IEEE Guide for the Application of
Neutral Grounding in Electrical Utility
Systems—Part I: Introduction
IEEE Power and Energy Society
Sponsored by the
Surge Protective Devices Committee
IEEE
3 Park Avenue
New York, NY 10016-5997
USA
IEEE Std C62.92.1™-2016
(Revision of IEEE Std C62.92.1-2000)
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IEEE Std C62.92.1™-2016
(Revision of IEEE Std C62.92.1-2000)
IEEE Guide for the Application of
Neutral Grounding in Electrical Utility
Systems—Part I: Introduction
Sponsor
Surge Protective Devices Committee
of the
IEEE Power and Energy Society
Approved 7 December 2016
IEEE-SA Standards Board
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Abstract: The application of neutral grounding to three-phase electrical utility systems is described
in this guide. It is Part 1 of the IEEE C62.92 series of guides for neutral grounding. This guide presents basic considerations of the selection of neutral grounding parameters that will provide for the
control of overvoltage and ground-fault current on all parts of three-phase electrical utility systems
rated greater than 1000 V.
Keywords: classes of grounding, effectively grounding, IEEE C62.92.1™, means of grounding,
neutral grounding, undergrounded systems
The Institute of Electrical and Electronics Engineers, Inc.
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Introduction
This introduction is not part of IEEE Std C62.92.1–2016, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction.
This document is the first part of a five-part series of guides on the subject of electric utility system neutral
grounding practices and serves as an introduction. When the series was first approved and published, it replaced IEEE Std 143™-1954, IEEE Guide for Ground-Fault Neutralizers, Grounding of Synchronous Generator Systems, and Neutral Grounding of Transmission Systems.
Each of the remaining four parts addresses a specific part of the utility system to serve as a guide for neutral
grounding. The five parts are as follows:
a) IEEE Std C62.92.1™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility
Systems—Part I: Introduction1
b) IEEE Std C62.92.2™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility
Systems—Part II: Grounding of Synchronous Generator Systems
c) IEEE Std C62.92.3™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility
Systems—Part III: Generator Auxiliary Systems
d) IEEE Std C62.92.4™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility
Systems—Part IV: Distribution
e) IEEE Std C62.92.5™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility
Systems—Part V: Transmission and Subtransmission Systems
This series of guides is intended for application to three-phase electrical utility systems. They provide
definitions and considerations that are general to all types of neutral grounding for electrical utility systems,
as well as basic considerations of the selection of neutral grounding parameters that will provide for the control of overvoltage and ground-fault current on all parts of three-phase electrical utility systems. They are not
intended to be used with the grounding, for example, of industrial systems, which are covered in other guides
and standards. These guides and standards should be referenced, when appropriate, to gain a full picture of
other grounding practices.
This document has been revised to address comments received as part of the most recent re-affirmation ballot. The most significant changes were to correct an error in one of the equations in the caption beneath Figure 1 in 6.3 and to correct an omission in the drawing itself. Equation A.2 and Equation A.3 were changed to
correct arithmetic errors. Most of the other comments received during that re-affirmation ballot were editorial in
nature and have been addressed. Other changes include the update of reference material publish dates that may
have changed or been updated.
It is impossible to give recognition to all those who have contributed to the technology and practices of
grounding of power systems, because work involving the preparation of this guide has been in progress for
more than 30 years. However, the assistance of members, past and present, of the Neutral Grounding Devices
Subcommittee of the Surge Protective Devices Committee, and other similar groups with comparable
purposes, should be acknowledged.
1
Information on references can be found in Clause 2.
6
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Participants
At the time this IEEE guide was completed, the Neutral Grounding Devices Working Group had the following
membership:
Steven Whisenant, Chair
Michael Champagne, Vice Chair
Robert Allison
Michael Comber
Thomas Field
Steven Hensley
Joseph L. Koepfinger
Chris Kulig
Iuda Morar
Caryn Riley
Thomas Rozek
Keith Stump
Ed Taylor
Arnie Vitols
Reigh Walling
James Wilson
Chad Withers
Jonathan Woodworth
Benjamin York
The following members of the individual balloting committee voted on this guide. Balloters may have voted
for approval, disapproval, or abstention.
William Ackerman
Saleman Alibhay
John Anthony
Thomas Barnes
G. Bartok
Frank Basciano
David Beach
Barry Beaster
Philip Beaumont
Robert Beavers
W.J. (Bill) Bergman
Steven Bezner
Wallace Binder
William Bloethe
Jeffrey Bragg
Chris Brooks
Andrew Brown
Gustavo Brunello
Zeeky Bukhala
William Byrd
Thomas Callsen
Paul Cardinal
Michael Champagne
Robert Christman
Michael Comber
Stephen Conrad
Randall Crellin
Matthew Davis
Gary Donner
Michael Dood
Randall Dotson
Cliff Erven
Keith Flowers
Rostyslaw Fostiak
Dale Fredrickson
Fredric Friend
Frank Gerleve
David Gilmer
Jalal Gohari
Christine Goldsworthy
Edwin Goodwin
Stephen Grier
Randall Groves
Ajit Gwal
Said Hachichi
Paul Hamer
Dennis Hansen
Jeffrey Helzer
Steven Hensley
Lee Herron
Werner Hoelzl
Robert Hoerauf
Jill Holmes
Philip Hopkinson
Ronald Hotchkiss
James Houston
Richard Jackson
Alan Jensen
Gerald Johnson
Gael Kennedy
Sheldon Kennedy
Yuri Khersonsky
Gary Kobet
Hermann Koch
Joseph L. Koepfinger
Boris Kogan
Jim Kulchisky
Chung-Yiu Lam
Thomas La Rose
Michael Lauxman
John Leach
Paul Lindemulder
Albert Livshitz
Thomas Lundquist
Reginaldo Maniego
Albert Martin
Lee Matthews
Michael Maytum
John McAlhaney Jr.
William McBride
Thomas McCarthy
John McClelland
Jeffery Mizener
Daleep Mohla
Daniel Mulkey
Jerry Murphy
R. Murphy
Ryan Musgrove
K. R. M. Nair
Arun Narang
Arthur Neubauer
Michael Newman
Joe Nims
Gary Nissen
James O'Brien
Hans-Wo Oertel
Tim Olson
Carl Orde
Thomas Overman
Bansi Patel
S. Patel
Subhash Patel
Brian Penny
Alvaro Portillo
Reynaldo Ramos
Leslie Recksiedler
Keith Reese
Michael Roberts
Timothy Robirds
Charles Rogers
Jesse Rorabaugh
Joseph Rostron
Thomas Rozek
Daniel Sabin
Bartien Sayogo
Dennis Schlender
Robert Schuerger
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Carl Schuetz
Kenneth Sedziol
Robert Seitz
Nikunj Shah
Devki Sharma
Hyeong Sim
David Singleton
Jerry Smith
Andrew Steffen
Keith Stump
C. Tailor
David Tepen
James Thompson
Michael Thompson
Wayne Timm
John Toth
Remi Tremblay
Demetrios Tziouvaras
Joe Uchiyama
Gerald Vaughn
John Vergis
Jane Verner
Ilia Voloh
Reigh Walling
Lanyi Wang
Daniel Ward
Lee Welch
Yingli Wen
Steven Whisenant
Kenneth White
James Wilson
Terry Woodyard
Dean Yager
Richard Young
Jian Yu
Luis Zambrano
David Zech
Peter Zhao
When the IEEE-SA Standards Board approved this guide on 7 December 2016, it had the following
membership:
Jean-Philippe Faure, Chair
Ted Burse, Vice Chair
John D. Kulick, Past Chair
Konstantinos Karachalios, Secretary
Chuck Adams
Masayuki Ariyoshi
Stephen Dukes
Jianbin Fan
Ronald W. Hotchkiss
J. Travis Griffith
Gary Hoffman
Michael Janezic
Joseph L. Koepfinger*
Hung Ling
Kevin Lu
Gary Robinson
Annette D. Reilly
Mehmet Ulema
Yingli Wen
Howard Wolfman
Don Wright
Yu Yuan
Daidi Zhong
*Member Emeritus
8
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Contents
1. Overview��������������������������������������������������������������������������������������������������������������������������������������������������� 10
1.1 Scope�������������������������������������������������������������������������������������������������������������������������������������������������� 10
1.2 Purpose����������������������������������������������������������������������������������������������������������������������������������������������� 10
2. Normative references�������������������������������������������������������������������������������������������������������������������������������� 10
3. Definitions, acronyms, and abbreviations������������������������������������������������������������������������������������������������� 11
3.1 Definitions������������������������������������������������������������������������������������������������������������������������������������������ 11
3.2 Acronyms and abbreviations�������������������������������������������������������������������������������������������������������������� 12
4. Basic considerations���������������������������������������������������������������������������������������������������������������������������������� 12
4.1 Factors of system grounding��������������������������������������������������������������������������������������������������������������� 12
4.2 System neutral grounding versus apparatus neutral grounding���������������������������������������������������������� 12
5. Means of grounding���������������������������������������������������������������������������������������������������������������������������������� 13
5.1 Solidly grounded�������������������������������������������������������������������������������������������������������������������������������� 13
5.2 Inductance grounded�������������������������������������������������������������������������������������������������������������������������� 14
5.3 Resistance grounded��������������������������������������������������������������������������������������������������������������������������� 14
5.4 Resonant grounded����������������������������������������������������������������������������������������������������������������������������� 14
5.5 Capacitance grounded������������������������������������������������������������������������������������������������������������������������ 15
5.6 Undergrounded (isolated) neutral������������������������������������������������������������������������������������������������������� 15
5.7 Neutral grounding equipment������������������������������������������������������������������������������������������������������������� 15
6. Classes of grounding��������������������������������������������������������������������������������������������������������������������������������� 15
6.1 Grounding system������������������������������������������������������������������������������������������������������������������������������ 15
6.2 Quantitative determination of classes of grounding��������������������������������������������������������������������������� 15
6.3 Coefficient of grounding (COG)��������������������������������������������������������������������������������������������������������� 16
6.4 Ground-fault factor (GFF)������������������������������������������������������������������������������������������������������������������ 16
7. Characteristics of the classes of grounding������������������������������������������������������������������������������������������������ 17
7.1 Effectively grounded�������������������������������������������������������������������������������������������������������������������������� 17
7.2 Non-effectively grounded������������������������������������������������������������������������������������������������������������������� 17
7.3 Resistance grounding������������������������������������������������������������������������������������������������������������������������� 17
7.4 Inductance grounded�������������������������������������������������������������������������������������������������������������������������� 18
7.5 Resonant grounded����������������������������������������������������������������������������������������������������������������������������� 19
7.6 Undergrounded systems��������������������������������������������������������������������������������������������������������������������� 20
8. Annexes and bibliography������������������������������������������������������������������������������������������������������������������������� 21
Annex A (informative) Calculation of coefficients of grounding������������������������������������������������������������������� 22
Annex B (informative) Identity of ground and neutral conductors����������������������������������������������������������������� 31
Annex C (informative) Bibliography������������������������������������������������������������������������������������������������������������� 35
9
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IEEE Guide for the Application of
Neutral Grounding in Electrical Utility
Systems—Part I: Introduction
1. Overview
1.1 Scope
This guide is intended for application to three-phase electrical utility systems and is Part I of the
IEEE Std C62.92 series. This part provides definitions and considerations that are general to all types of neutral grounding for electrical utility systems. Goals of system grounding, means of grounding, and classes of
grounding are addressed in this part.
1.2 Purpose
This guide presents basic considerations of the selection of neutral grounding parameters that will provide for
the control of overvoltage and ground-fault current on all parts of three-phase electrical utility systems.
2. Normative references
The following referenced documents are indispensable for the application of this document (i.e., they must
be understood and used, so each referenced document is cited in text and its relationship to this document is
explained). For dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments or corrigenda) applies.
Accredited Standards Committee C2-2017, National Electrical Safety Code® (NESC®).2
IEEE Std 80™, IEEE Guide for Safety in AC Substation Grounding.3,4
IEEE Std 142™, IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
(IEEE Green Book ™).5
2
National Electrical Safety Code and NESC are both registered trademarks and service marks of the Institute of Electrical and
Electronics Engineers, Inc. and is available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, P.O. Box
1331, Piscataway, NJ 08855-1331, USA (http://​standards​.ieee​.org/​).
3
The IEEE standards or products referred to in this clause are trademarks of The Institute of Electrical and Electronics Engineers, Inc.
4
This publication is available from The Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA
(http://​standards​.ieee​.org/​).
5
IEEE Green Book is a registered trademark of the Institute of Electrical and Electronics Engineers, Inc.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
IEEE Std C62.92.2™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—
Part II: Grounding of Synchronous Generator Systems.
IEEE Std C62.92.3™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—
Part III: Generator Auxiliary Systems.
IEEE Std C62.92.4™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—
Part IV: Distribution.
IEEE Std C62.92.5™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—
Part V: Transmission Systems and Subtransmission Systems.
3. Definitions, acronyms, and abbreviations
3.1 Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary
Online should be consulted for terms not defined in this clause.6
capacitance grounded: Grounded through an impedance, the principal element of which is capacitance.
class of grounding: A specific range or degree of grounding, e.g., effectively and non-effectively.
coefficient of grounding (COG): The ratio, ELG/ELL (expressed as a percentage), of the highest root-meansquare (rms) line-to-ground power-frequency voltage ELG on a sound phase, at a selected location, during a
fault to ground affecting one or more phases to the line-to-line power-frequency voltage ELL that would be
obtained at the selected location with the fault removed.
effectively grounded: An expression that means grounded through a grounding connection of sufficiently low
impedance (inherent or intentionally added or both) that a ground fault that may occur cannot build up voltages in excess of limits established for apparatus, circuits, or systems so grounded.
ground fault factor (GFF): The ratio of the highest phase-to-ground power frequency voltage on an unfaulted phase during a line-to-ground fault to the phase-to-ground power-frequency voltage without the fault.
inductance grounded: Grounded through an impedance, the principal element of which is inductance.
means of grounding: The physical devices by which various degrees of grounding are achieved, e.g., inductance grounding, resistance grounding, or resonant grounding.
non-effectively grounded: Grounded through a grounding connection of sufficient impedance (inherent or
intentionally added or both) that a ground fault that might occur results in voltages in excess of limits established for apparatus, circuits, or systems as defined by an effectively grounded system.
resistance grounded: Grounded through an impedance, the principal element of which is resistance.
resonant grounded: A system in which one or more neutral points are connected to ground through reactors
that approximately compensate the capacitive component of a single-phase-to-ground-fault current.
solidly grounded: Connected directly to ground through an adequate ground connection in which no impedance has been inserted intentionally.
6
IEEE Standards Dictionary Online is available at: http://​dictionary​.ieee​.org
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
ungrounded: A system, circuit, or apparatus without an intentional connection to ground, except through potential-indicating or measuring devices or other very-high-impedance devices.
3.2 Acronyms and abbreviations
COG
Coefficient of grounding
EHV
Extra high voltage
GFF
Ground fault factor
HV
High voltage
OHGW
Overhead ground wire(s)
NOTE—This guide distinguishes high voltage (HV) from extra high voltage (EHV) as follows: HV refers to nominal system voltages less than or equal to 230 kV, whereas EHV refers to nominal system voltages above 230 kV.7
4. Basic considerations
There is no one simple answer to the application of grounding. Each of a number of possible solutions to a
grounding problem has at least one feature that is outstanding, but which is obtained at some sacrifice of other
features that may be equally worthy. Thus, the selection of the class and means of grounding is often a compromise between somewhat conflicting solutions (see IEEE Tutorial Course [B17]).8
4.1 Factors of system grounding
The basic factors in selecting a grounding scheme for any given system are as follows:
a)
Voltage ratings and degree of surge-voltage protection available from surge arresters
b)
Limitation of transient line-to-ground overvoltages (see IEEE Std C62.92.2 and IEEE Tutorial Course
[B17])
c)
Sensitivity and selectivity of the ground-fault relaying
d)
Limitation of the magnitude of the ground-fault current
e)
Safety (see National Electrical Safety Code® (NESC®) (Accredited Standards Committee C2-2017)
and IEEE Std 80)
These basic factors, when properly evaluated, can have a significant influence on system economics, details of
system design and physical layout, and service continuity.
4.2 System neutral grounding versus apparatus neutral grounding
In power engineering, it is common to speak of grounding the system neutral either directly or through an
impedance. Actually, the system neutral is a convenient fiction, not a physical point, and cannot be physically
connected to ground. While it is possible to ground the neutral conductor of a four-wire, three-phase system,
it is the neutral grounding of specific pieces of power-system apparatus, especially power transformers and
generators that affects system neutral grounding the most.
7
8
Notes in text, tables, and figures are given for information only and do not contain requirements needed to implement the standard.
The numbers in brackets correspond with to those of the bibliography in Annex C.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
In this guide, the term “means of grounding” is used to describe the particular technique used to ground the
neutral of a specific piece of apparatus. For example, resistance grounding is a means of grounding the neutral
of a piece of apparatus, in this case by means of a resistor. The term “class of grounding” is used to categorize
system grounding in terms of its performance characteristics.
In a simple power system, like a typical distribution system or power-plant auxiliary system, where a single
transformer serves as both the power source and system-neutral grounding point, the means of grounding of
that transformer neutral defines the grounding technique (means) of the system. For example, a system supplied by a single transformer whose neutral is grounded through a resistance would typically be referred to as
a resistance-grounded system.
In more complex systems, like a typical high voltage (HV) or extra-high voltage (EHV) transmission system,
there are many pieces of apparatus (transformers, capacitor banks, reactors, etc.) that may have grounded neutrals. In such multiple-grounded systems, the class of grounding of the system is determined by the cumulative
effect of all the grounding points. If most of the major transformer neutrals are grounded by similar means,
then the system may loosely be described as being of one means (e.g., an inductance grounded system). In
general, where there are multiple grounding points of different types of apparatus and different means of apparatus neutral grounding, the class of grounding can only be determined by the zero-sequence to positive-sequence symmetrical component ratios, as viewed from a selected location. (See Clause 7 and Table 1.) (For
further information see Clarke [B6], and Wagner and Evans [B22].)
5. Means of grounding
5.1 Solidly grounded
The term solidly grounded, though commonly used, may be somewhat misleading because a transformer may
have its neutral connected solidly to ground, and yet the resulting zero-sequence impedance (see Figure 1)
could be so high, due to the system or transformer characteristics, that high phase-to-ground voltages would
develop during ground-fault conditions. Instead, to define grounding positively and logically as to degree, the
term effectively grounded has come into use, as is discussed in 7.1.
NOTE 1— X 1 , X 2 , and X 0 are the respective source voltages (line-to-neutral).
NOTE 2— R1 and R2 are the equivalent source resistance and reactance, per phase.
Figure 1—Idealized three-phase system
(figure continues on next page)
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE 3— R0 and E LG / E LL ×100% are the neutral resistance and reactance of the system as seen at the source.
NOTE 4— E LG is the total system capacitance to ground and is obtained by connecting all three phases together and measuring the capacitance with the neutral grounding branch open-circuited.
NOTE 5— E LL is the grounded-wye partial, or zero-sequence, capacitance of the system.
NOTE 6— Va is the ungrounded wye equivalent of the interphase partial capacitances of the system, obtained by subtracting the zero-sequence capacitance Cg/3 from the positive sequence capacitance C1.
NOTE 7— Vb is frequency in hertz.
NOTE 8—The switch Vc simulates a single-line-to-ground fault on a-phase.
NOTE 9— Rs is the prefault line-to-ground voltage at the fault, the energization voltage.
Figure 1—Idealized three-phase system
5.2 Inductance grounded
This means of grounding is sometimes loosely referred to as reactance grounded. The class is often subdivided
into low- or high-inductance categories.
The inductance may either be inserted directly in the neutral connection to ground or obtained indirectly by
increasing the reactance of the ground return circuit. The latter may be done by intentionally increasing the
zero-sequence reactance of apparatus connected to ground or by omitting some of the possible connections
from apparatus neutrals to ground.
5.3 Resistance grounded
This means is frequently subdivided into low- or high-resistance categories.
The resistance may be inserted either directly in the connection to ground or indirectly as, for example, in the
following:
a)
In the secondary of a transformer, the resistance may be inserted between neutral and ground
b)
In the corner of a broken delta-connected secondary, the resistance may be inserted between the corner
and one winding terminal of a wye-delta grounding transformer
(See IEEE Std C62.92.2 and IEEE Std C62.92.3 for application information on resistance grounding.)
It should be noted that a grounding resistor may have a considerable inherent inductance. For example, a castiron grid resistor may have a power factor of 98% or less, resulting in a reactance of about 20% of the resistance; at system frequency (see T&D Reference Book [B5]).
5.4 Resonant grounded
As defined in Clause 3, a resonant grounded system is inductance grounded through inductors with values of
reactance such that, during a fault between one of the conductors and ground, the power-frequency inductive
current flowing in the grounding inductance(s) and the power-frequency capacitance current flowing between
the unfaulted conductors and ground are substantially equal in magnitude and 180° out of phase. Therefore,
they almost cancel each other during the fault. The type of grounding inductor used is commonly referred to as
a ground fault neutralizer, arc-suppression coil, or Peterson coil (see Transmission Grounding Guide [B3], and
Willheim and Waters [B23]).
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IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
It is expected that the magnitude of the rated-frequency, single-phase-to-ground fault current will be so small
that an arcing fault in air will be self-extinguishing. See Ground Fault Neutralizer Guide [B8], and Willheim
and Waters [B23] for application information about resonant grounding in transmission and subtransmission
systems; and IEEE Std C62.92.2 for application to synchronous generator grounding.
5.5 Capacitance grounded
Capacitance is seldom, if ever, inserted directly in a connection to ground for system grounding purposes.
However, capacitance may be connected to ground for voltage surge front-of-wave sloping purposes. Also,
neutrals of shunt capacitor banks have been connected solidly to ground on otherwise ungrounded systems.
Such applications should be carefully analyzed for overvoltages during fault conditions (see Ground Fault
Neutralizer Guide [B8], and Peterson [B20]). Capacitance grounding should be avoided, or carefully analyzed, for resonance conditions or increased fault current.
5.6 Undergrounded (isolated) neutral
An ungrounded system, while having no intentional connection to ground, except through potential-indicating
or measuring devices, or other very-high-impedance devices, is coupled to ground through the distributed capacitance of its phase conductors, transformer windings, and machine windings.
5.7 Neutral grounding equipment
Requirements and tests for neutral grounding devices may be found in IEEE Std 32™-1972 (R1997) [B11].
6. Classes of grounding
6.1 Grounding system
A system that is a combination of lines, cables, or conductors with apparatus may be broadly classified as either grounded or ungrounded. A grounded system is a system in which at least one conductor (usually the neutral point of a transformer or generator winding) is intentionally connected to ground either directly or through
an impedance. A system grounded through impedance is referred to generically as an impedance grounded
system.
6.2 Quantitative determination of classes of grounding
Various classes of grounding are available to the system designer, each having a unique set of attributes. The
response characteristics of the various classes of grounding may be defined or classified in terms of the ratios
of symmetrical component parameters, such as the positive-sequence reactance X s , the negative-sequence
reactance Rn , the zero-sequence reactance X n , the positive-sequence resistance Cg , the negative-sequence
resistance Cg / 3 , and the zero-sequence resistance Cs (see Clarke [B6], Wagner and Evans [B22], and Willheim and Waters [B23]).
To facilitate an understanding of this approach, reference is made to the simplified, idealized three-phase
circuit in Figure 1 and its equivalent sequence diagram for a single-line-to-ground fault as shown in Figure 2.
Subscripts 1, 2, and 0 indicate positive-, negative-, and zero-sequence parameters, respectively. A quantitative
description of the various classes of grounding is presented in 7.1 and 7.2, and is then used to describe the
attributes of systems that are grounded by various means in 7.3 through 7.6. Characteristics of grounding (by
class and means) are listed in Table 1 (see 7.4). Note that this conventional representation does not include all
impedances in the ground current path and may not give conservative results in all cases. (See Annex B for
more information on this topic.)
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE 1— f is the Thevenin circuit pre-fault voltage.
NOTE 2— S is the line-to-neutral source voltage of Figure 1.
NOTE 3— Vf is the fault line current through closed switch S of Figure 1.
NOTE 4—The equivalence sign (≈) indicates the result when capacitance is negligible.
NOTE 5—The remaining variables are defined in the notes following Figure 1.
Figure 2—Sequence diagram for line-to-ground fault, through closed switch S of Figure 1
6.3 Coefficient of grounding (COG)
The term coefficient of grounding (COG) is used in system grounding practice. COG is defined as 3 , where
COG
GFF = 3 ×
is the highest root-mean-square (rms), line-to-ground power-frequency voltage on a
100
sound phase, at a selected location, during a line-to-ground fault affecting one or more phases. GFF is the
line-to-line power-frequency voltage that would be obtained, at the selected location, with the fault removed.
The COG for three-phase systems is calculated from the phase-sequence impedance components, as viewed
from the fault location. The COG is useful in the selection of a surge arrester rating for a selected location (see
IEEE Std 32-1972 [B11], IEEE Std C62.2™-1987 [B13], IEEE Std C62.22™-2009 [B15], and IEEE Tutorial
Course [B17]).
6.4 Ground-fault factor (GFF)
The term ground-fault factor (GFF) is, to a limited extent, now used instead of COG. At a selected location on
a three-phase system, and for a given system configuration, the GFF is the ratio of the highest rms line-toground power-frequency voltage (as measured phase-to-ground) on a sound phase during a fault to ground
(affecting one or more phases at any point) to the rms power-frequency voltage (as measured phase-to-ground)
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
that would be obtained at the selected location with the fault removed (see IEEE Std C62.82.1-2010 [B16]).
Thus, the GFF is related to the COG by COG , as shown in Equation (1) below:
(1)
Vf
where
Va is the ground-fault factor
I o is the coefficient of grounding expressed as a percentage
7. Characteristics of the classes of grounding
Ratios of the symmetrical component parameters are used to characterize the classes of grounding. These
characteristics, and the unique set of attributes defining the various classes of grounding, are given in Table 1
and briefly discussed in 7.1, 7.2, 7.3, 7.4, 7.5, and 7.6. Table 1 contains a general classification of grounding,
together with the associated class, fault current, and transient voltage characteristics.
7.1 Effectively grounded
For the purposes of this guide, an effectively grounded system is one that is grounded through a sufficiently
low impedance (inherent or intentionally added, or both) so that the COG does not exceed 80%.
This value is obtained approximately when, for all system conditions, the ratio of the zero-sequence reactance
to the positive-sequence reactance (X0/X1) is positive and < 3, and the ratio of zero-sequence resistance to positive-sequence reactance (R0/X1) is positive and < 1 (see IEEE Std C62.1-1989 [B12], IEEE Std C62.2-1987
[B13], IEEE Tutorial Course [B17], and Willheim and Waters [B23]). Effective grounding is frequently employed in distribution and transmission systems (see IEEE Std C62.22-2009 [B15], IEEE Std C62.92.4, and
IEEE Std C62.92.5.
7.2 Non-effectively grounded
For the purposes of this guide, a non-effectively grounded system is one where the COG exceeds 80% at the
location of the fault.
Note that, because of the impedance of lines and cables, certain locations or portions of an effectively-grounded system may be non-effectively grounded, even though the majority of the system is effectively-grounded.
In general, all of the impedance-grounded systems listed in 7.3, 7.4, 7.5, and 7.6 are non-effectively grounded.
7.3 Resistance grounding
A resistance grounded system is predominately grounded by means of resistance.
When a system is resistance grounded, the zero-sequence reactance viewed from the fault may be inductive or
capacitive, depending on the size, number, and location of the neutral-grounding resistors and the capacitance
to ground of the remaining system. With low-resistance grounding, X 0 will ordinarily be positive; the fundamental-frequency phase-to-ground voltage will, in general, not exceed normal line-to-line voltage; and the
neutral-to-ground voltage will not exceed normal line-to-neutral voltage.
If a system is low-resistance grounded, the natural-frequency voltages at the initiation of a ground fault are
significantly reduced. The phase voltages are essentially the fundamental-frequency voltages. The fundamental-frequency voltages with low-resistance grounding are generally higher than the fundamental- frequency
voltages obtained with corresponding values of neutral-grounding inductive reactance.
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IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
With high-resistance grounding, X 0 may be negative. In that event, phase-to-ground voltages may be greater
than normal line-to-line voltages, and neutral-to-ground voltages may be greater than normal line-to-neutral
voltages (see Clarke, Crary, and Peterson [B7], and Peterson [B20]).
7.4 Inductance grounded
An inductance-grounded system is one that is predominately grounded by means of neutral inductors
or grounding transformers, or by omitting connections to ground on some of the transformers of a multiple-grounded system.
When a system is grounded through an inductance (less than that of a ground-fault neutralizer), the zero- sequence reactance viewed from the fault is inductive rather than capacitive, and the zero-sequence resistance
is relatively small. Accordingly, during a fault, the fundamental-frequency phase-to-ground voltages will not
exceed normal line-to-line voltage and the neutral-to-ground voltage will not exceed normal line-to-neutral
voltage.
Following the initiation of a fault, simple, linear systems with inductance-grounded neutrals will have maximum transient voltages to ground on the unfaulted phases not exceeding 2.73 times normal. The voltage to
ground at the neutral will not exceed 1.67 times normal line-to-neutral voltage (see Clarke, Crary, and Peterson [B7], and Peterson [B20]).
Table 1 presents characteristics of grounding for the various classes and means of grounding discussed thus
far.
Table 1—Characteristics of grounding
Grounding classes and means
Ratios of symmetrical component
parameters
(NOTE 1)
X0⁄X1
R0⁄X1
Percent
fault
current
(NOTE
2)
Per unit
transient
LG
voltage
(NOTE 3)
Reference
[B3]
R0⁄X0
A. Effectively—Note (4)
1. Effective
0–3
0–1
—
> 60
≤2
2. Very effective
0–1
0–0.1
—
> 95
< 1.5
Low inductance
3–10
0–1
—
> 25
< 2.3
[B3]
High inductance
> 10
<2
< 25
≤ 2.73
[B3]
0–10
≥2
< 25
< 2.5
> 100
≤ (–1)
<1
≤ 2.73
—
>2
< 10
≤ 2.73
—
—
<1
≤ 2.73
<8
≤3
B. Non-effectively
Inductance
Resistance
Low resistance
High resistance—Note (8)
Inductance and resistance
> 10
Resonant—Note (5)
[B7], IEEE
Std C62.92.4
Ungrounded capacitance
Range A—Note (6)
– ∞ to –40
—
—
[B6], [B18]
Table continues
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Table 1—Characteristics of grounding (continued)
Grounding classes and means
Range B—Note (7)
Ratios of symmetrical component
parameters
(NOTE 1)
X0⁄X1
R0⁄X1
R0⁄X0
–40 to 0
—
—
Percent
fault
current
(NOTE
2)
Per unit
transient
LG
voltage
(NOTE 3)
Reference
>8
>3
[B6], [B18]
NOTE 1—Values of the COG corresponding to various combinations of these ratios are shown in the figures in Annex A. COG affects the selection of surge arrester ratings (see IEEE Std C62.2-1987 [B13] and IEEE Std C62.22-2009
[B15]).
NOTE 2—Ground fault current in percentage of the three-phase short circuit value (IEEE Std 142).
NOTE 3—Transient line-to-ground voltage, following the sudden application of a fault, in per unit of the crest of the
prefault line-to-ground operating voltage, from Clarke, Crary, and Peterson [B7], and Peterson [B20]) for a simple,
linear network.
NOTE 4—In linear circuits, Class A1 limits the fundamental line-to-ground voltage on an unfaulted phase to 138% of
the prefault voltage; Class A2 to less than 110%.
NOTE 5—See 7.5, Willheim and Waters [B23], and precautions given in IEEE Std C62.92.2, IEEE Std C62.92.3,
IEEE Std C62.92.4 and IEEE Std C62.92.5.
NOTE 6—Usual isolated neutral (ungrounded) system for which the zero-sequence reactance is capacitive (negative).
NOTE 7—Refer to 7.6. Each case should be evaluated on its own merits.
NOTE 8—In a high-resistance grounded system Rs , X s , and X n are negligible compared to Rn and X Cg .
R0 + jX 0 = 3Rn ( − j3X Cg ) / ( 3Rn − j3X Cg ) . Thus R0 / X 0 = − X Cg / Rn . For a properly designed high-resistance
system, Rn ″ X Cg , and therefore R0 / X 0 ≤ ( −1) .
7.5 Resonant grounded
A resonant-grounded system is one that is grounded through one or more ground-fault neutralizers (see Willheim and Waters [B23]). When a system is to be grounded through a ground-fault neutralizer, the neutral inductance provided by the neutralizer coil is adjusted as in Equation (2) below so that:
3 X Cg = 3 X n + X s
(2)
where
X Cg
is the capacitive reactance of the system to ground
Cg is the total capacitance of the system to ground
X n is the neutral reactance
X s is the system’s reactance
(See Figure 1 and Figure 2 for further definitions of X Cg and X n .)
For resonant-grounded systems, Rs and X s can generally be neglected because they are very small in comparison to X n and X Cg . Thus, when adjusted to resonance, X n = X Cg .
R0 + jX 0 is the equivalent series representation of the parallel inductance/capacitance network, as defined in
Figure 3. If Rs and X s are small in comparison with X n , X 0 approaches infinity, because the zero-sequence
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network is parallel resonant. With resistance Rs included (depending on its value), R0 may be large with respect to X 0 . Based on either assumption, the zero sequence network presents a very high impedance to the
flow of 60 Hz current through a fault to ground. The fundamental-frequency voltages on the unfaulted phases,
during a line-to-ground fault, are essentially line-to-line voltages. They are not increased by the presence of
fault resistance. The maximum transient voltages to ground of the unfaulted phases are less than 2.73 times
normal and the neutral-to-ground voltage is less than 1.67 times normal line-to-neutral voltage (see Clarke,
Crary, and Peterson [B7], and Peterson [B20]).
Figure 3—Resonant zero-sequence network
7.6 Undergrounded systems
An ungrounded system is one with no intentional connections to ground except for potential measuring or
surge protection apparatus. Though called ungrounded, this type of system is in reality coupled to ground
through the distributed capacitance of its phase windings and conductors. In the absence of a ground fault, the
neutral of an ungrounded system under reasonably balanced load conditions will usually be close to ground
potential, being held there by the balanced electrostatic capacitance between each phase conductor and ground.
In an ungrounded system, X 0 is negative and of the order of magnitude of the zero-sequence capacitive reactance, while R0 / X 0 is relatively small. Under fault conditions, the fundamental-frequency phase-to-ground
voltages may be in excess of normal line-to-line voltages, and in some cases, particularly when the system is
large in extent, these voltages may be considerably higher (see Clarke, Crary, and Peterson [B7], and Peterson
[B20]).
There are a number of economic factors and operating practices governing the choice between grounded and
ungrounded systems. Ungrounded systems may require higher insulation levels as a result of possible severe
transient overvoltages. Abnormal insulation stress may be reduced when X 0 / X 1 lies between –∞ and –40. In
addition, it can be difficult and time consuming to identify the location of ground faults, especially when the
system is large.
When X 0 / X 1 is in the range of –40 to 0, severe series-resonance overvoltages can occur if the circuit response is linear. However, in ferro-nonlinear circuits involving a saturating transformer, highly distorted, less
severe oscillations may be generated over a broad range of negative X 0 / X 1 (see Clarke, Crary, and Peterson
[B7], and Peterson [B20]). Generally, capacitance grounding and such circuit conditions should be avoided
and each case should be carefully analyzed (see Karlicek and Taylor [B18]). However, cases exist showing
that ungrounded systems can have satisfactory operating performance.
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8. Annexes and bibliography
The figures in Annex A give COG for the ranges of positive X 0 / X 1 and R0 / X 1 , and generally indicate the
ranges of each class. Annex B contains a discussion of the effects and the modeling of earth-return path impedances such as neutral conductors, overhead ground wires (OHGWs), tower footing resistances, and substation
ground-grid resistances. Annex C is a bibliography of related standards, texts, and technical papers, some of
which are referenced in this guide.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Annex A
(informative)
Calculation of coefficients of grounding
The term COG is defined as the ratio of ELG / ELL , expressed as a percentage, of the highest rms line-to-ground
power frequency voltage ( ELG ) on a sound phase, at a selected location, during a fault to earth affecting one or
more phases to the line-to-line power frequency voltage ( ELL ) that would be obtained, at the selected location,
with the fault removed. COG may be calculated from the known impedances of the system and the fault. For
this purpose, it is convenient to express the system impedances in terms of their equivalent symmetrical component impedances Z1 , Z 2 , and Z 0 (see Clarke [B6], and Wagner and Evans [B22]).
As defined above, the COG for a specific location may vary depending on the type of fault, the fault location,
and the impedance in the fault. For the purposes of constructing Figure A.2, Figure A.3, Figure A.4, Figure A.5, and Figure A.6, the following assumptions were made:
a)
The COG is to be determined for the same location that the fault is placed.
b)
The fault type (single or double phase to ground) is that which produces the highest COG.
c)
The fault impedance, if any is purely resistive and has the value which produces the highest COG.
d)
The negative sequence impedance of the system at the fault location is equal to the positive sequence
impedance ( Z 2 = Z1 ).
For a system with positive X 0 , coefficients of grounding for locations other than the fault location are generally the same as or lower than the coefficient for the fault location. In exceptional circumstances, coefficients
may be slightly higher at locations where appreciable capacitive zero sequence current flows through high inductance lines to reach the fault location.
In systems with negative X 0 , the COG is generally lowest at the fault location and higher at remote locations.
For this reason, the usefulness of the coefficient on such systems is limited to systems in which line impedances are negligibly small.
Given these assumptions and the definition above, the quantities that must be calculated to determine the COG
are the a-phase voltage, VaLLG , for a phase b- and c-phase-to-ground fault and the b- and c-phase voltages, VbLG
and VcLG , for an a-phase-to-ground fault. These faults and their symmetrical component-equivalent networks
are shown in Figure A.1.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Figure A.1—Symmetrical component equivalent networks for two fault types used in
calculations
From Clarke [B6], the significant voltages for these faults are given in Equation (A.1), Equation (A.2), and
Equation (A.3):
VaLLG = V f ×
3Z 2 ×( Z 0 + 2 Z F )
( Z1 × Z 2 ) + ( Z1 + Z 2 ) ×( Z 0 + 3Z F )
(A.1)
VbLG = V f ×
3a 2 Z F − j 3 ×( Z 2 − aZ 0 )
Z 0 + Z1 + Z 2 + 3Z F
(A.2)
VcLG = V f ×
3aZ F + j 3 ×( Z 2 − a 2 Z 0 )
Z 0 + Z1 + Z 2 + 3Z F
(A.3)
where
V f is the power frequency voltage line-to-ground, (kV)
VaLLG
is the line-to-ground voltage for a-phase during a line-to-line-to-ground fault of b and c
phases (kV)
VbLG and VcLG
are the respective line-to-ground voltage of each phase during an a-phase-toground fault (kV)
Z1 Z 2 , and Z 0 are the positive, negative, and zero sequence impedances, respectively (Ω)
Z F is the fault impedance (Ω)
a and a 2 are phasor operators
a = −0.5 + j
3
3
= 1∠120° ; a 2 = −0.5 − j
= 1∠240° .
2
2
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Letting Z 2 = Z1 , Z F = RF and rearranging, as seen in Equation (A.4), Equation (A.5), and Equation (A.6):
VaLLG = V f ×
3Z 0 + 6 RF
Z1 + 2 Z 0 + 6 RF
(A.4)


Z 0 − Z1

VbLG = V f ×a 2 −
2 Z1 + Z 0 + 3RF 

(A.5)


Z 0 − Z1

VcLG = V f ×a −

2 Z1 + Z 0 + 3RF 
(A.6)
where
V f is the power frequency voltage line-to-ground (kV)
VaLLG
is the line-to-ground voltage for a-phase during a line-to-line-to-ground fault of b and c
phases (kV)
VbLG and VcLG
are the respective line-to-ground voltage of each phase during an a-phase-toground fault (kV)
Z1 Z 2 , and Z 0 are the positive, negative, and zero sequence impedances, respectively (Ω)
RF is the fault resistance (Ω)
a and a 2 are phasor operators
By the definition of COG:
 Z 0 + 2 RF 

COGa = 3 ×
 Z1 + 2 Z 0 + 3RF 
(A.7)
COGb =

Z 0 − Z1
1  2

×a −
2 Z1 + Z 0 + 3RF 
3 
(A.8)
COGC =

Z 0 − Z1
1 

×a −
2 Z1 + Z 0 + 3RF 
3 
(A.9)
where
COGa , COGb , and COGc are the coefficients of grounding for each respective phase
Z1 , Z 2 , and Z 0
are the positive, negative, and zero sequence impedances, respectively (Ω)
RF is the fault resistance (Ω)
a and a 2
are phasor operators are a = −5 + j
3
3
= 1∠120 and a 2 = 0.5 − j
= 1∠240
2
2
To eliminate one parameter, it is convenient to divide each impedance by X 1 .
Defining the ratios R '1 = R1 / X 1 ; R '0 = R0 / X 1 , X '0 = X 0 / X 1 , R ' F = RF / X 1


( R '0 + 2 R ' F ) + jX '0

COGa = 3 ×
 R '1 + 2 R '0 + 6 R ' F + j (1 + 2 X '0 ) 
COGb =
(A.10)
( R '0 + R '1 ) + j ( X '0 −1) 
1  2

×a −
2 R '1 + R '0 + 3R ' F + j ( X '0 + 2) 
3 
(A.11)
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
COGc =
( R '0 + R '1 ) + j ( X '0 −1) 
1 

×a −
2 R '1 + R '0 + 3R ' F + j ( X '0 + 2) 
3 
(A.12)
where
COGa , COGb , and COGc
R '1 , R '2 , R ' F and R '0
are the coefficients of grounding for each respective phase
are defined above Equation (A.10)
In Figure A.2, Figure A.3, Figure A.4, Figure A.5, and Figure A.6, the curves were obtained by means of a digital computer analysis of Equation (A.10), Equation (A.11), and Equation (A.12). In particular, Equation (A.13), the computer was programmed to find pairs ( R '0 , X '0 ) for which:
COG = limitC and ∂COG/∂R' F =0; or R' F >0; or ∂COG/∂R' F ≤ 0;R' F =0
(A.13)
where
COG
is the maximum of COGa , COGb , and COGc
C is a selected limit for example 80%
R '1 was fixed at five discrete values between 0 and 2 to produce the five graphs
The partial derivative was simulated by a discrete differential.
It should be recognized that in some circumstances, the assumptions made in producing these curves will not
be valid and COG may be higher than shown. For example, a capacitive fault impedance that can be obtained
from a ground fault in a capacitor bank could yield a higher COG. Such cases can be analyzed individually by
the use of Equation (A.1), Equation (A.2), and Equation (A.3).
NOTE—Parameter values given against each curve indicate limiting value of COG within area circumscribed
by curve. Definitions of grounding class or means is indicated in each area.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Figure A.2—Boundaries for coefficients of grounding for ratio of positive-sequence
resistance R1 to positive-sequence reactance X1 of 0
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE—Parameter values given against each curve indicate limiting value of COG within area circumscribed by curve.
Definitions of grounding class or means is indicated in each area.
Figure A.3—Boundaries for coefficients of grounding and ratio of positive-sequence
resistance R1 to positive sequence reactance X1 of 0.2
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE—Parameter values given against each curve indicate limiting value of COG within area circumscribed by curve.
Definitions of grounding class or means is indicated in each area.
Figure A.4—Boundaries for coefficients of grounding for ratio of positive-sequence
resistance R1 to positive-sequence reactance X1 of 0.5
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE—Parameter values given against each curve indicate limiting value of COG within area circumscribed by curve.
Definitions of grounding class or means is indicated in each area.
Figure A.5—Boundaries for coefficients of grounding for ratio of positive-sequence
resistance R1 to positive-sequence reactance X1 of 1.0
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE—Parameter values given against each curve indicate limiting value of COG within area circumscribed by curve.
Definitions of grounding class or means are indicated in each area.
Figure A.6—Boundaries for for coefficients of grounding for ratio of positive-sequence
resistance R1 to positive-sequence reactance X1 of 2.0
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Annex B
(informative)
Identity of ground and neutral conductors
In the calculation of the zero-sequence impedance of power transmission lines (see IEEE Std C62.92.5 and
distribution lines (see IEEE Std C62.92.4, it has been a conventional practice to include simplifying assumptions about the grounding network to reduce the complexity of the calculations.
—— The impedance of grounding electrodes, such as ground rods, tower footings, and station ground grids
to remote ground has been assumed to be negligible. Grounded conductors such as multi- grounded
neutrals, cable sheaths, and OHGWs are assumed to be perfectly connected to earth.
—— The impedance of the earth-return path has been incorporated into the zero-sequence impedance of
the line conductors so that the earth can be considered an equipotential surface. A consequence of this
assumption is that zero-sequence voltages calculated with respect to local ground are correct; however,
the calculations cannot provide zero-sequence voltage at a given point with respect to ground at another point; likewise, differences in ground potential between two points cannot be calculated.
Even though computer programs could easily deal with the complexity, these simplifying assumptions persist
to the present day because of the following:
—— The data about the impedance of grounding electrodes is not as readily available as conventional line
impedance data.
—— It is generally believed that the results of ignoring the grounding impedances are conservative, i.e.,
fault studies ignoring grounding impedances will yield the highest fault currents and highest temporary overvoltages.
—— The distribution of fault currents in the grounding network and differences in grounding potential are
generally not considered significant for short-circuit duty and arrester application purposes.
To illustrate the significance of these simplifying assumptions, consider the system shown diagrammatically in Figure B.1. This system consists of a three-span transmission line, with OHGW, linking two substations with transformers whose neutrals are grounded through inductors. (The OHGW could also represent the
multi-grounded neutral conductor of a distribution line.)
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Figure B.1—Diagram of transmission line and substations
Figure B.2 shows the conventional zero-sequence impedance diagram for this system, assuming fully transposed phase conductors. Note that only the zero sequence impedance of the line, the transformers and the
neutral inductors are specifically represented. The effect of the ground or multi-grounded neutral conductor
and the impedance of the earth-return path are incorporated into the value of Z 0 L and do not appear directly.
Figure B.2—Conventional simplified zero-sequence network
Figure B.3 shows an expanded version of Figure B.2 in which the identity of the overhead neutral conductor
has been retained in the zero-sequence network. Note that only two towers are shown in Figure B.3; most lines
would have many more towers. Retaining the identity of the neutral conductor and towers permits separate
evaluation of phase-to-neutral and phase-to-ground voltages and fault currents at a particular location. For a
phase-to-ground fault, I f / 3 enters the network at g and exits at p; for a phase-to-neutral fault, I f / 3 enters at
n and exits at p. The only information lost by this representation is the difference of earth potential between
earth points.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
NOTE 1— Rgg is the substation ground mat resistance to ground.
NOTE 2— Rt is the tower footing or pole ground resistance to ground.
NOTE 3— Z 0 is the zero-sequence self-impedance of line section between two towers, ignoring neutral conductor.
NOTE 4— Z nn is the self-impedance, with earth return, of neutral conductor section between two towers.
NOTE 5— Z ph − n is the average mutual impedance, with earth return, from phase conductors to neutral conductor
Figure B.3—Zero sequence network with overhead ground wires (OHGWs) retained
Techniques for calculation of Z 0 , Z nn , and Z ph − n are provided in Anderson [B1], Clarke [B6], and Wagner
and Evans [B22].
Calculation methods for Rgg and Rt are presented in IEEE Std 80™-2013.
The method of calculating line impedances with earth return, as presented in Anderson [B1], Clarke [B6], and
Wagner and Evans [B22], and most other texts, is based on the work of John R. Carson [B4]. In Carson’s work,
the impedance of the earth-return path is incorporated into the calculated impedance for the line and equivalent circuits. Carson’s formulations do not give differences in potential between ground points.
Clarke [B6], using the work of Rüdenberg [B21], provides a method of separating the impedance of the
earth-return path from the impedance of the line. Figure B.4 shows a detailed zero-sequence diagram based on
the model proposed by Clarke. Note that the substation ground-grid resistances, Rgg , the tower footing resistances, Rt , the OHGW, n , and the earth-return impedance, Z g , are all specifically represented.
Z g = 0.0592 ×
D
f
f
+ j ×0.1736 ×log10 e  / km
60
60
h
(B.1)
where
Z g is the earth-return impedance in Ω/km
f is the frequency in Hz
ρ is the earth resistivity in Ω∙m
De is 658.4 × ρ / f in m
h is the average conductor height in m
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Figure B.4—Zero-sequence impedance diagram with earth-return impedance
The zero-sequence parameters, R0 and X 0 , calculated from Figure B.3 or Figure B.4 for a phase-to-tower
fault (terminals p-n) may be significantly different from those for a phase-to-ground fault (terminals p-g).
Likewise, the zero-sequence voltage seen by a piece of equipment connected from phase to the tower (p-n)
may be different from those seen phase to ground (p-g).
The curves of Annex A show how the COG increases rapidly for ( R0 / X 1 ) > 1 . Because the inclusion of Rgg
and Rt tends to increase R0 , detailed representation of a line may be necessary to establish accurate maximum
values of the COG.
In a paper by Mancao, Burke, and Myers [B19], which uses a three-phase analysis program with complete
representation of the neutral conductor and grounding impedances, the authors suggest that temporary overvoltages on distribution systems may be significantly higher when grounding impedances are represented.
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IEEE Std C62.92.1-2016
IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
Annex C
(informative)
Bibliography
Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only.
[B1] Anderson, P. M., Analysis of Faulted Power Systems. Ames: Iowa State University Press, 1973.
[B2] “Application Guide for the Grounding of Synchronous Generator Systems,” Transactions of the
American Institute of Electrical Engineers, June, pp. 517–530, 1953.
[B3] “Application Guide on Methods of Neutral Grounding of Transmission Systems,” Transactions of the
American Institute of Electrical Engineers, Aug, pp. 663–668, 1953.
[B4] Carson, J. R., “Wave Propagation in Overhead Wires with Ground Return,” Bell System Technical
Journal, vol. 5, pp. 539–544, 1926.
[B5] Central Station Engineers of the Westinghouse Electric Corporation, Electrical Transmission and
Distribution Reference Book, Chapter 19, “Grounding of Power System Neutrals.”
[B6] Clark, E., Circuit Analysis of AC Power Systems, Vol. 1. New York: John Wiley & Sons, Inc., 1943.
[B7] Clarke, E., S. B. Crary, and H. A. Peterson, “Overvoltages During Power System Faults,” Transactions of
the American Institute of Electrical Engineers, vol. 58, Aug, pp. 377–385, 1939.
[B8] “Guide for the Application of Ground Fault Neutralizers,” Transactions of the American Institute of
Electrical Engineers, Apr, pp. 183–190, 1953.
[B9] “IEEE Committee Report, “Voltage Rating Investigation for Application of Lightning Arresters on
Distribution Systems,” IEEE Transactions (Power Apparatus and Systems),” PAS, vol. 91, no. 3, pp. 1067–
1074, May/June 1972.
[B10] IEEE 100™ The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition.9
[B11] IEEE Std 32™-1972 (Reaff), IEEE Standard Requirements, Terminology, and Test Procedure for
Neutral Grounding Devices.
[B12] IEEE Std C62.1™-1989 (Reaff 1994), IEEE Standard for Gapped Silicon-Carbide Surge Arresters for
AC Power Circuits.10
[B13] IEEE Std C62.2™-1987 (Reaff 1994), IEEE Guide for the Application of Gapped Silicon-Carbide
Surge Arresters for Alternating Current Systems.
[B14] IEEE Std C62.11™-2012, IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current
Power Circuits (>1 kV).
9
IEEE publications are available for The Institute of Electrical and Electronics Engineers, 445 Hoes Lane Piscataway, NJ 08854, USA
(http://standards.ieee.org/).
10
IEEE Std C62.1-1989 has been withdrawn; however, copies can be obtained from The Institute of Electrical and Electronics Engineers,
445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/).
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IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part I: Introduction
[B15] IEEE Std C62.22™-2009, IEEE Guide for the Application of Metal-Oxide Surge Arresters for
Alternating Current Systems.
[B16] IEEE Std C62.82.1™-2011, IEEE Standard for Insulation Coordination—Definitions, Principles, and
Rules.
[B17] IEEE Tutorial Course, 79EH0144–6 PWR, Surge Protection in Power Systems.
[B18] Karlicek, R. F., and Taylor, E. R., Jr., “Ferroresonance of Grounded Potential Transformers on
Ungrounded Power Systems,” AIEE Transactions, vol. 78, Part IIIA, 1959, pp. 607–618.
[B19] Mancao, R. T., J. J. Burke, and A. Myers, “The Effect of Distribution System Grounding on MOV
Selection,” IEEE Transactions on Power Delivery, vol. 8, no. 1, pp. 139–145, January 1993.
[B20] Peterson, H. A., Transients in Power Systems. New York: Dover Publications Inc., 1966, Chapter 1.
[B21] Rüdenberg, Reinhold, Elektrische Schaltvorgänge und verwandte Störungserscheinungen in
Starkstromanlagen. Berlin: Verlag von Julius Springer, 1933, p. 231.
[B22] Wagner, C. F. and R. D. Evans, Symmetrical Components as Applied to the Analysis of Unbalanced
Electrical Circuits. New York: McGraw-Hill, 1933.
[B23] Willheim, R. and M. Waters, Neutral Grounding in High-Voltage Transmission. New York: Elsevier Co.
(D. Van Nostrand Co.), 1956.
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