles will be muchhigher than the substation earth resistance.

The OHEW is not continuous to the supplier. OHEW current will dissipate in the last electrode.The case study will discuss two different scenarios and

explain how that impacts the substation.

VI. CASE STUDYThe case study will be conducted under the following terms:

Stand alone substation earth grid is 0.5 OhmPole electrode resistance of 10 OhmsBee OHEW AAC 7/4.9Pole separation of 100 metersSL-Ground fault current of 2000ASplit factor of 0.4The Bee OHEW has a resistance of 0.268 Ohms/km. The

maximum EPR at the substation can be computed to be 400V.

Figure 9 shows the studied circuit for one side of the

substation. Using circuit analysis considering that Ie is 1000A,

the 200A assumed to be utilised by the OHEW on the other

side.

OHEWea ZIEPRV = (17)

OHEWa

eab ZRt

VIVV

= (18)

gwOHEW ZZ = (19)


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gwR is the self resistance of the OHEW in Ohms/m. Similar

analysis will be carried out on the circuit in figure 9 using the

400 EPR at the substation as the voltage source:

VVa 322=

VVb 261=

VVc 86

AIFinal 759

The voltage at point c

VRtIV Finalc 759010759

This proves that the maximum EPR under the single line to

ground fault is 7590 V and not the 400 volts drop at the

substation. Under the condition where eI is forced to the

OHEW, equation 20 will determine the number of poles

needed to have electrode and OHEW,

OHEW

i

k

k

eii ZRt

V

IVV

=

=

1

11 (20)

RtIV ii = (21)

Rt

V

II

i

k

k

eFinal

==

1

1 (22)

==n

k

keFinal VRtIV1

(23)

i : represent the pole numbers

iI : represent the current in i the electrode

FinalI : represent the current in the last electrode

The voltage drop at the substation must be bigger than the

drop voltage at the electrode as shown in equation 24

=

>>1

1

i

k

ke VRtIEPR (24)

This condition can occur under the following conditions

(otherwise the maximum EPR will occur on the electrode and

not the substation): Existing of a large number or electrode Small return current Small electrode resistance

Under the assumption that the voltage drop across the

electrode is almost the same for the ten electrodes, the number

of electrode is ten, figure 10 shows the relation between the

OHEW current and the number of electrode needed to

dissipate the current. This figure is under the assumption

where the voltage drop across the electrodes is of the EPR

at the substation on average

n number or electrode

0

50

100

150

200

250

300

350

400

0 500 1000 1500 2000 2500 3000

OHEW current (A)

NumberofElect

rode

Fig. 10 the number of electrode against the OHEW current

VII. CONCLUSIONIn conclusion this paper shows that the design, of an earthing

system for a traction substation with few hundred meters of

OHEW at each end, needs to be assessed as part of the grid tooptimize the design and to eliminate any dangerous situations.

In Addition, this paper shoes that the attempt to compute the

EPR in the substation using the stand alone earth grid

resistance will lead to an over engineered design or to unsafe

environment and create a false assumption that the maximum

EPR that can be transferred to each electrode at each pole of

the feeder.

REFERENCES

[1].IEEE guide to safety in AC substation grounding, 2000 (IEEE, NewYork, 2000)

[2].AS/NZS 4853:2000 electrical hazards on metallic pipelines.[3].M. Nassereddine, A. Hellany, How to design an effective earthingsystem to ensure the safety of the people ACTEA conference, conference

proceedings pp. 112-116, July 2009.

[4].M. Nassereddine, A. Hellany, AC interference study on pipeline: theimpact of the OHEW under full load and fault current computer andelectrical engineering, 2009. ICCEE, Vol 1, 28-30 Dec. 2009 Pages:497-501

[5].M. Nassereddine, A. Hellany, Designing a lightning protection systemusing the rolling sphere method computer and electrical engineering,2009. ICCEE, Vol 1, 28-30 Dec. 2009 Pages:502-506

[6].R. Hans, A practical approach for computation of grid current, IEEEtransactions on power delivery, Vol. 14, No. 3, July 1999.

[7].C.N. Chang, computation of current-division factors and assessment ofearth-grid safety at 161/69kV indoor-type and outdoor-type substations

IEE proc.-Gener. Transm. Distrib., Vol. 152, No. 6, November 2005.

[8].A. Campoccia, A method to evaluate voltages to earth during an earthfault in an HV network in a system of interconnected earth electrodes of

MV/LV substations IEEE transactions on power delivery, Vol. 23, No. 4,

Oct 2008.

[9].S. Mangione, A simple method for evaluation ground fault currenttransfer at the transition station of a combined overhead-cable line, IEEE

transaction on power delivery, Vol. 23, No, 3, July 2008.

[10]. E. Viel, Fault current distribution in HV cable systems IEE proc-Gener. Transmission distribution, Vol. 147, No. 4, July 2000.

[11]. L. D. Grcev, Transient electromagnetic fields near large earthingsystems IEEE transactions on magnetic, VOL. 32 No. 3, May 1996.

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Earthing Desing v42-255