Reduction of Ground Potential Rise of Earthing Grid for

230/33/11 kV Substation (THAPYAYWA)

Shwe Myint1, Yan Aung Oo2
Electrical Power Engineering Department
Mandalay Technological University

Abstract – Ground Potential Rise (GPR) is the most GPR = IGRG (1)
important factor for the safety of substation earthing Where, IG = current flowing between the ground and the
system. The factors influenced on the performance of
substation earthing grid are Touch and Step voltage, GPR surrounding earth,
and total grid resistance. The variant of touch and step
RG = resistance of the station grounding system.
voltage is mainly depended on the variant of ground
potential rise (GPR). The performance of substation The burial depth of the grid affects the Ground
earthing grid can be improved by reducing GPR. The value Potential Rise (GPR) to some extent as well as the ground
of GPR can be determined using Ohm’s Law and changed rod, connected with the grid, and also plays an important
due to many factors, such as fault current, horizontal grid
resistance, vertical grid resistance and numbers of ground role in GPR reduction.
rod, soil resistivity and so on. This paper presents reduction
of GPR for a substation earthing grid by increasing the
length of vertical earth rods using MULTISIM simulation
software.

Keywords – Ground Potential Rise (GPR), Horizontal and

Vertical grid resistance, MULTISIM simulation
software, Step and Touch voltage.

I. INTRODUCTION
A reliable grounding system must have low earth
resistance to reduce the excessive voltages, known as
ground potential rise, which develop during a fault
condition that could be hazardous to a being in the
vicinity of the substation. In other words a good path to
Figure 1—Basic shock situations [1]
earth is essential in order for the grounding system to
operate as required. An ideal grounding grid should have
zero resistance to the earth mass. The ground potential II. DETERMINATION OF SIMULATION CIRCUIT
rise (GPR) is calculated using Ohms law, thus, the ground PARAMETERS
potential rise increases proportionally to the fault current.
There are a number of mathematical and
Therefore, a lower of total grounding system resistance
must be obtained. Typical values for ground resistance computer models of the processes with respect to the
are 1.0 Ω or less with respect to large substations. For grounding grids potential exposure to touch and step
smaller distribution substations, the accepted value ranges potentials. More often they are based on the circuit theory
form 1-5Ω, depending on the safety margins that have to or electromagnetic field theory of the processes. The
be achieved. The volume of earth near the earth electrode common approach of these models is to firstly calculate
has the most impact on the ground resistance and hence, currents following which an evaluation of the potentials
this is where the most dangerous step voltages are usually
above the grounding grids conductors can be made. It is
found. The ground potential rise is equal to the product of
the station grid impedance and the total fault current that possible to evaluate grounding performance parameters
flows through it. When a ground fault occurs, fault such as current distribution, potentials and touch and step
current will divide among all circuit paths back to the voltages at various parts of a power plant. However there
source including metallic, earth return paths include are no specifics about possible changes of the earth
overhead ground wires, multi-grounded neutrals, bonding surface potentials in case of breaks or damages to the
conductors, station ground grids, messenger wires, bonding of the horizontal elements
metallic cable shields and other conducting materials. The
calculated values of GPR may also be used in estimation
of step and touch voltages. However, if the calculated R hor(ver)  R me  R cont  R soil (2)
GPR is higher than the safety margin for step and touch
potential then further evaluations and investigations
But the first two elements (Rme and Rcont) have
should be conducted. The ground potential rise (GPR) at
very small values because both resistivity of these two
the substation is given by equation given below:
elements are 8.62 x 10-8 Ωm and often can be neglected.
 230kV Thapyaywa (Meiktila)Substation Single Line Diagram

Myingyan

Yeywa 2

Yeywa 1
Belin 2

Belin 1
Bus Coupler
Meas; Bus 2
Main Tr;2

Main Bus2

Main Bus1

Meas; Bus 1
Main Tr;1

Thazi
Taungdwingyi
33kV Bus

Figure 2. Equivalent scheme of the horizontal and

vertical elements in the soil Figure 3. 230/33/11kV Thapyaywa (Meiktila) substation single line
diagram
The resistance of the vertical and horizontal
elements in the soil can be calculated as:
ρ ev  2.l ver 4.l  7.t  263m
R ver  . ln  0.5ln ver 
6000 6500 7000 8000 9000 10000 11000 13000 15000 15000 15000 15000 15000 15000 15000 15000 15000 13000 11000 10000 9000 8000 6500
`

l ver  7.t 
(3) 6000 6000

2 l ver  d ver
`
8400 8400
`
6100 10000

75.5m
`

Where, ρe.v.– equivalent resistance of earth for 16200 Control Room 10000
`
9700

the vertical elements, Ωm; t – depth of the grounding grid;

12000
`
12000 12000

lver. – length of the vertical element; dver. – diameter of the 116.8m 10000
`
10000
`

vertical element. 15000

` 7900
7100
`

ρ e.h l 2 12000 12000

R hor 

41.3m
`
hor
.ln (4) 8000 8000

2 l hor t.dhor
`
7000
7000
`
6400 6400
15000 15000 11000`

Where, ρe.h.– equivalent resistance of the earth

6000 6500 7000 8000 9000 10000 11000 13000 13000 9000 8000 6500 6000

N 154m

for the horizontal elements, Ωm; lhor. –length of the W

E
Vertical earthing rod
horizontal element, dhor – diameter of the horizontal
`
Horizontal earthing conductor
S

element.
The resistance of earth Rsoil depends on the Figure 4. Existing condition earth grid layout plan for THAPYAYWA
resistivity of different soil stratums (ρ1, ρ2, ρ3 etc.). The substation
number of layers (stratums) can differ from one area to
another depending on the soil structure [3]. TABLE II
DIFFERENT VALUES OF RHOR FOR UNEQUAL SPACING EARTHING GRID
Length of horizontal Horizontal element resistance in
III. ASSESSING PERFORMANCE OF EXISTING SUBSTATION
element in m Ω
EARTH GRID
6 12.548
TABLE I 6.1 12.393
6.4 11.952
EARTHING GRID PARAMETERS FOR THAPYAYWA SUBSTATION 6.5 11.812
7 11.165
1 Area of the grid (A) in m2 25178.7
7.1 11.045
2 Total buried length of horizontal grid 5602.4
7.9 10.178
conductor (L ) in m
c 8 10.080
3 Total buried length of vertical earth rods 86 x 2.5 =215 8.4 9.708
(L ) in m 9 9.204
R

Total buried length of the grid (L ) in m 9.7 8.683

4 5817.4
T 10 8.479
5 The peripherial length of the grid (L ) in m 913.6 11 7.870
p

6 The length of each ground rod(L ) in m 2.5 12 7.349

r 13 6.898
7 Grid burial depth (h or t) in m 1 15 6.156
8 Diameter of horizontal conductor (d) in m 0.011 16.2 5.788
9 Cross-sectional area of horizontal 95
2
TABLE III
conductor in mm (copper-clad steel)
THE DIFFERENT VALUES OF VERTICAL RESISTANCE ACCORDING TO
10 L and L in m 116.8 and 263 LENGTH OF VERTICAL RODS
x y

11 Apparent soil resistivity (ρ ) in Ωm 58.45

ev Lver
12 Diameter of vertical ground rod in m(d ) 0.018 in m 2.5 5 7.5 10 12.5 17.5 22.5
ver
(copper-clad steel) Rver 22.0 12.5 8.92 6.99 5.78 3.48
in Ω 4.329
2 13 3 7 4 1
A. Performance of Ground Potential Rise at Centre In figure 6, the maximum possible GPR case for
Injection Point
Case D
corner injection is case (S). Also those for both the centre
6000
6000 6500 7000 8000 9000 10000 11000 13000 15000 15000 15000

`
`
15000 15000 15000 15000 15000 15000 13000 11000 10000 9000 8000 6500
6000 and corner injections are case (S). Therefore, the value of
8400 8400
`
6100

16200 Control Room

`
10000

10000 Case C
GPR for case (S) must be reduced by increasing length of
`
Case A 9700 12000

12000
`
12000
vertical earthing rods in this substation.
`
10000 10000
`
15000
7100
Case S
` 7900
`
Case P
12000 12000 60 0 650 70 0 80 0 90 0 10 0 100 130 0 150 0 150 0 150 0 150 0 150 0 150 0 150 0 150 0 150 0 130 0 100 10 0 90 0 80 0 650
` `
8000 60 0 60 0
8000 `
`
7000 840 840
7000
` `
6400 6400
610 10 0
6000 6500 7000 8000 9000 10000 11000 13000 15000 15000 13000 11000` 9000 8000 6500 6000

`
1620 Control Room 10 0
Case B `
970
Vertical earthing rod Horizontal earthing conductor `
`
120 0

120 0 120 0
Figure 5. Current injection at center point of grid peripheral earth `
10 0
10 0
grid layout plan for THAPYAYWA substation `
710
150 0
`
When the current injected at the centre point of 120 0
` 790

120 0

the grid peripheral, GPR can be measured with volts 80 0

`
80 0
Case R
`

meter (or) measured probe from instrument toolbar in the 70 0

640
`
70 0
640
80 0 90 0 10 0 100 130 0 150 0 150 0 130 0 1 `0 0 90 0 80 0 650 60 0
MULTISIM circuit simulation software (13). The 60 0 650 70 0
Case T
Case Q
following table presents the values of grounding potential
Vertical earthing rod Horizontal earthing conductor `
rise (GPR) at every current injected point of all centre
injection cases according to injected current rating. Figure 7. Current injection at corner points of grid peripheral earth grid
layout plan for THAPYAYWA substation
In figure 4, The value of GPR for case (A) is
more than those for another cases. Thus, maximum TABLE V
THE VALUES OF GPR IN VOLTS MEASURED FORM THE MULTISIM
possible GPR value case for the centre injection is case
SIMULATION CORNER CURRENT INJECTION CIRCUIT
(A).
TABLE IV Injected Case (P) Case (Q) Case ( R ) Case Case
current (s) (T)
GROUND POTENTIAL RISE IN VOLTS FOR CURRENT I NJECTION AT (A)
CENTRE POINTS OF THE GRID 10000 70900 74900 57300 85300 60900
Injected 20000 142000 150000 115000 171000 122000
Current Case (A) Case (B) Case ( C ) Case (D)
(A) 30000 213000 225000 172000 256000 183000
10000 81300 51500 48900 34900 40000 284000 299000 229000 341000 244000
20000 163000 103000 97900 69800 50000 354000 374000 287000 427000 305000
30000 244000 155000 147000 105000 60000 425000 449000 344000 512000 365000
40000 325000 206000 196000 140000 70000 496000 524000 401000 597000 425000
50000 406000 258000 245000 175000 80000 567000 599000 459000 683000 487000
60000 488000 309000 294000 210000 90000 638000 674000 516000 768000 548000
70000 568000 360000 343000 245000 100000 709000 749000 573000 853000 609000
80000 650000 412000 392000 279000
90000 732000 464000 441000 313000
100000 813000 515000 489000 349000

Figure 6. Performance of GPR at all center injected cases Figure 8. Performance of GPR at all corner injected cases
 IV. REDUCING GPR AT MAXIMUM POSSIBLE POINT OF The next method is increasing number of
CURRENT INJECTION vertical earth rods. So, 86 earth rods are added to existing
The performance of GPR is enhanced by increasing the numbers of earth rods to become 172 rods and 2.5 m case.
length and numbers of vertical ground rods. The But, this case can’t reduce to cover the mutual affect in
the calculation of actual touch and step voltage although
following table shows the different values of GPR for existing GPR values are reduced to 39.74%.Thus,
defined injected currents related to length of vertical considering 172 rods and 5m case, vertical resistance
ground rods (2.5 to 12.5 m) respectively at case (S). This values becomes less than that of the former case and also
case is chosen to enhance case because this case can GPR values are greatly reduced to 53.82%. Moreover,
affect maximum possible GPR according to article III-A reducing % of 172 rods and 5m case is 47.13%less than
the same useful burial length of the vertical rod, 86 and
and III-B. 10m case. Therefore, the letter case is the best suitable
Table-VI shows different values of GPR for case for this substation earthing grid to reduce GPR
THAPYAYWA substation earthing grid with the variety which is greatly influence in the performing of actual
of vertical rods length at maximum possible values case. touch and step voltage. The following table shows
For 100kA current injection, the value of GPR with rods different values of GPR for two improvement cases and
length 5m is 8.79% less than the value of GPR with rods existing case.
length 2.5m or existing case and also the value of GPR TABLE VII
with rods length 7.5m is 13.60% less than those of 2.5m.
COMPARISON OF GPR BETWEEN TWO IMPROVEMENT CASES AND
Thus, the longer length of vertical ground rods, the more EXISTING SYSTEM
reducing of ground potential rise (GPR).
GPR (V)
Injected
TABLE VI Current 86 rods 86 rods and 172 rods and
(A) and 2.5m 5 m at Case 2.5m at Case
DIFFERENT VALUES OF GPR IN VOLTS AT CASE (S) RELATED TO
at Case (S) (S) (R )
VARIOUS LENGTH OF GROUND RODS
10000 85300 77800 51400

Current Length of vertical rods (Lver)

20000 171000 156000 103000
injected
(A) 2.5m 5m 7.5 m 10 m 12.5 m 30000 256000 233000 154000

5000 42700 38900 36800 35400 34500 40000 341000 311000 206000

10000 85300 77800 73600 70900 68900 50000 427000 389000 257000

60000 512000 467000 308000

20000 171000 156000 147000 142000 138000

70000 597000 545000 360000

30000 256000 233000 221000 213000 207000
80000 683000 623000 411000
40000 341000 311000 294000 284000 276000
90000 768000 700000 463000
50000 427000 389000 368000 354000 345000
100000 853000 778000 514000
60000 512000 467000 442000 425000 413000

80000 683000 623000 589000 567000 551000

100000 853000 778000 737000 709000 689000

Figure 10. Comparison of GPR between two improvement cases and

existing system

Figure 9. Performance of GPR with various length of ground rods at Table VIII. shows comparison of GPR in V between 172
case (S) for THAPYAYWA substation earth grid rods and 5m case and 86 rods and 10 m case.
 TABLE VIII vertical earth rods under each and every injected fault
COMPARISON OF GPR BETWEEN THREE IMPROVEMENT CASES AND
EXISTING SYSTEM current levels. It can be clearly seen that the increase
length of vertical earth rod from 2.5m to 5m, 7.5m, 10m
GPR (V)
Injected Current and 12.5m caused the decrease percent in GPR of 8.9%, -
(A) 86 rods and 86 rods 172 rods and
2.5m and10 m 5m
13.56%, 16.9% and 19.23% respectively. So the value of
GPR can be reduced by increasing length of vertical rods.
10000 85300 74500 39388 Another way to reduce the value GPR is adding
number of vertical ground rods to existing value. But, it is
20000 171000 149000 78777
necessary to have associated length of the rods to cover
30000 256000 224000 118165 the affect of mutual impedance in each other.

40000 341000 298000 157554 ACKNOWLEDGEMENT

The author would like to acknowledge the support and
50000 427000 373000 196942 the encouragement of Prof. Dr. Myint Thein, Pro-rector,
Mandalay Technological University. The author is deeply
60000 512000 447000 236330
indebted to Dr. Khin Thuzar Soe, Associate Professor and
70000 597000 522000 275719 Head of the Department of Electrical Power Engineering,
Mandalay Technological University, for her
80000 683000 596000 315107
accomplished supervision, encouragement, enthusiastic
90000 768000 671000 354496
instructions, invaluable help, and indispensable guidance
and editing this paper. The author wishes to extend
100000 853000 745000 393884 grateful thanks to her supervisor, Dr. Yan Aung Oo,
Associate Professor, Department of Electrical Power
Engineering, Mandalay Technological University, for his
supervision, critical reading of manuscript, and tolerance
helped in all the time of this research work. The author
specially thanks to all her teachers from Department of
Electrical Power Engineering, Mandalay Technological
University and her family for their supports and
encouragement and also thanks to all her friends.

REFERENCES

[1] IEEE Std.80-2000:”IEEE Guide for Safety in AC Substation

Figure11.Comparison of GPR between two improvement cases and existing Grounding”. The Institute of Electrical and Electronic
system Engineers , New York, 2000.
[2] Li, Z., Chen, W., Fan, J., Lu, J. “A Novel Mathematical
Modeling of Grounding System Buried in Multilayer Earth”.
V. DISCUSSIONS AND CONCLUSIONS
IEEE Transactions on Power Delivery, v. 21(3), 2006.
The performance of Earthing grid system is very
[3] El Sherbiny M., “Simple Formulas for Calculating the
important to ensure the human and protective devices in
Grounding Resistance of Rod bed Buried in Non-Uniform Soil”
safe environment. Actual Step and Mesh voltage of a
Proceedings of the 37th Midwest. Symposium on Circuits and
substation must keep under the maximum allowable
Systems University of Waterloo, Waterlo, Ontario, Canada
limits under fault condition. Ground potential rise, GPR is
N2L 3Gl, 1954, pp. 1281-1284.
greatly influence on actual step and mesh voltage of
substation grounding system. Ground potential rise also [4] Yury Chikarov., “Safety Parameters of Grounding Devices at

mainly depends on the length and numbers of ground an Electric Power Plant”. Department of Electrical and

rods and grid spacing and the amount of injected fault Electronic Engineering, Auckland University of Technology,

current level. In this study, performance of GPR and GPR Auckland, New Zealand.
reduction are presented under constant grid spacing and [5] Shwe Myint, “Performance Analysis of Actual Step and Mesh
various vertical earth rods lengths. Since GPR is the Voltage of Substation Grounding System with the variation of
product of the injected fault current and earth resistance, Length and Number of Ground Rod”, Department of Electrical
the variance of the GPR is directly proportional to the Power Engineering , Mandalay Technological University,
variance of the grid resistance caused by the changes of Myanmar, www.ijsea.com.