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P1627/D4.1
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Draft Standard for Grounding Practices for DC
Electrification Overhead Contact Systems,
including Application of Lightning Arresters for
Transit Systems
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Prepared by Working Group 17 of the Overhead Contact System Subcommittee
Sponsored by the Rail Vehicle Transit Interface Standards Committee
of the IEEE Vehicular Technology Society
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IEEE
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Copyright © 2008 by the Institute of Electrical and Electronics Engineers, Inc.
Three Park Avenue
New York, New York 10016-5997, USA
All rights reserved.
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This document is an unapproved draft of a proposed IEEE Standard. As such, this document is
subject to change. USE AT YOUR OWN RISK! Because this is an unapproved draft, this
document must not be utilized for any conformance/compliance purposes. Permission is hereby
granted for IEEE Standards Committee participants to reproduce this document for purposes of
IEEE standardization activities only. Prior to submitting this document to another standards
development organization for standardization activities, permission must first be obtained from the
Manager, Standards Intellectual Property, and IEEE Standards Activities Department. Other
entities seeking permission to reproduce this document, in whole or in part, must obtain
permission from the Manager, Standards Intellectual Property, IEEE Standards Activities
Department.
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IEEE Standards Activities Department
Manager, Standards Intellectual Property
445 Hoes Lane, P.O. Box 1331
Piscataway, NJ 08855-1331, USA
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Abstract
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This standard is a basis for the design and application of dc surge protection devices to protect
Overhead Contact System (OCS) from transient overvoltages associated with lightning and
switching surges. Switching surges are inherent characteristics of electrification system and are
generally considered low energy transient overvoltages. Lightning surges are natural, caused by
electrical discharge that occurs in the atmosphere between clouds or between clouds and ground.
Lightning surges are known for causing very high energy transient overvoltages by direct or
indirect coupling with OCS. This standard covers transient overvoltage protection of OCS used in
dc transit rail electrification systems. Transient overvoltage protection of third rail dc and the
running rails can be achieved by the application of same type of surge protection devices as are
recommended for the OCS, although these rails are less likely to be affected by lightning transient
overvoltages due to their proximity to earth where flash-over can occur to drain surge energy
away from the rails without the application of surge overvoltage protection devices. The word
surge arrester instead of lightning arrester has been used in this standard without affecting the
technical contents of the standard.
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Keywords
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{INSERT DATE}
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P<designation>D<number>
Surge arrester, Lightning, Overvoltage, Switching surges.
Copyright © 2007 IEEE. All rights reserved.
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Introduction
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The majority of the present operating dc electrified rail systems use OCS or third rail to supply
power to the vehicles. The probability of lightning surge hitting an OCS, or third rail depends on
its geometry, its own height, and length, and its relative location with respect to the presence of
buildings, trees, towers etc. in its vicinity, and the lightning flash-to-ground density (number of
lightning-to-ground strokes per square kilometer per year) of the area [B23][B24].
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If after the lightning risk assessment, the expected frequency of direct lightning to the OCS (ND) to
be protected exceeds its tolerable frequency of lightning (NC), as established by applicable
standards and codes (NFPA 780, IEC 62305), lightning protection system (LPS) should be
designed and installed. In this case OCS poles should be equipped with appropriate surge
protection devices grounded by use of low resistance ground rods and OCS poles should be
grounded by their separate ground rods.
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Should the calculated average annual number of direct lightning strokes (ND) to the OCS be below
the permissible strokes (NC), no LPS (with the exception of application of dc transient surge
protection devices of low energy capability ) is necessary and the OCS is defined as self protected
by IEC 62305 and NFPA780.
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Arbitrary installation of direct stroke diverters such as lightning rods and ground wires above the
OCS changes the geometry and may increase its chances of more lightning exposure. Based
upon such reasoning it appears that even in the area of higher lightning strokes, the application of
ground wires and ground rods above the OCS should not be used. Then this standard’s approach
is to develop an OCS transient overvoltage protection that focuses on the basic approach of
selection and application of dc surge arresters, their grounding configuration, and grounding of
OCS metallic poles. This consists of combination of the following:
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
Selection of appropriately rated dc surge protection (lightning arrester) devices to
protect OCS from transient overvoltages caused by switching and lightning
phenomenon.
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
Grounding of surge arresters by individually dedicated shortest possible grounding
conductor connected to ground rods.
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Use of dedicated grounding of OCS poles by application of ground rods separate from
surge arrester ground rods to keep the structure potential below insulation breakdown
level of positive and negative feeder cables and to minimize damage to the vehicle.
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Use of an additional surge arrester to protect the traction power substation equipment
as necessary.
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Use of OCS basic insulation levels to minimize outages due to switching and lightning
surges.
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Setting basic insulation levels for the OCS is in the scope of work of another subcommittee and
thus reference [B22] on IEEE Draft P1626 standard and [B26] are included in this standard. DC
electrification OCS and associated structure should not be compared to electric utility
transmission lines as the physical configuration and location of the two are quite different whereby
concept of lightning protection by use of ground/shield wire may not be applicable to dc
electrification OCS. However, in case of single phase ac electrification OCS, an overhead ground
wire which carries return traction ac current also performs the function of shield wire.
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Prior to and during the development of this standard, there were reports of surge arrester failures
on several transit systems resulting in service interruption and equipment failures. Questions
were being raised regarding the grounding of OCS support poles, and the pros and cons of using
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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IEEE P1627/D1.3
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surge arresters in the OCS. Most importantly, there was no understanding of the cause of failures
of surge arrester and their proper application to dc electrification OCS. Although, so far no
personnel injuries have been reported involving failure of dc surge arresters at any of the
operating transit properties in USA.
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To establish lightning protection design measures, the derivation of lightning intensity and
lightning stroke surge energy is established based upon the typical available lightning data. There
are equal chances of a lightning strike hitting any of the system components due to their proximity,
and thus flashover is certain if lightning strikes the OCS components since poles and OCS
supporting members appear to be practically grounded for the surge voltage. Flashover appears
to drain large amounts of lightning surge energy to ground, with the remainder of the surge energy
relieved by the dc surge arresters placed at appropriate locations.
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When considering lightning protection for OCS, numerous questions arise, such as:
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a)
Will dc surge arrester handle surge energy if a lightning flash (strike) directly hits the
OCS wire near the arrester location?
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b)
If a lightning flash directly hits the OCS wire between two traction power substations,
what will be the energy discharged through the surge arresters at feeder poles?
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c)
Is there a need to apply dc surge arresters at the mid point of two adjacent substations to
enhance the lightning protection of the dc rail transit OCS?
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d)
Do we need to apply surge arresters at the connection points of underground dc
supplementary feeder cable to OCS contact wire located approximately every 400 feet,
where overhead messenger wire design is not possible in downtown areas due to
aesthetics and other restrictions of height of the messenger wires?
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e)
Should there be shield/ground wire above the messenger to enhance lightning
protection?
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Responses to the above questions are discussed in this standard.
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IEEE P1627 “Standard for Transient Overvoltage Protection of dc Electrification Overhead
Contact System”, including application of dc surge protection devices was prepared by Working
Group 17 of the Overhead Contact Systems Sub-Committee of the Rail Transit Vehicle Interface
Standards Committee of the IEEE Vehicular Technology Society.
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Origin and Development of IEEE P1627
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The Overhead Contact Systems Sub-Committee was formed in 2001 with the purpose of
developing standards governing the design, construction, and maintenance of the OCS and
current collection system.
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Working group 17 was established to develop standards governing the transient overvoltage
protection of OCS for the dc rail transit systems. The primary concern of the working group was a
lack of uniform practices and a lack of understanding of the proper application of dc surge
arresters and their grounding configuration to protect OCS and associated poles including dc
traction power system and vehicles. Precise information of the transient environment, expected
magnitude, duration and frequency of transient surges is very unpredictable and there has been
no recorded data available to guide application engineers in the selection and application of dc
surge protection devices. However, it is known that certain degree of threat of both the internal
switching surges and the external lightning surges exist that could occur at random causing
damage to OCS and dc system without proper application of surge overvoltage protection
devices.
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It appears that the lack of knowledge of the transient environment and understanding the
parameters and rating of dc surge arresters lead to use of guesswork in the selection and
application of such devices causing failures at some installations. At some transit properties,
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This is an unapproved IEEE Standards Draft, subject to change.
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failures of dc lighting arresters and equipment damage associated with those failures have been
reported. Lack of understanding of characteristics of transient overvoltages, dc surge arrester
ratings, their proper installation and grounding configurations, lack of their clear test and
application data from the manufacturers has been a major concern in the industry that lead to the
development of this standard. Proper application of dc surge protection devices to divert surge
energy associated with the switching and lightning surges away from the OCS relates directly to
their effect on equipment and personnel protection.
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Patents
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Attention is called to the possibility that implementation of this standard may require use of subject
matter covered by patent rights. By publication of this standard, no position is taken with respect
to the existence or validity of any patent rights in connection therewith. The IEEE shall not be
responsible for identifying patents for which a license may be required by an IEEE standard or for
conducting inquiries into the legal validity or scope of those patents that are brought to its
attention.
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Participants
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At the time this standard was completed, the working group had the following membership:
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Dev Paul, Chair
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Ramesh Dhingra, Vice Chair
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Alan Blatchford
Butch Campbell
Ron Clark
Ian Hayes
Albert Hoe
Gordon MacDonald
Dev Paul
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Chris Pagni
Steve Mitan
Jay Sender
Jeffrey N. Sisson
Vish Mawley
Edward Rowe
Suresh Shrimavle
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Carl Wessel
Paul White
Kelvin Zan
Ethan Kim
Ramesh Dhingra
Stuart Kuritzky
The following members of the balloting committee voted on this standard. Balloters may have
voted for approval, disapproval, or abstention. (To be provided by IEEE editor at time of
publication.)
_____________________________________________________________________________
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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IEEE P1627/D1.3
Contents
1.
Overview ................................................................................................................................. 1
1.1
Scope ......................................................................................................................... 1
1.2
Purpose ...................................................................................................................... 1
1.3
Format of Standard .................................................................................................... 1
2.
References ............................................................................................................................. 2
3.
Definitions, abbreviations, and acronyms ............................................................................... 2
4.
5.
3.1
Definitions .................................................................................................................. 2
3.2
Abbreviations and Acronyms ..................................................................................... 4
Transient Surges ..................................................................................................................... 4
4.1
Lightning and Switching Surges ................................................................................. 5
4.2
Surge Characteristics - Propagation .......................................................................... 6
4.3
Magnetic Stored Energy of Surge .............................................................................. 7
Surge Environment – DC Electrification System ..................................................................... 8
5.1
6.
Lightning Stroke Terminology ............................................................................................... 16
6.1
7.
Underground Supplementary Cable Connections to OCS....................................... 23
Grounding ............................................................................................................................ 26
8.1
9.
Lightning Intensity Estimation .................................................................................. 17
Lightning Stroke to OCS system ........................................................................................... 22
7.1
8.
DC Surge Protection Device Requirements............................................................... 8
OCS Pole Grounding ............................................................................................... 26
DC Surge Arresters .............................................................................................................. 27
9.1
Application................................................................................................................ 27
9.2
Surge arrester Rating............................................................................................... 28
10.
DC Surge Arrester Service Requirements ............................................................................ 29
11.
DC Surge Arrester Testing.................................................................................................... 29
11.1
Design Tests ............................................................................................................ 29
11.2
In Service (Field) Tests ............................................................................................ 30
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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11.3
IEEE P1627/D1.3
Bibliography ............................................................................................................. 31
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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IEEE P1627/D1.3
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Draft Standard for DC Electrification Overhead
Contact Systems, including Application of
Lightning Arresters for Transit Systems
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1. Overview
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1.1 Scope
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The scope of this standard covers application of transient overvoltage surge protection devices
and their grounding configurations to protect OCS used in dc traction electrification for heavy rail,
light rail, and trolleybus systems.
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1.2 Purpose
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The purpose of this standard is to establish minimum design requirements for application of dc
surge protection devices and their grounding configurations to protect OCS and associated
traction power system components from transient overvoltages caused by switching and lightning
surges. Such a design will provide a reasonable degree of protection to equipment by diverting
surge energy and related hazards away from the OCS system. At the present, there are no
uniform practices for application of properly rated dc surge arresters and their grounding
configurations to protect OCS used in dc traction electrification. The use of this standard is
intended to provide understanding of the transient overvoltage environment that exists at a
particular dc electrification system and then a systematic approach to protect OCS and associated
dc traction power system from transient overvoltages by use of appropriately rated dc surge
protection devices.
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1.3 Format of Standard
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First the basic characteristics of transient surges, their origin, and energy and propagation
behavior are described in Sections 4 and 7. This is followed by a brief description of the surge
environment expected at a dc electrification OCS included in Sections 5 and 7. In Sections 6 and
7, this standard addresses analysis of lightning strike to the dc traction power system
components.
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This standard addresses OCS, the running rails, OCS supporting structure, metallic poles,
messenger wire, underground supplementary conductors, traction power substations, and
vehicles. This also includes lightning waveform parameters.
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This standard also addresses the mismatch between the actual number of lightning strikes and
specificity of the lightning parameters required in performing lightning protection analysis for an
electric transit system. At present there is no data for the lightning effects and design experience
available from the operating transit properties. Experience from operational lightning location
systems [B18] [B19] [B20] [B24], ongoing research, and scientific ability to measure lightning
parameters can avoid guesswork in the lightning protection design of electric transit systems.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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IEEE P1627/D1.3
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From a cause and effect standpoint, the maximum rate-of-rise, the peak current, and the wave
front rise-time are associated with determining the maximum voltage that will be seen on the OCS
subjected to unpredictable threat of a direct or nearby ground lightning discharge. The probability
of a lightning strike is greater when the electric transit system is located in a high isokeraunic level
area. [B2].
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Lightning may be of concern when the dc system is in a relatively high isokeraunic area.
Isokeraunic area map of the world can be seen in reference [B9]. In addition, dealing with a transit
system involving the general public and, more importantly, the havoc due to interruption of system
operations caused by lightning is a design concern for dc transit systems. Thus to minimize the
effect of lightning and switching surge voltages to a typical dc traction power system, application
of appropriately rated dc surge arresters should be included in the design.
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2. References
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This standard shall be used in conjunction with the following publications. If the following
standards are superseded by an approved revision or new version, the latest revision shall apply.
In case of conflict between this standard and the referenced documents, this standard shall take
precedence. Those provisions of the referenced standard that are not in conflict with this
standard shall apply as referenced. (all referenced standards need to be referred in the text of the
standard and they need to be in alpha numerical order)
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National Electrical Safety Code
(NESC)
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National Electrical Code
(NEC)
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Canadian Electrical Code
(CEC)
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British Standard BSN EN 50124-1:2001 Railway Applications – Insulation coordination
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IEEE Standard Dictionary of Electrical and Electronics Terms, IEEE Std 100
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IEEE Guide for the Application of Metal Oxide Surge Arresters for Alternating Current Systems,
ANSI/IEEE Standard C62.22,
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IEEE Guide for Application of Gapped Silicon Carbide Surge Arresters for Alternating Current
Systems, ANSI/IEEE Standard C62.2 – 1987
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EN 50526-1 Draft Feb. 2009, “Railway applications – Fixed installations – D.C. Surge arresters
and voltage limiting devices – Part 1: Surge arresters”
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EN 50526-2 Draft March 17, 2009, ““Railway applications – Fixed installations – D.C. Surge
arresters and voltage limiting devices – Part 2: Voltage limiting devices”
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3. Definitions, abbreviations, and acronyms
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3.1 Definitions
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Review all IEEE Dictionary terms For the purpose Insulation Standard IEEE P1626/D1
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IEEE Glossary of OCS
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3.1.1 Clearance: Shortest distance in air between two conductive materials.
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3.1.2 Creepage Distance: Shortest distance along the surface of the insulating material between
two conductive materials.
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3.1.3 Electrical Section: Part of an electrical circuit having its own voltage rating for insulation
coordination.
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3.1.4 Grounded: Electrical section intentionally connected to earth that cannot be interrupted.
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3.1.5 Insulated: All components isolated from the energized OCS conductors by at least one level
of insulation. An insulated section may be under the influence of adjacent energized circuits. An
insulated section may be considered as an electrical section.
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3.1.6 Surge arrester: A device typically mounted on OCS poles and connected to the OCS,
designed to protect insulated feeder cables against lightning, by providing a path to ground
through a spark-gap, with or without variable resistance elements.
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3.1.7 Nominal voltage: Value assigned to a circuit or system approximately equivalent to the
working voltage for designating the voltage class.
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3.1.8 Overvoltage: Voltage having a peak value exceeding the maximum steady state voltage at
normal operating conditions.
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3.1.9 Rated Voltage: Value of voltage assigned to a component, device or equipment.
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3.1.10 Rated Impulse Voltage: Value of voltage assigned to the equipment referring to the
specified withstand capability of the insulation against transient overvoltages.
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3.1.11 Rated Insulation Voltage: RMS withstand voltage assigned to the equipment referring to
the specified permanent (over five minutes) withstand capability of the insulation between
energized components and earth.
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3.1.12 Residual Discharge Voltage (VIR): Is the voltage across the surge arrester at the instant
of surge peak current discharge of a specific surge current wave shape due to lightning or
switching phenomenon. Same magnitude of surge peak current with different wave-shape may
result in different value of VIR depending upon the type and manufacturer of the surge arrester.
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3.1.13 Maximum Continuous Operating Voltage (MCOV): The maximum designated rootmean-square (rms) continuous operating voltage value in dc volts that many be applied
continuously between the terminals of the arrester.
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3.1.14 Surge Arrester: See Surge arrester.
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3.1.15 Isokeraunic level: Number of thunderstorm days per year in an area is called isokeraunic
level
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3.1.16 Overhead Contact System (OCS): Contact wire and messenger wire electrically in
parallel connected to traction power substation (TPSS) by underground feeders. Contact wire
makes contact with vehicle pantograph and is acting as positive polarity contact point for the dc
power supply delivered to the vehicles from TPSS.
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3.1.17 Lightning flash: Very bright light stream seen in the sky between the cloud and the ground
on a stormy day.
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3.1.18 Lightning stroke: Same as lightning flash.
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3.1.19 Lightning strike: Same as lightning flash.
Copyright © 2007 IEEE. All rights reserved.
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3.1.20 Lightning Intensity: Number of lightning strikes/per square meter/year in an area is called
its lightning intensity.
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3.1.21 Temporary Overvoltage (ETOV): The maximum root-mean-square (rms) temporary
overvoltage value in dc volts that may occur at the OCS due to vehicle regeneration or utility
power supply voltage regulation.
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3.1.22 Supplementary Cable: Cable connected in parallel with OCS is called positive
supplementary cable. Cable connected in parallel with running rail is called negative
supplementary cable.
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3.1.23 Voltage Margin-of-Protection (VSA): The voltage seen at the OCS (voltage between the
OCS and the local ground) when the lightning surge arrester conducts lightning surge peak
current and associated energy to ground.
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3.2 Abbreviations and Acronyms
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AC
ANSI
AREMA
ASTM
AWG
DC
ETB
FRA
ICLP
IEEE
ISO
LRV
NEC
NEMA
NESC
NETA
NFPA
OCS
OSHA
RMS
ROW
TES
UBC
UL
USASI
USDOT
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4. Transient Surges
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Transient overvoltage surges are caused by lightning, switching phenomenon within the system,
induced and impressed voltages and become superimposed over the dc power system voltage.
They are unpredictable and could be brief and quick. Their wave-shape, magnitude, and energy
content may vary considerably depending upon their cause of origin, system configuration, and
surge impedance parameters. Such transient overvoltages and associated energy shall be
diverted away from the system by application of appropriate dc surge protection devices at
specific locations within the overall electrification system. Lightning surges, called external surges
can impinge on the electrification system directly or indirectly, and can induce transient
overvoltages with considerable energy to create hazards. Switching surges, called internal surges,
Alternating Current
American National Standards Institute
American Railway Engineering and Maintenance of way Association
American Society for Testing and Materials
American Wire Gauge
Direct Current
Electrified Trolley Bus
Federal Railroad Administration
International Conference on Lightning Protection
Institute of Electrical and Electronics Engineers
International Organization for Standards
Light Rail Vehicle
National Electrical Code (NFPA-70)
National Electrical Manufacturers Association
National Electrical Safety Code
National Electrical Testing Association
National Fire Protection Association
Overhead Contact System
Occupational Safety and Health Administration Act
Root Mean Square
Right-of-way
Traction Electrification System
Uniform Building Code
Underwriters Laboratories, Inc.
United States of America Standards Institute
United States Department of Transportation
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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are caused by sudden changes in the electrification system, such as dc breaker operation, and
may not be as severe as lightning due to their relatively low energy.
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4.1 Lightning and Switching Surges
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The standards make a distinction between switching and lightning surges on the basis of the
duration of the front or rise-time from zero-to-peak value. Surges with fronts of up to 20 s are
defined as lightning surges, and those with longer fronts are defined as switching surges [B15].
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Lightning surge intensity depends upon the isokeraunic level of the area. The probability of direct
stroke can be estimated by the various factors listed in NFPA 780. To understand the
characteristics and nature of such surges, considerable published data is available [B1] [B2].
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For lightning surge application purposes, industry has standardized voltage impulse waveform
1.2/50 s indicating crest is reached in 1.2 s and it decays to half the crest in 50 s. Similarly, a
current impulse wave of 8/20s is used where the crest is reached in 8 s and decays half the
crest value in 20s.
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A steep-fronted surge is one with a rise time of 0.1-0.5 s and a virtual time to half value of
around 5 s. An impulse current wave shape of 10/1000 s (long wave) is more representative of
the high energy surges usually experienced from the inductive elements [B4] [B5].
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4.1.1 Causes of Lightning Surges
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Lighting is an electrical discharge that occurs in the atmosphere between clouds or between
clouds and ground. It is very high-energy phenomenon and can be a source of harm for transit
system. Lightning flashes, in fact, can release many hundreds of mega-joules of energy.
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A typical cloud-to-cloud lightning begins with a preliminary breakdown due to the intense electric
field initiated in the lower part of the cloud, and is generally negatively charged. The polarity of the
lightning, in fact, which is a function of the local territory, can be statistically assumed as negative
in 90% of the cases.
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The process is followed by a discharge, or leader, which creates a highly conductive channel,
which advances, in a “zigzag” path, towards the earth and meets an upward advancing leader.
This stepped discharge is caused by the non-uniformity of electric field. The medium between
clouds and earth, in fact, is not uniform, as the air characteristics (density, pollution, humidity, etc.)
continuously vary, hence the non-linearity of the lightning path.
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As the discharge progresses towards the earth, the electric field existing between the advancing
channels and the earth or objects on earth i.e. buildings, poles, and trees etc, increases. In fair
weather conditions, the electric field at the ground is quite high, while in the presence of stormy
clouds in the sky, the electric field ranges between 30 and 40 kV/m. At the point of strike, the field
assumes the value of 400 kV/m for the duration of the lightning.
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The dielectric strength of the air, during stormy weather, may be well below its strength of 3000
kV/m in dry weather conditions. Such circumstance can facilitate the cloud-to-earth electric charge
to reach the surrounding air critical breakdown value and cause (due to corona effect) upward
directed discharges (upward leaders), basically at earth potential.
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An “attachment” process takes place between the two channels. The ground potential propagates
upwards, circulating to earth the negative charge accumulated in the downward directed channel.
This process, usually called a return stroke, causes the circulation of the high-intensity, impulse
lightning current to ground. A very rapid rise to the peak within a few microseconds (s) and, then,
a relatively slow decay, are the typical characteristics of a lightning surge current wave.
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IEEE P1627/D1.3
1
4.1.2 Causes of Switching Surges in the DC traction power system
2
3
4
Switching operations that cause overvoltages are mainly due to trapped energy in part of the
circuit and subsequent release of that energy. The following system operations in dc transit
system may cause switching surges of varying degrees.
5
6
a)
DC Circuit Breaker Operation: Interruption of dc current generates an arc with transient
overvoltage in the order of 2.5 times the dc system voltage [B21].
7
8
9
10
b)
Pantograph Arcing: An uneven pantograph contact with the OCS wire causes arcing.
The intensity of such an arc and associated voltage/current surge may depend upon
various factors, such as load current, vehicle speed, air gap clearance, and weather
conditions.
11
12
13
c)
AC vacuum Breaker Operation: In rare circumstances, current chopping during
interruption and pre-ignition during closing of the vacuum breaker may lead to transient
overvoltage [B17]
14
15
d)
Voltage Transient due to dv/dt across diodes: Surges are generated due to inherent
characteristics of the circuit.
16
17
e)
Induced switching surges from ac power system: Heavy direct lightning strike to an ac
power line in close vicinity to the OCS system may induce surges in the dc system.
18
19
f)
Current Limiting Fuse Blowing: Current-limiting fuses protecting the rectifier diodes may
generate transient arc overvoltage.
20
21
g)
Feeder Cable Arcing Fault: Loose cable connections arcing may lead to transient
surges.
22
AC line making contact with the OCS wire can lead to voltage surge to dc system
23
4.2 Surge Characteristics - Propagation
24
25
The surge Impedance (Z) and the surge propagation velocity (v) are defined by the following
expressions [B3].
26
Z=
27
v  LC
28
Where:
29
Z
Surge impedance in ohms
30
L
Inductance in henries per unit length
31
C
Capacitance in farads per unit length
32
33
V
Surge velocity will be in meter/sec if the if the units of L are in henries/meter and units of
C are in farads/meter
34
35
The surge current (I), the surge voltage (V) and the surge impedance (Z) are related by the
expression below [B2].
L
C
(1)
(2)
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1
V = IZ
(3)
2
3
The rate-of-rise of surge voltage (dv/dt) and rate-of-rise of surge current (di/dt) are related to
surge impedance (Z) as follows [B1]
4
di/dt = dv/dt (1/Z)
5
6
7
8
Surge propagates at the speed of light, 1000ft/s, (304.8m/s) in the OCS wire and approximately
half this speed 500ft/s (152.4m/s) in the dc feeder cables [B2] [B3]. Surge experiences a surge
reflection and refraction at a junction point due to change in surge impedance values. Surge
propagation theory is well-documented [B2] [B10].
9
10
The following expression [B15] [B21] may be used to derive the surge voltage that could
propagate towards dc switchgear via feeder cables.
11
VFC = 2 VI (ZC/n)/ [ ZOCS + (ZC/n)]
12
Where:
13
VFC :Surge voltage through feeder cables
14
VI : Incident surge voltage at the OCS feeder pole junction point, say 35 kV flash over value
15
ZOCS: OCS wire surge impedance, 400  [B3]
16
ZC: Each feeder cable surge impedance, 40  [B3]
17
n: Number of feeder cables in parallel, say 3
18
4.3 Magnetic Stored Energy of Surge
19
20
Switching surge energy (w) is exchanged between system inductance L and capacitance C
parameters [B2] [B5] during its propagation defined by the expression below.
21
W = (1/2) L I2 joules (watt-seconds)
22
L
Inductance in henries
23
I
Current in rms. amperes
24
Estimation of the energy trapped in the OCS wire using expression (6) is as follows:
25
L= 2.0 mH/mile Ind. of the OCS wire [B2]
26
I = 2000 A (assumed surge current)
27
D= 2 miles assumed distance between adjacent traction power substations
28
29
It is reasonable to consider that 1/2 [B2] of the surge current will propagate towards each adjacent
substation.
30
W = ½(2x10-3 )(1000)2 = 1000 joules
31
32
It shall be noted that the above calculated energy will become 25 kilo-joules if the current surge is
assumed as 10 kA.
Copyright © 2007 IEEE. All rights reserved.
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(4)
(5)
(6)
7
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IEEE P1627/D1.3
1
5. Surge Environment – DC Electrification System
2
3
4
5
6
It is evident from rail electrification systems using OCS that there are two paths for the surges to
impact the substation equipment, one from OCS and other from the ac primary power supply
system. The expected upper limit of the surge without the dc surge arrester that could propagate
through the OCS system will be equal to the dry flashover value of the OCS system, which is
typically 35 kV peak for 800 V dc system [B27].
7
8
9
10
The incoming primary surge will see the doubling effect at the rectifier transformer due to change
in surge impedance. The intensity of incoming surge will be related to distribution system
parameters and the impinging surge. Internal switching surges and surges at the OCS may be
related to any of the system operational causes listed under Section 4.1.2 earlier.
11
12
13
14
15
Due to proximity of the OCS positive wire and the running rails (or negative wire in case of ETB
system) there are equal chances that the lightning may strike both of these components
simultaneously. Any surge voltage impinging the OCS wire is free to propagate towards the dc
switchgear, as well as to the vehicle. The associated surge current may divide according to the
surge impedance paths and may experience some attenuation.
16
5.1 DC Surge Protection Device Requirements
17
18
For dc application, ac surge arresters with nonlinear resistors are re-rated. The basic
requirements of a dc surge protection device include the following:
19
20
a)
At highest working voltage, the device shall be essentially non-conducting, with a
minimal leakage current.
21
22
b)
At an overvoltage moderately above the working voltage, the device while conducting
shall permit only small increases in its own terminal voltage.
23
24
c)
Device shall have an adequate energy absorption capability to handle the stored energy
in the dc system.
25
26
d)
Upon suppressing the system transient overvoltage, the device shall quickly interrupt the
normal dc voltage follow current.
27
28
e)
Device shall be suitable for outdoor and indoor application subjected to harsh weather
without degradation in performance.
29
30
31
f)
The residual voltage at the OCS at maximum expected surge current magnitude shall
be less than the damaging voltage of the equipment to be protected. It is affected by the
lead length inductance and rate of rise of surge current.
32
33
g)
Device shall accommodate dc system temporary overvoltage condition as described in
Section 5.1.3 without its failure due to repeated occurrence of ETOV at the OCS.
34
35
h)
Device shall be capable of providing a close margin of protection without excessive
maintenance and damage.
36
i)
Device shall have indication of failure
37
j)
Device testing procedure is desirable
38
39
DC Surge arrester application configuration is shown in Fig. 1. The MOV dc surge arrester with
characteristics shown in Fig. 2 (b) appears to meet above considerations and closely matches
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IEEE P1627/D1.3
1
2
with the characteristics of an ideal device shown in Fig. 2 (a). However, its energy absorbing
capabilities should be checked in dc system application.
3
4
5
6
7
8
9
10
The gapped MOV surge arrester shown in Fig. 2 (d) may not provide a close margin of voltage
protection as the triggering of the gap will occur at relatively higher peak overvoltage condition.
Spark gap type arrester with characteristics of Fig. 2(c) may not reseal causing system follow
current flow to ground. Some manufacturers promote magnetically blown spark gaps with series
connected non-linear resistors in a single stack. To improve the internal voltage distribution, a
grading resistor and capacitor is provided for each spark gap. Such a device could provide
adequate surge absorbing capability, as well as close margin of protection. This standard makes
a recommendation to use properly rated MOV type surge arrester.
11
5.1.1 DC Surge Arrester Test Data and Energy Capability
12
13
14
15
16
17
18
19
20
21
Without a standard on the manufacture of dc surge arresters, the testing and rating method
among the various suppliers may vary. The energy absorbing capability varies, depending upon
the quality and quantity of basic material (zinc oxide) used in the development of surge arrester. It
appears that the manufacturers rely on the test data provided by the suppliers of the basic surge
arrester material. For example, the data shown in Table 2 by the surge arrester manufacturer is
the same as shown by Harris Semiconductor Corporation [B5] for the Type CA Metal-Oxide
Varistors. However, there is no indication how the individual units were tested after their assembly
in their housing. The design of connection terminals may change the test data. The unit shall be
tested after assembly. It shall be noted that the terminology used in Table 4 is in accordance with
ANSI/IEEE Std. C62.33 [B11], however, it differs from the terminology used in Tables 1, 2 and 3.
22
23
24
25
26
27
Manufacturer's technical data shown in Table 1 does not list all required test data including the
energy capability for the surge arrester as seen in Tables 2 and 3. Fig.3 from another
manufacturer does not indicate the test surge wave shape. Fig. 4 shows the generic relationship
of surge arrester MCOV rating and energy capability. Surge arrester’s energy capability can be
increased by using a series parallel combination of the basic MOV discs without increasing its
MCOV rating which may be required for a dc surge arrester [B1].
28
29
30
Two different nomenclatures, discharge voltage (VIR) and clamping voltage (VC), have been used
by the dc surge arrester manufacturers as shown in Tables 1, 2, 3 and 4. This may create
confusion to the application engineer.
31
32
The lack of test data and test procedures for dc surge arresters has created a challenge for the
application engineer to evaluate their protection capabilities.
33
Table 1: DC Surge Arrester Parameters from Published Catalogue
MCOV
Volts
DC
0.5s, 10kA
Maximum
Discharge
Voltage - VIR
(kV peak)*
900
3.4
1800
5.8
500A Switching Surge
Maximum Discharge
Voltage - VIR
(kV peak)**
8/20s Impulse Wave peak current
maximum discharge voltage - VIR
(kV peak)
1.5 kA
3 kA
5 kA
10 kA
20 kA
2.2
2.5
2.6
2.8
3.0
3.5
4.4
5.0
5.1
5.5
6.0
7.0
34
35
* Maximum discharge voltage for a 10 kA impulse current wave, which produces a current wave
cresting in 0.5s.
36
** Based upon a current surge of 45s time to crest, 500A peak.
37
Copyright © 2007 IEEE. All rights reserved.
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August 28, 2009
1
IEEE P1627/D1.3
Table 2: DC Surge Arrester Parameters from Published Catalogue
Nominal
Voltage
Rated
Voltage
kV
0.75
1.50
3.00
MCOV
kV
1
2
4
kV
1
2
4
Thermal
Energy
absorbing
capability
kJ
10
20
40
Max. values of the residual voltages in kV (discharge
Voltage-VIR) at peak discharge currents of impulses
30/60s
0.5kA
kV
1.9
3.8
7.6
1kA
kV
2
4
8
8/20s
5kA
10kA
kV
kV
2.3
2.4
4.5
4.8
9.0
9.6
20kA
kV
2.7
5.3
10.6
1/2s
20kA
kV
2.6
5.1
10.2
2
3
Table 3: DC Surge Arrester Parameters from Published Catalogue
Nom.
Volt.
kV
(Type)
1.0
1.5
2.0
2.5
3.0
4.2
4
MCOV
Max. Values of the residual voltage in kV (discharge Voltage-VIR) at peak
discharge currents of impulses. Energy capability, 2 impulses – 10.5 kJ/kV MCOV
30/60s
kV
250A
500A
1 kA
1.0
1.5
2.0
2.5
3.0
4.2
1.96
2.92
3.89
4.95
5.84
8.10
2.01
2.99
3.99
5.07
5.98
8.30
2.06
3.06
4.08
5.19
6.12
8.50
2 kA
1kA
2.13
3.19
4.25
5.41
6.38
8.85
2.10
3.15
4.20
5.34
6.30
8.75
8/20s
1.5kA 5kA 10kA
2.16
3.22
4.29
5.45
6.43
8.92
2.31
3.46
4.61
5.86
6.92
9.60
2.40
3.60
4.80
6.10
7.20
10.0
20kA
2.64
3.96
5.28
6.71
7.92
11.0
1/2s
10
20
kA
kA
2.67
4.04
5.38
6.84
8.07
11.2
3.00
4.47
5.96
7.57
8.93
12.4
Table 4: Metal-Oxide Disc Varistors (CA Series) from basic MOV material [B5]
Maximum Ratings (85 C )
Cont.
Peak
Transient
DC
Amp
Energy with Wave
Volt.
Wave
10/1000s
8/20s
Vm(dc)
W tm
Itm
(Volts)
(Joules)
(kA)
Characteristics (25 C)
Varistor Voltage at
Max. Peak Clamping
1 ma dc test current
Voltage (VC) with
surge wave
200A peak, 8/20s
Min.
VN(dc)
Max
VC
(Volt) (Volt) (Volt)
(Volt)
Typical
Cap. @
1MHz
970
1150
1400
1750
2600
3200
3200
5000
70
70
70
70
1080
1290
1620
2020
1200
1500
1800
2200
1320
1650
2060
2550
1880
2340
2940
3600
Picofarads
(pf)
3500
2700
2200
1800
2150
6000
70
2500
2700
3030
4300
1500
2500
7500
70
2970
3300
3630
5200
1200
3000
8600
70
3510
3900
4290
6200
1000
3500
10000
70
4230
4700
5170
7400
800
5
W tm : Rated Single Pulse Transient Energy,
VN(dc): Nominal varistor dc voltage
6
7
Itm : Single Pulse Transient Peak Current,
Vm(dc): Max. Cont. Operating Voltage (MCOV)
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This is an unapproved IEEE Standards Draft, subject to change.
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1
2
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This is an unapproved IEEE Standards Draft, subject to change.
11
August 28, 2009
1
IEEE P1627/D1.3
Fig. 2: Characteristics of Various Surge Arresters
2
3
Fig. 3: Clamping Voltage v/s Peak current (check copy right)
4
5
Fig. 4: Surge Arrester Energy Capability (copy right check)
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IEEE P1627/D1.3
1
2
5.1.2 DC Surge Arrester Application: Surge arresters configuration and surge impedance
parameters are shown in Fig. 1. There is a need for research for surge arrester application.1
3
4
5
6
A systematic approach shall be applied based upon the system configuration, surge parameters,
and the expected intensity of surges and careful review of the manufacturer’s data on arresters.
The selection of an arrester will require establishing its MCOV rating, energy handling capability,
and the arrester discharge voltage.
7
5.1.3 DC Surge Arrester MCOV Rating
8
9
10
DC surge arrester maximum continuous operating voltage (MCOV) will definably be greater than
the system operating voltage, but should also be greater than the temporary overvoltage value
(ETOV) determined by the following expression [B21].
11
ETOV = (F) (VR)(RG)E
12
Where:
13
E:
14
15
VR: Specified Voltage Regulation of Transformer Rectifier Unit in Per Unit, which will be (1.06)
for 6% voltage regulation
16
17
F: Upper limit of utility primary power supplies voltage regulation factor, suggest this value not to
be less than 1.10.
18
RG: Vehicle Regeneration Factor, use 1.15
19
20
21
22
23
24
25
26
27
28
29
30
Using these values, ETOV will be 1073 volts, thus dc surge arrester MCOV should be greater than
1073 V in this example. This statement is based upon the consideration that in a dc transit system
temporary overvoltage may appear at the OCS as many times as the operating trains go through
the regeneration mode especially if there are no other trains nearby to absorb the regenerated
power. Such ETOV may also occur during night time when utility voltage may go up and dc load
(trains) is relatively less. To assure that selected surge arrester MCOV should be greater than
ETOV , an engineering analysis of the actual dc power system performance parameters listed in
equation (7) above and adequate voltage-margin of protection is required. This standard
recommends MCOV ≥ ETOV to avoid surge arrester premature failure due to ETOV especially when
indication of the failed (degraded without enclosure rupture) surge arrester is not easily available
unless field testing of the installed surge arresters is conducted. Degraded surge arrester may be
conducting harmful dc stray current to earth.
31
5.1.4 DC Surge Arrester Voltage – Margin-of-Protection
32
33
34
35
36
37
38
Surge arrester voltage-margin-of-protection (Vsa) above remote earth is defined as the peak
voltage seen at the OCS by conduction of a surge current that results in maximum front of the
wave internal discharge voltage (VIR), and maximum voltage drop across its grounding leads on
both sides. For correction application of a dc surge arrester to OCS, it’s this peak voltage that
should be as close to OCS dc voltage as possible such that no damage should occur to the dc
traction power system components, including dc feeder cables, vehicles and substations. This
voltage can be calculated by the following equation:
39
Vsa = VIR + L. di/dt + IZSG
1
(7)
System No Load Voltage, 800V
(8)
There is a room for research to establish similar concept of High-Low or the Low-High cascaded
arresters [B14] at the OCS pole and the vehicle
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IEEE P1627/D1.3
1
Where:
2
3
ZSG Surge Arrester ground rod (electrode) impedance measured in ohms at 60 Hz, usually less
than 5 ohms
4
VIR Front of the wave maximum IR discharge, voltage drop of arrester in kV peak
5
L
Inductance in henries of surge arrester leads
6
I
Peak surge current in amperes
7
di/dt Rate-of-rise of the surge current in kA/sec
8
9
10
11
12
13
It shall be noted that the voltage drop across the ground electrode impedance (IZSA) does not
affect the dc equipment protection margin as the system negative, which acts as a reference point
is grounded via leakage resistance to ground of the running rails insulators. In addition, all metallic
components, including the OCS pole, are grounded to the same earth near to surge arrester
ground electrode. Thus, for the surge arrester Vsa calculation, as shown in the equation (9) below
the factor IZSG should not be used.
14
Vsa = VIR + L. di/dt
15
16
17
18
19
20
21
22
23
Without complete knowledge of the surge environment and test data on the dc surge arresters,
one application approach is to apply a lower voltage surge arrester, such as 970 MCOV dc for the
800V dc system, knowing it has relatively lower energy capability, but better voltage margin of
protection. In case of its failure in actual application, it will provide the measure of surge
environment. To increase surge energy handling capability of the surge arrester without increasing
its MCOV rating, another approach may be to apply two surge arresters of low MCOV rating in
parallel with individual ground leads and grounding electrode connections . However, the concern
of increased leakage current under normal system operation with parallel arrester approach
should be checked based upon this analysis.
24
25
26
27
28
Based upon above described analysis, this standard recommends MCOV ≥ ETOV to avoid surge
arrester premature failure due to ETOV especially when indication of the failed (degraded without
enclosure rupture) surge arrester is not easily available unless field testing of the installed surge
arresters is conducted. Degraded surge arrester at the OCS system may be conducting harmful
dc stray current to earth until it is replaced with a new surge arrester.
(9)
29
30
5.1.5 DC Surge Arrester Energy Discharge Capability
31
Surge arrester surge energy (W) in joules can be calculated by the following expression below:
32
W=
t
 V.I.dt
(10)
0
33
Where:
34
V: Surge arrester front of the wave protective level in volts
35
I: Peak discharge current in amperes
36
t: Time in seconds the surge reaches voltage (V)
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August 28, 2009
IEEE P1627/D1.3
1
2
If the surge wave shape is known, then another easier expression for the energy discharged
through an arrester may be calculated by using the equation (11) below [B5].
3
W = KVCIτ
4
5
K = Constant, 0.5 for triangular wave, 1.0 for rectangular wave and 1.4 for exponential decaying
wave
6
W = Energy in joules
7
VC = Clamping voltage in volts
8
I = Impulse current in amperes
9
τ = Impulse duration in seconds
(11)
10
5.1.6 DC Surge Arrester Application Analysis Calculation
11
12
13
An engineering analysis calculation for application of dc surge arrester to OCS operating at 800 V
nominal dc voltage is discussed. Calculations are based upon using surge arresters with MCOV
rating of 900 V and 1800 V and other parameters published by their manufacturer.
14
15
16
a)
Using rate-of-rise of incoming surge voltage (dv/dt) at the OCS surge arrester location of
11 kV/s per kV MCOV [B1], the calculated values of dv/dt will be 9.90 kV/s and 19.8
kV/s for the 900 V and 1800 V arresters respectively.
17
18
b)
Surge arrester lead lengths inductance using 0.4 H/ft with 25 feet length will be in the
order of 10 H for each arrester.
19
20
21
c)
Rate-of-rise of the surge current at the arrester location by using equation (4) and surge
impedance of arrester leads of 400  will be 0.025 kA/s and 0.050 kA/s respectively
for each arrester.
22
23
24
d)
Using the published data shown in Table 1 for surge arrester front of wave protective
level at 0.5 s, 10 kA peak discharge current, discharge voltage (VIR) will be 3.4 kV and
5.8 kV respectively for each arrester.
25
26
e)
Margin of protection voltage (Vsa) by using equation (9) without considering (IZSG) factor
will be:
27
900V : Vsa = 3.4+0.25 =3.65 kV
28
1800V: Vsa = 5.8+0.50 =6.30 kV
29
30
31
32
33
f)
Energy (W) in Joules discharged by the surge arrester for a switching surge may
conservatively be estimated by the equation (11). The values indicated in Table 4 are in
the order of 2600 and 5000 joules respectively for 900V and 1800V arresters for the
long wave 10/1000s and may be compared with the calculations in this step for
45/1000s wave:
34
900V DC MCOV Rated Surge Arrester:
35
W = 0.5 * 2.2 * 0.5 * 45 +1.4 * 2.2* 0.5* 0.5*1 000
36
W = 24.75 + 770.00 = 794.75 Joules
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15
August 28, 2009
IEEE P1627/D1.3
1
1800V DC MCOV Rated Surge Arrester:
2
W=0.5 *4.4.2*0.5*45+1.4*4.4*0.5*0.5*1000 =1589.5 Joules
3
4
5
6
7
8
It should be noted that the calculated values of energy for each surge arrester using 45/1000s
wave parameters are lower than the published data of energy discharge capability of the arrester
using 10/1000s wave. Since the characteristics of the two wave shapes are different, therefore
manufacturer should be consulted to provide discharge energy capability for (45/1000s) wave.
g)
Voltage surge through feeder cable (VFC) in kV at the dc switchgear can be calculated
by use of equation (5):
9
900V MCOV Surge Arrester: VFC=2x3.65x(40/3)/[400+40/3] = 0.24 kV
10
1800V MCOV Surge Arrester: VFC=2x6.30x(40/3)/ [400+40/3] = 0.41kV
11
12
13
14
It appears that without the effect of surge impedances, the conservative voltage impressed at the
equipment may be 3.65 kV and 6.30 kV, respectively as indicated in item e) above.
h)
Current surge (IS) associated with voltage surge derived in item g) above can be
calculated by use of expression (3)
15
900V MCOV Surge Arrester: IS = ES/ZS = 0.24x(3/40) = 0.018 kA
16
1800V MCOV Surge Arrester: IS = ES/ZS = 0.41x(3/40) = 0.0.031 kA
17
18
19
20
21
22
Although the energy capability of 1800V arrester is higher than the energy capability of 900V
arrester, however, the 900V arrester provides better voltage protection if it can withstand system
energy capability. It shall be noted that 900V MCOV rating appears to be lower than the system
temporary overvoltage (ETOV) value of 1073 volts derived by using equation (7) and thus may lead
to premature failures in actual installation. Thus the surge arrester with MCOV rating of 1150 V
may be the proper choice for 800 V dc system.
23
6. Lightning Stroke Terminology
24
25
26
27
28
29
30
31
32
33
Perhaps it is best to clarify the terminology; reference [B20] makes a distinction between the
traditionally used term “stroke” and a more precise reference to the term, “flash”. A flash
describes the entire electrical discharge to the stricken object. Stroke, on the other hand,
describes only the high-current components of a flash. Because of the observed multiplicity of
strokes, the relationship between the terms “flash” and “stroke” is that there can be many strokes
in a single flash. Research into flash characteristics indicates that 55 percent of all flashes contain
multiple strokes, with an average value of three strokes [B20]. This information is important
because of the differences in wave shape of the successive strokes. The term “flashover” is
described as an electrical discharge completed from an energized conductor to a grounded
support structure, which will be OCS poles in case of an LRT system.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
16
August 28, 2009
IEEE P1627/D1.3
1
2
Fig.5 Surge Arresters Configuration – Surge Impedance Diagram2
3
4
6.1 Lightning Intensity Estimation
5
6
7
Lightning intensity within a specific area is generally based upon the ground flash density, N g, in
flashes per km 2/year. At present, this data is not available in the United States and thus, lightning
intensity must be based upon the isokeraunic level, or the number of thunderstorms per year, T d.
2
Future research may provide a software program to provide surge current distribution
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
17
August 28, 2009
IEEE P1627/D1.3
1
2
The value of Ng may be approximated by using the following empirical expression [B4] [B19]. With
more research data available in the future such an expression may change.
3
Ng = 0.04 Td1.25
4
5
6
7
8
9
For example consider the area of a light rail transit (LRT) where T d is in the order of 40-60
[B3][B4]. Using expression (12) and Td of 60, the calculated value of Ng will be near 6.68. It is
noted that the exponent value of 1.25 in expression (12) is somewhat uncertain, for some
published literature indicates this value to be 1.35. However, 1.25 has been accepted by the
committee responsible for the development of the standard [B19] and thus, for this example, OCS
lightning protection analysis will be based upon the value of Ng to be around 6.68.
10
11
12
13
This calculated number for Ng provides some measure of likelihood of lightning strike to ground in
the area. The actual number of lightning flashes/year, NOCS, that may strike the light rail OCS or
nearby ground inducing direct or indirect lightning surge waves, may be calculated by using the
following expression:
14
NOCS = wLNg
15
Where:
16
L = length of LRT system in kilometers
17
w = Width of area covering LRT tracks in kilometers
18
19
Assuming a double track dc system with width near 0.015 kilometer and Ng of 6.68, by expression
(13), calculated value of NOCS will be: L/10.
20
NOCS = L/10
21
22
23
Assuming the probability of direct hit of lightning strike to OCS (N D) is 20% of the value calculated
for the actual number of lightning flashes/year, NOCS in that case, for the above example following
relationship applies.
24
ND = 1/5 (NOCS) = L/50
25
Where: L is the dc electrification system length in kilo-meters
26
27
28
29
Assumed low probability of 20% of direct lightning hit to OCS indicated above is based upon the
reasoning that there are equal chances that lightning may hit any of the OCS support structures,
nearby buildings, trees, substation structures, communication and control cabinets including
running rails.
30
31
32
33
34
35
Thus in the above example for a dc system with 10 km length, the calculated number of lightning
flashes per year (NOCS) that may strike the OCS system is one (1) and perhaps 50 km length is
needed for direct hit to OCS. For 10 km length of OCS system, the expected single lightning flash
per year may not be a direct hit to the OCS system. In addition, the expected single lightning flash
may or may not be of concern, depending upon the severity and energy associated with the
lightning stroke (surge) contained in the flash.
36
6.1.1 Lightning Stroke – OCS Flash Over
37
38
39
40
This discussion is intended to establish the lightning overvoltage intensity to the OCS
components, especially the contact wire, which is generally protected by dc surge arresters. The
various components of the OCS, including messenger wire, contact wire, feeder cables and
supporting structure (which consists of metallic poles, cross-arms, and running rails), are relatively
(12)
(13)
(14)
Copyright © 2007 IEEE. All rights reserved.
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(15)
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IEEE P1627/D1.3
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2
close to each other. There are equal chances that the lightning strike may hit any of the abovedescribed OCS components.
3
4
5
6
7
8
The messenger wire, cross-arms, and grounded metallic poles may provide some measure of
shielding of direct lightning strike to the OCS contact wire. In rare circumstances, if the lightning
strikes directly to the OCS wire, flashover is almost certain since the insulated air gaps and
clearances from the grounded metallic components including the poles is relatively low with wet
and dry flashover values near 20 kV to 35 kV peak respectively for 750 V nominal dc. Lightning
strike energy after the flashover at the OCS pole will go to ground via grounding path of the poles.
9
10
11
12
13
14
After the flashover, the maximum voltage expected at the OCS contact wire would not be more
than the actual dry flash over value of the insulator. The time to flashover from stroke, the energy
contained in the remaining surge wave at the OCS, and its propagation away from the point of
strike will depend upon the rate-of-rise of the incoming surge waves of the lightning flash strokes.
As indicated earlier, 60 percent of the strokes may strike the OCS poles and the remainder at
mid-span of poles.
15
16
17
18
19
20
21
The maximum distance that a lightning surge will need to travel before hitting the grounded pole
for flashover phenomenon is ½ of the pole spacing distance, which in terms of the surge wave
propagation time is relatively small. Without the application of dc surge arrester at each OCS pole,
the metallic grounded OCS poles will provide adequate path to the lightning strokes with peak
voltages exceeding dry flash over voltage of the insulators. This OCS poles flashover to ground
will cease automatically once the OCS surge voltage falls below the insulator’s actual flash over
voltage. The flashover may occur again if there are repeated lightning strokes in a particular flash.
22
23
24
25
26
27
28
If the flashover occurs near the dc feeder poles with dc surge arresters, the dc surge arrester may
also start discharging during the pole flashing. It is also apparent that as the propagation time of
the surge to adjacent feeder pole towards the next substation is small, the surge arrester on
adjacent substation will also start conducting. In addition, the surge wave will also propagate via
an underground feeder cable to the dc switchgear with a reduced surge magnitude indicated by
equation (5) in section 4.2 earlier. Thus, surge arresters applied at the dc feeder breakers will
reduce the effect of surge propagation on feeder cables and the substation equipment.
29
30
31
32
33
For a LRT system in a high isokeraunic area, if the flashover occurs to OCS poles or rails, then
the induced surge voltage will get into the running rails, or if the surge strikes directly to the
running rails then the surge may propagate to the substation negative bus via the negative
underground dc feeders. Therefore in such areas surge arresters should also be applied at the dc
negative bus or the running rails.
34
35
36
37
38
39
40
In case of high isokeraunic lightning stroke, a concern of damage to the surge arrester rises due
to its limited surge energy handling capability. However, it appears that for such a severe lightning
stroke, flashover across the outer surface of the surge arrester may occur due to its short length.
Such flashovers will drain the surge energy to ground leaving lesser surge current and energy to
be discharged through the surge arrester. If there is still some concern of the dc surge arrester to
be inadequate in handling the surge energy, then properly rated surge arrester with higher surge
energy handling capability should be applied.
41
42
43
44
45
46
47
48
The dc surge arresters applied at the dc feeder poles or other locations should be adequate to
handle the discharge current of the lightning surge wave deposited by the lightning flash strokes
after the OCS flash over occurs. In addition, the dc surge arrester discharge voltage should be
such that it provides adequate voltage margin of protection to the operating Light Rail Vehicle
(LRV) and the traction power substations. Since these surge arresters at the OCS contact wire
are first lines of defense to trap the lightning/switching surge voltage below the protection level of
the connected equipment, an engineering analysis of surge arrester voltage ratings should be
performed for proper selection of the surge arresters [B21].
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
19
August 28, 2009
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IEEE P1627/D1.3
6.1.2 Lightning Stroke Magnitude
2
3
4
5
6
7
8
9
10
11
12
Research on the stroke current peak amplitude reported that the mean value of first stroke is near
31 kA, with a 95 percent probability of the stroke magnitude being between 10 and 100 kA [B20].
The first stroke wave shape mean value just before the current peak has been reported to be near
24.3 kA/s, which is helpful in understanding the impulse voltages that can occur for discharges
through inductances. It is necessary to indicate that although the average value of the peak
magnitude of the subsequent stroke(s) is generally less than the first stroke, the wave front(s) of
the subsequent stroke(s) are typically faster. The average value is near 39.9 kA/s, although
values in excess of 70 kA/s have been reported. The above mentioned stroke parameters relate
to the flash itself and much of the data was obtained from mountaintop observatories [B28]. It is
also reported that 60 percent of the direct flashes hit the tower where they would flashover to the
ground and the remainder hit on the spans between the towers.
13
14
The above listed current lightning waves develop very high corresponding voltage waves based
upon their relationship provided by the equations (3) and (4) described under Section 4.2 earlier.
15
16
17
18
19
20
Consideration must be given to some modification of the flash characteristics striking OCS,
especially when tracks may be surrounded by urban development. Any high-rise buildings
including the trees and street light poles that are taller than the OCS poles will provide some
degree of lightning flash shielding to the OCS. However, since there is no measured research
data specifically for the dc transit OCS, the conservative approach is to use the data available for
the transmission towers for the OCS.
21
6.1.3 Lightning Stroke Induced Over-voltage
22
23
24
25
26
27
28
29
30
31
Lightning overvoltages are also possible due to electric and magnetic fields induced from nearby
lightning, often referred to as indirect or induced surges. For transmission lines, peak
overvoltages induced by first strokes varied between +150 kV and –40 kV, the mean being 23 kV.
The mean rise time for these voltage surges was 6 s. This provides rate of rise of the voltage
wave to be approximately 4 kV/s. The study further revealed that induced overvoltges caused by
subsequent lightning strokes had 11 kV peaks, with a mean rise time of 4 s. This provides a rate
of rise of the voltage wave to be approximately 3 kV/s, which is much lower than the values
reported for the direct lightning flash hitting the transmission lines. Such lightning wave
parameters may be used for LRT system design and engineering analysis of lightning protection,
which is the purpose of this standard.
32
6.1.4 Lightning Stroke Surge Energy
33
Surge energy (J) may be calculated by the expression [B1]:
t
34
J=
t
dv di
 V .I .dt   dt . dt t
0
2
dt Joules
(16)
0
35
Assume lightning stroke with the following parameters:
36
dv
 200 kV/ s (2x1011 V/ sec)
dt
37
38
Assume surge impedance for surge voltage to be near 40 ohms, parallel combination of OCS with
supplementary feeder cable. Thus surge current wave by use of expression (18) will be as follows
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
20
August 28, 2009
IEEE P1627/D1.3
1
di
 5kA / s (5x109 A/sec)
dt
2
3
4
The maximum flashover kV peak for OCS is near 35 kV (dry weather condition), thus within
35/200 s (0.18s), OCS poles will flashover to ground with or without the application of dc surge
arresters.
5
6
Thus the lightning stroke energy that may pose threat of OCS damage or the dc surge arresters
will be for flashover time of 0.18 s with calculated stroke energy value indicated below.
7
t = 0.18 s (18x10-8 sec)
8
J = 2x1011 x5x109.x [t
9
J = 1021x 10-24 x183 x 1/3 joules = 5.83/3 kJ
3
x10
/ 3]18
0
8
joules
(17)
10
11
12
13
14
15
The OCS system appears to get self-relief from the heavy lightning stroke energy (responsible for
damage to dc surge arresters and other OCS equipment) due to flashover near 35 kV peak surge
magnitude without the help of surge arresters. However, 35 kV peak voltages are quite damaging
to the system components, such as dc switchgear and also LRV components. Thus dc surge
arresters of proper rating should be applied. These surge arresters will discharge current and will
handle energy as indicated in expressions (20) and (21) below.
16
6.1.5 Arrester Discharge Energy
17
18
19
20
21
22
23
24
Arrester discharge current is a function of many interrelated parameters, including:

Surge impedance of the OCS

Stroke current characteristics, wave shape, peak current magnitude, and its rate-of-rise

Distance of the surge arrester from the point of stroke

Ground resistance at the location of stroke

Number and locations of flashovers

Flashover characteristics of the OCS insulators

Arrester discharge voltage
25
26
The following expression [B4] is used for power distribution overhead lines and may be used for
the OCS system:
27
IA = (ES - EA)/ Z
28
Where:
29
EA = Arrester impulse discharge voltage (kV) for current IA (kA)
30
ES = Prospective surge voltage (kV)
31
Z = Surge impedance of the conducting path of the surge arrester ()
32
IA = Impulse current (kA) associated with impulse voltage
33
34
Energy discharged by the arrester, J, in kilojoules (kJ), may be conservatively estimated by the
following expression [B18]:
35
J = 2 DL EA IA /v
(18)
(19)
Copyright © 2007 IEEE. All rights reserved.
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August 28, 2009
IEEE P1627/D1.3
1
Where:
2
EA = Arrester discharge voltage (kV)
3
IA = Switching impulse current (kA)
4
DL = Line length (miles) or (km)
5
v = the speed of light (190 miles/ms) or (300 km/ms)
6
7
The expression assumes that the entire line is charged to a prospective switching surge voltage
and is discharged through the arrester during twice the travel time of the line.
8
9
If the surge wave shape is known, then another easier expression for the energy discharged
through an arrester may be calculated by using the equation (11) described under section 5.1.5.
10
7. Lightning Stroke to OCS system
11
12
13
14
15
16
17
For analysis purposes, assume maximum distance between the substations to be near 1 1/2
miles. Assume that there are surge arresters installed only at the feeder poles adjacent to each
traction power substation, and there are no other poles between the substations that are equipped
with surge arresters as shown in Fig. 5. A lightning strike hitting the OCS wire midway between
the two substations will propagate equally with 1/2 the impinging surge current magnitude to each
substation [B2]. Thus, in this example the surge will travel maximum distance of 3/4 mile before
reaching a pole with dc surge arresters.
18
19
20
21
For a 750V dc nominal LRT system voltage, consider a dc surge arrester rated at 2 kV duty cycle
with MCOV rating near 1800 V dc with discharge voltage rated at 7.0 kV. This discharge voltage is
the surge arrester test voltage, which is based upon 20 kA peak current of a standard 8/20s
wave.
22
23
Time in milliseconds to travel 3/4 mile will be 3/4x1/190 (4s). The energy discharged through the
surge arrester in kJ using equation (19) may be calculated as shown in equation (21).
24
25
26
27
28
Assume surge wave is magnified to twice its magnitude (2 times 35 kV dry flashover value of
OCS) due to open circuit condition of a sectionalizing dc disconnect switch. Using OCS surge
impedance of 40 ohms (surge impedance of OCS wire in parallel with underground
supplementary cable), and assuming surge arrester discharge voltage (7 kV) to be the test
voltage at 20 kA peak, the discharge current IA and surge energy discharge will be as follows:
29
IA = (2x35 – 7.0)/40 = 1.575 kA
(20)
30
J = 2 x ¾x(1/190) x 7.0 x 1.575 kJ = 0.088 kJ
(21)
31
32
33
34
35
36
37
38
39
40
41
42
It should be noted that the time to travel 3/4 mile distance by the lightning stroke is very small and
it is possible that the lightning stroke time may be longer than two times the travel time for 3/4 mile
distance. Under such circumstances, the maximum estimated time for the lightning stroke should
be used for estimating the energy discharge through the surge arrester. The time 3/4x2/190 ms (8
s) used in calculating energy J in kilojoules should be increased to a reasonable value, say 300
s, the maximum estimated time the lightning flash containing more than an average of three
strokes may exist. This will lead to calculated energy of 3.30 kJ, which will still be below 4.4 kJ
(2.2 kJ/kV) value for a 2000V dc surge arrester. This estimation of surge energy is very
conservative as the assumed value of surge time and arrester discharge voltage seems to be on
the high side. However, dc surge arrester selection based upon such high energy discharge
requirements will assure that the arrester will not be damaged by high isokeraunic lightning flash
hitting very close to its location.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
22
August 28, 2009
IEEE P1627/D1.3
1
2
3
4
5
6
7
8
9
10
11
In the above calculation it has been assumed that lightning surge impinging the OCS wire will
flash over to grounded metallic OCS poles once the surge wave voltage reaches 35 kV peak.
Time to reach 35 kV peak will depend upon the rate of rise of lightning surge wave. If the rate of
rise for example is near 200 kV/µs, then the time to reach 35 kV peak will be 35/200 µs, which is
far less than the travel time of 4µs for 3/4 mile distance. Thus, OCS wire will not charge more than
35 kV peak voltage unless the surge comes across an open circuit caused by open position of a
disconnect switch. However, as the surge propagation time to reach open circuit location is quite
higher than the time to develop 35 kV peak voltages at lightning flash striking location, the OCS
pole flash will occur before double peak voltage (2x35 kV) is impressed upon the OCS contact
wire. Thus, flashover phenomena will reduce the surge energy that will be discharged through the
dc surge arrester.
12
13
14
To provide assurance that OCS wire flashing over occurs in case of a direct lightning strike
impinging the system in high isokeraunic areas horn air gap type arresters should be considered
midway between adjacent traction power substations.3
15
7.1 Underground Supplementary Cable Connections to OCS
16
17
18
19
20
21
22
23
24
25
In certain sections of the OCS, it is assumed that there will be underground supplementary cable.
For discussion purposes, it is assumed that the average distance between the OCS contact wire
and the underground supplementary cable tap connections are in the order of approximately 400
feet. For calculation purposes, assume 4000 feet length of such supplementary cable which will
then require a total of eleven (11) OCS contact wire-to-supplementary cable tap connections. This
configuration of OCS and supplementary cable connections will have total of nine (9) underground
cable splices. Such underground supplementary cable installation, electrically in parallel to OCS
will require cable splice connections to be located in the underground manholes. Lightning surge
withstand capability of such underground cable splices in the manholes and the tap point
connection of cable to overhead OCS contact wire are of concern.
26
27
28
29
Analysis of the cable splices and cable connections to the OCS contact wire would require a
derivation of the peak value of the lightning surge voltage expected at these connection points
and cable splices. Then this surge voltage peak value will be compared to the tolerable values of
basic surge withstand impulse voltages (BSL) of the cable splices and cable-to-OCS connections.
30
31
The basic switching surge level (BSL) of the 2 kV cable is near 75 kV peak. Underground cable
splice BSL levels to match with the cable BSL level are also available.
32
Assume the following:
33
ZOCS = 400 ohms (OCS contact wire surge impedance) [B2]
34
ZC =
40 ohms (cable surge impedance) [B2]
35
36
Vi =
Voltage magnitude of the incident lightning wave at the impedance junction point
(connection of cable to OCS contact wire or at the underground cable splice)
37
Ii =
Current magnitude of the incident lightning wave at the impedance junction point
38
Vr =
Voltage magnitude of the reflected lightning wave at the impedance junction point
3
Such air gap horn type surge arresters do not pose any threat of dc leakage current or uncertainty of their
damage due to ambient temperature. They shall be bonded to the OCS poles that are grounded with its own
grounding electrode with low resistance-grounding impedance by appropriately sized wire (not less than #6
AWG) 600V insulated cable to avoid jeopardizing the OCS double insulators criteria. Another alternate to
the horn gap is to consider surge arrester with higher duty cycle voltage.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
23
August 28, 2009
IEEE P1627/D1.3
1
Ir =
Current magnitude of the reflected lightning wave at the impedance junction point
2
V=
Total voltage magnitude (refracted voltage) at the impedance junction point
3
I=
Total current magnitude (refracted current) at the impedance junction point
4
The following expressions are well documented [B1].
5
V =Vi + Vr
(22)
6
I = Ii + Ir
(23)
7
8
At the interface of two surge impedances Z1 and Z2, the expressions for the above indicated surge
voltage and current are related by the following expressions:
9
V = [2x Z2 /(Z1 + Z2)] Vi
(24)
10
I = [2x Z1/(Z1 + Z2)] Ii
(25)
11
12
13
14
15
16
17
For the sake of completeness, expressions for the surge current as well as the surge voltage
have been described. However, analysis of the surge wave voltage is more critical for the cable
insulation protection when compared to surge current. It is well understood that the cable can
tolerate excessive magnitude of surge current for short duration without appreciable heat rise to
create damage to cable insulation. Hence, only the surge voltage analysis is presented under
Sections 7.1.1 and 7.1.2 below. Single Line diagram indicated in Fig.1 shows basic elements of
the power system.
18
7.1.1 Lightning Hits OCS ahead of the Supplementary Cable Connections
19
20
21
22
23
The initial and final surge voltages at the junction points of cable to OCS or splice point of
supplementary cable may be calculated by using expression (24) which requires knowledge
incident voltage magnitude (Vi) through surge impedance Z1 before hitting the junction point of
surge impedances Z1 and Z2. In the case of OCS and supplementary cable, the surges
impedances are as follows.
24
Z1 = 400 Ώ, for OCS and Z2 = 40 Ώ for the supplementary cable.
25
26
27
28
If all cable to OCS taps is spaced equally and the installation is uniform, then, for practical
purposes neglecting the effect surge impedances of cable splices and the OCS/cable connection
tap points, the combined surge impedance (Z) of underground supplementary cable and OCS
contact wire may be represented by expression (26) shown below.
29
Z = Z1 x Z2 / (Z1 + Z2) ≈ Z2 (since Z1 >> Z2)
30
31
32
33
If voltages (V1), (V2), (V3) and (V11) are successively represented as the surge voltages at the first,
second, third and last (eleventh) junction points when the surge voltage travels along the OCS
section with underground supplementary positive cable, the expressions for these surge voltages
will be as follows:
34
V1 = [2x Z2/(Z1 + Z2)] Vi
(27)
35
V2 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)] Vi
(28)
36
V3 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)]2 Vi
(29)
37
V10 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)]9 Vi
(30)
Copyright © 2007 IEEE. All rights reserved.
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(26)
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August 28, 2009
IEEE P1627/D1.3
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2
3
Using values indicated for the surge impedances in ohms, surge voltages represented by
expressions (27) through (30) will be practically equal in magnitude, approximately 18% of the
incident stroke surge voltage magnitude.
4
5
The final (eleventh) point will be end of the supplementary cable where the surge impedance will
become again Z1 and the surge voltage will be escalated as follows:
6
V11 = [2x40/(400+40)].[2x400/(400+40)]Vi
7
8
This final voltage appears to be approximately 33% of the initial surge voltage that appeared when
the surge entered parallel combination of OCS and supplementary cable.
9
10
11
12
13
If installation of the underground supplementary cable riser feeders and OCS connections is
uniform, then the surge impedance will be practically the same, slightly less than 40 ohms. The
above calculations indicate that voltage will never be more than the striking voltage unless there is
a switch that may be in an open position to make this voltage two times the initial surge voltage.
This twice the initial surge voltage can be derived by using the expression (24) as shown below.
14
V = [2x Z2 /(Z1 + Z2)] Vi = [2/(Z1/Z2+ 1)] Vi
15
16
17
Since Z2 at open switch will be infinite, thus Z1/Z2 will become zero in expression (32) making
surge voltage V as two times the initial voltage Vi. These surge voltage calculations do not take
into account the effect of surge attenuation due to cable inductance and capacitance effects.
18
19
20
21
If we assume that the striking voltage is limited to 35 kV by the flashover phenomenon, then it
appears that the underground supplementary cable splices and the OCS-to-supplementary feeder
cable tap connections may not require surge protection, except at the first and the last connection
points at the OCS.
22
7.1.2 Lightning Hits OCS within Supplementary Cable Connections Zone
23
24
25
26
27
28
29
The surge voltage analysis for this case will be identical to the analysis presented under Section
7.1.1 above, with the exception that the incident surge in the air at the OCS will propagate to each
side traversing the OCS/cable tap connections and underground supplementary cable splices.
From a theoretical point of view, the current surge that propagates in each direction will practically
be half the magnitude of the incident surge stroke current. The final maximum surge voltage will
be at the outermost cable-to-OCS connection tap points, and it will practically become double the
traveling surge voltage as indicated by the following calculation.
30
V = [2x 400 /(400 + 40)] Vi = 1.82x Vi
31
32
33
34
This voltage V will be equal to the initial lightning stroke surge voltage, which initially split into half
the magnitude at the strike locations. All intermediate tap points will see a lesser surge voltage in
the order of 9% (from analysis in 7.1.1) magnitude of the initial lightning stroke before it splits into
half the magnitude.
35
36
37
Thus, if the OCS flashover voltage is near 35 kV without the application of the dc surge arresters,
then the maximum surge voltage will be near 35 kV peak or 70 kV if the design installation
involves dc disconnect switch which is open.
38
7.1.3 Lightning Surge Propagation Discussion for Supplementary Cable
39
40
41
From a theoretical point of view, more surge current may tend to propagate through the
underground supplementary cable as compared to the OCS wire due to difference in the values of
their respective surge impedances. However, the speed of surge propagation through OCS wire is
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
(31)
(32)
(33)
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IEEE P1627/D1.3
1
2
3
two times the speed of surge through supplementary cable and feeder taps [B2]. This may lead to
balancing out the surge energy propagation through OCS and underground supplementary cable
and feeder taps.
4
5
6
7
8
It should also be mentioned that the underground supplementary cable switching surge peak
impulse voltage withstand level far exceeds 35 kV peak surge wave that can be expected without
considering the doubling effect. Thus it is not necessary that dc surge arresters be applied at
every 400 feet at supplementary cable-to-OCS connection locations if there is no dc disconnect
connection.
9
10
11
However, doubling peak voltage effect cannot be avoided at the dc disconnect switches when
they are in open position. Thus at disconnect switch locations, dc surge arresters are required on
each side of the switch.
12
13
In addition, it is prudent to perform surge analysis based upon actual configuration of the
OCS/supplementary cable to optimize the design of surge arrester applications to the dc system.
14
15
16
17
18
19
20
In specific configurations of OCS under the high voltage utility overhead lines, consideration shall
be made to prevent live ac wire contacting the OCS by use of guard wire or some other means.
Additional surge arrester at the OCS in the vicinity of OCS/high voltage utility lines should not be
used due to its misapplication for such a hazard. If the high voltage line touches the OCS, then
the dc system will be exposed to a sustained over voltage condition equal to dc voltage plus the
peak ac system voltage leading to damage of the surge arresters and other dc system
components until the utility fault is cleared
21
22
23
It is a general opinion that guesswork and overconcern of lightning protection without performing
surge analysis indicated in this standard has led to a design of applying surge arresters at each
OCS-to-supplementary cable taps. 4
24
8. Grounding
25
8.1 OCS Pole Grounding
26
27
28
29
30
8.1.1 OCS support poles with surge arresters shall have dedicated ground rod (ground electrode)
for connecting the surge arrester to ground by use of exothermic weld. Measured grounding
resistance of surge arrester ground electrode should not exceed 5 ohms to avoid excessive surge
arrester residual voltage at OCS when a heavy lightning surge current discharge current takes
place either through flash over across the arrester or by current discharge through the arrester.
31
32
33
34
35
36
37
38
Surge arrester ground rod should not be bonded to OCS pole foundation re-bars to avoid the
uncertainly of damage to the footing due to energy dissipation of a lightning surge at a rapid
speed. Bonding the pole with surge arrester grounding system may also transfer surge hazard to
the pole which is not desirable. In addition, separate dedicated grounding rod is needed to ground
OCS pole. All OCS poles with surge arresters shall be grounded individually by a maximum of 25
ohms grounding system. For those poles that are accessible to public the grounding resistance
shall be 5 ohms or less. [B27] Following the installation of the pole mounted surge arrester its
grounding system resistance shall be measured and recorded for future reference.
4
Such a design should be avoided based upon the surge voltage analysis presented in this standard.
Addition of such excessive number of dc surge arresters to the OCS is an application concern, especially
when such surge arresters do not have any indication that the arrester is in a degraded mode and may be
injecting undesirable dc stray current to ground.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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August 28, 2009
IEEE P1627/D1.3
1
9. DC Surge Arresters
2
9.1 Application
3
4
5
9.1.1 Surge arresters shall be provided and shall be connected to the OCS as a minimum at the
feeder poles. Two (2) parallel surge arresters with separate ground conductor and separate
ground rods may be needed at feeder poles for high intensity lightning area. [B27]
6
7
8
9
10
11
12
13
14
15
16
17
a)
Considering the low profile of OCS, proximity of all components, inherently grounded
poles, and major portion of the dc rail transit system close to high-rise structures and
trees, the probability of lightning striking the OCS is very low. With this configuration,
application of the ground shield wire above the messenger and contact wire does not
appear to provide any greater degree of protection, especially when the lightning strike
tends to flashover the grounded structures.
MOV surge arresters are sensitive to ambient temperature. In the summer when ambient
temperature is high, metallic tip of the MOV dc surge arrester may become hot leading to transfer
of heat to the surge arrester material. This may cause premature surge arrester failures. In
addition, the surge arrester provides leakage current under TOV conditions leading to heat
dissipation as well degradation of its internal MOV elements. Thus the installation should consider
excessive temperature effect on performance and selection of MOV surge arresters.
18
19
a)
20
Wire lead from arrestor to positive (or negative OCS) shall not exceed 610 mm where
possible.
MOV dc surge arresters should be installed at the following locations:
21
22

At positive (and negative for electric trolley bus) feeder poles, close to pole-mounted or
pad-mounted dc disconnect switches on load side of the switches.
23
24

At positive (and negative for electric trolley bus) pole-mounted or pad-mounted OCS
sectioning switches. Arresters shall be installed on both sides of the switch.
25
26

The application of dc surge arrester for dc switchgear is not covered in this standard;
however, review of surge arrester at the OCS and dc switchgear should be coordinated.
27
28

The application of dc surge arrester for vehicle is not covered in this standard; however,
review of surge arrester at the OCS and vehicle should be coordinated.
29
30
31

For OCS system within high isokeraunic areas, consider installing surge arresters at the
negative bus to protect the equipment under rare circumstances of lightning surge
reaching the negative bus via running rails and dc negative feeder cables.5
32
33
34

Install dc surge arrester at positive (and negative for electric trolley bus) underground
feeder tap location. As a minimum at the first and last OCS to underground positive
(negative) supplementary feeder cable tap location.
35
36

Surge arresters shall be considered at locations where surge impedance changes due to
OCS configuration, such as bridges and end points of the tunnels.
5
Surge arrester at appropriate locations on running rail may be applied in elevated guideways in areas of
high isokeraunic levels
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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August 28, 2009
IEEE P1627/D1.3
1
2

Surge arresters shall be considered for high isokeraunic areas, at the midway between
the adjacent substations.
3
4

Surge arrestors shall be considered in the above applications for negative contact wires
and feeder cables in electric trolley bus systems.
5
6
7
8
b)
An engineering analysis should be performed to determine appropriate voltage and
energy capability rating of the dc surge arresters. The analysis should take into
consideration the OCS location, ambient environment and operating voltage
characteristics.
9
10
c)
MOV dc surge arresters continuously conduct milli-amperes level of current to ground.
This current may increase if the internal material becomes defective. 6
11
9.1.2 Name Plate Information
12
The following information shall be provided:
13

DC Surge Arrester
14

Rated voltages, duty cycle voltage and MCOV
15

Nominal peak discharge current with current waves of 8/20 s, and 100/1000 s
16

Surge discharge capability kA peak
17

Energy discharge capability (klo-joules)
18

Manufacturer’s name
19

Year of manufacture
20

Serial number
21
9.2 Surge arrester Rating
22
23
9.2.1 Surge arresters discharge voltage shall be no more than 80% of the BIL of the equipment
that is being protected.
24
25
26
9.2.2 MCOV voltage rating of surge arresters shall be greater than Temporary Overvoltage TOV
of the OCS system. For dc surge arrester ratings refer to Tables 1, 2 and 3 under Section 5 and
Table 5 below.
27
6
Future development of dc surge arresters should provide visual indication when the surge
arrester becomes defective or fails so that it can be removed to avoid the uncertainty of draining
continuous dc stray current to ground.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
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August 28, 2009
IEEE P1627/D1.3
1
2
Table 5: OCS System Surge Arrester Voltage Levels
System Nominal Voltage
(Volts) dc
System Max.
Continuous Voltage
(Volts) dc
Temporary overvoltage
voltage (ETOV )
(Volts) dc
600 to 850
1,020
1,150
1,500
1,800
2,000
3,000
3,600
4,000
3
4
5
9.2.3 DC Traction Power System Components’ Voltage Withstand Capability. DC Traction Power
System components’ including OCS and dc switchgear voltage withstand capability listed in Table
6 is the per IEEE draft Standard P1626 [B22] and IEEE STD C37.14 [B7].
6
Table 6: DC Traction Power System Components’ Voltage Withstand Capability
DC Switchgear
OCS minimum dc Spark over Voltage
Rated Voltage
BIL
Dry Weather
Wet Weather
(Volts dc)*
(kV Peak)*
(kV Peak)**
(kV Peak)**
1,300
65
45
25
2,100
65
45
25
4,200
65
45
25
7
*
Per IEEE STD C37.14 [B7]
8
**
Per IEEE Draft STD P1626 [B22]
9
10. DC Surge Arrester Service Requirements
10
11
12
13
Operating ambient temperature shall be within the range of the operating environment between 40 degrees Fahrenheit and + 104 degrees Fahrenheit. System OCS to ground voltage shall be
within the rating of the arrester under all system operating conditions. For OCS systems above
1,800 feet consult the surge arrester manufacturer for revised ratings.
14
11. DC Surge Arrester Testing
15
11.1 Design Tests
16
Surge arresters shall be subjected to the following design tests:
17

Insulation withstand test between the terminals
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
29
August 28, 2009
IEEE P1627/D1.3
1

DC dry and wet spark over test
2

Discharge voltage test
3

Impulse protective level voltage time characteristics
4

Accelerated aging procedure
5

Pressure relief test
6
7

Leakage current at MCOV and 5% voltage in access of MCOV, leakage current v/s
voltage
8

Effect of higher temperature on leakage current, current v/s temperature graph
9
10

Peak discharge voltage under 8/20 µs lightning surge, with peak current of 200 A, 500
A, 5 kA and 10 kA
11
12

Peak discharge voltages under 45/1000 µs switching surge, with peak currents of 100
A, 200 A and 500 A.
13
14

Energy absorbing capability for the 45/1000 µs switching surge with 100A, 200A and
500A peak current
15
16

Energy absorbing capability for the 8/20 µs lightning surge with 200A, 500A , 5 kA and
10 kA peak
17
18
New and clean arresters shall be used for each design test. For additional test requirements, see
IEC Draft Standard NE 50562-1.
19
The arrester shall be mounted in the position(s) in which it is designed to be used.
20
Ambient temperature for test shall be -40 to +104 degrees Fahrenheit.
21
11.2 In Service (Field) Tests
22
All surge arresters shall be visually inspected per IEEE Std. P1628 recommendations.
23
Surge arresters shall be subjected to electrical tests per manufacturer’s recommendations.
24
25
26
27
28
29
30
31
32
33
Apply dc test voltage in very slow increments of 20V dc each till the voltage reaches MCOV rating
of the surge arrester. At each voltage step, measure leakage current through the arrester as
excessive leakage current will result if arrester is defective. Do not apply voltage above MCOV
rating for longer than manufacture’s recommended time to avoid damaging the surge arrester
during testing. Voltage above TOV rating of OCS system shall only be surge waves and not the
dc voltage. Surge wave shall be 8/20s with peak current magnitudes of 0.5kA, 1.0kA and 1.5
kA. Such surge waves if applied should be for a very short duration less than 25s. Review such
testing requirement with the manufacturer of the surge arrester under test before application of
the suggested surge current for testing.
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
30
August 28, 2009
IEEE P1627/D1.3
1
11.3 Bibliography
2
[B1] Dr. Allen Greenwood, Electrical Transients in Power Systems, New York, Wiley 1971 Edition
3
4
[B2] IEEE Standard 141 – 1986, (Red Book), IEEE recommended Practice for Electric Power
Distribution for Industrial Plants
5
6
[B3] Dev Paul and S.I Venugopalan, " Power Distribution System Equipment Overvoltage
Protection", IEEE Trans. Ind. Appl., vol. 30, pp. 1290-1297, Sept./Oct. 1994
7
8
[B4] ANSI/IEEE Standard C62.22-1997, IEEE Guide for the Application of Metal -Oxide Surge
Arresters for Alternating Current Systems
9
10
[B5] Transient Voltage Suppression Devices Data Book .1992 Edition by Harris Semiconductor
Corporation
11
12
[B6] ANSI/IEEE Standard C37.20.1- 1987, IEEE Standard for Metal - Enclosed Low - Voltage
Power Circuit-Breaker Switchgear.
13
14
[B7] ANSI/IEEE Standard C37.14 - 1999, IEEE Standard for Low-Voltage DC Power Circuit
Breakers Used in Enclosures
15
16
[B8] NEMA Standard, Publication No. RI9-1968 (1978), Silicon Rectifier Units for Transportation
Power Supplies.
17
18
[B9] Tseng WU Liao and Thomas H. Lee “Surge Suppressors for the Protection of Solid-State
Devices," IEEE Trans. Ind. Appl., vol. IGA-2, pp.44-52, Jan. /Feb. 1966
19
20
[B10] Luke Yu, “ Quick Evaluation of Voltage Surge in Electrical Power Systems,” IEEE Trans.
Ind. Appl., vol 31, pp. 379-383 Mar./Apr. 1995
21
22
[B11] ANSI/IEEE Standard C62.33-1982, IEEE Standard Test Specifications for Varistor SurgeProtective Devices
23
24
[B12] ANSI/IEEE Standard C62.35-1987, IEEE Standard Test Specifications for Avalanche
Junction Semiconductor Surge Protective devices.
25
26
[B13] Roger C. Dugan, Mark F. McGranaghan, H. Wayne Beaty, “Electrical Power Systems
Quality” Chapter 4, McGraw-Hill Edition.
27
28
[B14] Jih-sheng Lai and Francois D. MartZloff, " Coordinating Cascaded Surge Protection
Devices: High-Low versus Low-High", IEEE Trans. Ind. App. vol. 29, pp. 680-687, July/Aug. 1993
29
30
[B15] Nirmal K. Ghai, “Design and Application Considerations for Motors in Steep- Fronted Surge
Environments”, IEEE Trans. Ind. App. Vol. 33, pp. 177-187, Jan./Feb. 1997
31
32
[B16] Keith W. Eilers, Mark Wingate, “Application and Safety Issues for Transient Voltage Surge
Suppressors” IEEE Trans. Ind. App. vol. 36, pp. 1734-1740, Nov/Dec. 2000
Copyright © 2007 IEEE. All rights reserved.
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August 28, 2009
IEEE P1627/D1.3
1
2
[B17] Paul G. Shade, “Vacuum Interrupters: The New Technology for Switching and Protection
Distribution Circuits” IEEE Trans. Ind. App. vol. 33, pp. 1501-1511, Nov/Dec. 1997
3
4
5
[B18] Edward A. Bardo, Kenneth L. Cummins, William A. Brooks, “Lightning Current Parameters
Derived From Lightning Location Systems: What Can We Measure?” 18th International Lightning
Detection Conference June 6-8, 2004, Helsinki, Finland www.vaisala.com/ILDC2004
6
7
8
[B19] IEEE Committee Report, “A Simplified Method of Estimating Lightning Performance of
Transmission Lines”, IEEE Transactions on Power Apparatus and Systems, PAS – 104, pp 919 –
932, 1985.
9
10
[B20] Transmission Line Reference Book, 345 kV and Above, Second Edition, Electric Power
Research Institute, pp .545-552.
11
12
[B21] Dev Paul, “Light Rail Transit DC Traction Power System Surge Overvoltage Protection”
IEEE Trans. Industry Application, Vol. 38, pp 21- 28, Jan. /Feb. 2002
13
14
[B22] IEEE Draft Standard P1626, “Standard for DC Overhead Contact System Insulation
Requirements for Transit Systems”.
15
[B23] NFPA 780 “Standard for the Installation of Lightning Protection Systems”
16
17
[B24] IEC 62305, “Protection against Lightning”, 2006-01
18
19
[B25] IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, IEEE
Std C.62.41 – 1991, pp 34.
20
[B26] British Standard BSN EN 50124-1:2001 Railway Applications – Insulation coordination
21
22
[B27] Dev Paul “DC Rapid Transit System: OCS Pole Grounding Technical Analysis and Safety,
APTA 2004”
23
24
[B28] Dev Paul “Lightning Protection Analysis of Light Rail Transit DC Overhead Contact System”,
IEEE I&CPS Conference 2005, pp.160-169
Copyright © 2007 IEEE. All rights reserved.
This is an unapproved IEEE Standards Draft, subject to change.
32