Ground potential rise at overhead AC transmission line structures during power frequency faults

1. controlling of surrounding soil potential gradient under
line fault conditions.
Scope
The scope of the Technical Brochure is to examine the
ground potential rise in the close proximity to the line
structures during phase-to-ground fault on overhead AC
transmission lines. The following areas are discussed:
• Sources of fault current, the current magnitude and
its flow through the line structure grounding system;
• Analytical models and approach for fault current and
GPR calculations;
• Soil characteristics and effect;
• Safety hazards and the acceptable levels of step and
touch voltages;
• Design consideration of potential hazard mitigation
solutions;
• Standards and industry practices associated with
specific aspects of the safety criteria;
• Line design considerations, such as structure footing
resistance, shield wire, cross-bonding, insulation,
etc., that will impact on line performance under fault
conditions;
• Impact on electrical facilities and installations
bonded to or in the vicinity of the line structure
location;
• Conductive and transfer effects associated with the
fault current;
• Practical case studies on GPR effect.
Some countries may have specific codes or regulations
that could differ from the recommendations or models
described in this document. For example, aspects related to
soil model, assumptions on design parameters or statistical
approaches may be different or not permitted in some
regulation authorities. •••
Introduction
In many urban areas, residential sub-divisions, etc.,
overhead line structures co-exist with many infrastructures.
The general public may spend a considerable amount of
time in the vicinity of the overhead line facilities. Such
locations typically arise from industrial and residential
development near older power lines where the development
may not have been foreseen when the lines were built. The
situation presents specific issues along the power line that
needs to be addressed from an electrical safety point of view
as the key design requirements.
The objective of this Technical Brochure (TB) is to raise
the awareness of potential safety hazard from touch, step
and transferred voltages due to Ground Potential Rise
(GPR) that cannot be ignored or under-estimated in the
safe operation of overhead transmission lines.
The GPR in this case is the result of ground current from
a line fault flowing through the line structure grounding
system and the surrounding soil. It is important to assess the
GPR and the associated touch, step and transferred voltage
hazards at locations where public safety is deemed to be a
concern. The grounding studies should also evaluate high
GPR that could effect on other grounding systems in close
vicinity to ensure safety.
The main goals of the grounding system designed for
the line structure and any metallic objects bonded to it
or in in close proximity is to provide a mean of diverting
fault currents into the ground without causing damage
to the facilities and to protect the general public from
the hazard of electrical shock. Effective grounding of line
facilities is typically met through proper sizing of shield
wires, maintaining effective line structure grounding and
Ground potential rise at overhead AC transmission
line structures during power frequency faults
Members
G. Watt, Convenor (CA), L. Figueroa, Secretary (FR), W. A. Chisholm (CA),
C. Crew (AU), M. Fairhurst (UK), T. De Grauw (AU), M. Janssen (NL), J. Jardini (BR),
J. Kelleher (IE), M. Kvarngren (SE), D. Liebhaber (US),
J. Lundquist (SE), D. Mateus (PT), P. Moran (IE), V. Naidoo (ZA), O. Regis (BR),
P. H. Pretorius (ZA), R. Puffer (DE), H. Stegeman (NL), C. Wang (CA)
694WG B2.56
technical brochure
No. 293 - August 2017 ELECTRA 73

2. The fault current distribution calculation is highlighted
with an example of applying to a transmission line with and
without shield wire. The presence of shield wires reduces the
zero sequence impedance and results in a higher magnitude
of fault currents. In general, the current flowing into the local
grounding/earthing through the structure at or near the fault
location is lower for the line equipped with shield wire than
without.
The GPR at a given structure location is presented by
the formula: GPR = I * R where I is the portion of the fault
current, at the given structure, flowing through the structure
togroundandRisthefootingresistanceof thegivenstructure.
The footing resistance is normally obtained by measurement.
This Chapter also briefly reviews the typical transmission
line grounding systems including a combination of ground
rod, buried conductor and concrete footings with steel
reinforcing rods. A number of empirical formulae available
for estimating the resistance of ground electrodes are shown.
Chapter 5 describes the soil characteristics which are
fundamental parameters in the design of line grounding
systems and calculating ground fault current distribution
along the transmission line.
Soil is a complex system consisting of solid, liquid and
voids. The resistivity of soil is strongly influenced by minerals
and moisture content. Soil resistivity varies not only with
composition and chemistry of soil but also with temperature,
mineral content and compactness. Consequently, soil
resistivity may vary considerably from one location to
another, as well as seasonally. To determine soil resistivity
for the line grounding design, it is necessary to perform
field measurements at multiple locations along the line. A
number of measuring techniques including the basic Wenner
four-point method to ground penetration radar suitability
method are highlighted. •••
Technical Brochure Contents
Chapter 1 sets the context of the effect of GPR and to raise
the awareness of the potential safety hazard resulting from
GPR due to faults occurring on overhead lines.
Chapter 2 introduces public safety concerns and to aid
engineers in understanding of the complex design situation
associated with line grounding problems and available
solutions.
Chapter 3 examines the source of the problem, the fault
current. The fault current distribution is influenced by
parameters that include the shield wire impedance, the
structure footing impedance, the distance between faulted
structure and the connecting substation ground/earth
electrode resistance. Typically, the fault current flows along
the line away from the fault through the shield wires.
System fault level is usually highest at line terminals
(substation) depending on the network configuration. The
dc offset current component can have an effect of increasing
the fault current magnitude.
Figure A illustrates that the fault current decreases along
the line due to the impedance of the line.
In general, the fault current magnitude is higher when the
structure footing resistance is low. The effect on GPR needs
to be studied thoroughly when lowering footing resistance.
To determine the frequency of fault, an estimation of the
probability that a fault may occur at a given structure can be
performed.
Chapter 4 gives an overview of the well-known classical
analytical model for short circuit analysis and calculation
of fault current distribution along the line using equivalent
network models.
Figure A - Variation of the fault current (3I0 = 3 times zero sequence current) along a line between two Stations, A (at
0 km) and B (at 354 km), dependent of the position of the fault
694WG B2.56
technical brochure
No. 293 - August 2017 ELECTRA 75

3. gradient control rings and mitigating against transferred
potential. This standard has to be read in conjunction with
National Normative Aspects (NNA) also named Part 2. Each
NNA is related to one specific country and reflects national
practices.
For establishing safety criteria, the fundamental
consideration is human physiological responses to electrical
shock resulted from step and touch voltages. The criteria
are defined as the tolerable or acceptable safe level of voltage
difference that can occur between a human body and a
grounded metallic structure or between the extremities of a
human body. The limits of electric current flow through the
human body are stated in IEC TS 60479-1. An assessment of
safety conditions is demonstrated by a case study.
The implementation of overhead transmission line
grounding/earthing systems should take into account the
specific locations where the structures are erected. Different
solutions will depend on the perceivable risk to individuals,
associated to ground potential rise and resulting expected step
and touch voltages.
Standards adopted by many utilities for design requirements
of transmission line grounding are:
• CENELEC EN 50341-1 European Committee for
Electrotechnical Standardization: Overhead electrical
lines exceeding AC 1 kV - Part 1: General requirements -
Common Specifications, 2012.
• IEEE Standard 80 “IEEE Guide for Safety in AC Substation
Grounding. Institute of Electrical and Electronics
Engineers,” 2000.
• DIN VDE 0141/7.76 Verband Deutscher Elektrotechniker,
1976.
• EATS 41-24 Electricity Association, TS 41-24: Guidelines
for the Design, Installation.
The risk concerning events that pose problems of electrical
safety related to GPR can be analysed based on a multitude
of assumptions using either a deterministic or a probabilistic
approach. The deterministic approach is commonly followed
to assess electrical shock safety criteria. It involves assumptions
which contain parameters of statistical nature.
The probabilistic method identifies risky situations for
human and then estimates the likelihood of a dangerous
event occurring when an individual or a group of individuals
is present. The probabilistic approach is to calculate the
probability distributions of system conditions and potential
ground fault events which are subject to random variations of
the system performance and operation history. The tolerable
step and touch voltages are functions of various factors, such
as body impedance, duration of exposure, etc., that influence
the human physiological response that may lead to fatality. All
these parameters can be treated as random variables with certain
probability distributions rather than deterministic values.
When the probability distributions of the system •••
Soil modeling is a methodology to mathematically
represent thelocalsoilprofilefordesigningagroundingsystem.
For simplicity, the soil can be considered homogeneous but in
reality, it usually is multi-layered because geological features
cause soil types to vary from location to location, and the
presence of bedrock or ground water can result in significant
changes in resistivity as a function of depth.Therefore,a multi-
layer soil model should be used for the line grounding design.
Chapter 6 provides an overview of safety considerations
with respect to hazards of step, touch and transferred voltages.
Public safety is paramount to the safe operation of overhead
transmission lines. Safety criteria, safety standards and
practices, and risk assessment are also discussed.
The area surrounding a structure during a fault is subjected
to a temporary surface ground potential rise. Unless proper
precautions are taken in design, the maximum potential
gradients along the ground surface may be of sufficient
magnitude during the fault to become a safety hazard to people
in the area.
The circumstances that create potential electrical shock
hazard are identified as follow:
• Relatively high fault current to ground in relation to the
area of the grounding system and its resistance to remote
ground;
• Soil resistivity and distribution of ground fault currents
such that high potential gradients may occur at points on
the ground surface;
• Presence of human at such a point, time and position
that the body is bridging two points of high potential
difference;
• Absence of such contact resistance or other series
resistance to limit current through the body to a safe
value;
• Duration of the fault and human contact, and hence, the
flow of current through the human body for a sufficient
time to cause harm.
The relative infrequency of safety hazard incidence is
due largely to the low probability of coincidence of all the
unfavorable conditions listed above.
The design criteria for quantifying safety considerations
include the predicted Surface Ground Potential Rise (SGPR) in
conjunction with the probability of occurrence and occupancy
(remoteness of structure). The aforementioned criteria are
used to assess the risk of electrical safety hazard. The risks are
scrutinized using measures such as touch, step and transferred
potentials. These measures are further explained in this
Chapter.
The CENELEC standard EN 50341-1 dealing with general
requirements for overhead line design requires that touch
voltages are limited to certain permissible levels at pre-defined
types of locations, i.e., Often Frequented Structures (OFT’s).
However, conflicts may occur between installing ground
694WG B2.56
technical brochure
No. 293 - August 2017 ELECTRA 77

4. typically flow through the shield wires. However, a significant
portion still flows into ground through ground electrodes.
The provision of an effective grounding system on
overhead lines can be challenging and costly. The design
of line grounding is primarily determined by the lightning
performance requirement of the line. However, GPR due to
power frequency line faults could dictate the line grounding
system design and layout requirements to limit step and touch
voltages.
The safety risk mitigation may require stringent measures
at locations where public access is frequent and to a lesser
degree when the access is extremely rare. The GPR mitigation
measures should be designed to ensure any potential hazards
be kept to acceptable levels.
There are plenty technical resources already available
for a variety of ground electrode designs suitable for GRP
mitigation. Other guides and standards listed in the Reference
section, provide further details on managing fault currents and
coordinating power system fault current with nearby railroads,
pipelines, or telecommunication facilities.
One of the crucial roles of transmission line grounding is to
minimizetheelectricalsafetyriskexposure.Designs,procedures
and operations are governed by standards, regulations and
company policy. Power utilities are responsible for establishing
design standards and practices for their organizations to meet
the specific statutory needs and environment. 
conditions (line fault) and the tolerable step and touch voltages
are available, the probability of exceeding the tolerable voltages
can be evaluated.This hazard probability and the probability of
human presence near a line structure location or a line section
where the line fault would occur are convolved to ascertain the
overall safety risk probability.
This Chapter also examines the level of risk to the general
public and utility workers from touch and step voltages and
transferred potentials. The ALARP (As Low as Reasonably
Practicable) concept and risk assessment are discussed.
Chapter 7 describes line design considerations for GPR
mitigation. Typical mitigation methods for step, touch and
transferred voltages hazard include but are not limited to the
following:
• System fault current level reduction;
• Fault clearing time reduction;
• Shield wire installation;
• Structure footing impedance reduction;
• Grounding grid installation;
• Ground surface treatment;
• Physical barrier installation;
• Insulated structural member of tower;
• Isolation of voltage transfer device.
Two practical case studies are presented to illustrate the
actual design considerations.
Chapter 8 presents the concept of conductive coupling
and its impacts. The potential rise of the electrode and of
the neighbouring soil with respect to remote ground occurs
due to the fault current. The magnitude of the conductive
interference depends on the fault current and line grounding
impedance plus a number of factors including separation
distance between grounding electrode and installations,
grounding of the installations and electrical characteristics of
the surrounding soil.
The various mechanisms by which the conductive coupling
canaffectvariousinstallationsarebrieflydiscussedandmeasures
to reduce the conductive coupling are purposed. The measures
used to reduce the conductive coupling should be so chosen that,
they are technically, economically and socially viable.
Annex A provides supplementary information on public
facilities that are built close to transmission line structures.
Annex B presents a case study of mapping GPR on site. The
basic line parameters calculations is shown in Annex C.
Conclusions
Line phase-to-ground faults can result in high current flow
inthegroundingnetwork.Themajorityof thefaultcurrentwill
BROCHURE N° 694
(en anglais seulement)
(in English only)
Disponible sur / Available on:
www.e-cigre.org
Prix non-Membres / Non-Member Price:
160 e
Purchase (non-members)
Free download (members)
694WG B2.56
technical brochure
No. 293 - August 2017 ELECTRA 79