![International Journal of Trend in Scientific Research and Development (IJTSRD)
Volume 3 Issue 5, August 2019 Available Online: www.ijtsrd.com e-ISSN: 2456 – 6470
@ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1798
Simplified Method for Substation Grounding System Design
Aye Myo Thant1, Cho Cho Win2, Thant Zaw Oo2
1Department of Electrical Power Engineering, Technological University, Mandalay, Myanmar
2Department of Electrical Power Engineering, Technological University, Kalay, Myanmar
How to cite this paper: Aye Myo Thant |
Cho Cho Win | Thant Zaw Oo "Simplified
Method for Substation Grounding System
Design" Published
in International
Journal of Trend in
Scientific Research
and Development
(ijtsrd), ISSN: 2456-
6470, Volume-3 |
Issue-5, August
2019, pp.1798-1801,
https://doi.org/10.31142/ijtsrd26754
Copyright © 2019 by author(s) and
International Journalof Trendin Scientific
Research and Development Journal. This
is an Open Access article distributed
under the terms of
the Creative
CommonsAttribution
License (CC BY 4.0)
(http://creativecommons.org/licenses/by
/4.0)
ABSTRACT
This paper focused 230/66 kV, substation grounding system and calculation
results of required parameters at location of Kalay region. The grounding
system is essential to protect people working or walking in the vicinity of
earthed facilities and equipments against the danger of electric shock. It
provides the floor surface either assures an effective insulation from earth
potential or effectively equipment to a close mesh grid. Calculations of
grounding grid system in the substation area where the top soil-layer
resistivity is less than the bottom-layer resistivity can be less the number of
ground rod used in the grid because the value of Ground Potential Rise (GPR)
is insignificantly different. 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 are designed in detail consideration.
KEYWORDS: substation grounding, earth resistance, soil layer resistivity, round
potential rise, mesh gird
1. INTRODUCTION
Substation grounding provides a low impedance path and carries current into
ground under normal and fault conditions without adversely affecting
continuity of service. Under a fault condition, the ground voltage may rise to a
level that may endanger the public outside the vicinity of the substation. So,
grounding system is required essentially for all power system [1].
The original purpose of the protective earth was to ensure
the safety of people and property within the zone served by
the earthing system. This requires a high current capacity
path with relatively low impedance at the fundamental
frequency so that voltages developed under high fault
current conditions are not hazardous. The intent of thesis is
to provide guidance and information pertinent to safe
grounding practices in AC substation design. The specific
purposes of this thesis are to establish, as a basic for design,
the safe limits of potential differences that can exists in a
substation under fault conditionsbetween pointsthat can be
contacted by the human body. Develop criteria for safe
design, practical grounding systems, based on these
criteria[3]. Lightning an short circuit: the earthing system
must protect the occupants, prevent direct damage such as
fire, flashover or explosions due to a direct lightning strike
and overheating due to a short-circuit current.
2. Ground Mat (earth-mat)
The term ground mat applies only that of path of grid which
is burried in the soil. A solid metallic plate, rod orasystemof
closely spaced bare conductors that are connected to and
often placed in shallow depths above a ground grid or
elsewhere at the earth’s surface, in order to obtain an extra
protective measure minimizing the danger of the exposure
to high step or touch voltages in a critical operating area or
places that are frequently used by people. Grounded metal
gratings placed on or above the soil surface, or wire mesh
placed directly under the surface material, are common
forms of a ground mat [4].
Figure1. Ground Grid Mat (earth-mat)
Busbar structures and equipment structures was earthed at
two points. Marshalling boxes, cubiclesand allothermetallic
enclosures, which are normally not carrying any current,
were earthed. In the substation Disconnecting switch with
earth-switch, circuit breaker, Lightning arrester, Potential
transformer, Current transformer, Power transformer,
Equipment’s body and structures, Cubicles are installed
compactly. All of these are grounded atthebodyof structure.
All other equipments such as CircuitBreakers,CTs, Isolators,
Post Insulators, etc. were earthed at two points.
IJTSRD26754](https://image.slidesharecdn.com/344simplifiedmethodforsubstationgroundingsystemdesign-190916073532/85/Simplified-Method-for-Substation-Grounding-System-Design-1-320.jpg)
![International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470
@ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1799
3. Design Characteristics
For 230/66kV substation, 100MVA outdoor AIS substation.
Need to calculate the deliverable single line to ground fault
current of substation. And related transmission line
impedance Z1, Z2, Z0(R+jXL) values from hand calculation.
Soil resistivity can be determine with visual inspection or
proper measurement devices (Four Pin Earth Tester)[3]. It
plotted to measure the following model. For square shaped
Grid/Mat as bellows;
Figure2. Six Locations for Earth Resistivity
Measurement
Table1. Soil Resistivity Measurement Record Table
Location
Test Pin
Spacing
Meter Display
Ohm-meter
Remark
Direction 1 4 65
Avg 48
Ω-m
Direction 2 4 33
Direction 3 4 60
Direction 4 4 36
Direction 5 4 56
Direction 6 4 40
Substation Available Area,
A = 60m × 60m = 3600m2
Grid Spacing, D = 6m
Grid burial depth, h = 0.8-1 m (for 230kV and above level)
Length of each Ground rod, Lr = 4m
Max Line to Ground Fault Current, If = 7.0kV, 66kV Bus side
Duration of fault in sec, tc = 1 sec
Primary Line Voltage = 230,000V
Secondary Line Voltage = 66,000V
Soil resistivity, = 48 Ω-m (Wenner Four Pin Method)
Surface crushed rock resistivity, = 2500 Ω-m
Number of Earth rod = 46 Nos
Figure3. Preliminary Calculation of Touch and Step
Voltage
ctfKIkcmilA ×=
Equation (1)
Switchyard surface layer is crushed rock 4”(0.102m)
thickness, with resistivity of 2500 Ω m, and for an actual
inner resistivity of 48Ω.m by four pin tester. The reflection
factor K is computed using the following equation;
sρρ
sρρ
K
+
−
=
Equation (2) (1)
Linemen or operator body’s weight can be expected at
least70kg, (assume RB:1000Ω). So that, consider the
permissible/ tolerable step and touch voltage initially.
Fault current, If = 3I0,
)
3
X
2
X
1
j(X)
0
R
2
R
1
(R
f
R3
3
V3
++++++×
×
=
where; Km = geometrical factor,
ρ = the soil resistivity, a
corrective factor, IG = maximum grid current For square
shaped mat design
( )
( )( )
−⋅
⋅+
⋅
−
⋅⋅
+
+
⋅⋅
=
1n2π
8
ln
hK
iiK
d4
h
dD8
2
2hD
dh16
2
D
lnh
2π
1
mK
1.0iiK =
(with rods),
1h0 =
(usually constant)
0h
h
1hK +=
Irregularity factor;
( )n0.1480.644iK ⋅+=
Mesh Voltage;
R
2
y
2
x
r
C
imG
m
L
LL
L
1.221.55L
KKIρ
E
×
+
×++
×××
=
The effective buried conductor length (Ls); (with or without
ground rods)
( )R0.85LC0.75LsL +=
( )
−
−+
+
+
⋅
=
2n
0.51
D
1
hD
1
h2
1
π
1
sK
Equation (3) (1)
Step Voltage; Es
sL
GItKsKρ
sE
⋅⋅⋅
=
Equation (4)
Earthing grid resistance
+
++=
h A201
1
1
20A
1
L1
1
ρRg
Equation (5)
The Schwarz equations that are modeling the effect of
earthing rods or electrodes shown as below[2].
m2R2R1R
2
mR2R1R
g
R
−+
−
=
Equation (6)
Touch voltage limit-the maximum potential differene
between the surface potential and potential of an earth
conducting structure during a fault due to ground potential
rise.](https://image.slidesharecdn.com/344simplifiedmethodforsubstationgroundingsystemdesign-190916073532/85/Simplified-Method-for-Substation-Grounding-System-Design-2-320.jpg)
![International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470
@ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1801
who are working in Electrical Power Engineering
Department, Technological University (Kalay) for their
willingness to share of ideas and helpful suggestions on
throughout the research.
References
[1] Donald, N.L.IEEE Guide for safety in AC Substation
Grounding, IEEE-SA Stand board, New York, (2000).
[2] IEEE guide for safety in AC substation Grounding, IEEE
80-2000.
[3] John D. Mc Donald, Electrical Power Substation
Engineering, (2003).
[4] Garrett, D.L. Guideline for the Design, Installation,
Testing and Maintenance of Main Earthing System in
Substation, Electricity Association, (1992)
[5] Chairman of IEEE Standards Board; IEEE Guide for
Measuring Earth Resistivity, Ground Impedance, and
Earth Surface Potentials of Ground System, (1983)](https://image.slidesharecdn.com/344simplifiedmethodforsubstationgroundingsystemdesign-190916073532/85/Simplified-Method-for-Substation-Grounding-System-Design-4-320.jpg)
Simplified Method for Substation Grounding System Design
- 1. International Journal of Trend in Scientific Research and Development (IJTSRD) Volume 3 Issue 5, August 2019 Available Online: www.ijtsrd.com e-ISSN: 2456 – 6470 @ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1798 Simplified Method for Substation Grounding System Design Aye Myo Thant1, Cho Cho Win2, Thant Zaw Oo2 1Department of Electrical Power Engineering, Technological University, Mandalay, Myanmar 2Department of Electrical Power Engineering, Technological University, Kalay, Myanmar How to cite this paper: Aye Myo Thant | Cho Cho Win | Thant Zaw Oo "Simplified Method for Substation Grounding System Design" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456- 6470, Volume-3 | Issue-5, August 2019, pp.1798-1801, https://doi.org/10.31142/ijtsrd26754 Copyright © 2019 by author(s) and International Journalof Trendin Scientific Research and Development Journal. This is an Open Access article distributed under the terms of the Creative CommonsAttribution License (CC BY 4.0) (http://creativecommons.org/licenses/by /4.0) ABSTRACT This paper focused 230/66 kV, substation grounding system and calculation results of required parameters at location of Kalay region. The grounding system is essential to protect people working or walking in the vicinity of earthed facilities and equipments against the danger of electric shock. It provides the floor surface either assures an effective insulation from earth potential or effectively equipment to a close mesh grid. Calculations of grounding grid system in the substation area where the top soil-layer resistivity is less than the bottom-layer resistivity can be less the number of ground rod used in the grid because the value of Ground Potential Rise (GPR) is insignificantly different. 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 are designed in detail consideration. KEYWORDS: substation grounding, earth resistance, soil layer resistivity, round potential rise, mesh gird 1. INTRODUCTION Substation grounding provides a low impedance path and carries current into ground under normal and fault conditions without adversely affecting continuity of service. Under a fault condition, the ground voltage may rise to a level that may endanger the public outside the vicinity of the substation. So, grounding system is required essentially for all power system [1]. The original purpose of the protective earth was to ensure the safety of people and property within the zone served by the earthing system. This requires a high current capacity path with relatively low impedance at the fundamental frequency so that voltages developed under high fault current conditions are not hazardous. The intent of thesis is to provide guidance and information pertinent to safe grounding practices in AC substation design. The specific purposes of this thesis are to establish, as a basic for design, the safe limits of potential differences that can exists in a substation under fault conditionsbetween pointsthat can be contacted by the human body. Develop criteria for safe design, practical grounding systems, based on these criteria[3]. Lightning an short circuit: the earthing system must protect the occupants, prevent direct damage such as fire, flashover or explosions due to a direct lightning strike and overheating due to a short-circuit current. 2. Ground Mat (earth-mat) The term ground mat applies only that of path of grid which is burried in the soil. A solid metallic plate, rod orasystemof closely spaced bare conductors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth’s surface, in order to obtain an extra protective measure minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people. Grounded metal gratings placed on or above the soil surface, or wire mesh placed directly under the surface material, are common forms of a ground mat [4]. Figure1. Ground Grid Mat (earth-mat) Busbar structures and equipment structures was earthed at two points. Marshalling boxes, cubiclesand allothermetallic enclosures, which are normally not carrying any current, were earthed. In the substation Disconnecting switch with earth-switch, circuit breaker, Lightning arrester, Potential transformer, Current transformer, Power transformer, Equipment’s body and structures, Cubicles are installed compactly. All of these are grounded atthebodyof structure. All other equipments such as CircuitBreakers,CTs, Isolators, Post Insulators, etc. were earthed at two points. IJTSRD26754
- 2. International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470 @ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1799 3. Design Characteristics For 230/66kV substation, 100MVA outdoor AIS substation. Need to calculate the deliverable single line to ground fault current of substation. And related transmission line impedance Z1, Z2, Z0(R+jXL) values from hand calculation. Soil resistivity can be determine with visual inspection or proper measurement devices (Four Pin Earth Tester)[3]. It plotted to measure the following model. For square shaped Grid/Mat as bellows; Figure2. Six Locations for Earth Resistivity Measurement Table1. Soil Resistivity Measurement Record Table Location Test Pin Spacing Meter Display Ohm-meter Remark Direction 1 4 65 Avg 48 Ω-m Direction 2 4 33 Direction 3 4 60 Direction 4 4 36 Direction 5 4 56 Direction 6 4 40 Substation Available Area, A = 60m × 60m = 3600m2 Grid Spacing, D = 6m Grid burial depth, h = 0.8-1 m (for 230kV and above level) Length of each Ground rod, Lr = 4m Max Line to Ground Fault Current, If = 7.0kV, 66kV Bus side Duration of fault in sec, tc = 1 sec Primary Line Voltage = 230,000V Secondary Line Voltage = 66,000V Soil resistivity, = 48 Ω-m (Wenner Four Pin Method) Surface crushed rock resistivity, = 2500 Ω-m Number of Earth rod = 46 Nos Figure3. Preliminary Calculation of Touch and Step Voltage ctfKIkcmilA ×= Equation (1) Switchyard surface layer is crushed rock 4”(0.102m) thickness, with resistivity of 2500 Ω m, and for an actual inner resistivity of 48Ω.m by four pin tester. The reflection factor K is computed using the following equation; sρρ sρρ K + − = Equation (2) (1) Linemen or operator body’s weight can be expected at least70kg, (assume RB:1000Ω). So that, consider the permissible/ tolerable step and touch voltage initially. Fault current, If = 3I0, ) 3 X 2 X 1 j(X) 0 R 2 R 1 (R f R3 3 V3 ++++++× × = where; Km = geometrical factor, ρ = the soil resistivity, a corrective factor, IG = maximum grid current For square shaped mat design ( ) ( )( ) −⋅ ⋅+ ⋅ − ⋅⋅ + + ⋅⋅ = 1n2π 8 ln hK iiK d4 h dD8 2 2hD dh16 2 D lnh 2π 1 mK 1.0iiK = (with rods), 1h0 = (usually constant) 0h h 1hK += Irregularity factor; ( )n0.1480.644iK ⋅+= Mesh Voltage; R 2 y 2 x r C imG m L LL L 1.221.55L KKIρ E × + ×++ ××× = The effective buried conductor length (Ls); (with or without ground rods) ( )R0.85LC0.75LsL += ( ) − −+ + + ⋅ = 2n 0.51 D 1 hD 1 h2 1 π 1 sK Equation (3) (1) Step Voltage; Es sL GItKsKρ sE ⋅⋅⋅ = Equation (4) Earthing grid resistance + ++= h A201 1 1 20A 1 L1 1 ρRg Equation (5) The Schwarz equations that are modeling the effect of earthing rods or electrodes shown as below[2]. m2R2R1R 2 mR2R1R g R −+ − = Equation (6) Touch voltage limit-the maximum potential differene between the surface potential and potential of an earth conducting structure during a fault due to ground potential rise.
- 3. International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470 @ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1800 Etouch(50kg) = (RB+1.5ρ)*IB(50) Equation (7) Etouch(50kg) =(1000+1.5Csρs)*IB(50) Equation (8) Etouch(70kg) = (RB+1.5ρ)*IB (70) Equation (9 ) Etouch(70kg) = (1000+1.5Csρs)*IB (70) Equation(10) Step voltage limit-the maximum difference in surface potential experienced by a person bridging a distance of 1m with the feet without contact to any earthed object. For 50kg, s t 0.116 B I = Equation (11) For 70kg, s t 0.157 B I = Equation (12) Estep(50kg) = (RB+6ρ)*IB(50) Equation (13) Estep(50kg) = (1000+6Csρs)*IB(50) Equation (14) Estep(70kg) = (RB+6ρ)*IB(50) Equation (15) Estep(70kg) = (1000+6Cs ρs)*IB(50) Equation (16) The minimum conductor size capable of withstanding adiabatic temperature rise associated with earth fault, Amm2 + +−× ×= T aK o T mK 0Ln ρ rα rtc 410TCPA 1 If Equation (17) Where; TCAP = temperature capacity per unit volume, αr = thermal co-eff of resistivity, ρr = resistivity of ground conductor, Ta = ambient temperature,Tm = maximum allowable temperature The diameter of the grid conductor, π A 2mm4 d × = Equation (18) 4. Results and Discussion The large substation, the ground resistance is usually about 1Ω of less. In smaller distribution substation the usually acceptable range is from 1 to 5Ω, depending on local conditions in table (1).The designation of step voltage,mesh voltage and ground resistance are expressed in comparison situation. The effective buried conductor length; (with or without ground rods), length or grid conductor, ground conductor, grid current, grid resistance, ground potential rise are focused consideration on existingsubstation data. In this calculation, a tolerance of %5± is assumed in all parameters. The result GPR value is very important and it should be less then permissible touch voltage in table (2). Because of consideration, switchyard surface layer is crushed rock 4”(0.102m) thickness, with resistivity of 2500 Ω m, and for an actual inner resistivity of 48Ω.m by four pin tester and Resistivity of surface crushed rock causes to a reduction factor Cs . Linemen/Operator body’s weight can be expected at least70kg, (Assume RB:1000Ω). So that, consider the permissible/ tolerable step and touch voltage initially. Finally, this allowed value will decide the obtained a safe limit design. Amm 2 based on this computation, a copper wire as small as size 4.74mm could be used but due to the mechanical strength and ruggedness requirements, a larger size, d=15mm is usually as a minimum for future fault level and future generation capacity. In this case, dmin= 15-16 mm, standard copper wire is should be choice for better grid. In view of the above results, the earth mat design and configuration is successfulandcompleted asshown in tables. Table2. Comparison of Permissible and Designed Values Designed Permissible/ Allowed Step Voltage (Es) 119.46V <2553V Mesh Voltage(Em) 179.44V <804.7V Grid Resistance, Rg 0.38 Ω 1 Ω Earth Mat Design is safe and Completed Table3. Result Table for Substation Grounding System Reflection factor (K) -0.96 Reduction factor (Cs) 0.7 Step voltage (Estep70) 2553V Touch voltage (Etouch70) 804.7V Total length of all earth rod (LR) 184m Total length of horizontal conductor (LC) 1320m Parameter length of grid conductor (Lg) 240m Total ground conductor length (LT) 1504m Grid resistance (Rg) 0.38Ω Grid current (IG) 4200A Ground Potential Rise (GPR) 1596V Geometrical factor (Km) 0.633 Irregularity factor (Ki) 2.272 Mesh voltage (Em) 179.44V Effective buried conductor length (Ls) 1153.2 Step voltage (Estep) 119.46V Conclusions The grounding system in a substation consistsof aminimum of four earth electrodes installed around the inside perimeter of the substation and connected together withthe earth mesh the exact spacing of the electrodes which will be based on local conditions, resistivity of the area and space available for electrodes. The spacing between should be greater than the electrodes’ length. Although the earth mesh will often result in a low enough resistance without the use of electrodes, fifty electrodes are still necessary in this to ensure the fault level capability and forty-six electrodes are used for neutral ground grid. Electrodes are also required in case of the drying out of the soil at the depth of the earth mesh in long dry spells. The size of the high voltage grounding conductors is determined by the earth fault level but in any case shall be not smaller than 70 mm2 copper. The size of the main grounding conductor shall not be smaller than 25 mm2 copper. Acknowledgements The author is deeply grateful to Prof. Dr. Yadana Aung, Head of Electrical Power Engineering Department, Technological University (Mandalay) for her kindly permission and encouragement. The authorisdeeplygratefultoherteachers
- 4. International Journal of Trend in Scientific Research and Development (IJTSRD) @ www.ijtsrd.com eISSN: 2456-6470 @ IJTSRD | Unique Paper ID – IJTSRD26754 | Volume – 3 | Issue – 5 | July - August 2019 Page 1801 who are working in Electrical Power Engineering Department, Technological University (Kalay) for their willingness to share of ideas and helpful suggestions on throughout the research. References [1] Donald, N.L.IEEE Guide for safety in AC Substation Grounding, IEEE-SA Stand board, New York, (2000). [2] IEEE guide for safety in AC substation Grounding, IEEE 80-2000. [3] John D. Mc Donald, Electrical Power Substation Engineering, (2003). [4] Garrett, D.L. Guideline for the Design, Installation, Testing and Maintenance of Main Earthing System in Substation, Electricity Association, (1992) [5] Chairman of IEEE Standards Board; IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of Ground System, (1983)