earthing mat design and calculation in myanmar

Abstract—This paper presents the design of earthing system for

230 kV substation and simulation for calculation of required

parameters. In substation, earthing system is essential not only to

provide the protection of people working or walking in the vicinity of

earthed facilities and equipments against the danger of electric shock

but also to maintain the proper function of the electrical system. By

using proper conductor and electrode side, earthing system may be

able to lightning effects. This paper is to provide information

pertinent to safe earthing practices in AC substation design and to

establish the safe limits of potential differences under normal and

fault conditions.

Standard equations are used in the design of earthing system to get

desired parameters such as touch and step voltage criteria for safety,

earth resistance, grid resistance, maximum grid current, minimum

conductor size and electrode size, maximum fault current level and

resistivity of soil. By selection the proper horizontal conductor size,

vertical electrode size and soil resistivity, the best choice of the

project for safety is performed. This paper mentions the calculation

of the desired parameters which are simulated by MATLAB program.

Some simulated results are evaluated. The goal of this paper is to be a

safe earthing system for substations.

Keywords—Safe earthing system, AC substation, MATLAB

program.

I. INTRODUCTION

HIS paper is concerned with earthing practices and design

for outdoor AC substation include distribution,

transmission substations for power frequency in the range of

50 Hz. The intent of this paper is to provide information

pertinent to safe earthing practices in AC substation design.

DC substation and the effect of lightning surges are not

covered by this paper. With paper caution, the method

described in here is also applicable to indoor portion of such

substation. And the design procedure is presented in here.

The specific purpose of this paper is:

1) To review substation earthing practices with

references to safety

2) As a basic for design, to establish the safe limits of

step, touch, mesh voltage etc, potential difference

under fault conditions between points that can bee

contacted by human body

3) To provide a procedure for the design of practical

earthing system.

Authors are with Mandalay Technological University, Myanmar (e-mails:

eieicho2006@ gmail.com, [email protected]).

This paper briefly discusses the field tests required to

evaluate soil resistivity. The procedures for measuring the

resistance of the installed earthing system and the continuity of

the grid conductor are described in here. It covers some of the

practical aspects of earthing in more detail. The goal of this

paper is to design earthing grid to safety, to evaluate and to

simulate grid conductor size, vertical electrode size, criteria

permissible potential difference, grid resistance, maximum

grid current, grid potential size and required facts for design

procedure by a computer program. Although there are two

methods namely Trench Method and Conductor Flowing

Method for grid construction, the second are in used in here.

II. EARTHING SYSTEM FOR SUBSTATION

An effective substation earthing system typically consists of

earth rods, connecting cables from the buried earthing grid to

metallic parts of structures and equipment, connections to

earthed system neutrals, and the earth surface insulating

covering material. Current flowing into the earthing grid from

lightning arrester operation, impulse or switching surge

flashover of insulators, and line to ground fault current from

the bus or connected transmission lines all cause potential

differences between earthed points in the substation. Without a

properly designed earthing system, large potential differences

can exist between different points within the substation itself.

Under normal circumstances, it is the current constitutes the

main threat to personal.

An effective earthing system has the following objectives:

1) Ensure such a degree of human safety that a person

working or walking in the vicinity of earthed facilities

is not expressed to the danger of a critical electric

shock. The touch and step voltage produced in a fault

condition have to be at safe values. A safe value is

one that will not produce enough current within a

body to cause ventricular fibrillation.

2) Provide means to carry and dissipate electric currents

into earth under normal and fault conditions without

exceeding any operation and equipment limits or

adversely affecting continuity of services.

3) Provide earthing for lightning impulses and the surges

occurring from the switching of substation equipment,

which reduces damage to equipment and cables.

4) Provide a low resistance for the protective relays to

see and clear ground faults, which improves

Design of Earthing System for New Substation

Project (Shwe Sar Yan) in Myanmar

Ei Ei Cho, and Marlar Thein Oo

T

World Academy of Science, Engineering and Technology 18 2008

430

protective equipment performance, particularly at

minimum fault.

Fig. 1 Earthing grid design

For step voltage criteria the limit is

s

sssstept

.),k)ρ(hC(E

11606100050 += (1)

s

sssstept

.),k)ρ(hC(E

15706100070 += (2)

Where

Cs = 1 for no protective surface layer

ρs = the resistivity of the surface material in Ω-m

ts = duration of shock circuit in sec

The touch voltage limit is

s

sstoucht

.),k)ρCs(h.(E

116051100050 += (3)

s

sstoucht

.),k)ρCs(h.(E

157051100070 += (4)

The minimum conductor size formula is mentioned below-

Amm2 =

+−

+

×××

×

)T(K

)T(T

TCAP

ραt

I

a

am

rrc

0

4

1ln

10

(5)

Acmils =

+−

+

×××

×

)T(K

)T(T

TCAP

ραt

I.

a

am

rrc

0

4

1ln

10

521973 (6)

Where

I = rms value in kA

Amm2

= conductor sectional size in mm2

Tm = maximum allowable temperature in ˚C

Ta = ambient temperature for material constants in

˚C

α0 = thermal coefficient of resistivity at 0˚C

αr = thermal coefficient of resistivity at reference

temperature Tr

ρr = the resistivity of the ground conductor at

reference temperature Tr in uΩ/cm3

K0 = 1/α0 or 1/α0 - Tr

tc = time of current flow in sec

TCAP = thermal capacity factor

For grounding resistance, the following formula is used

Rg =

+

++

Ah

ALρ

t 201

11

20

11 (7)

Where

ρ = soil resistivity

Lt = total length of grid conductor

A = total area enclosed by earth grid

h = depth of earth grid conductor

For calculation of grid current, equation (8) is used

Ig = Sf x 3I0 (8) Ig = maximum grid current

3I0 = symmetrical fault current in substation for

conductor sizing in A

Sf = current diversity factor

Equation (9) is expressed for grid potential rise (GPR)

GPR = IgRg (9)

Formula for calculation of mesh and step voltage are

Em =

rrg

gimm

NL.L

IKρK

151+ (10)

Es =

rrg

gisi

NL.L

IKρK

151+ (11)

Where


431

Em = mesh voltage at the center of corner mesh

Es = step voltage between point

Km = spacing factor for mesh voltage

Ks = spacing factor of step voltage

Ki = correct factor for grid geometry

Equation for Km is described below

−+

−

++

)nπ(Kh

K

d

h

Dd

h)(D

hd

D

m

ii

12

8ln

48

2

16ln

2π

1 22

(12)

Spacing factor for step voltage equation is presented as this

Kis =

−++

+ −).(

DhDhπsn 2

50111

2

11 (13)

Where

D = spacing between adjacent grid conductor

H = depth of burial grid conductor

D = diameter of grid conductor

For soil resistivity evaluation, below equation is used

ρ = 2πaR (14)

Where

ρ = soil resistivity

a = distance between earth spikes

R = earth resistance measured with the Metraterr2

TABLE I

CONSTANT VALUES FOR DESIGN CALCULATION

Symbol Quantity Values

Ta ambient temperature 35˚C

Tm maximum allowable

temperature

1084 ˚C

Ts fault duration time 1 sec

K0 temperature of

thermal coefficient of

resistivity at 0˚C

245 ˚C

αr thermal coefficient of

resistivity at reference

temperature

0.00378

ρr resistivity of each

conductor reference

temperature

4.397 uΩ/cm

TCAP thermal capacity factor 3.8466 J/cm3/˚C

h depth of burial grid

conductor

0.6 m

h0 reference depth of grid 1 m

ρs surface layer resistivity 3000 Ω-m

d diameter of grid

conductor

16 mm

Lr length of one earth rod 3 m

TABLE II

OUTPUT RESULT FOR GRID CONSTRUCTION DESIGN

Symbol Quantity values

Ac Earth conductor size 103 mm2

d Earth electrode

diameter

16mm

Ig Max: grid current 1.957kA

Rg Ground resistance 0.838Ω

GPR Grid potential rise 1650V

Km Spacing factor for

mesh voltage

0.653

Kim Correct factor for Em 3.838

Ks Spacing factor for

step voltage

0.342

Kis Correct factor for Es 3.752

Etouch Touch voltage

criterior

651.55V

Estep Step voltage criterior 2135.2

Em Calculation mesh

voltage

201.177V

Es Calculation of step

voltage

103.546V

Lt Total grid current 5972m

ρ Soil resistivity 250 Ω

These data described above table are obtained by using

MATLAB program.

III. CALCULATION

STEP 1 Conduction design

Minimum cross section area (Ac) = 95 mm2

STEP 2 Touch and step voltage criterior

Etouch (70) 65.55 V

Estep (70) 2135.2 V

STEP 3 Design of ground mesh

Na = Number of conductor in X-axis = 18 Nos

Nb = Number of conductor in Y-axis = 19 Nos

Lg = Mesh conductor length = 5132 m

Nr = Quantity of ground rod = 280 rods

D = Ground rod spacing = 8 m

h = Depth of burial grid conductor = 0.6 m

Lt = Total length of conductor = 5972 m

STEP 4 Substation grid resistance (Rg)

Rg = 0.843 Ω

STEP 5 Grid current (Ig)

Ig = 1.957 kA

STEP 6 Grid potential rise (GPR)

GPR = Ig x Rg

GPR > Etouch

STEP 7 Mesh and step voltage

Kim for Em

Kis for Es

Km

Ks


432

STEP 8 Check touch voltage and step voltage

Em < Etouch OK

Es < Estep OK

Block Diagram of Design Procedure

These results are obtained by MATLAB program. These

figures prove that this earth grid design is safe for 230 kV

substation in the range of soil resistivities 100-350 Ωm.

Fig. 1 Grounding Resistance with Different Values of Soil Resistivity

Fig. 2 Grid Potential Rise with Different Values of Soil Resistivity

Fig. 3 Grid Conductor Size with Different Values of Fault Current

(For Copper conductor)


433

Fig. 4 Calculated Mesh Voltages with Different Values of Soil

Resistivity

Fig. 5 Calculated Step Voltages with Different Values of Soil

Resistivity

IV. CONCLUSION

This paper has focus on design of earthing system for

230kV AC substation but not DC substation. The results for

earthing system design are obtained by MATLAB program.

For earthing conductor and vertical earth electrode, copper

clad steel and aluminium clad steel are used. The step by step

approach to designing a substation earthing system is

presented . The exact fault current level is not described but

fault current is assumed 10 times of normal current. The

various kinds of conductor sizes for earth equipment are

mentioned in this paper. Construction of earthing grid drawing

is expressed in here.

ACKNOWLEDGMENT

The author is grateful to her Excellency U Thaung,

Minister, Ministry of Science and Technology, Dr. Zaw Min

Aung, Rector of Mandalay Technological University, Dr. Myo

Myint Aung, Head of Electrical Power Engineering

Department, Dr. Salai Tluang Za Thang, Mandalay

Technological University, U Zaw Ye Myint, Project Manager

from Shwe Sar Yan Substation.

REFERENCES

[1] Geri. A, “Behavior of grounding system excited by high impulse

current: the model and its validation,” IEEE Trans, Power Deliver,

1999.

[2] Gagg. G. F, Earth Resistances, New York, 1964.

[3] Tharpar. B and Gross. E. T. B, Ground Grid of High Voltage Stations.

1963.

[4] Tiri G. Sverak and Donald N. Laird, “IEEE Guide for Safety in AC

Substation Grounding,” 1986.


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earthing mat design and calculation in myanmar

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