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PAGE NO. 2 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
TABLE OF CONTENTS
1.0 SCOPE
2.0 PURPOSE OF TRANSMISSION LINE GROUNDING
3.0 GROUNDING IN GENERAL
4.0 FUNDAMENTAL CONSIDERATIONS
4.1 Earth
4.2 Power System Network
4.3 Grounding Performance of Transmission Line Structures
4.4 Types of Disturbance
5.0 SAFETY
5.1 General
5.2 Earth Surface Potential
5.3 Thresholds of Current
5.4 Ground Potential Mitigating Conductors
6.0 GROUNDING OF STRUCTURES
6.1 Single Pole Wood Structures
6.2 Multipole Wood Structures
6.3 Steel Lattice Structures
6.4 Steel Pole Structures
7.0 GROUNDING REQUIREMENTS FOR ADJACENT FACILITIES
8.0 GROUNDING CONDUCTORS
8.1 Grounding Conductor Material
8.2 Grounding Conductor Size
8.3 Grounding Conductor Connections
8.4 Guarding and Protection
PAGE NO. 3 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
9.0 GROUNDING ELECTRODE
9.1 Function of Grounding Electrode
9.2 Types of Grounding Electrodes
9.3 Methods of Connection to Electrodes
10.0 GROUNDING RESISTANCE
10.1 Requirements
10.2 Grounding Resistance Tests
10.3 Grounding Resistance of Driven Rods
10.4 Grounding Resistance of Counterpoise
11.0 EARTH RESISTIVITY
11.1 General
11.2 Earth Resistivity Test
12.0 FAULT CURRENT DISTRIBUTION
12.1 General
12.2 Importance of Fault Current Distribution
12.3 Overhead Ground Wire
13.0 CORROSION
13.1 Cause of Corrosion
13.2 Rate of Corrosion
13.3 Control of Corrosion
14.0 RECOMMENDED SIZES OF GROUNDING CONDUCTOR
15.0 BIBLIOGRAPHY
APPENDIX I: SAMPLE CALCULATIONS FOR SIZE OF COPPER-CLAD STEEL
COUNTERPOISE CONDUCTORS
PAGE NO. 4 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
1.0 SCOPE
This Standard covers the general requirements and methods to be used in the design of
grounding of transmission line structures for the overhead transmission line system of Saudi
Electricity Company (SEC), Saudi Arabia.
2.0 PURPOSE OF TRANSMISSION LINE GROUNDING
Grounding systems serve four main functions:
2.1 To safeguard a person from electric shock by ensuring that, under fault conditions,
all surfaces including those of metallic equipment and the ground with which he is in
simultaneous contact, remain at safe relative potentials.
2.2 To dissipate both voltages and currents under fault conditions without exceeding
predictable limits or adversely affecting continuity of service.
2.3 To provide a path to dissipate surges induced by lightning or switching surges and to
provide a path to ground from the overhead ground wire.
2.4 To reduce the possibility of static discharge caused by natural electric disturbances in
the atmosphere.
3.0 GROUNDING IN GENERAL
3.1 A grounding system consists of grounding conductors, connecting together all items
to be grounded, and a grounding electrode or electrodes. The grounding electrode
forms the medium of contact with the body of earth and may consist of a single or a
combination of buried conductors/ground rods and may include other elements such
as foundation steel connected to the grounding system.
3.2 Factors influencing the design of the grounding system are as follows:
3.2.1 The magnitude and duration of ground-fault current which can pass
between the fault location and the system neutral point(s) (which will
influence the size of the grounding conductors).
3.2.2 The portion of this current which will pass between the grounding system
and the body of earth.
3.2.3 The site earth resistivity.
3.2.4 The degree of exposure to mechanical damage and corrosion (which will
influence the choice of materials and their manner of installation).
PAGE NO. 5 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
4.0 FUNDAMENTAL CONSIDERATIONS
The analysis of a power system subject to a phase to ground fault or lightning stroke at a
transmission line structure requires specific data on the following variables which play a
critical role in the grounding performance of transmission lines:
4.1 Earth
Typically earth consists of several layers of soil, each having a different resistivity.
For transmission line grounding purposes, the earth can be reasonably approximated
by a two-layered soil structure. However, for calculation of ground resistance of a
structure, uniform soil resistivity may be assumed.
The layer resistivity and thickness are generally determined by interpreting the
apparent resistivity values measured using the Wenner four electrode method.
4.2 Power System Network
It is assumed that the fault or disturbance is at a structure on a transmission line
section between two substations. Each substation and the power system network to
which it is connected is designated as a terminal.The terminals can be described by
equivalent circuit of the network which exists at either extremity of the transmission
line. Typical power system network is shown in Figure TE-2210-0100-00.
The transmission line is assumed to consist of several zones and each zone
comprises of one or more transmission line section.
A transmission line section is defined as: the phase conductors, ground wires
between two grounded transmission line structures and one of these two grounded
structures. Each transmission line zone and consequently any transmission line
section is characterized by the self and mutual impedances of conductors and ground
impedance of transmission line structure. More detalis can be found in the EPRI
Report, EL-2699, “Transmission Line Grounding” (Refernce 4).
These impedances can be determined with the help of simplified formulae or
computer programs, usually designated as line parameter programs.
4.3 Grounding Performance of Transmission Line Structures
The distribution of fault current between a structure and the overhead ground wires
connected to it is greatly dependent on the ground resistance of the structure. A
knowledge of surface potentials around the faulted structure may also be needed to
assess the overall grounding performance of the structure. These earth potentials are
usually calculated for local soil characteristics.
The ground resistance of a structure and the earth potentials around it may be
calculated using simplified analytical expressions under the assumption of uniform
PAGE NO. 6 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
soil resistivity ‘ρ’and simple structure grounding systems such as a hemispherical
electrode, a vertical rod and a horizontal cylindrical conductor.
A very useful approach to transmission line grounding design is to initially consider
the structure base and associated grounding elements as equivalent to a
hemispherical electrode with an appropriate radius ‘r’. Under this assumption, the
ground resistance is:
R
r
Ohms=
ρ
π2
(Eq.10-1)
Where: ρ = Soil resistivity in ohm-meter
r = electrode radius in meter
The surface potential ‘V’ at a distance x (meter) from the center of the hemisphere
injecting a current I (amperes) in the earth is:
V
I
2 x
Volts=
ρ
π (Eq.10-2)
4.4 Types of Disturbance
The disturbance is assumed to be localized at a specific structure of the transmission
line. The disturbance could be a surge caused by a lightning stroke to the structure or
a power frequency ground fault at the structure. The ground fault could be initiated
by various causes such as insulator pollution or a back-flash resulting from a
lightning stroke.
4.4.1 Power Frequency Ground Faults
The structure fault may be of the single, double or three phase-to-ground
type but the majority of transmission line power frequency ground faults
are single phase-to-ground.
4.4.2 Lightning Strokes
The lightning response of the transmission line structure is certainly the
most difficult subject in the area of transmission line design. Nevertheless,
the lightning response of the structure may be represented by the following
equation, so called "transient surge impedance".
Zs
ct
a
=
⎡
⎣⎢
⎤
⎦⎥60 ln 2
(Eq.10-3)
Where: c = velocity of light (3 x 108
m/s)
t = time, in seconds.
a = radius of the equivalent structure, in meters.
Zs = surge impedance, in Ohms
PAGE NO. 7 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
5.0 SAFETY
5.1 General
A survey conducted by IEEE (Reference 6) among various utilities around the world
indicates that there have been no fatal accidents due to excessive touch or step
voltages in the vicinity of transmission line structures. Mostly the accidents are
caused by lightning or involve low voltage circuitry and household electrical
devices. A low accident rate is not the result of transmission line structure
grounding design but rather is a consequence of a very low probability of human
exposure to hazardous situations. For an electrocution incident to occur, the
following events must occur simultaneously:
- A ground fault at a transmission line structure sufficient in magnitude to
generate hazardous step and touch potentials.
- A person must be in a dangerous position near this structure at the time of
fault.
Because of the infrequent presence of people near high voltage transmission line
structures and because a fault at any specific structure is very unlikely, the
electrocution incident is an extremely low probability event. This favorable situation
may not exist at some exposed locations along the transmission line or may change
in future because of joint use of transmission line right of way in populated urban
areas and progressively higher fault current magnitudes. For such conditions it shall
be responsibility of the design engineer to determine the locations which may
present high human exposure risks and if necessary a suitable grounding system to
control hazardous earth surface voltages shall be designed.
5.2 Earth Surface Potential
5.2.1 Tolerable Limits of Body Current
The effects of an electric shock depend on the frequency, magnitude and
duration of the current flowing through the vital organs of the body. In
transmission line grounding, the normal frequency of 60 Hz is considered.
The threshold of perception is generally at a current flow of about one to
eight milliamperes. Following equation, developed by Dalziel, is generally
used to show the relationship between time and current that a person
weighing 50 kg can safely withstand without ventricular fibrillation.
I
t
b =
0116.
(Eq.10-4)
Where: Ib = rms current passing through the body, in
amperes.
t = time duration of shock, in seconds.
0.116 = an energy constant for a 50 kg person.
PAGE NO. 8 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
The equation is based on tests limited to three-second duration and is not
valid for longer durations. It indicates that higher current can be allowed
where fast operating protective devices can be relied upon to limit fault
duration.
5.2.2 Tolerable Potential Differences
Using the value of tolerable body current established by equation (10-4)
and the appropriate circuit constant, it is possible to calculate the tolerable
potential difference between possible points of contact. The two types of
potential differences that have to be given consideration are:
a. Step Potential: the potential difference between any two points on the
ground surface which can be touched simultaneously by the feet.
b. Touch Potential: the potential difference between any point on the
ground where a man may stand and any point which can be touched
simultaneously by either hand.
Using the appropriate circuit constant and the expression for the tolerable
current from equation (10-4), the tolerable potential differences between
the two points which may be touched simultaneously become:
V
t
Step =
+116 0 7. sρ
(Eq.10-5)
V
t
Touch
s
=
+116 017. ρ
(Eq.10-6)
Where: ρs = the resistivity of the soil near the surface, in
ohm-meters.
t = the duration of the shock, in seconds.
Consideration of a foot to ground resistance and high speed clearing of
faults may then be most helpful in achieving safety.
5.3 Thresholds of Current
A current in the order of 0.5 to 1 mA is the minimum current which can be
perceived by the fingers. As the current increases, a tingling sensation occurs
followed quickly by a painful muscular contraction. At a current of 6 to 7 mA for
women and 9 to 10 mA for men (at 50 to 60 Hz), a threshold is reached beyond
which it is no longer possible to release the energized conductor. This current is
called the let-go-current. For higher currents (20 to 50 mA) breathing difficulties
may occur if the shock duration lasts for minutes. Whereas currents of 100 to 200
mA may cause death if the shock duration is in the order of 0.5 second.
PAGE NO. 9 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
5.4 Ground Potential Mitigating Conductors
If the usual grounding system of a transmission line and its overhead ground wires
are not sufficient to provide a safe environment at critically located transmission line
structures, the installation of additional ground conductors shall be considered to
reduce step and touch voltages.
A practical and widely used arrangement of such conductors consists of a ring (for
tubular/circular steel structures) or a square loop (for latticed steel structures) buried
at a shallow depth around the structure base. If necessary, several concentric grading
rings or square loops buried at progressively increasing depth can be installed in
difficult cases. These rings are usually designated as ground potential control rings or
loops. Typical rings arrangements are shown in figure TE-2210-0200-00, whereas
the size of control rings shall be determined per Clause 14 of this standard. The
number of control rings or loops will depend on the magnitude of step and touch
potentials to be reduced.
The ring provides beneficial effects on the overall performance of the grounding
system as follows:
5.4.1 It reduces the ground resistance and, in most cases, the potential rise of the
structure.
5.4.2 It reduces the step potential gradients which exist near the buried part of
the structure or structure foundation.
6.0 GROUNDING OF STRUCTURES
6.1 Single Pole Wood Structures
6.1.1 For minimum grounding on single wood pole structures, a 7 No. 10 (7.77
mm diameter & 36.83 mm² cross-section) copper-clad steel conductor shall
be installed 10 meters above ground level and extended to the butt of the
pole and wrapped five (5) complete turns 1100 mm from the pole butt. All
hardware on the wood pole structure shall be bonded to a 9.525 mm (3/8
in.) galvanized steel wire and connected to copper-clad steel grounding
conductor.
6.1.2 When the required structure footing resistance is not attained, additional
grounding shall be required. The most economical system for providing
additional grounding, when required in conjunction with the butt-wrap,
shall be ground rod located 3000 mm from structure footing or foundation.
6.1.3 When rock is encountered, other systems for providing additional
grounding are radial counterpoise and continuous counterpoise terminated
by a ground rod at the end.
PAGE NO. 10 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
6.2 Multipole Wood Structures
6.2.1 Grounding methods on a multipole wood structure shall be similar to the
single pole structure. Ground wires of the individual pole shall be
connected together at the top of each structure to improve the structure
footing resistance.
6.2.2 When the required structure footing resistance is not attained, additional
ground rods shall be added to each pole ground. On three-pole structures,
an additional ground rod is not required on the middle pole.
6.3 Steel Lattice Structures
6.3.1 The reinforcing steel cage of the concrete foundation shall be bonded to the
respective tower stub angle at atleast two points by using a suitable size of
steel wire. The connection of wire to the stub angle shall be preferably
through a mechanical connector (clamp) and buried inside the concrete.
6.3.2 Two opposite legs of lattice steel structures shall be grounded by means of
counterpoise conductors and two exothermic coupled copper clad ground
rods. In case the resistance is more than the required value, deep
grounding or additional counterpoise conductors and ground rods may be
required on the remaining legs of lattice steel structures.
6.3.3 When rock is encountered, other systems for providing additional
grounding such as radial counterpoise or continuous counterpoise
arrangements terminated by a ground rod at the end may be required.
6.3.4 Wherever possible ground rods shall be installed underneath the footing
(spread foundations) to a minimum depth of 3 meter. For other types of
foundations the ground rod shall be placed at a distance of 8 m from the
centre of footing.
6.4 Steel Pole Structures
6.4.1 The reinforcing steel cage of the concrete foundation shall be bonded to the
anchor bolts (two bolts), as an economical way to lower the structure
footing resistance.
6.4.2 Two exothermic coupled ground rods shall be connected at the base of
steel pole structures. Where greater resistance is encountered, multiple
ground rods may also be required. The spacing between the two rods shall
be in the range of one to two times the length of rod.
7.0 GROUNDING REQUIREMENTS FOR ADJACENT FACILITIES
7.1 All metallic facilities such as communication towers, fences and pipelines running in
close proximity to SEC transmission lines shall be grounded by the facility owner to
PAGE NO. 11 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
insure safety to the personnel during steady-state and fault conditions. Guidelines
specified in TES-P-122.09 shall be followed by the facility owner while requesting
SEC approval for allowing such facilities in the vicinity of transmission lines.
7.2 Grounding of pipelines shall be coordinated with cathodic protection of pipelines to
avoid a negative effect on each other.
7.3 Terminal tower on each end of transmission line shall be connected with substation
grounding grid.
8.0 GROUNDING CONDUCTORS
8.1 Grounding Conductor Material
All grounding conductors shall be of annealed copper-clad steel wires with a
conductivity of 40% IACS (International Annealed Copper Standard) conforming to
the requirements of 10-TMSS-05.
8.2 Grounding Conductor Size
8.2.1 Each element of the grounding system (including connecting ground leads
and electrodes) shall be so designed that it will:
a. Resist fusing and deterioration of electric joints under the most
adverse combinations of fault current magnitude and fault duration to
which it might be subjected.
b. Be mechanically reliable and rugged to a high degree, especially on
locations exposed to corrosion and physical damage.
c. Have sufficient conductivity so that it will not contribute
substantially to dangerous local potential differences.
8.2.2 Methods of meeting the above requirements in paragraph 8.2.1 are
discussed in order below.
a. Adequacy of a copper-clad steel conductor size and its joints against
fusing can be determined from the following formula for fusing
current:
A I
t
TCAP
n
Tm Ta
K Ta
c r r
o
=
+
−
+
⎡
⎣
⎢
⎤
⎦
⎥
α ρ .10
1 1
4
(Eq.10-7)
PAGE NO. 12 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
Where:
I = A portion of fault current flowing in the ground
conductor, in kA
= S.If
S = Current distribution factor, to be calculated from fault
current distribution model or taken as 0.50 for fault
near the substation and 0.30 for fault away from the
substation whichever is greater
If = Fault current in kA, 63 kA for 380kV& 230kV system
and 40 kA for 132kV,115kV,110kV& 69kV system
A = Conductor cross-section in mm²
Tm = Maximum allowable temperature in °C for copper-clad
steel conductors
Ta = Ambient temperature in °C
αr = Thermal coefficient of resistivity of conductor material
at reference temperature Tr for copper-clad steel
ρr = Resistivity of ground conductor at reference
temperature in micro-ohm/cm for copper-clad steel
tc = Fault clearing time
TCAP = Thermal Capacity factor in J/cm³/°C
K0 =
1
α αo
rTor K =
1
0
r
−
Where
Tr = Reference temperature for material constants in °C
α0 = Thermal coefficient of resistivity of conductor material
at °C
K0 = Thermal coefficient of conductivity of conductor
material at °C
The material constants for copper-clad steel conductor of 40%
conductivity are given in the following table, whereas for other type
of conductors these constants are given in IEEE standard 80. In case
of bolted connections, the maximum allowable temperature Tm shall
be taken as 300°C.
PAGE NO. 13 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
Table 10-1: Material Constants
Tm Ta αr ρr tc TCAP K0
(°C) (°C) @ 20°C @ 20°C
(μΩ/m)
(Sec) (J/cm³/°C
)
1084 40 0.00378 4.397 0.50 3.846 245
b. The short-time fusing current of copper-clad steel conductor on a
size-for-size basis is within 20% of that of copper.
c. The size of wire required where lightning may impinge directly upon
it is considerably greater than that required to conduct the lightning
surge through it. The minimum size selected for fault current shall be
more than adequate for conducting lightning currents. In practice the
requirements on mechanical reliability will set a minimum conductor
size. Since it is impractical to observe or inspect buried portions of
grounding system, the calculated size of ground conductor shall be
enhanced for mechanical ruggedness as well as to account for future
growth of fault current levels and relay malfunctioning and human
errors which can result in fault durations in excess of desired clearing
times.
8.3 Grounding Conductor Connections
8.3.1 Connection of grounding conductors shall be made by means matching the
characteristics of both the material to be grounded and grounding
conductors, and shall be suitable for the environmental exposure. These
means include welding, mechanical and compression connections.
8.3.2 All buried connections of grounding conductor to ground rods shall be
made by exothermic welding, whereas all above ground connections (such
as with tower stub angle etc.) shall be of approved compression and/or
mechanical connectors. To prevent theft of ground conductor, the
connection with the stub angle shall be made on its inner flange with
ground conductor embedded in the concrete foundation.
8.3.3 Dissimilar types of grounding conductors shall be bonded together by a
bimetallic connector treated with a manufacture recommended inhibitor to
prevent corrosion.
8.4 Grounding Conductor Protection
8.4.1 The grounding conductor shall be guarded from mechanical damage only
in areas where they are readily accessible to the public.
8.4.2 Where protection is required, grounding conductors shall be protected by
conduit or molding suitable for such exposure and shall be extended for not
less than 2.45 meters above the ground.
PAGE NO. 14 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
8.4.3 In areas where special protection is not required, grounding conductors
shall be attached closely and securely to the surface of the pole or other
structure in position where they are least exposed to mechanical damage.
9.0 GROUNDING ELECTRODE
9.1 Function of Grounding Electrode
A suitable grounding electrode is the only feasible means of maintaining the various
noncurrent carrying metal structures of a power installation at safe potential levels
and keeping the earth surface potential gradient in the vicinity of the electrode
within tolerable values.
9.2 Types of Grounding Electrodes
9.2.1 Existing Electrodes
Existing electrodes consist of conducting items installed for purposes other
than grounding.
a. Steel Reinforcing Bars in Concrete Foundations and Footings
The steel reinforcing bar system of a concrete foundation or footing
which is not insulated from direct contact with earth and which
extends at least one (1) meter below grade constitutes an effective
and acceptable type of grounding electrode. Where a steel structure
supported on this foundation is to be used as the grounding
conductor, it shall be interconnected by bonding anchor bolts or stub
angles and reinforcing bars.
The normal applied steel ties are considered to provide adequate
bonding between bars of the reinforcing cage.
b. Steel Casing of Extended and Cased Foundation
A steel casing of an extended and cased foundation which extends at
least one (1) meter below grade, constitutes an effective and
acceptable type of grounding electrode. The grounding conductor of a
structure supported on the foundation shall be bonded to the steel
casing.
9.2.2 Made Electrodes
Where made electrodes are used, these shall as far as practical penetrate
into the permanent soil moisture level. Made electrodes shall be of a metal
or combination of metals which do not corrode excessively under existing
conditions for the expected service life. All outer surfaces of made
PAGE NO. 15 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
electrodes shall be conductive that is, not having paint, enamel or other
insulating type covering.
a. Driven Ground Rods
i. Driven rods are generally more satisfactory and
economical where bedrock is 3 meter or more below the
surface. Where soil conditions permit, few deep rods are
usually more satisfactory than multiple short rods since
the volume of soil affected increases directly with the
length of rod below the surface and the soil resistivity also
decreases with depth due to increased moisture content.
For ordinary soil conditions 3 m length of copper clad rod
shall be adopted as a minimum standard length to be
driven into the soil with upper end of the rod at the bottom
of spread footing type foundation and 0.6 m below the
surface for other types of foundations.
ii. The effect of the rod diameter on the resistance of ground
connection to earth is small. The diameter of the ground
rod is determined mainly by the mechanical rigidity
required for driving. Rod diameter of 16 mm (5/8 inches)
nominal shall be considered as the minimum requirement
for driving in all types of soil.
iii. In case rods are to be driven to a depth more than 3 m to
achieve the desired resistance value, sectional rods may be
used. The individual rods shall be connected by couplings
and a removable stud shall be used to drive the rods.
iv. Multiple rods may also be used to reduce the ground
resistance. Spacing between multiple rods shall not be less
than 3 meter.
b. Counterpoise
i. In case of rocky areas or shallow bedrock, or where lower
resistance is required than attainable with driven rods,
counterpoise shall be used.
ii. Counterpoise made of bare copper-clad steel wire of a
minimum size of 7 No. 10 (7.77mm diameter) with a
minimum length of 30 meters shall be buried at a depth of
not less than 0.45 meter and shall be laid approximately
straight. Counterpoise may be in single length, or may be
several lengths connected at the ends or at some point
away from the ends.
PAGE NO. 16 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
iii. Where rock surfaces are encountered, the rock shall be cut
150 mm deep. The counterpoise shall be laid in the trench
and then filled with concrete.
iv. Other lengths or configurations may be used if their
suitability is supported by a qualified engineering study.
c. Pole Butt Wire Wraps
i. Made electrodes shall be of grounding conductor wrapped
at the pole butt previous to the setting of the pole. The
ground conductor shall be of copper-clad steel conductor
and shall have a continuous bare or exposed length below
ground level of not less than 4 meters.
ii. The size of grounding conductor wraps shall not be
smaller than 7 No. 10 (7.77 mm diameter).
9.3 Methods of Connection to Electrodes
9.3.1 Buried ground connections to existing electrodes (connection of
reinforcing cage to stub angle and anchor bolts) shall be made by means of
approved compression/bolted type connectors, whereas connections to the
made electrode shall be made by means of exothermic welding.
9.3.2 Above ground connections to steel structures shall be made by
compression/bolted connectors. Connections to steel casings may be made
by exothermic welding or compression/bolted type connectors whichever
is more suitable.
9.3.3 For concrete encased reinforcing bar electrodes, a steel rod similar to the
reinforcing bar shall be used to join, by welding, a main vertical
reinforcing bar to an anchor bolt or stub angle.
10.0 GROUND RESISTANCE
10.1 Requirements
The grounding electrode system may consist of one or more inter-connected
electrodes and shall have a resistance to ground low enough to minimize hazards to
personnel and flashovers across the insulator assemblies. Low resistance reduces
the amount of insulation required to minimize flashover and it is usually more
economical to reduce ground resistance than to increase insulation.
10.1.1 A structure ground resistance of 10 Ohms or less shall be obtained for
69kV, 110kV, 115kV and 132kV transmission line structures located
within a distance of 1.6km from the substation.
PAGE NO. 17 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
For a distance of more than 1.6km from the substation, the required
structure ground resistance shall be 20 Ohms or less.
10.1.2 For 230 and 380 kV transmission lines, the required structure ground
resistance shall be 3 Ohms or less within a distance of 3 km from the
substation and 10 Ohms or less for a distance of more than 3 km from the
substation for all types of soil except where rock is encountered at shallow
depths for which the requirement shall be 10 ohm or less and 20 ohm or
less for the respective distances of 3 km and more than 3 km.
10.2 Ground Resistance Tests
The fall-of-potential method shall be used to measure the structure ground resistance
along the transmission line route or right-of-way. The detailed description of this
method is given in ANSI/IEEE Std.81.
The megger earth tester (null balance) or clamp-on ground resistance tester shall be
used in making all ground resistance measurements.
10.3 Ground Resistance of Driven Rods
Structure ground resistance is an extremely important parameter in the determination
of lightning flashover rates. It is a fluctuating statistical variable the magnitude of
which is governed not only by geography but also by other conduction physics in the
earth.
The resistance of a driven ground rod has been derived as:
R
L
n
L
a
=
⎛
⎝
⎜
⎞
⎠
⎟ −
⎡
⎣
⎢
⎤
⎦
⎥
ρ
π2
1
4
1 When L > a (Eq.10-8)
Where: R = ground resistance, in Ohms
ρ = ground resistivity, in ohm-meters
L = length of rod, in meters
a = rod radius, in meters
The diameter of the rod is of some significance because it affects the logarithmic
term 4L/a, but the length is more important.
Ground resistance may be lowered by connecting driven ground rods in parallel. If
the spacing between rods is great compared with the length of the individual rods,
the resistance will be reduced in proportion to the number of rods. If the rods are
close together, each rod will be in the intense electrical field of its neighbor, then the
overall resistance (in Ohms) becomes:
R
L
n
L
a
=
⎛
⎝
⎜
⎞
⎠
⎟
ρ
π2
1
2
(Eq.10-9)
PAGE NO. 18 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
10.4 Ground Resistance of Counterpoise
The continuous counterpoise consists of one or two conductors buried continuously
under the transmission line or along certain sections of the line. The conductors are
interconnected to the overhead ground wire and ground system (if any) at each
transmission line supporting structure.
When the counterpoise is buried close to the surface of a uniform earth, the ground
resistance (in Ohms) is approximately equal to:
R n
ae
= −
⎡
⎣
⎢
⎤
⎦
⎥
ρ
πl
l
1
2
2
1 (Eq.10-10)
a = radius of counterpoise, in meters
e = depth of burial below existing ground level, in meters
11.0 EARTH RESISTIVITY
11.1 General
The ability of a group of buried metallic conductors such as transmission structure
grounds and counterpoise, to conduct current into the soil is significantly dependent
on the resistivity of the soil. Earth resistivity can even affect the susceptibility of a
particular location to lightning strikes. A knowledge of soil resistivities is of
fundamental importance in accurate prediction of transmission line performance
during ground fault or lightning conditions.
It is not practical to determine soil resistivity everywhere along the route of a
transmission line. There are certain circumstances which require that accurate
knowledge of soil structure be determined at specific sites. At most other sites,
accurate values are not necessary and an order of magnitude estimate is sufficient.
Fortunately, there are usually indirect sources of information from which it is
possible to secure a qualitative knowledge of the soil structure. This section is
directed at the measurement and interpretation techniques most generally used to
determine soil structure and resistivity.
11.2 Earth Resistivity Test
Electrical resistivity tests shall be made to determine the earth resistivity along the
transmission line route or right-of-way. These shall be preferably made at a number
of places within the right-of-way and with different probe spacings, to get an
indication of any important variations of resistivity with location and depth.
The Wenner four-electrode method shall be used to measure earth resistivity. Earth
resistivity tests shall be made as part of the geotechnical investigation that is
metersin,secounterpoioflength:Where =l
PAGE NO. 19 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
performed for foundation design information. If earth resistivity tests are not made, a
resistivity of 1000 Ohm-meter shall be assumed.
The method of measurement in general use is based on the following equation:
2222
4A
2A
+1
4
BA
A
B
AR
+
−
+
=
π
ρ (Eq.10-11)
Where: ρ = resistivity of soil, in Ohm-meter.
A = distance between adjacent electrodes, in meters.
R = resistance in Ohm resulting from dividing the voltage between
the potential probes by the current flowing between the
current electrodes.
B = depth of the electrodes, in meter.
If "B" is small compared to "A", as in the case of probes penetrating the ground a
short distance only, the above equation can be simplified as follows:
ρ π= 2 AR (Eq.10-12)
12.0 FAULT CURRENT DISTRIBUTION
12.1 General
The majority of transmission line faults are to ground and generally occur between a
phase conductor and a transmission line structure, as the result of an insulator
flashover. In some cases, the presence of foreign objects between a phase conductor
and the overhead ground wire or a grounded structure may cause a ground fault
somewhere along one span of the transmission line. Occasionally, the ground fault is
caused by a phase conductor in direct contact with the overhead ground wire or the
earth’s surface. In all these cases, the current return paths include earth and,
therefore, present an impedance which at most is equal to the equivalent ground
impedance of the grounded structures which carry the fault current. If in addition to
the earth current, part of the total fault current returns to the generating sources via
metallic return circuits (such as overhead ground wires), the impedance value will
be even less.
12.2 Importance of Fault Current Distribution
The transmission line performance at the faulted structure and/or at other locations
on the transmission line is significantly influenced by the fault current distribution
between the structure and the overhead ground wires connected to the structure.
Figure TES-2210-0300-00 shows a typical system which illustrates this aspect. R
represents the faulted structure ground resistance and Zs the equivalent power
generation (source) impedance. It is also assumed that the value of Zs is higher than
PAGE NO. 20 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
the structure resistance value R so that the total fault current (≈ V/Zs) is
approximately constant when R varies within a relatively narrow range. Figure TES-
2210-0300-00 leads to the following conclusions:
12.2.1 If one of the ground wire impedances Zl or Zr is small relative to R, then
very little current enters the faulted structure and the potential rise of the
structure is proportional to the metallic return circuit path impedance, i.e.
Zl in parallel with Zr.
12.2.2 If in contrast, R is small compared to Zl and Zr, then most of the fault
current flows through the structure, causing a ground potential rise
proportional to the structure ground resistance R.
This simple example illustrates the importance of fault current distribution.
In a real transmission line there are hundreds of grounded transmission
structures. The metallic return circuits are not only bonded to the structures
but also are electromagnetically coupled to the phase conductors. The
ground resistance of the structures is not constant and generally varies
between wide limits along the route of the transmission line. The fault
current distribution is therefore more complex to calculate and will vary
with the type of transmission line and location of fault on the line.
12.2.3 There are various methods which have been used for calculation of fault
current distribution between the faulted structure and the metallic return
conductors. One of the most suitable method is a double-sided elimination
which is described in “EPRI Report EL-2699, Transmission Line
Grounding”. This method shall be adopted for determining fault current
distribution and consequently the current to be considered for the
calculation of size of grounding conductor. Computer programs such as
SPLIT and GATL may be used for solving complex analytical expressions.
12.2.4 When a ground fault occurs on a transmission line equipped with ground
wires, a significant portion of the fault current is diverted to the structures
on each side of the faulted structure. Consequently both the fault current
and potential rise at the faulted structure are decreased.
A survey by IEEE Working group 78.1 (IEEE paper F 79 632-1) conducted
among various utilities around the world regarding power system
grounding practices indicates that 30% to 70% of fault current is
considered to flow in the substation grounding grid depending upon the
overhead network configuration, location of fault and ground resistance.
Typically 50% division of current between groundwires and grounding
grid is used by the utilities.
Similar survey was also conducted by EPRI, USA among North American
Power Electric Utility Companies regarding grounding practices for
overhead transmission lines. The survey indicates that majority of the
utilities consider 10 to 60% of fault current to flow in the grounding grid.
PAGE NO. 21 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
12.2.5 SEC fault current distribution study indicates that a current of about 25 to
30% of standard maximum system fault current (at the source substation)
flows in the structure when the resistance of the faulted structure is one (1)
ohm and the fault is located with in 3km distance from the substation.
When the resistance of the faulted structures in the range of 3 to 10 ohms,
the maximum current flow in the structure is about 13 to 15% of standard
maximum system fault current..
For design of grounding system, 50% of standard maximum system fault
current (at source substation) shall be considered for the structures located
within a distance of 3km from the substation on both ends of the
transmission line and 30% of standard maximum system fault current for
the structures located beyond 3km from substation.
12.3 Overhead Ground Wire
Overhead transmission lines shall be provided with overhead ground wires
throughout their entire length. These shall be electrically bonded at each structure as
well as tied solidly to the grounding system of the substation so that when a tower
fault or lightning stroke does occur, the effect of the connected station grounding
system shall decrease the magnitude of gradients near the tower bases.
12.3.1 Overhead ground wires terminated at steel structures shall be electrically
bonded to the structure. The mechanical deadend hardware shall not be
used as the electrical path for conducting lightning surges or fault currents
to the deadend structure. Steel deadend structures are considered adequate
for solidly grounding properly terminated overhead ground wires to the
ground mat.
12.3.2 Overhead ground wires terminated at structural wood or other
nonconducting material shall be electrically bonded to the ground wire. A
continuous electrical path shall be made between the overhead ground wire
and the station ground mat or grid with a down conductor and
counterpoise.
12.3.3 Overhead ground wires with sufficient mechanical strength shall be located
to shield the line conductors adequately from lightning direct strokes.
13.0 CORROSION
13.1 Cause of Corrosion
The corrosion of metal is caused principally by electrochemical reactions, which are
accompanied by a flow of electric current between a portion of the metal and the
moisture in its environment. A metallic connection between the two metals allows
current to flow from one (the cathode) to the other (the anode) and forces the ions of
the anode to migrate to the electrolyte causing corrosion to the anode.
PAGE NO. 22 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
13.2 Rate of Corrosion
The amount of corrosion by electrolysis depends on the type of soil or air, the
currentand the electrochemical equivalent of the metal. High resistivity soil reduces
corrosion currents and thereby reduces corrosion rates.
13.3 Control of Corrosion
Precautions to prevent corrosion shall include, but not be limited to, the following:
13.3.1 Grounding conductors shall be copper or copper-clad steel conductors.
13.3.2 Copper or copper clad steel ground rods shall be used.
13.3.3 Where possible, route the grounding conductor at least 6 meters away from
buried steel work.
13.3.4 The use of dissimilar metals electrically connected together shall be
avoided.
13.3.5 If dissimilar metals are used, these shall be bonded together by a bimetal
connector and treated with proper inhibitor to reduce the rate of corrosion.
14.0 RECOMMENDED SIZES OF GROUNDING CONDUCTOR
On the basis of short circuit levels established for various transmission line voltages in the
system of SEC and the approach described in preceding paragraphs, sample calculations
were made to determine the size of grounding conductor to be used for counterpoise
arrangements. While sample calculations are included as an Appendix-I to this Standard,
the recommended sizes for various transmission line voltages are given in Table 10-2. The
various parameters used in the calculations are given below:
Short circuit level = 63 kA for 230kV and 380kV System
= 40 kA for 69kV, 110kV, 115kV and 132kV System
Maximum Allowable Temperature = 300°C for bolted connections
Ambient Temperature = 40°C
Current flow in the
grounding conductor = 50% of fault current, when fault is within 3km
of substation
= 30% of fault current when fault is at a distance
of more than 3 km from the substation
Fault clearing time = 0.50 Seconds (30 cycles)
PAGE NO. 23 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
Conductivity of grounding conductor= 40% of IACS (International Annealed Copper
Standard)
Table 10-2 : Recommended Sizes of Grounding Conductor
Ground
Conductor Size
Sr.
No.
Transmission
Line
Voltage
Fault
Current
(kA)
Current Flow
in the ground
conductors (kA) (AWG) Dimeter
mm
Application
31.5 19 No. 7 18.31 Within 3 km from Substation
1. 380kV 63
18.9 7 No.5 13.87 Beyond 3 km from substation
31.5 19 No. 7 18.31 Within 3 km from Substation
2. 230kV 63
18.9 7 No. 5 13.87 Beyond 3 km from substation
20 7 No. 5 13.87 Within 3 km from Substation
3. 132kV 40
12 7 No. 7 11.00 Beyond 3 km from substation
20 7 No. 5 13.87 Within 3 km from Substation
4. 115kV 40
12 7 No. 7 11.00 Beyond 3 km from substation
20 7 No. 5 13.87 Within 3 km from Substation
5. 110kV 40
12 7 No. 7 11.00 Beyond 3 km from substation
20 7 No. 5 13.87 Within 3 km from Substation
6. 69kV 40
12 7 No. 7 11.00 Beyond 3 km from substation
PAGE NO. 24 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
FIG. TE-2210-0100-00 : EQUIVALENT POWER SYSTEM NETWORK
PAGE NO. 25 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
FIG. TE-2210-0200-00 : TYPICAL GROUND POTENTIAL MITIGATING RINGS
FIG. TE-2210-0300-00 : FAULT CURRENT DISTRIBUTION
PAGE NO. 26 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
15.0 BIBLIOGRAPHY
1. ANSI/IEEE Std. 80-2000, “IEEE Guide for Safety in Substation Grounding”.
2. ANSI C2-2007, “National Electrical Safety Code”.
3. SEC Engineering Report by Chas T. Main Int. “380 kV Transmission Line
Grounding Details”.
4. EPRI Report, EL-2699, “Transmission Line Grounding”.
5. Engineering Publication by Copperweld Bimetallic Group, “Copperweld Ground
Wire”.
6. IEEE paper published in PAS-99, No.4, “Survey on Power System Grounding
Design Practices”.
PAGE NO. 27 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
APPENDIX I
SAMPLE CALCULATIONS FOR SIZE OF COPPER-CLAD
STEEL COUNTERPOISE CONDUCTORS
1. Case A: 230kV and 380kV Lines
Fault current If = 63 kA
Current flow in faulted structure:
I = 50% of If near S/S = 31.5 kA
I = 30% of If away from S/S = 18.9 kA
Fault clearing time tc = 0.5 sec. (30 cycles)
Maximum allowable temperature Tm = 250-350°C for bolted connections
(Say 300°C average)
Ambient temperature, Ta = 40°C
Conductivity of conductor = 40%
Acmils = 1973.52 I
t P
TCAP
Tm Ta
K Ta
c r r
O
α .
ln
10
1
4
+
−
+
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎤
⎦
⎥
Acmils = 1973.52 x I
Tm
05 0 00378 4 397 10
3846
1
40
245 40
4
. . .
.
ln
× × ×
+
−
+
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎤
⎦
⎥
Acmils = 1973.52 x I
Tm
216077
1
40
285
.
ln +
−⎡
⎣⎢
⎤
⎦⎥
(a) Fault near Substation
Acmils = 197352 315
216077
1
260
285
. .
.
ln
×
+
⎡
⎣⎢
⎤
⎦⎥
Acmils = 358,897, say 396 kcmil or 19 No. 7 would be appropriate.
PAGE NO. 28 OF 28TESP122.10R0/MSO
TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0
Date of Approval: February 18, 2007
(b) Fault away from Substation
Acmils = 197352 18 9
216077
1
260
285
. .
.
ln
×
+
⎡
⎣⎢
⎤
⎦⎥
= 215,338
say 232 kcmil or 7 No. 5 would be appropriate.
2. Case B: 69kV, 110kV, 115kV and 132kV Lines
Fault Current = 40 kA
Current flow in the faulted tower near the substation = 20 kA
Current flow in the faulted tower away from substation = 12 kA
Sizes for conditions a & b would be as below:
(a) A = 227,871 cmil, say 232 kcmil or 7 No. 5
(b) A = 136,723 cmil, say 146 kcmil or 7 No.7
on Lines".

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Tes p-122.10-r0 (1)

  • 1.
  • 2. PAGE NO. 2 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 TABLE OF CONTENTS 1.0 SCOPE 2.0 PURPOSE OF TRANSMISSION LINE GROUNDING 3.0 GROUNDING IN GENERAL 4.0 FUNDAMENTAL CONSIDERATIONS 4.1 Earth 4.2 Power System Network 4.3 Grounding Performance of Transmission Line Structures 4.4 Types of Disturbance 5.0 SAFETY 5.1 General 5.2 Earth Surface Potential 5.3 Thresholds of Current 5.4 Ground Potential Mitigating Conductors 6.0 GROUNDING OF STRUCTURES 6.1 Single Pole Wood Structures 6.2 Multipole Wood Structures 6.3 Steel Lattice Structures 6.4 Steel Pole Structures 7.0 GROUNDING REQUIREMENTS FOR ADJACENT FACILITIES 8.0 GROUNDING CONDUCTORS 8.1 Grounding Conductor Material 8.2 Grounding Conductor Size 8.3 Grounding Conductor Connections 8.4 Guarding and Protection
  • 3. PAGE NO. 3 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 9.0 GROUNDING ELECTRODE 9.1 Function of Grounding Electrode 9.2 Types of Grounding Electrodes 9.3 Methods of Connection to Electrodes 10.0 GROUNDING RESISTANCE 10.1 Requirements 10.2 Grounding Resistance Tests 10.3 Grounding Resistance of Driven Rods 10.4 Grounding Resistance of Counterpoise 11.0 EARTH RESISTIVITY 11.1 General 11.2 Earth Resistivity Test 12.0 FAULT CURRENT DISTRIBUTION 12.1 General 12.2 Importance of Fault Current Distribution 12.3 Overhead Ground Wire 13.0 CORROSION 13.1 Cause of Corrosion 13.2 Rate of Corrosion 13.3 Control of Corrosion 14.0 RECOMMENDED SIZES OF GROUNDING CONDUCTOR 15.0 BIBLIOGRAPHY APPENDIX I: SAMPLE CALCULATIONS FOR SIZE OF COPPER-CLAD STEEL COUNTERPOISE CONDUCTORS
  • 4. PAGE NO. 4 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 1.0 SCOPE This Standard covers the general requirements and methods to be used in the design of grounding of transmission line structures for the overhead transmission line system of Saudi Electricity Company (SEC), Saudi Arabia. 2.0 PURPOSE OF TRANSMISSION LINE GROUNDING Grounding systems serve four main functions: 2.1 To safeguard a person from electric shock by ensuring that, under fault conditions, all surfaces including those of metallic equipment and the ground with which he is in simultaneous contact, remain at safe relative potentials. 2.2 To dissipate both voltages and currents under fault conditions without exceeding predictable limits or adversely affecting continuity of service. 2.3 To provide a path to dissipate surges induced by lightning or switching surges and to provide a path to ground from the overhead ground wire. 2.4 To reduce the possibility of static discharge caused by natural electric disturbances in the atmosphere. 3.0 GROUNDING IN GENERAL 3.1 A grounding system consists of grounding conductors, connecting together all items to be grounded, and a grounding electrode or electrodes. The grounding electrode forms the medium of contact with the body of earth and may consist of a single or a combination of buried conductors/ground rods and may include other elements such as foundation steel connected to the grounding system. 3.2 Factors influencing the design of the grounding system are as follows: 3.2.1 The magnitude and duration of ground-fault current which can pass between the fault location and the system neutral point(s) (which will influence the size of the grounding conductors). 3.2.2 The portion of this current which will pass between the grounding system and the body of earth. 3.2.3 The site earth resistivity. 3.2.4 The degree of exposure to mechanical damage and corrosion (which will influence the choice of materials and their manner of installation).
  • 5. PAGE NO. 5 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 4.0 FUNDAMENTAL CONSIDERATIONS The analysis of a power system subject to a phase to ground fault or lightning stroke at a transmission line structure requires specific data on the following variables which play a critical role in the grounding performance of transmission lines: 4.1 Earth Typically earth consists of several layers of soil, each having a different resistivity. For transmission line grounding purposes, the earth can be reasonably approximated by a two-layered soil structure. However, for calculation of ground resistance of a structure, uniform soil resistivity may be assumed. The layer resistivity and thickness are generally determined by interpreting the apparent resistivity values measured using the Wenner four electrode method. 4.2 Power System Network It is assumed that the fault or disturbance is at a structure on a transmission line section between two substations. Each substation and the power system network to which it is connected is designated as a terminal.The terminals can be described by equivalent circuit of the network which exists at either extremity of the transmission line. Typical power system network is shown in Figure TE-2210-0100-00. The transmission line is assumed to consist of several zones and each zone comprises of one or more transmission line section. A transmission line section is defined as: the phase conductors, ground wires between two grounded transmission line structures and one of these two grounded structures. Each transmission line zone and consequently any transmission line section is characterized by the self and mutual impedances of conductors and ground impedance of transmission line structure. More detalis can be found in the EPRI Report, EL-2699, “Transmission Line Grounding” (Refernce 4). These impedances can be determined with the help of simplified formulae or computer programs, usually designated as line parameter programs. 4.3 Grounding Performance of Transmission Line Structures The distribution of fault current between a structure and the overhead ground wires connected to it is greatly dependent on the ground resistance of the structure. A knowledge of surface potentials around the faulted structure may also be needed to assess the overall grounding performance of the structure. These earth potentials are usually calculated for local soil characteristics. The ground resistance of a structure and the earth potentials around it may be calculated using simplified analytical expressions under the assumption of uniform
  • 6. PAGE NO. 6 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 soil resistivity ‘ρ’and simple structure grounding systems such as a hemispherical electrode, a vertical rod and a horizontal cylindrical conductor. A very useful approach to transmission line grounding design is to initially consider the structure base and associated grounding elements as equivalent to a hemispherical electrode with an appropriate radius ‘r’. Under this assumption, the ground resistance is: R r Ohms= ρ π2 (Eq.10-1) Where: ρ = Soil resistivity in ohm-meter r = electrode radius in meter The surface potential ‘V’ at a distance x (meter) from the center of the hemisphere injecting a current I (amperes) in the earth is: V I 2 x Volts= ρ π (Eq.10-2) 4.4 Types of Disturbance The disturbance is assumed to be localized at a specific structure of the transmission line. The disturbance could be a surge caused by a lightning stroke to the structure or a power frequency ground fault at the structure. The ground fault could be initiated by various causes such as insulator pollution or a back-flash resulting from a lightning stroke. 4.4.1 Power Frequency Ground Faults The structure fault may be of the single, double or three phase-to-ground type but the majority of transmission line power frequency ground faults are single phase-to-ground. 4.4.2 Lightning Strokes The lightning response of the transmission line structure is certainly the most difficult subject in the area of transmission line design. Nevertheless, the lightning response of the structure may be represented by the following equation, so called "transient surge impedance". Zs ct a = ⎡ ⎣⎢ ⎤ ⎦⎥60 ln 2 (Eq.10-3) Where: c = velocity of light (3 x 108 m/s) t = time, in seconds. a = radius of the equivalent structure, in meters. Zs = surge impedance, in Ohms
  • 7. PAGE NO. 7 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 5.0 SAFETY 5.1 General A survey conducted by IEEE (Reference 6) among various utilities around the world indicates that there have been no fatal accidents due to excessive touch or step voltages in the vicinity of transmission line structures. Mostly the accidents are caused by lightning or involve low voltage circuitry and household electrical devices. A low accident rate is not the result of transmission line structure grounding design but rather is a consequence of a very low probability of human exposure to hazardous situations. For an electrocution incident to occur, the following events must occur simultaneously: - A ground fault at a transmission line structure sufficient in magnitude to generate hazardous step and touch potentials. - A person must be in a dangerous position near this structure at the time of fault. Because of the infrequent presence of people near high voltage transmission line structures and because a fault at any specific structure is very unlikely, the electrocution incident is an extremely low probability event. This favorable situation may not exist at some exposed locations along the transmission line or may change in future because of joint use of transmission line right of way in populated urban areas and progressively higher fault current magnitudes. For such conditions it shall be responsibility of the design engineer to determine the locations which may present high human exposure risks and if necessary a suitable grounding system to control hazardous earth surface voltages shall be designed. 5.2 Earth Surface Potential 5.2.1 Tolerable Limits of Body Current The effects of an electric shock depend on the frequency, magnitude and duration of the current flowing through the vital organs of the body. In transmission line grounding, the normal frequency of 60 Hz is considered. The threshold of perception is generally at a current flow of about one to eight milliamperes. Following equation, developed by Dalziel, is generally used to show the relationship between time and current that a person weighing 50 kg can safely withstand without ventricular fibrillation. I t b = 0116. (Eq.10-4) Where: Ib = rms current passing through the body, in amperes. t = time duration of shock, in seconds. 0.116 = an energy constant for a 50 kg person.
  • 8. PAGE NO. 8 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 The equation is based on tests limited to three-second duration and is not valid for longer durations. It indicates that higher current can be allowed where fast operating protective devices can be relied upon to limit fault duration. 5.2.2 Tolerable Potential Differences Using the value of tolerable body current established by equation (10-4) and the appropriate circuit constant, it is possible to calculate the tolerable potential difference between possible points of contact. The two types of potential differences that have to be given consideration are: a. Step Potential: the potential difference between any two points on the ground surface which can be touched simultaneously by the feet. b. Touch Potential: the potential difference between any point on the ground where a man may stand and any point which can be touched simultaneously by either hand. Using the appropriate circuit constant and the expression for the tolerable current from equation (10-4), the tolerable potential differences between the two points which may be touched simultaneously become: V t Step = +116 0 7. sρ (Eq.10-5) V t Touch s = +116 017. ρ (Eq.10-6) Where: ρs = the resistivity of the soil near the surface, in ohm-meters. t = the duration of the shock, in seconds. Consideration of a foot to ground resistance and high speed clearing of faults may then be most helpful in achieving safety. 5.3 Thresholds of Current A current in the order of 0.5 to 1 mA is the minimum current which can be perceived by the fingers. As the current increases, a tingling sensation occurs followed quickly by a painful muscular contraction. At a current of 6 to 7 mA for women and 9 to 10 mA for men (at 50 to 60 Hz), a threshold is reached beyond which it is no longer possible to release the energized conductor. This current is called the let-go-current. For higher currents (20 to 50 mA) breathing difficulties may occur if the shock duration lasts for minutes. Whereas currents of 100 to 200 mA may cause death if the shock duration is in the order of 0.5 second.
  • 9. PAGE NO. 9 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 5.4 Ground Potential Mitigating Conductors If the usual grounding system of a transmission line and its overhead ground wires are not sufficient to provide a safe environment at critically located transmission line structures, the installation of additional ground conductors shall be considered to reduce step and touch voltages. A practical and widely used arrangement of such conductors consists of a ring (for tubular/circular steel structures) or a square loop (for latticed steel structures) buried at a shallow depth around the structure base. If necessary, several concentric grading rings or square loops buried at progressively increasing depth can be installed in difficult cases. These rings are usually designated as ground potential control rings or loops. Typical rings arrangements are shown in figure TE-2210-0200-00, whereas the size of control rings shall be determined per Clause 14 of this standard. The number of control rings or loops will depend on the magnitude of step and touch potentials to be reduced. The ring provides beneficial effects on the overall performance of the grounding system as follows: 5.4.1 It reduces the ground resistance and, in most cases, the potential rise of the structure. 5.4.2 It reduces the step potential gradients which exist near the buried part of the structure or structure foundation. 6.0 GROUNDING OF STRUCTURES 6.1 Single Pole Wood Structures 6.1.1 For minimum grounding on single wood pole structures, a 7 No. 10 (7.77 mm diameter & 36.83 mm² cross-section) copper-clad steel conductor shall be installed 10 meters above ground level and extended to the butt of the pole and wrapped five (5) complete turns 1100 mm from the pole butt. All hardware on the wood pole structure shall be bonded to a 9.525 mm (3/8 in.) galvanized steel wire and connected to copper-clad steel grounding conductor. 6.1.2 When the required structure footing resistance is not attained, additional grounding shall be required. The most economical system for providing additional grounding, when required in conjunction with the butt-wrap, shall be ground rod located 3000 mm from structure footing or foundation. 6.1.3 When rock is encountered, other systems for providing additional grounding are radial counterpoise and continuous counterpoise terminated by a ground rod at the end.
  • 10. PAGE NO. 10 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 6.2 Multipole Wood Structures 6.2.1 Grounding methods on a multipole wood structure shall be similar to the single pole structure. Ground wires of the individual pole shall be connected together at the top of each structure to improve the structure footing resistance. 6.2.2 When the required structure footing resistance is not attained, additional ground rods shall be added to each pole ground. On three-pole structures, an additional ground rod is not required on the middle pole. 6.3 Steel Lattice Structures 6.3.1 The reinforcing steel cage of the concrete foundation shall be bonded to the respective tower stub angle at atleast two points by using a suitable size of steel wire. The connection of wire to the stub angle shall be preferably through a mechanical connector (clamp) and buried inside the concrete. 6.3.2 Two opposite legs of lattice steel structures shall be grounded by means of counterpoise conductors and two exothermic coupled copper clad ground rods. In case the resistance is more than the required value, deep grounding or additional counterpoise conductors and ground rods may be required on the remaining legs of lattice steel structures. 6.3.3 When rock is encountered, other systems for providing additional grounding such as radial counterpoise or continuous counterpoise arrangements terminated by a ground rod at the end may be required. 6.3.4 Wherever possible ground rods shall be installed underneath the footing (spread foundations) to a minimum depth of 3 meter. For other types of foundations the ground rod shall be placed at a distance of 8 m from the centre of footing. 6.4 Steel Pole Structures 6.4.1 The reinforcing steel cage of the concrete foundation shall be bonded to the anchor bolts (two bolts), as an economical way to lower the structure footing resistance. 6.4.2 Two exothermic coupled ground rods shall be connected at the base of steel pole structures. Where greater resistance is encountered, multiple ground rods may also be required. The spacing between the two rods shall be in the range of one to two times the length of rod. 7.0 GROUNDING REQUIREMENTS FOR ADJACENT FACILITIES 7.1 All metallic facilities such as communication towers, fences and pipelines running in close proximity to SEC transmission lines shall be grounded by the facility owner to
  • 11. PAGE NO. 11 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 insure safety to the personnel during steady-state and fault conditions. Guidelines specified in TES-P-122.09 shall be followed by the facility owner while requesting SEC approval for allowing such facilities in the vicinity of transmission lines. 7.2 Grounding of pipelines shall be coordinated with cathodic protection of pipelines to avoid a negative effect on each other. 7.3 Terminal tower on each end of transmission line shall be connected with substation grounding grid. 8.0 GROUNDING CONDUCTORS 8.1 Grounding Conductor Material All grounding conductors shall be of annealed copper-clad steel wires with a conductivity of 40% IACS (International Annealed Copper Standard) conforming to the requirements of 10-TMSS-05. 8.2 Grounding Conductor Size 8.2.1 Each element of the grounding system (including connecting ground leads and electrodes) shall be so designed that it will: a. Resist fusing and deterioration of electric joints under the most adverse combinations of fault current magnitude and fault duration to which it might be subjected. b. Be mechanically reliable and rugged to a high degree, especially on locations exposed to corrosion and physical damage. c. Have sufficient conductivity so that it will not contribute substantially to dangerous local potential differences. 8.2.2 Methods of meeting the above requirements in paragraph 8.2.1 are discussed in order below. a. Adequacy of a copper-clad steel conductor size and its joints against fusing can be determined from the following formula for fusing current: A I t TCAP n Tm Ta K Ta c r r o = + − + ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ α ρ .10 1 1 4 (Eq.10-7)
  • 12. PAGE NO. 12 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 Where: I = A portion of fault current flowing in the ground conductor, in kA = S.If S = Current distribution factor, to be calculated from fault current distribution model or taken as 0.50 for fault near the substation and 0.30 for fault away from the substation whichever is greater If = Fault current in kA, 63 kA for 380kV& 230kV system and 40 kA for 132kV,115kV,110kV& 69kV system A = Conductor cross-section in mm² Tm = Maximum allowable temperature in °C for copper-clad steel conductors Ta = Ambient temperature in °C αr = Thermal coefficient of resistivity of conductor material at reference temperature Tr for copper-clad steel ρr = Resistivity of ground conductor at reference temperature in micro-ohm/cm for copper-clad steel tc = Fault clearing time TCAP = Thermal Capacity factor in J/cm³/°C K0 = 1 α αo rTor K = 1 0 r − Where Tr = Reference temperature for material constants in °C α0 = Thermal coefficient of resistivity of conductor material at °C K0 = Thermal coefficient of conductivity of conductor material at °C The material constants for copper-clad steel conductor of 40% conductivity are given in the following table, whereas for other type of conductors these constants are given in IEEE standard 80. In case of bolted connections, the maximum allowable temperature Tm shall be taken as 300°C.
  • 13. PAGE NO. 13 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 Table 10-1: Material Constants Tm Ta αr ρr tc TCAP K0 (°C) (°C) @ 20°C @ 20°C (μΩ/m) (Sec) (J/cm³/°C ) 1084 40 0.00378 4.397 0.50 3.846 245 b. The short-time fusing current of copper-clad steel conductor on a size-for-size basis is within 20% of that of copper. c. The size of wire required where lightning may impinge directly upon it is considerably greater than that required to conduct the lightning surge through it. The minimum size selected for fault current shall be more than adequate for conducting lightning currents. In practice the requirements on mechanical reliability will set a minimum conductor size. Since it is impractical to observe or inspect buried portions of grounding system, the calculated size of ground conductor shall be enhanced for mechanical ruggedness as well as to account for future growth of fault current levels and relay malfunctioning and human errors which can result in fault durations in excess of desired clearing times. 8.3 Grounding Conductor Connections 8.3.1 Connection of grounding conductors shall be made by means matching the characteristics of both the material to be grounded and grounding conductors, and shall be suitable for the environmental exposure. These means include welding, mechanical and compression connections. 8.3.2 All buried connections of grounding conductor to ground rods shall be made by exothermic welding, whereas all above ground connections (such as with tower stub angle etc.) shall be of approved compression and/or mechanical connectors. To prevent theft of ground conductor, the connection with the stub angle shall be made on its inner flange with ground conductor embedded in the concrete foundation. 8.3.3 Dissimilar types of grounding conductors shall be bonded together by a bimetallic connector treated with a manufacture recommended inhibitor to prevent corrosion. 8.4 Grounding Conductor Protection 8.4.1 The grounding conductor shall be guarded from mechanical damage only in areas where they are readily accessible to the public. 8.4.2 Where protection is required, grounding conductors shall be protected by conduit or molding suitable for such exposure and shall be extended for not less than 2.45 meters above the ground.
  • 14. PAGE NO. 14 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 8.4.3 In areas where special protection is not required, grounding conductors shall be attached closely and securely to the surface of the pole or other structure in position where they are least exposed to mechanical damage. 9.0 GROUNDING ELECTRODE 9.1 Function of Grounding Electrode A suitable grounding electrode is the only feasible means of maintaining the various noncurrent carrying metal structures of a power installation at safe potential levels and keeping the earth surface potential gradient in the vicinity of the electrode within tolerable values. 9.2 Types of Grounding Electrodes 9.2.1 Existing Electrodes Existing electrodes consist of conducting items installed for purposes other than grounding. a. Steel Reinforcing Bars in Concrete Foundations and Footings The steel reinforcing bar system of a concrete foundation or footing which is not insulated from direct contact with earth and which extends at least one (1) meter below grade constitutes an effective and acceptable type of grounding electrode. Where a steel structure supported on this foundation is to be used as the grounding conductor, it shall be interconnected by bonding anchor bolts or stub angles and reinforcing bars. The normal applied steel ties are considered to provide adequate bonding between bars of the reinforcing cage. b. Steel Casing of Extended and Cased Foundation A steel casing of an extended and cased foundation which extends at least one (1) meter below grade, constitutes an effective and acceptable type of grounding electrode. The grounding conductor of a structure supported on the foundation shall be bonded to the steel casing. 9.2.2 Made Electrodes Where made electrodes are used, these shall as far as practical penetrate into the permanent soil moisture level. Made electrodes shall be of a metal or combination of metals which do not corrode excessively under existing conditions for the expected service life. All outer surfaces of made
  • 15. PAGE NO. 15 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 electrodes shall be conductive that is, not having paint, enamel or other insulating type covering. a. Driven Ground Rods i. Driven rods are generally more satisfactory and economical where bedrock is 3 meter or more below the surface. Where soil conditions permit, few deep rods are usually more satisfactory than multiple short rods since the volume of soil affected increases directly with the length of rod below the surface and the soil resistivity also decreases with depth due to increased moisture content. For ordinary soil conditions 3 m length of copper clad rod shall be adopted as a minimum standard length to be driven into the soil with upper end of the rod at the bottom of spread footing type foundation and 0.6 m below the surface for other types of foundations. ii. The effect of the rod diameter on the resistance of ground connection to earth is small. The diameter of the ground rod is determined mainly by the mechanical rigidity required for driving. Rod diameter of 16 mm (5/8 inches) nominal shall be considered as the minimum requirement for driving in all types of soil. iii. In case rods are to be driven to a depth more than 3 m to achieve the desired resistance value, sectional rods may be used. The individual rods shall be connected by couplings and a removable stud shall be used to drive the rods. iv. Multiple rods may also be used to reduce the ground resistance. Spacing between multiple rods shall not be less than 3 meter. b. Counterpoise i. In case of rocky areas or shallow bedrock, or where lower resistance is required than attainable with driven rods, counterpoise shall be used. ii. Counterpoise made of bare copper-clad steel wire of a minimum size of 7 No. 10 (7.77mm diameter) with a minimum length of 30 meters shall be buried at a depth of not less than 0.45 meter and shall be laid approximately straight. Counterpoise may be in single length, or may be several lengths connected at the ends or at some point away from the ends.
  • 16. PAGE NO. 16 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 iii. Where rock surfaces are encountered, the rock shall be cut 150 mm deep. The counterpoise shall be laid in the trench and then filled with concrete. iv. Other lengths or configurations may be used if their suitability is supported by a qualified engineering study. c. Pole Butt Wire Wraps i. Made electrodes shall be of grounding conductor wrapped at the pole butt previous to the setting of the pole. The ground conductor shall be of copper-clad steel conductor and shall have a continuous bare or exposed length below ground level of not less than 4 meters. ii. The size of grounding conductor wraps shall not be smaller than 7 No. 10 (7.77 mm diameter). 9.3 Methods of Connection to Electrodes 9.3.1 Buried ground connections to existing electrodes (connection of reinforcing cage to stub angle and anchor bolts) shall be made by means of approved compression/bolted type connectors, whereas connections to the made electrode shall be made by means of exothermic welding. 9.3.2 Above ground connections to steel structures shall be made by compression/bolted connectors. Connections to steel casings may be made by exothermic welding or compression/bolted type connectors whichever is more suitable. 9.3.3 For concrete encased reinforcing bar electrodes, a steel rod similar to the reinforcing bar shall be used to join, by welding, a main vertical reinforcing bar to an anchor bolt or stub angle. 10.0 GROUND RESISTANCE 10.1 Requirements The grounding electrode system may consist of one or more inter-connected electrodes and shall have a resistance to ground low enough to minimize hazards to personnel and flashovers across the insulator assemblies. Low resistance reduces the amount of insulation required to minimize flashover and it is usually more economical to reduce ground resistance than to increase insulation. 10.1.1 A structure ground resistance of 10 Ohms or less shall be obtained for 69kV, 110kV, 115kV and 132kV transmission line structures located within a distance of 1.6km from the substation.
  • 17. PAGE NO. 17 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 For a distance of more than 1.6km from the substation, the required structure ground resistance shall be 20 Ohms or less. 10.1.2 For 230 and 380 kV transmission lines, the required structure ground resistance shall be 3 Ohms or less within a distance of 3 km from the substation and 10 Ohms or less for a distance of more than 3 km from the substation for all types of soil except where rock is encountered at shallow depths for which the requirement shall be 10 ohm or less and 20 ohm or less for the respective distances of 3 km and more than 3 km. 10.2 Ground Resistance Tests The fall-of-potential method shall be used to measure the structure ground resistance along the transmission line route or right-of-way. The detailed description of this method is given in ANSI/IEEE Std.81. The megger earth tester (null balance) or clamp-on ground resistance tester shall be used in making all ground resistance measurements. 10.3 Ground Resistance of Driven Rods Structure ground resistance is an extremely important parameter in the determination of lightning flashover rates. It is a fluctuating statistical variable the magnitude of which is governed not only by geography but also by other conduction physics in the earth. The resistance of a driven ground rod has been derived as: R L n L a = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ρ π2 1 4 1 When L > a (Eq.10-8) Where: R = ground resistance, in Ohms ρ = ground resistivity, in ohm-meters L = length of rod, in meters a = rod radius, in meters The diameter of the rod is of some significance because it affects the logarithmic term 4L/a, but the length is more important. Ground resistance may be lowered by connecting driven ground rods in parallel. If the spacing between rods is great compared with the length of the individual rods, the resistance will be reduced in proportion to the number of rods. If the rods are close together, each rod will be in the intense electrical field of its neighbor, then the overall resistance (in Ohms) becomes: R L n L a = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ρ π2 1 2 (Eq.10-9)
  • 18. PAGE NO. 18 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 10.4 Ground Resistance of Counterpoise The continuous counterpoise consists of one or two conductors buried continuously under the transmission line or along certain sections of the line. The conductors are interconnected to the overhead ground wire and ground system (if any) at each transmission line supporting structure. When the counterpoise is buried close to the surface of a uniform earth, the ground resistance (in Ohms) is approximately equal to: R n ae = − ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ρ πl l 1 2 2 1 (Eq.10-10) a = radius of counterpoise, in meters e = depth of burial below existing ground level, in meters 11.0 EARTH RESISTIVITY 11.1 General The ability of a group of buried metallic conductors such as transmission structure grounds and counterpoise, to conduct current into the soil is significantly dependent on the resistivity of the soil. Earth resistivity can even affect the susceptibility of a particular location to lightning strikes. A knowledge of soil resistivities is of fundamental importance in accurate prediction of transmission line performance during ground fault or lightning conditions. It is not practical to determine soil resistivity everywhere along the route of a transmission line. There are certain circumstances which require that accurate knowledge of soil structure be determined at specific sites. At most other sites, accurate values are not necessary and an order of magnitude estimate is sufficient. Fortunately, there are usually indirect sources of information from which it is possible to secure a qualitative knowledge of the soil structure. This section is directed at the measurement and interpretation techniques most generally used to determine soil structure and resistivity. 11.2 Earth Resistivity Test Electrical resistivity tests shall be made to determine the earth resistivity along the transmission line route or right-of-way. These shall be preferably made at a number of places within the right-of-way and with different probe spacings, to get an indication of any important variations of resistivity with location and depth. The Wenner four-electrode method shall be used to measure earth resistivity. Earth resistivity tests shall be made as part of the geotechnical investigation that is metersin,secounterpoioflength:Where =l
  • 19. PAGE NO. 19 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 performed for foundation design information. If earth resistivity tests are not made, a resistivity of 1000 Ohm-meter shall be assumed. The method of measurement in general use is based on the following equation: 2222 4A 2A +1 4 BA A B AR + − + = π ρ (Eq.10-11) Where: ρ = resistivity of soil, in Ohm-meter. A = distance between adjacent electrodes, in meters. R = resistance in Ohm resulting from dividing the voltage between the potential probes by the current flowing between the current electrodes. B = depth of the electrodes, in meter. If "B" is small compared to "A", as in the case of probes penetrating the ground a short distance only, the above equation can be simplified as follows: ρ π= 2 AR (Eq.10-12) 12.0 FAULT CURRENT DISTRIBUTION 12.1 General The majority of transmission line faults are to ground and generally occur between a phase conductor and a transmission line structure, as the result of an insulator flashover. In some cases, the presence of foreign objects between a phase conductor and the overhead ground wire or a grounded structure may cause a ground fault somewhere along one span of the transmission line. Occasionally, the ground fault is caused by a phase conductor in direct contact with the overhead ground wire or the earth’s surface. In all these cases, the current return paths include earth and, therefore, present an impedance which at most is equal to the equivalent ground impedance of the grounded structures which carry the fault current. If in addition to the earth current, part of the total fault current returns to the generating sources via metallic return circuits (such as overhead ground wires), the impedance value will be even less. 12.2 Importance of Fault Current Distribution The transmission line performance at the faulted structure and/or at other locations on the transmission line is significantly influenced by the fault current distribution between the structure and the overhead ground wires connected to the structure. Figure TES-2210-0300-00 shows a typical system which illustrates this aspect. R represents the faulted structure ground resistance and Zs the equivalent power generation (source) impedance. It is also assumed that the value of Zs is higher than
  • 20. PAGE NO. 20 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 the structure resistance value R so that the total fault current (≈ V/Zs) is approximately constant when R varies within a relatively narrow range. Figure TES- 2210-0300-00 leads to the following conclusions: 12.2.1 If one of the ground wire impedances Zl or Zr is small relative to R, then very little current enters the faulted structure and the potential rise of the structure is proportional to the metallic return circuit path impedance, i.e. Zl in parallel with Zr. 12.2.2 If in contrast, R is small compared to Zl and Zr, then most of the fault current flows through the structure, causing a ground potential rise proportional to the structure ground resistance R. This simple example illustrates the importance of fault current distribution. In a real transmission line there are hundreds of grounded transmission structures. The metallic return circuits are not only bonded to the structures but also are electromagnetically coupled to the phase conductors. The ground resistance of the structures is not constant and generally varies between wide limits along the route of the transmission line. The fault current distribution is therefore more complex to calculate and will vary with the type of transmission line and location of fault on the line. 12.2.3 There are various methods which have been used for calculation of fault current distribution between the faulted structure and the metallic return conductors. One of the most suitable method is a double-sided elimination which is described in “EPRI Report EL-2699, Transmission Line Grounding”. This method shall be adopted for determining fault current distribution and consequently the current to be considered for the calculation of size of grounding conductor. Computer programs such as SPLIT and GATL may be used for solving complex analytical expressions. 12.2.4 When a ground fault occurs on a transmission line equipped with ground wires, a significant portion of the fault current is diverted to the structures on each side of the faulted structure. Consequently both the fault current and potential rise at the faulted structure are decreased. A survey by IEEE Working group 78.1 (IEEE paper F 79 632-1) conducted among various utilities around the world regarding power system grounding practices indicates that 30% to 70% of fault current is considered to flow in the substation grounding grid depending upon the overhead network configuration, location of fault and ground resistance. Typically 50% division of current between groundwires and grounding grid is used by the utilities. Similar survey was also conducted by EPRI, USA among North American Power Electric Utility Companies regarding grounding practices for overhead transmission lines. The survey indicates that majority of the utilities consider 10 to 60% of fault current to flow in the grounding grid.
  • 21. PAGE NO. 21 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 12.2.5 SEC fault current distribution study indicates that a current of about 25 to 30% of standard maximum system fault current (at the source substation) flows in the structure when the resistance of the faulted structure is one (1) ohm and the fault is located with in 3km distance from the substation. When the resistance of the faulted structures in the range of 3 to 10 ohms, the maximum current flow in the structure is about 13 to 15% of standard maximum system fault current.. For design of grounding system, 50% of standard maximum system fault current (at source substation) shall be considered for the structures located within a distance of 3km from the substation on both ends of the transmission line and 30% of standard maximum system fault current for the structures located beyond 3km from substation. 12.3 Overhead Ground Wire Overhead transmission lines shall be provided with overhead ground wires throughout their entire length. These shall be electrically bonded at each structure as well as tied solidly to the grounding system of the substation so that when a tower fault or lightning stroke does occur, the effect of the connected station grounding system shall decrease the magnitude of gradients near the tower bases. 12.3.1 Overhead ground wires terminated at steel structures shall be electrically bonded to the structure. The mechanical deadend hardware shall not be used as the electrical path for conducting lightning surges or fault currents to the deadend structure. Steel deadend structures are considered adequate for solidly grounding properly terminated overhead ground wires to the ground mat. 12.3.2 Overhead ground wires terminated at structural wood or other nonconducting material shall be electrically bonded to the ground wire. A continuous electrical path shall be made between the overhead ground wire and the station ground mat or grid with a down conductor and counterpoise. 12.3.3 Overhead ground wires with sufficient mechanical strength shall be located to shield the line conductors adequately from lightning direct strokes. 13.0 CORROSION 13.1 Cause of Corrosion The corrosion of metal is caused principally by electrochemical reactions, which are accompanied by a flow of electric current between a portion of the metal and the moisture in its environment. A metallic connection between the two metals allows current to flow from one (the cathode) to the other (the anode) and forces the ions of the anode to migrate to the electrolyte causing corrosion to the anode.
  • 22. PAGE NO. 22 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 13.2 Rate of Corrosion The amount of corrosion by electrolysis depends on the type of soil or air, the currentand the electrochemical equivalent of the metal. High resistivity soil reduces corrosion currents and thereby reduces corrosion rates. 13.3 Control of Corrosion Precautions to prevent corrosion shall include, but not be limited to, the following: 13.3.1 Grounding conductors shall be copper or copper-clad steel conductors. 13.3.2 Copper or copper clad steel ground rods shall be used. 13.3.3 Where possible, route the grounding conductor at least 6 meters away from buried steel work. 13.3.4 The use of dissimilar metals electrically connected together shall be avoided. 13.3.5 If dissimilar metals are used, these shall be bonded together by a bimetal connector and treated with proper inhibitor to reduce the rate of corrosion. 14.0 RECOMMENDED SIZES OF GROUNDING CONDUCTOR On the basis of short circuit levels established for various transmission line voltages in the system of SEC and the approach described in preceding paragraphs, sample calculations were made to determine the size of grounding conductor to be used for counterpoise arrangements. While sample calculations are included as an Appendix-I to this Standard, the recommended sizes for various transmission line voltages are given in Table 10-2. The various parameters used in the calculations are given below: Short circuit level = 63 kA for 230kV and 380kV System = 40 kA for 69kV, 110kV, 115kV and 132kV System Maximum Allowable Temperature = 300°C for bolted connections Ambient Temperature = 40°C Current flow in the grounding conductor = 50% of fault current, when fault is within 3km of substation = 30% of fault current when fault is at a distance of more than 3 km from the substation Fault clearing time = 0.50 Seconds (30 cycles)
  • 23. PAGE NO. 23 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 Conductivity of grounding conductor= 40% of IACS (International Annealed Copper Standard) Table 10-2 : Recommended Sizes of Grounding Conductor Ground Conductor Size Sr. No. Transmission Line Voltage Fault Current (kA) Current Flow in the ground conductors (kA) (AWG) Dimeter mm Application 31.5 19 No. 7 18.31 Within 3 km from Substation 1. 380kV 63 18.9 7 No.5 13.87 Beyond 3 km from substation 31.5 19 No. 7 18.31 Within 3 km from Substation 2. 230kV 63 18.9 7 No. 5 13.87 Beyond 3 km from substation 20 7 No. 5 13.87 Within 3 km from Substation 3. 132kV 40 12 7 No. 7 11.00 Beyond 3 km from substation 20 7 No. 5 13.87 Within 3 km from Substation 4. 115kV 40 12 7 No. 7 11.00 Beyond 3 km from substation 20 7 No. 5 13.87 Within 3 km from Substation 5. 110kV 40 12 7 No. 7 11.00 Beyond 3 km from substation 20 7 No. 5 13.87 Within 3 km from Substation 6. 69kV 40 12 7 No. 7 11.00 Beyond 3 km from substation
  • 24. PAGE NO. 24 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 FIG. TE-2210-0100-00 : EQUIVALENT POWER SYSTEM NETWORK
  • 25. PAGE NO. 25 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 FIG. TE-2210-0200-00 : TYPICAL GROUND POTENTIAL MITIGATING RINGS FIG. TE-2210-0300-00 : FAULT CURRENT DISTRIBUTION
  • 26. PAGE NO. 26 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 15.0 BIBLIOGRAPHY 1. ANSI/IEEE Std. 80-2000, “IEEE Guide for Safety in Substation Grounding”. 2. ANSI C2-2007, “National Electrical Safety Code”. 3. SEC Engineering Report by Chas T. Main Int. “380 kV Transmission Line Grounding Details”. 4. EPRI Report, EL-2699, “Transmission Line Grounding”. 5. Engineering Publication by Copperweld Bimetallic Group, “Copperweld Ground Wire”. 6. IEEE paper published in PAS-99, No.4, “Survey on Power System Grounding Design Practices”.
  • 27. PAGE NO. 27 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 APPENDIX I SAMPLE CALCULATIONS FOR SIZE OF COPPER-CLAD STEEL COUNTERPOISE CONDUCTORS 1. Case A: 230kV and 380kV Lines Fault current If = 63 kA Current flow in faulted structure: I = 50% of If near S/S = 31.5 kA I = 30% of If away from S/S = 18.9 kA Fault clearing time tc = 0.5 sec. (30 cycles) Maximum allowable temperature Tm = 250-350°C for bolted connections (Say 300°C average) Ambient temperature, Ta = 40°C Conductivity of conductor = 40% Acmils = 1973.52 I t P TCAP Tm Ta K Ta c r r O α . ln 10 1 4 + − + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ Acmils = 1973.52 x I Tm 05 0 00378 4 397 10 3846 1 40 245 40 4 . . . . ln × × × + − + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ Acmils = 1973.52 x I Tm 216077 1 40 285 . ln + −⎡ ⎣⎢ ⎤ ⎦⎥ (a) Fault near Substation Acmils = 197352 315 216077 1 260 285 . . . ln × + ⎡ ⎣⎢ ⎤ ⎦⎥ Acmils = 358,897, say 396 kcmil or 19 No. 7 would be appropriate.
  • 28. PAGE NO. 28 OF 28TESP122.10R0/MSO TRANSMISSION ENGINEERING STANDARD TES-P-122.10, Rev. 0 Date of Approval: February 18, 2007 (b) Fault away from Substation Acmils = 197352 18 9 216077 1 260 285 . . . ln × + ⎡ ⎣⎢ ⎤ ⎦⎥ = 215,338 say 232 kcmil or 7 No. 5 would be appropriate. 2. Case B: 69kV, 110kV, 115kV and 132kV Lines Fault Current = 40 kA Current flow in the faulted tower near the substation = 20 kA Current flow in the faulted tower away from substation = 12 kA Sizes for conditions a & b would be as below: (a) A = 227,871 cmil, say 232 kcmil or 7 No. 5 (b) A = 136,723 cmil, say 146 kcmil or 7 No.7 on Lines".