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1August 28, 2009 IEEE 2P1627/D1.3

1Draft Standard for DC Electrification Overhead 2Contact Systems, including Application of 3Lightning Arresters for Transit Systems

4Prepared by Working Group 17 of the Overhead Contact System Subcommittee 5Sponsored by the Rail Vehicle Transit Interface Standards Committee 6of the IEEE Vehicular Technology Society

11This document is an unapproved draft of a proposed IEEE Standard. As such, this document is 12subject to change. USE AT YOUR OWN RISK! Because this is an unapproved draft, this 13document must not be utilized for any conformance/compliance purposes. Permission is hereby 14granted for IEEE Standards Committee participants to reproduce this document for purposes of 15IEEE standardization activities only. Prior to submitting this document to another standards 16development organization for standardization activities, permission must first be obtained from 17the Manager, Standards Intellectual Property, and IEEE Standards Activities Department. Other 18entities seeking permission to reproduce this document, in whole or in part, must obtain 19permission from the Manager, Standards Intellectual Property, IEEE Standards Activities 20Department.

21IEEE Standards Activities Department 22Manager, Standards Intellectual Property 23445 Hoes Lane, P.O. Box 1331 24Piscataway, NJ 08855-1331, USA

25Abstract

26This standard is a basis for the design and application of dc surge protection devices to protect 27Overhead Contact System (OCS) from transient overvoltages associated with lightning and 28switching surges. Switching surges are inherent characteristics of electrification system and are 29generally considered low energy transient overvoltages. Lightning surges are natural, caused by 30electrical discharge that occurs in the atmosphere between clouds or between clouds and 31ground. Lightning surges are known for causing very high energy transient overvoltages by direct 32or indirect coupling with OCS. This standard covers transient overvoltage protection of OCS used 33in dc transit rail electrification systems. Transient overvoltage protection of third rail dc and the 34running rails can be achieved by the application of same type of surge protection devices as are 35recommended for the OCS, although these rails are less likely to be affected by lightning 36transient overvoltages due to their proximity to earth where flash-over can occur to drain surge 37energy away from the rails without the application of surge overvoltage protection devices. The 38word surge arrester instead of lightning arrester has been used in this standard without affecting 39the technical contents of the standard.

40Keywords

41Surge arrester, Lightning, Overvoltage, Switching surges.

3 i 1August 28, 2009 IEEE P1627/D1.3

1Introduction 2The majority of the present operating dc electrified rail systems use OCS or third rail to supply 3power to the vehicles. The probability of lightning surge hitting an OCS, or third rail depends on 4its geometry, its own height, and length, and its relative location with respect to the presence of 5buildings, trees, towers etc. in its vicinity, and the lightning flash-to-ground density (number of 6lightning-to-ground strokes per square kilometer per year) of the area [B23][B24].

7If after the lightning risk assessment, the expected frequency of direct lightning to the OCS (N D) to 8be protected exceeds its tolerable frequency of lightning (NC), as established by applicable 9standards and codes (NFPA 780, IEC 62305), lightning protection system (LPS) should be 10designed and installed. In this case OCS poles should be equipped with appropriate surge 11protection devices grounded by use of low resistance ground rods and OCS poles should be 12grounded by their separate ground rods.

13Should the calculated average annual number of direct lightning strokes (N D) to the OCS be 14below the permissible strokes (NC), no LPS (with the exception of application of dc transient surge 15protection devices of low energy capability ) is necessary and the OCS is defined as self 16protected by IEC 62305 and NFPA780.

17Arbitrary installation of direct stroke diverters such as lightning rods and ground wires above the 18OCS changes the geometry and may increase its chances of more lightning exposure. Based 19upon such reasoning it appears that even in the area of higher lightning strokes, the application of 20ground wires and ground rods above the OCS should not be used. Then this standard’s approach 21is to develop an OCS transient overvoltage protection that focuses on the basic approach of 22selection and application of dc surge arresters, their grounding configuration, and grounding of 23OCS metallic poles. This consists of combination of the following:

24  Selection of appropriately rated dc surge protection (lightning arrester) devices to protect 25 OCS from transient overvoltages caused by switching and lightning phenomenon.

26  Grounding of surge arresters by individually dedicated shortest possible grounding 27 conductor connected to ground rods.

28  Use of dedicated grounding of OCS poles by application of ground rods separate from 29 surge arrester ground rods to keep the structure potential below insulation breakdown 30 level of positive and negative feeder cables and to minimize damage to the vehicle.

31  Use of an additional surge arrester to protect the traction power substation equipment as 32 necessary.

33  Use of OCS basic insulation levels to minimize outages due to switching and lightning 34 surges.

35Setting basic insulation levels for the OCS is in the scope of work of another subcommittee and 36thus reference [B22] on IEEE Draft P1626 standard and [B26] are included in this standard. DC 37electrification OCS and associated structure should not be compared to electric utility 38transmission lines as the physical configuration and location of the two are quite different 39whereby concept of lightning protection by use of ground/shield wire may not be applicable to dc 40electrification OCS. However, in case of single phase ac electrification OCS, an overhead ground 41wire which carries return traction ac current also performs the function of shield wire.

42Prior to and during the development of this standard, there were reports of surge arrester failures 43on several transit systems resulting in service interruption and equipment failures. Questions 44were being raised regarding the grounding of OCS support poles, and the pros and cons of using 45surge arresters in the OCS. Most importantly, there was no understanding of the cause of

2 Copyright © 2007 IEEE. All rights reserved. 3 This is an unapproved IEEE Standards Draft, subject to change. ii 1August 28, 2009 IEEE P1627/D1.3 1failures of surge arrester and their proper application to dc electrification OCS. Although, so far no 2personnel injuries have been reported involving failure of dc surge arresters at any of the 3operating transit properties in USA.

4To establish lightning protection design measures, the derivation of lightning intensity and 5lightning stroke surge energy is established based upon the typical available lightning data. There 6are equal chances of a lightning strike hitting any of the system components due to their 7proximity, and thus flashover is certain if lightning strikes the OCS components since poles and 8OCS supporting members appear to be practically grounded for the surge voltage. Flashover 9appears to drain large amounts of lightning surge energy to ground, with the remainder of the 10surge energy relieved by the dc surge arresters placed at appropriate locations.

11When considering lightning protection for OCS, numerous questions arise, such as:

12 a) Will dc surge arrester handle surge energy if a lightning flash (strike) directly hits the 13 OCS wire near the arrester location? 14 b) If a lightning flash directly hits the OCS wire between two traction power substations, 15 what will be the energy discharged through the surge arresters at feeder poles?

16 c) Is there a need to apply dc surge arresters at the mid point of two adjacent substations 17 to enhance the lightning protection of the dc rail transit OCS?

18 d) Do we need to apply surge arresters at the connection points of underground dc 19 supplementary feeder cable to OCS contact wire located approximately every 400 feet, 20 where overhead messenger wire design is not possible in downtown areas due to 21 aesthetics and other restrictions of height of the messenger wires?

22 e) Should there be shield/ground wire above the messenger to enhance lightning 23 protection?

24Responses to the above questions are discussed in this standard.

25IEEE P1627 “Standard for Transient Overvoltage Protection of dc Electrification Overhead 26Contact System”, including application of dc surge protection devices was prepared by Working 27Group 17 of the Overhead Contact Systems Sub-Committee of the Rail Transit Vehicle Interface 28Standards Committee of the IEEE Vehicular Technology Society.

29Origin and Development of IEEE P1627 30The Overhead Contact Systems Sub-Committee was formed in 2001 with the purpose of 31developing standards governing the design, construction, and maintenance of the OCS and 32current collection system.

33Working group 17 was established to develop standards governing the transient overvoltage 34protection of OCS for the dc rail transit systems. The primary concern of the working group was a 35lack of uniform practices and a lack of understanding of the proper application of dc surge 36arresters and their grounding configuration to protect OCS and associated poles including dc 37traction power system and vehicles. Precise information of the transient environment, expected 38magnitude, duration and frequency of transient surges is very unpredictable and there has been 39no recorded data available to guide application engineers in the selection and application of dc 40surge protection devices. However, it is known that certain degree of threat of both the internal 41switching surges and the external lightning surges exist that could occur at random causing 42damage to OCS and dc system without proper application of surge overvoltage protection 43devices.

44It appears that the lack of knowledge of the transient environment and understanding the 45parameters and rating of dc surge arresters lead to use of guesswork in the selection and 46application of such devices causing failures at some installations. At some transit properties, 47failures of dc lighting arresters and equipment damage associated with those failures have been 48reported. Lack of understanding of characteristics of transient overvoltages, dc surge arrester 2 Copyright © 2007 IEEE. All rights reserved. 3 This is an unapproved IEEE Standards Draft, subject to change. iii 1August 28, 2009 IEEE P1627/D1.3 1ratings, their proper installation and grounding configurations, lack of their clear test and 2application data from the manufacturers has been a major concern in the industry that lead to the 3development of this standard. Proper application of dc surge protection devices to divert surge 4energy associated with the switching and lightning surges away from the OCS relates directly to 5their effect on equipment and personnel protection.

6Patents

7Attention is called to the possibility that implementation of this standard may require use of 8subject matter covered by patent rights. By publication of this standard, no position is taken with 9respect to the existence or validity of any patent rights in connection therewith. The IEEE shall not 10be responsible for identifying patents for which a license may be required by an IEEE standard or 11for conducting inquiries into the legal validity or scope of those patents that are brought to its 12attention.

13Participants

14At the time this standard was completed, the working group had the following membership:

15 Dev Paul, Chair

16 Ramesh Dhingra, Vice Chair

17 18Alan Blatchford 26Chris Pagni 34Carl Wessel 19Butch Campbell 27Steve Mitan 35Paul White 20Ron Clark 28Jay Sender 36Kelvin Zan 21Ian Hayes 29Jeffrey N. Sisson 37Ethan Kim 22Albert Hoe 30Vish Mawley 38Ramesh Dhingra 23Gordon MacDonald 31Edward Rowe 39Stuart Kuritzky 24Dev Paul 32Suresh Shrimavle 25 33

40The following members of the balloting committee voted on this standard. Balloters may have 41voted for approval, disapproval, or abstention. (To be provided by IEEE editor at time of 42publication.) 43______

21. Overview...... 1

3 1.1 Scope...... 1

4 1.2 Purpose...... 1

5 1.3 Format of Standard...... 1

62. References...... 2

73. Definitions, abbreviations, and acronyms...... 2

8 3.1 Definitions...... 2

9 3.2 Abbreviations and Acronyms...... 4

104. Transient Surges...... 4

11 4.1 Lightning and Switching Surges...... 5

12 4.2 Surge Characteristics - Propagation...... 6

13 4.3 Magnetic Stored Energy of Surge...... 7

145. Surge Environment – DC Electrification System...... 8

15 5.1 DC Surge Protection Device Requirements...... 8

166. Lightning Stroke Terminology...... 16

17 6.1 Lightning Intensity Estimation...... 17

187. Lightning Stroke to OCS system...... 22

19 7.1 Underground Supplementary Cable Connections to OCS...... 23

208. Grounding...... 26

21 8.1 OCS Pole Grounding...... 26

229. DC Surge Arresters...... 27

23 9.1 Application...... 27

24 9.2 Surge arrester Rating...... 28

2510. DC Surge Arrester Service Requirements...... 29

2611. DC Surge Arrester Testing...... 29

27 11.1 Design Tests...... 29

28 11.2 In Service (Field) Tests...... 30

29 11.3 Bibliography...... 31

1Draft Standard for DC Electrification Overhead 2Contact Systems, including Application of 3Lightning Arresters for Transit Systems

41. Overview

5Scope

6The scope of this standard covers application of transient overvoltage surge protection devices 7and their grounding configurations to protect OCS used in dc traction electrification for heavy rail, 8light rail, and trolleybus systems.

9Purpose

10The purpose of this standard is to establish minimum design requirements for application of dc 11surge protection devices and their grounding configurations to protect OCS and associated 12traction power system components from transient overvoltages caused by switching and lightning 13surges. Such a design will provide a reasonable degree of protection to equipment by diverting 14surge energy and related hazards away from the OCS system. At the present, there are no 15uniform practices for application of properly rated dc surge arresters and their grounding 16configurations to protect OCS used in dc traction electrification. The use of this standard is 17intended to provide understanding of the transient overvoltage environment that exists at a 18particular dc electrification system and then a systematic approach to protect OCS and 19associated dc traction power system from transient overvoltages by use of appropriately rated dc 20surge protection devices.

21Format of Standard

22First the basic characteristics of transient surges, their origin, and energy and propagation 23behavior are described in Sections 4 and 7. This is followed by a brief description of the surge 24environment expected at a dc electrification OCS included in Sections 5 and 7. In Sections 6 and 257, this standard addresses analysis of lightning strike to the dc traction power system 26components.

27This standard addresses OCS, the running rails, OCS supporting structure, metallic poles, 28messenger wire, underground supplementary conductors, traction power substations, and 29vehicles. This also includes lightning waveform parameters.

30This standard also addresses the mismatch between the actual number of lightning strikes and 31specificity of the lightning parameters required in performing lightning protection analysis for an 32electric transit system. At present there is no data for the lightning effects and design experience 33available from the operating transit properties. Experience from operational lightning location 34systems [B18] [B19] [B20] [B24], ongoing research, and scientific ability to measure lightning 35parameters can avoid guesswork in the lightning protection design of electric transit systems.

1From a cause and effect standpoint, the maximum rate-of-rise, the peak current, and the wave 2front rise-time are associated with determining the maximum voltage that will be seen on the OCS 3subjected to unpredictable threat of a direct or nearby ground lightning discharge. The probability 4of a lightning strike is greater when the electric transit system is located in a high isokeraunic 5level area. [B2].

6Lightning may be of concern when the dc system is in a relatively high isokeraunic area. 7Isokeraunic area map of the world can be seen in reference [B9]. In addition, dealing with a 8transit system involving the general public and, more importantly, the havoc due to interruption of 9system operations caused by lightning is a design concern for dc transit systems. Thus to 10minimize the effect of lightning and switching surge voltages to a typical dc traction power 11system, application of appropriately rated dc surge arresters should be included in the design.

122. References

13This standard shall be used in conjunction with the following publications. If the following 14standards are superseded by an approved revision or new version, the latest revision shall apply. 15In case of conflict between this standard and the referenced documents, this standard shall take 16precedence. Those provisions of the referenced standard that are not in conflict with this 17standard shall apply as referenced. (all referenced standards need to be referred in the text of the 18standard and they need to be in alpha numerical order) 19National Electrical Safety Code (NESC)

20National Electrical Code (NEC)

21Canadian Electrical Code (CEC)

22British Standard BSN EN 50124-1:2001 Railway Applications – Insulation coordination

23IEEE Standard Dictionary of Electrical and Electronics Terms, IEEE Std 100

24IEEE Guide for the Application of Metal Oxide Surge Arresters for Alternating Current Systems, 25ANSI/IEEE Standard C62.22,

26IEEE Guide for Application of Gapped Silicon Carbide Surge Arresters for Alternating Current 27Systems, ANSI/IEEE Standard C62.2 – 1987

28EN 50526-1 Draft Feb. 2009, “Railway applications – Fixed installations – D.C. Surge arresters 29and voltage limiting devices – Part 1: Surge arresters”

30EN 50526-2 Draft March 17, 2009, ““Railway applications – Fixed installations – D.C. Surge 31arresters and voltage limiting devices – Part 2: Voltage limiting devices”

323. Definitions, abbreviations, and acronyms

33Definitions

34Review all IEEE Dictionary terms For the purpose Insulation Standard IEEE P1626/D1 35IEEE Glossary of OCS

36 3.1.1 Clearance: Shortest distance in air between two conductive materials.

1 3.1.2 Creepage Distance: Shortest distance along the surface of the insulating material 2 between two conductive materials.

3 3.1.3 Electrical Section: Part of an electrical circuit having its own voltage rating for 4 insulation coordination. 5 3.1.4 Grounded: Electrical section intentionally connected to earth that cannot be 6 interrupted.

7 3.1.5 Insulated: All components isolated from the energized OCS conductors by at least 8 one level of insulation. An insulated section may be under the influence of adjacent 9 energized circuits. An insulated section may be considered as an electrical section.

10 3.1.6 Surge arrester: A device typically mounted on OCS poles and connected to the OCS, 11 designed to protect insulated feeder cables against lightning, by providing a path to ground 12 through a spark-gap, with or without variable resistance elements.

13 3.1.7 Nominal voltage: Value assigned to a circuit or system approximately equivalent to 14 the working voltage for designating the voltage class.

15 3.1.8 Overvoltage: Voltage having a peak value exceeding the maximum steady state 16 voltage at normal operating conditions.

17 3.1.9 Rated Voltage: Value of voltage assigned to a component, device or equipment.

18 3.1.10 Rated Impulse Voltage: Value of voltage assigned to the equipment referring to the 19 specified withstand capability of the insulation against transient overvoltages.

20 3.1.11 Rated Insulation Voltage: RMS withstand voltage assigned to the equipment 21 referring to the specified permanent (over five minutes) withstand capability of the insulation 22 between energized components and earth.

23 3.1.12 Residual Discharge Voltage (VIR): Is the voltage across the surge arrester at the 24 instant of surge peak current discharge of a specific surge current wave shape due to 25 lightning or switching phenomenon. Same magnitude of surge peak current with different 26 wave-shape may result in different value of VIR depending upon the type and manufacturer 27 of the surge arrester.

28 3.1.13 Maximum Continuous Operating Voltage (MCOV): The maximum designated 29 root-mean-square (rms) continuous operating voltage value in dc volts that many be applied 30 continuously between the terminals of the arrester.

31 3.1.14 Surge Arrester: See Surge arrester.

32 3.1.15 Isokeraunic level: Number of thunderstorm days per year in an area is called 33 isokeraunic level

34 3.1.16 Overhead Contact System (OCS): Contact wire and messenger wire electrically in 35 parallel connected to traction power substation (TPSS) by underground feeders. Contact 36 wire makes contact with vehicle pantograph and is acting as positive polarity contact point 37 for the dc power supply delivered to the vehicles from TPSS.

38 3.1.17 Lightning flash: Very bright light stream seen in the sky between the cloud and the 39 ground on a stormy day.

40 3.1.18 Lightning stroke: Same as lightning flash.

1 3.1.19 Lightning strike: Same as lightning flash.

2 3.1.20 Lightning Intensity: Number of lightning strikes/per square meter/year in an area is 3 called its lightning intensity.

4 3.1.21 Temporary Overvoltage (ETOV): The maximum root-mean-square (rms) temporary 5 overvoltage value in dc volts that may occur at the OCS due to vehicle regeneration or utility 6 power supply voltage regulation.

7 3.1.22 Supplementary Cable: Cable connected in parallel with OCS is called positive 8 supplementary cable. Cable connected in parallel with running rail is called negative 9 supplementary cable.

10 3.1.23 Voltage Margin-of-Protection (VSA): The voltage seen at the OCS (voltage between 11 the OCS and the local ground) when the lightning surge arrester conducts lightning surge 12 peak current and associated energy to ground.

13Abbreviations and Acronyms 14ANSI American National Standards Institute 15AREMA American Railway Engineering and Maintenance of way Association 16ASTM American Society for Testing and Materials 17AWG American Wire Gauge 18DC Direct Current 19ETB Electrified Trolley Bus 20FRA Federal Railroad Administration 21ICLP International Conference on Lightning Protection 22IEEE Institute of Electrical and Electronics Engineers 23ISO International Organization for Standards 24LRV Light Rail Vehicle 25NEC National Electrical Code (NFPA-70) 26NEMA National Electrical Manufacturers Association 27NESC National Electrical Safety Code 28NETA National Electrical Testing Association 29NFPA National Fire Protection Association 30OCS Overhead Contact System 31OSHA Occupational Safety and Health Administration Act 32RMS Root Mean Square 33ROW Right-of-way 34TES Traction Electrification System 35UBC Uniform Building Code 36UL Underwriters Laboratories, Inc. 37USASI United States of America Standards Institute 38USDOT United States Department of Transportation

394. Transient Surges

40Transient overvoltage surges are caused by lightning, switching phenomenon within the system, 41induced and impressed voltages and become superimposed over the dc power system voltage. 42They are unpredictable and could be brief and quick. Their wave-shape, magnitude, and energy 43content may vary considerably depending upon their cause of origin, system configuration, and 44surge impedance parameters. Such transient overvoltages and associated energy shall be 45diverted away from the system by application of appropriate dc surge protection devices at 46specific locations within the overall electrification system. Lightning surges, called external surges 47can impinge on the electrification system directly or indirectly, and can induce transient 48overvoltages with considerable energy to create hazards. Switching surges, called internal

1surges, are caused by sudden changes in the electrification system, such as dc breaker 2operation, and may not be as severe as lightning due to their relatively low energy.

3Lightning and Switching Surges

4The standards make a distinction between switching and lightning surges on the basis of the 5duration of the front or rise-time from zero-to-peak value. Surges with fronts of up to 20 s are 6defined as lightning surges, and those with longer fronts are defined as switching surges [B15].

7Lightning surge intensity depends upon the isokeraunic level of the area. The probability of direct 8stroke can be estimated by the various factors listed in NFPA 780. To understand the 9characteristics and nature of such surges, considerable published data is available [B1] [B2].

10For lightning surge application purposes, industry has standardized voltage impulse waveform 111.2/50 s indicating crest is reached in 1.2 s and it decays to half the crest in 50 s. Similarly, a 12current impulse wave of 8/20s is used where the crest is reached in 8 s and decays half the 13crest value in 20s.

14A steep-fronted surge is one with a rise time of 0.1-0.5 s and a virtual time to half value of 15around 5 s. An impulse current wave shape of 10/1000 s (long wave) is more representative of 16the high energy surges usually experienced from the inductive elements [B4] [B5].

175. Causes of Lightning Surges

18Lighting is an electrical discharge that occurs in the atmosphere between clouds or between 19clouds and ground. It is very high-energy phenomenon and can be a source of harm for transit 20system. Lightning flashes, in fact, can release many hundreds of mega-joules of energy.

21A typical cloud-to-cloud lightning begins with a preliminary breakdown due to the intense electric 22field initiated in the lower part of the cloud, and is generally negatively charged. The polarity of the 23lightning, in fact, which is a function of the local territory, can be statistically assumed as negative 24in 90% of the cases.

25The process is followed by a discharge, or leader, which creates a highly conductive channel, 26which advances, in a “zigzag” path, towards the earth and meets an upward advancing leader. 27This stepped discharge is caused by the non-uniformity of electric field. The medium between 28clouds and earth, in fact, is not uniform, as the air characteristics (density, pollution, humidity, 29etc.) continuously vary, hence the non-linearity of the lightning path.

30As the discharge progresses towards the earth, the electric field existing between the advancing 31channels and the earth or objects on earth i.e. buildings, poles, and trees etc, increases. In fair 32weather conditions, the electric field at the ground is quite high, while in the presence of stormy 33clouds in the sky, the electric field ranges between 30 and 40 kV/m. At the point of strike, the field 34assumes the value of 400 kV/m for the duration of the lightning.

35The dielectric strength of the air, during stormy weather, may be well below its strength of 3000 36kV/m in dry weather conditions. Such circumstance can facilitate the cloud-to-earth electric 37charge to reach the surrounding air critical breakdown value and cause (due to corona effect) 38upward directed discharges (upward leaders), basically at earth potential.

39An “attachment” process takes place between the two channels. The ground potential propagates 40upwards, circulating to earth the negative charge accumulated in the downward directed channel. 41This process, usually called a return stroke, causes the circulation of the high-intensity, impulse 42lightning current to ground. A very rapid rise to the peak within a few microseconds (s) and, 43then, a relatively slow decay, are the typical characteristics of a lightning surge current wave.

16. Causes of Switching Surges in the DC traction power system

2Switching operations that cause overvoltages are mainly due to trapped energy in part of the 3circuit and subsequent release of that energy. The following system operations in dc transit 4system may cause switching surges of varying degrees.

5 a) DC Circuit Breaker Operation: Interruption of dc current generates an arc with transient 6 overvoltage in the order of 2.5 times the dc system voltage [B21].

7 b) Pantograph Arcing: An uneven pantograph contact with the OCS wire causes arcing. 8 The intensity of such an arc and associated voltage/current surge may depend upon 9 various factors, such as load current, vehicle speed, air gap clearance, and weather 10 conditions.

11 c) AC vacuum Breaker Operation: In rare circumstances, current chopping during 12 interruption and pre-ignition during closing of the vacuum breaker may lead to transient 13 overvoltage [B17]

14 d) Voltage Transient due to dv/dt across diodes: Surges are generated due to inherent 15 characteristics of the circuit.

16 e) Induced switching surges from ac power system: Heavy direct lightning strike to an ac 17 power line in close vicinity to the OCS system may induce surges in the dc system.

18 f) Current Limiting Fuse Blowing: Current-limiting fuses protecting the rectifier diodes may 19 generate transient arc overvoltage.

20 g) Feeder Cable Arcing Fault: Loose cable connections arcing may lead to transient 21 surges.

22AC line making contact with the OCS wire can lead to voltage surge to dc system

23Surge Characteristics - Propagation 24The surge Impedance (Z) and the surge propagation velocity (v) are defined by the following 25expressions [B3].

L 26 Z = C (1)

27v  LC (2)

28Where:

29Z Surge impedance in ohms

30L Inductance in henries per unit length

31 C Capacitance in farads per unit length

32V Surge velocity will be in meter/sec if the if the units of L are in henries/meter and units of 33 C are in farads/meter 34The surge current (I), the surge voltage (V) and the surge impedance (Z) are related by the 35expression below [B2].

1V = IZ (3)

2The rate-of-rise of surge voltage (dv/dt) and rate-of-rise of surge current (di/dt) are related to 3surge impedance (Z) as follows [B1]

4di/dt = dv/dt (1/Z) (4)

5Surge propagates at the speed of light, 1000ft/s, (304.8m/s) in the OCS wire and 6approximately half this speed 500ft/s (152.4m/s) in the dc feeder cables [B2] [B3]. Surge 7experiences a surge reflection and refraction at a junction point due to change in surge 8impedance values. Surge propagation theory is well-documented [B2] [B10].

9The following expression [B15] [B21] may be used to derive the surge voltage that could 10propagate towards dc switchgear via feeder cables.

11VFC = 2 VI (ZC/n)/ [ ZOCS + (ZC/n)] (5)

12Where:

13VFC :Surge voltage through feeder cables

14VI : Incident surge voltage at the OCS feeder pole junction point, say 35 kV flash over value

15ZOCS: OCS wire surge impedance, 400  [B3]

16ZC: Each feeder cable surge impedance, 40  [B3]

17n: Number of feeder cables in parallel, say 3

181 Magnetic Stored Energy of Surge 19Switching surge energy (w) is exchanged between system inductance L and capacitance C 20parameters [B2] [B5] during its propagation defined by the expression below.

21W = (1/2) L I2 joules (watt-seconds) (6)

22L Inductance in henries

23I Current in rms. amperes

24Estimation of the energy trapped in the OCS wire using expression (6) is as follows:

25L= 2.0 mH/mile Ind. of the OCS wire [B2]

26I = 2000 A (assumed surge current)

27D= 2 miles assumed distance between adjacent traction power substations

28It is reasonable to consider that 1/2 [B2] of the surge current will propagate towards each 29adjacent substation.

30W = ½(2x10-3 )(1000)2 = 1000 joules

31It shall be noted that the above calculated energy will become 25 kilo-joules if the current surge is 32assumed as 10 kA.

17. Surge Environment – DC Electrification System 2It is evident from rail electrification systems using OCS that there are two paths for the surges to 3impact the substation equipment, one from OCS and other from the ac primary power supply 4system. The expected upper limit of the surge without the dc surge arrester that could propagate 5through the OCS system will be equal to the dry flashover value of the OCS system, which is 6typically 35 kV peak for 800 V dc system [B27].

7The incoming primary surge will see the doubling effect at the rectifier transformer due to change 8in surge impedance. The intensity of incoming surge will be related to distribution system 9parameters and the impinging surge. Internal switching surges and surges at the OCS may be 10related to any of the system operational causes listed under Section 4.1.2 earlier.

11Due to proximity of the OCS positive wire and the running rails (or negative wire in case of ETB 12system) there are equal chances that the lightning may strike both of these components 13simultaneously. Any surge voltage impinging the OCS wire is free to propagate towards the dc 14switchgear, as well as to the vehicle. The associated surge current may divide according to the 15surge impedance paths and may experience some attenuation.

162 DC Surge Protection Device Requirements 17For dc application, ac surge arresters with nonlinear resistors are re-rated. The basic 18requirements of a dc surge protection device include the following:

19 a) At highest working voltage, the device shall be essentially non-conducting, with a 20 minimal leakage current.

21 b) At an overvoltage moderately above the working voltage, the device while conducting 22 shall permit only small increases in its own terminal voltage.

23 c) Device shall have an adequate energy absorption capability to handle the stored energy 24 in the dc system.

25 d) Upon suppressing the system transient overvoltage, the device shall quickly interrupt 26 the normal dc voltage follow current.

27 e) Device shall be suitable for outdoor and indoor application subjected to harsh weather 28 without degradation in performance.

29 f) The residual voltage at the OCS at maximum expected surge current magnitude shall 30 be less than the damaging voltage of the equipment to be protected. It is affected by the 31 lead length inductance and rate of rise of surge current.

32 g) Device shall accommodate dc system temporary overvoltage condition as described in 33 Section 5.1.3 without its failure due to repeated occurrence of ETOV at the OCS.

34 h) Device shall be capable of providing a close margin of protection without excessive 35 maintenance and damage.

36 i) Indication of device failure is desirable

37 j) Device testing procedure is desirable

38DC Surge arrester application configuration is shown in Fig. 1. The MOV dc surge arrester with 39characteristics shown in Fig. 2 (b) appears to meet above considerations and closely matches

1with the characteristics of an ideal device shown in Fig. 2 (a). However, its energy absorbing 2capabilities should be checked in dc system application.

3The gapped MOV surge arrester shown in Fig. 2 (d) may not provide a close margin of voltage 4protection as the triggering of the gap will occur at relatively higher peak overvoltage condition. 5Spark gap type arrester with characteristics of Fig. 2(c) may not reseal causing system follow 6current flow to ground. Some manufacturers promote magnetically blown spark gaps with series 7connected non-linear resistors in a single stack. To improve the internal voltage distribution, a 8grading resistor and capacitor is provided for each spark gap. Such a device could provide 9adequate surge absorbing capability, as well as close margin of protection. This standard makes 10a recommendation to use properly rated MOV type surge arrester.

118. DC Surge Arrester Test Data and Energy Capability

12Without a standard on the manufacture of dc surge arresters, the testing and rating method 13among the various suppliers may vary. The energy absorbing capability varies, depending upon 14the quality and quantity of basic material (zinc oxide) used in the development of surge arrester. It 15appears that the manufacturers rely on the test data provided by the suppliers of the basic surge 16arrester material. For example, the data shown in Table 2 by the surge arrester manufacturer is 17the same as shown by Harris Semiconductor Corporation [B5] for the Type CA Metal-Oxide 18Varistors. However, there is no indication how the individual units were tested after their 19assembly in their housing. The design of connection terminals may change the test data. The unit 20shall be tested after assembly. It shall be noted that the terminology used in Table 4 is in 21accordance with ANSI/IEEE Std. C62.33 [B11], however, it differs from the terminology used in 22Tables 1, 2 and 3.

23Manufacturer's technical data shown in Table 1 does not list all required test data including the 24energy capability for the surge arrester as seen in Tables 2 and 3. Fig.3 from another 25manufacturer does not indicate the test surge wave shape. Fig. 4 shows the generic relationship 26of surge arrester MCOV rating and energy capability. Surge arrester’s energy capability can be 27increased by using a series parallel combination of the basic MOV discs without increasing its 28MCOV rating which may be required for a dc surge arrester [B1].

29Two different nomenclatures, discharge voltage (VIR) and clamping voltage (VC), have been used 30by the dc surge arrester manufacturers as shown in Tables 1, 2, 3 and 4. This may create 31confusion to the application engineer.

32The lack of test data and test procedures for dc surge arresters has created a challenge for the 33application engineer to evaluate their protection capabilities.

34 Table 1: DC Surge Arrester Parameters from Published Catalogue 500A Switching Surge 8/20s Impulse Wave peak current 0.5s, 10kA MCOV Maximum Discharge maximum discharge voltage - V Maximum IR Volts Voltage - VIR (kV peak) DC Discharge (kV peak)** Voltage - VIR 1.5 kA 3 kA 5 kA 10 kA 20 kA (kV peak)* 900 3.4 2.2 2.5 2.6 2.8 3.0 3.5

1800 5.8 4.4 5.0 5.1 5.5 6.0 7.0

35* Maximum discharge voltage for a 10 kA impulse current wave, which produces a current wave 36 cresting in 0.5s.

37**Based upon a current surge of 45s time to crest, 500A peak.

1 2 Table 2: DC Surge Arrester Parameters from Published Catalogue Max. values of the residual voltages in kV (discharge Thermal Nominal Rated Voltage-VIR) at peak discharge currents of impulses MCOV Energy Voltage Voltage absorbing 30/60s 8/20s 1/2s capability 0.5kA 1kA 5kA 10kA 20kA 20kA kV kV kV kJ kV kV kV kV kV kV 0.75 1 1 10 1.9 2 2.3 2.4 2.7 2.6 1.50 2 2 20 3.8 4 4.5 4.8 5.3 5.1 3.00 4 4 40 7.6 8 9.0 9.6 10.6 10.2 3 4 Table 3: DC Surge Arrester Parameters from Published Catalogue

Max. Values of the residual voltage in kV (discharge Voltage-VIR) at peak Nom. discharge currents of impulses. Energy capability, 2 impulses – 10.5 kJ/kV MCOV MCOV Volt. 30/60s 8/20s 1/2s kV (Type) 2 kA 1kA 1.5k 5kA 10kA 20kA 10 20 kV 250A 500A 1 kA A kA kA 1.0 1.0 1.96 2.01 2.06 2.13 2.10 2.16 2.31 2.40 2.64 2.67 3.00 1.5 1.5 2.92 2.99 3.06 3.19 3.15 3.22 3.46 3.60 3.96 4.04 4.47 2.0 2.0 3.89 3.99 4.08 4.25 4.20 4.29 4.61 4.80 5.28 5.38 5.96 2.5 2.5 4.95 5.07 5.19 5.41 5.34 5.45 5.86 6.10 6.71 6.84 7.57 3.0 3.0 5.84 5.98 6.12 6.38 6.30 6.43 6.92 7.20 7.92 8.07 8.93 4.2 4.2 8.10 8.30 8.50 8.85 8.75 8.92 9.60 10.0 11.0 11.2 12.4

5Table 4: Metal-Oxide Disc Varistors (CA Series) from basic MOV material [B5]

Maximum Ratings (85 C ) Characteristics (25 C) Cont. Peak Varistor Voltage at Max. Peak Clamping Transient Typical DC Amp 1 ma dc test current Voltage (V ) with Energy with Wave C Cap. @ Volt. Wave surge wave 10/1000s 1MHz 8/20s 200A peak, 8/20s

Vm(dc) Wtm Itm Min. VN(dc) Max VC Pico- (Volts) (Joules) (kA) (Volt) (Volt) (Volt) (Volt) farads (pf) 970 2600 70 1080 1200 1320 1880 3500 1150 3200 70 1290 1500 1650 2340 2700 1400 3200 70 1620 1800 2060 2940 2200 1750 5000 70 2020 2200 2550 3600 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

6Wtm : Rated Single Pulse Transient Energy, VN(dc): Nominal varistor dc voltage

7Itm : Single Pulse Transient Peak Current, Vm(dc): Max. Cont. Operating Voltage (MCOV)

1Fig. 2: Characteristics of Various Surge Arresters

3Fig. 3: Clamping Voltage v/s Peak current (check copy right)

4 5 Fig. 4: Surge Arrester Energy Capability (copy right check)

1 8.1.1 DC Surge Arrester Application: Surge arresters configuration and surge impedance 2 parameters are shown in Fig. 1. There is a need for research for surge arrester application.1 3A systematic approach shall be applied based upon the system configuration, surge parameters, 4and the expected intensity of surges and careful review of the manufacturer’s data on arresters. 5The selection of an arrester will require establishing its MCOV rating, energy handling capability, 6and the arrester discharge voltage.

79. DC Surge Arrester MCOV Rating

8DC surge arrester maximum continuous operating voltage (MCOV) will definably be greater than 9the system operating voltage, but should also be greater than the temporary overvoltage value 10(ETOV) determined by the following expression [B21].

11ETOV = (F) (VR)(RG)E (7)

12Where:

13E: System No Load Voltage, 800V

14VR: Specified Voltage Regulation of Transformer Rectifier Unit in Per Unit, which will be (1.06) 15 for 6% voltage regulation

16F: Upper limit of utility primary power supplies voltage regulation factor, suggest this value not to 17 be less than 1.10.

18RG: Vehicle Regeneration Factor, use 1.15

19Using these values, ETOV will be 1073 volts, thus dc surge arrester MCOV should be greater than 201073 V in this example. This statement is based upon the consideration that in a dc transit 21system temporary overvoltage may appear at the OCS as many times as the operating trains go 22through the regeneration mode especially if there are no other trains nearby to absorb the 23regenerated power. Such ETOV may also occur during night time when utility voltage may go up 24and dc load (trains) is relatively less. To assure that selected surge arrester MCOV should be 25greater than ETOV , an engineering analysis of the actual dc power system performance 26parameters listed in equation (7) above and adequate voltage-margin of protection is required. 27This standard recommends MCOV ≥ ETOV to avoid surge arrester premature failure due to ETOV 28especially when indication of the failed (degraded without enclosure rupture) surge arrester is not 29easily available unless field testing of the installed surge arresters is conducted. Degraded surge 30arrester may be conducting harmful dc stray current to earth.

31 9.1.1 DC Surge Arrester Voltage – Margin-of-Protection

32Surge arrester voltage-margin-of-protection (Vsa) above remote earth is defined as the peak 33voltage seen at the OCS by conduction of a surge current that results in maximum front of the 34wave internal discharge voltage (VIR), and maximum voltage drop across its grounding leads on 35both sides. For correction application of a dc surge arrester to OCS, it’s this peak voltage that 36should be as close to OCS dc voltage as possible such that no damage should occur to the dc 37traction power system components, including dc feeder cables, vehicles and substations. This 38voltage can be calculated by the following equation:

39Vsa = VIR + L. di/dt + IZSG (8)

40Where: 21 There is a room for research to establish similar concept of High-Low or the Low-High 3 cascaded arresters [B14] at the OCS pole and the vehicle 4 Copyright © 2007 IEEE. All rights reserved. 5 This is an unapproved IEEE Standards Draft, subject to change. 13 1August 28, 2009 IEEE P1627/D1.3

1ZSG Surge Arrester ground rod (electrode) impedance measured in ohms at 60 Hz, usually less 2 than 5 ohms

3VIR Front of the wave maximum IR discharge, voltage drop of arrester in kV peak

4L Inductance in henries of surge arrester leads

5I Peak surge current in amperes

6di/dt Rate-of-rise of the surge current in kA/sec

7It shall be noted that the voltage drop across the ground electrode impedance (IZSA) does not 8affect the dc equipment protection margin as the system negative, which acts as a reference 9point is grounded via leakage resistance to ground of the running rails insulators. In addition, all 10metallic components, including the OCS pole, are grounded to the same earth near to surge 11arrester ground electrode. Thus, for the surge arrester Vsa calculation, as shown in the equation 12(9) below the factor IZSG should not be used.

13Vsa = VIR + L. di/dt (9) 14Without complete knowledge of the surge environment and test data on the dc surge arresters, 15one application approach is to apply a lower voltage surge arrester, such as 970 MCOV dc for the 16800V dc system, knowing it has relatively lower energy capability, but better voltage margin of 17protection. In case of its failure in actual application, it will provide the measure of surge 18environment. To increase surge energy handling capability of the surge arrester without 19increasing its MCOV rating, another approach may be to apply two surge arresters of low MCOV 20rating in parallel with individual ground leads and grounding electrode connections. However, the 21concern of increased leakage current under normal system operation with parallel arrester 22approach should be checked based upon this analysis.

23Based upon above described analysis, this standard recommends MCOV ≥ ETOV to avoid surge 24arrester premature failure due to ETOV especially when indication of the failed (degraded without 25enclosure rupture) surge arrester is not easily available unless field testing of the installed surge 26arresters is conducted. Degraded surge arrester at the OCS system may be conducting harmful 27dc stray current to earth until it is replaced with a new surge arrester. 28

2910. DC Surge Arrester Energy Discharge Capability

30Surge arrester surge energy (W) in joules can be calculated by the following expression below:

t 31W=  V.I.dt (10) 0

32Where:

33V: Surge arrester front of the wave protective level in volts

34I: Peak discharge current in amperes

35t: Time in seconds the surge reaches voltage (V)

36If the surge wave shape is known, then another easier expression for the energy discharged 37through an arrester may be calculated by using the equation (11) below [B5].

1W = KVCIτ (11)

2K = Constant, 0.5 for triangular wave, 1.0 for rectangular wave and 1.4 for exponential decaying 3 wave

4W = Energy in joules

5VC = Clamping voltage in volts

6I = Impulse current in amperes

7τ = Impulse duration in seconds 811. DC Surge Arrester Application Analysis Calculation

9An engineering analysis calculation for application of dc surge arrester to OCS operating at 800 V 10nominal dc voltage is discussed. Calculations are based upon using surge arresters with MCOV 11rating of 900 V and 1800 V and other parameters published by their manufacturer.

12 a) Using rate-of-rise of incoming surge voltage (dv/dt) at the OCS surge arrester location 13 of 11 kV/s per kV MCOV [B1], the calculated values of dv/dt will be 9.90 kV/s and 14 19.8 kV/s for the 900 V and 1800 V arresters respectively.

15 b) Surge arrester lead lengths inductance using 0.4 H/ft with 25 feet length will be in the 16 order of 10 H for each arrester.

17 c) Rate-of-rise of the surge current at the arrester location by using equation (4) and surge 18 impedance of arrester leads of 400  will be 0.025 kA/s and 0.050 kA/s respectively 19 for each arrester.

20 d) Using the published data shown in Table 1 for surge arrester front of wave protective

21 level at 0.5 s, 10 kA peak discharge current, discharge voltage (VIR) will be 3.4 kV and 22 5.8 kV respectively for each arrester.

23 e) Margin of protection voltage (Vsa) by using equation (9) without considering (IZSG) factor 24 will be:

25 900V : Vsa = 3.4+0.25 =3.65 kV

26 1800V: Vsa = 5.8+0.50 =6.30 kV

27 f) Energy (W) in Joules discharged by the surge arrester for a switching surge may 28 conservatively be estimated by the equation (11). The values indicated in Table 4 are in 29 the order of 2600 and 5000 joules respectively for 900V and 1800V arresters for the 30 long wave 10/1000s and may be compared with the calculations in this step for 31 45/1000s wave:

32 900V DC MCOV Rated Surge Arrester:

33 W = 0.5 * 2.2 * 0.5 * 45 +1.4 * 2.2* 0.5* 0.5*1 000

34 W = 24.75 + 770.00 = 794.75 Joules

35 1800V DC MCOV Rated Surge Arrester:

1 W=0.5 *4.4.2*0.5*45+1.4*4.4*0.5*0.5*1000 =1589.5 Joules 2It should be noted that the calculated values of energy for each surge arrester using 45/1000s 3wave parameters are lower than the published data of energy discharge capability of the arrester 4using 10/1000s wave. Since the characteristics of the two wave shapes are different, therefore 5manufacturer should be consulted to provide discharge energy capability for (45/1000s) wave.

6 g) Voltage surge through feeder cable (VFC) in kV at the dc switchgear can be calculated 7 by use of equation (5):

8 900V MCOV Surge Arrester: VFC=2x3.65x(40/3)/[400+40/3] = 0.24 kV

9 1800V MCOV Surge Arrester: VFC=2x6.30x(40/3)/ [400+40/3] = 0.41kV 10It appears that without the effect of surge impedances, the conservative voltage impressed at 11the equipment may be 3.65 kV and 6.30 kV, respectively as indicated in item e) above.

12 h) Current surge (IS) associated with voltage surge derived in item g) above can be 13 calculated by use of expression (3)

14 900V MCOV Surge Arrester: IS = ES/ZS = 0.24x(3/40) = 0.018 kA

15 1800V MCOV Surge Arrester: IS = ES/ZS = 0.41x(3/40) = 0.0.031 kA 16Although the energy capability of 1800V arrester is higher than the energy capability of 900V 17arrester, however, the 900V arrester provides better voltage protection if it can withstand system 18energy capability. It shall be noted that 900V MCOV rating appears to be lower than the system 19temporary overvoltage (ETOV) value of 1073 volts derived by using equation (7) and thus may lead 20to premature failures in actual installation. Thus the surge arrester with MCOV rating of 1150 V 21may be the proper choice for 800 V dc system.

2212. Lightning Stroke Terminology 23Perhaps it is best to clarify the terminology; reference [B20] makes a distinction between the 24traditionally used term “stroke” and a more precise reference to the term, “flash”. A flash 25describes the entire electrical discharge to the stricken object. Stroke, on the other hand, 26describes only the high-current components of a flash. Because of the observed multiplicity of 27strokes, the relationship between the terms “flash” and “stroke” is that there can be many strokes 28in a single flash. Research into flash characteristics indicates that 55 percent of all flashes contain 29multiple strokes, with an average value of three strokes [B20]. This information is important 30because of the differences in wave shape of the successive strokes. The term “flashover” is 31described as an electrical discharge completed from an energized conductor to a grounded 32support structure, which will be OCS poles in case of an LRT system.

1 2Fig.5 Surge Arresters Configuration (add isolators-Not needed by Dev Paul)

3Change SA of the vehicle surge arrester to rail. Add rail to ground arrester (will 4revise Fig. 5 later by Dev Paul) 5Lightning Intensity Estimation

6Lightning intensity within a specific area is generally based upon the ground flash density, Ng, in 7flashes per km2/year. At present, this data is not available in the United States and thus, lightning 8intensity must be based upon the isokeraunic level, or the number of thunderstorms per year, T d. 9The value of Ng may be approximated by using the following empirical expression [B4] [B19]. With 10more research data available in the future such an expression may change.

1.25 1Ng = 0.04 Td (12)

2For example consider the area of a light rail transit (LRT) where T d is in the order of 40-60 [B3] 3[B4]. Using expression (12) and Td of 60, the calculated value of Ng will be near 6.68. It is noted 4that the exponent value of 1.25 in expression (12) is somewhat uncertain, for some published 5literature indicates this value to be 1.35. However, 1.25 has been accepted by the committee 6responsible for the development of the standard [B19] and thus, for this example, OCS lightning 7protection analysis will be based upon the value of Ng to be around 6.68.

8This calculated number for Ng provides some measure of likelihood of lightning strike to ground in 9the area. The actual number of lightning flashes/year, NOCS, that may strike the light rail OCS or 10nearby ground inducing direct or indirect lightning surge waves, may be calculated by using the 11following expression:

12NOCS = wLNg (13)

13Where:

14L = length of LRT system in kilometers

15w = Width of area covering LRT tracks in kilometers

16Assuming a double track dc system with width near 0.015 kilometer and Ng of 6.68, by expression 17(13), calculated value of NOCS will be: L/10.

18NOCS = L/10 (14)

19Assuming the probability of direct hit of lightning strike to OCS (N D) is 20% of the value calculated 20for the actual number of lightning flashes/year, NOCS in that case, for the above example following 21relationship applies.

22ND = 1/5 (NOCS) = L/50 (15)

23Where: L is the dc electrification system length in kilo-meters 24Assumed low probability of 20% of direct lightning hit to OCS indicated above is based upon the 25reasoning that there are equal chances that lightning may hit any of the OCS support structures, 26nearby buildings, trees, substation structures, communication and control cabinets including 27running rails.

28Thus in the above example for a dc system with 10 km length, the calculated number of lightning 29flashes per year (NOCS) that may strike the OCS system is one (1) and perhaps 50 km length is 30needed for direct hit to OCS. For 10 km length of OCS system, the expected single lightning flash 31per year may not be a direct hit to the OCS system. In addition, the expected single lightning flash 32may or may not be of concern, depending upon the severity and energy associated with the 33lightning stroke (surge) contained in the flash.

13.34 Lightning Stroke – OCS Flash Over

35This discussion is intended to establish the lightning overvoltage intensity to the OCS 36components, especially the contact wire, which is generally protected by dc surge arresters. The 37various components of the OCS, including messenger wire, contact wire, feeder cables and 38supporting structure (which consists of metallic poles, cross-arms, and running rails), are 39relatively close to each other. There are equal chances that the lightning strike may hit any of the 40above-described OCS components.

1The messenger wire, cross-arms, and grounded metallic poles may provide some measure of 2shielding of direct lightning strike to the OCS contact wire. In rare circumstances, if the lightning 3strikes directly to the OCS wire, flashover is almost certain since the insulated air gaps and 4clearances from the grounded metallic components including the poles is relatively low with wet 5and dry flashover values near 20 kV to 35 kV peak respectively for 750 V nominal dc. Lightning 6strike energy after the flashover at the OCS pole will go to ground via grounding path of the poles.

7After the flashover, the maximum voltage expected at the OCS contact wire would not be more 8than the actual dry flash over value of the insulator. The time to flashover from stroke, the energy 9contained in the remaining surge wave at the OCS, and its propagation away from the point of 10strike will depend upon the rate-of-rise of the incoming surge waves of the lightning flash strokes. 11As indicated earlier, 60 percent of the strokes may strike the OCS poles and the remainder at 12mid-span of poles.

13The maximum distance that a lightning surge will need to travel before hitting the grounded pole 14for flashover phenomenon is ½ of the pole spacing distance, which in terms of the surge wave 15propagation time is relatively small. Without the application of dc surge arrester at each OCS 16pole, the metallic grounded OCS poles will provide adequate path to the lightning strokes with 17peak voltages exceeding dry flash over voltage of the insulators. This OCS poles flashover to 18ground will cease automatically once the OCS surge voltage falls below the insulator’s actual 19flash over voltage. The flashover may occur again if there are repeated lightning strokes in a 20particular flash.

21If the flashover occurs near the dc feeder poles with dc surge arresters, the dc surge arrester may 22also start discharging during the pole flashing. It is also apparent that as the propagation time of 23the surge to adjacent feeder pole towards the next substation is small, the surge arrester on 24adjacent substation will also start conducting. In addition, the surge wave will also propagate via 25an underground feeder cable to the dc switchgear with a reduced surge magnitude indicated by 26equation (5) in section 4.2 earlier. Thus, surge arresters applied at the dc feeder breakers will 27reduce the effect of surge propagation on feeder cables and the substation equipment.

28For a LRT system in a high isokeraunic area, if the flashover occurs to OCS poles or rails, then 29the induced surge voltage will get into the running rails, or if the surge strikes directly to the 30running rails then the surge may propagate to the substation negative bus via the negative 31underground dc feeders. Therefore in such areas surge arresters should also be applied at the dc 32negative bus or the running rails.

33In case of high isokeraunic lightning stroke, a concern of damage to the surge arrester rises due 34to its limited surge energy handling capability. However, it appears that for such a severe 35lightning stroke, flashover across the outer surface of the surge arrester may occur due to its 36short length. Such flashovers will drain the surge energy to ground leaving lesser surge current 37and energy to be discharged through the surge arrester. If there is still some concern of the dc 38surge arrester to be inadequate in handling the surge energy, then properly rated surge arrester 39with higher surge energy handling capability should be applied.

40The dc surge arresters applied at the dc feeder poles or other locations should be adequate to 41handle the discharge current of the lightning surge wave deposited by the lightning flash strokes 42after the OCS flash over occurs. In addition, the dc surge arrester discharge voltage should be 43such that it provides adequate voltage margin of protection to the operating Light Rail Vehicle 44(LRV) and the traction power substations. Since these surge arresters at the OCS contact wire 45are first lines of defense to trap the lightning/switching surge voltage below the protection level of 46the connected equipment, an engineering analysis of surge arrester voltage ratings should be 47performed for proper selection of the surge arresters [B21].

1 13.1.1 Lightning Stroke Magnitude 2Research on the stroke current peak amplitude reported that the mean value of first stroke is near 331 kA, with a 95 percent probability of the stroke magnitude being between 10 and 100 kA [B20]. 4The first stroke wave shape mean value just before the current peak has been reported to be 5near 24.3 kA/s, which is helpful in understanding the impulse voltages that can occur for 6discharges through inductances. It is necessary to indicate that although the average value of the 7peak magnitude of the subsequent stroke(s) is generally less than the first stroke, the wave 8front(s) of the subsequent stroke(s) are typically faster. The average value is near 39.9 kA/s, 9although values in excess of 70 kA/s have been reported. The above mentioned stroke 10parameters relate to the flash itself and much of the data was obtained from mountaintop 11observatories [B28]. It is also reported that 60 percent of the direct flashes hit the tower where 12they would flashover to the ground and the remainder hit on the spans between the towers.

13The above listed current lightning waves develop very high corresponding voltage waves based 14upon their relationship provided by the equations (3) and (4) described under Section 4.2 earlier.

15Consideration must be given to some modification of the flash characteristics striking OCS, 16especially when tracks may be surrounded by urban development. Any high-rise buildings 17including the trees and street light poles that are taller than the OCS poles will provide some 18degree of lightning flash shielding to the OCS. However, since there is no measured research 19data specifically for the dc transit OCS, the conservative approach is to use the data available for 20the transmission towers for the OCS.

2114. Lightning Stroke Induced Over-voltage

22Lightning overvoltages are also possible due to electric and magnetic fields induced from nearby 23lightning, often referred to as indirect or induced surges. For transmission lines, peak 24overvoltages induced by first strokes varied between +150 kV and –40 kV, the mean being 23 kV. 25The mean rise time for these voltage surges was 6 s. This provides rate of rise of the voltage 26wave to be approximately 4 kV/s. The study further revealed that induced overvoltges caused by 27subsequent lightning strokes had 11 kV peaks, with a mean rise time of 4 s. This provides a rate 28of rise of the voltage wave to be approximately 3 kV/s, which is much lower than the values 29reported for the direct lightning flash hitting the transmission lines. Such lightning wave 30parameters may be used for LRT system design and engineering analysis of lightning protection, 31which is the purpose of this standard.

3215. Lightning Stroke Surge Energy

33Surge energy (J) may be calculated by the expression [B1]:

t dv di t . 34J = V.I.dt   dt dt 2 dt Joules (16)  0 t 0

35Assume lightning stroke with the following parameters:

dv 36  200 kV/ s (2x1011 V/ sec) dt 37Assume surge impedance for surge voltage to be near 40 ohms, parallel combination of OCS 38with supplementary feeder cable. Thus surge current wave by use of expression (18) will be as 39follows

di 1  5kA/ s (5x109 A/sec) dt 2The maximum flashover kV peak for OCS is near 35 kV (dry weather condition), thus within 335/200 s (0.18s), OCS poles will flashover to ground with or without the application of dc surge 4arresters.

5Thus the lightning stroke energy that may pose threat of OCS damage or the dc surge arresters 6will be for flashover time of 0.18 s with calculated stroke energy value indicated below.

7t = 0.18 s (18x10-8 sec)

8 11 9. 3 18x10 8J = 2x10 x5x10 x[t / 3]0 joules

9J = 1021x 10-24 x183 x 1/3 joules = 5.83/3 kJ (17) 10The OCS system appears to get self-relief from the heavy lightning stroke energy (responsible for 11damage to dc surge arresters and other OCS equipment) due to flashover near 35 kV peak surge 12magnitude without the help of surge arresters. However, 35 kV peak voltages are quite damaging 13to the system components, such as dc switchgear and also LRV components. Thus dc surge 14arresters of proper rating should be applied. These surge arresters will discharge current and will 15handle energy as indicated in expressions (20) and (21) below.

1616. Arrester Discharge Energy

17Arrester discharge current is a function of many interrelated parameters, including: 18 Surge impedance of the OCS 19 Stroke current characteristics, wave shape, peak current magnitude, and its rate-of-rise 20 Distance of the surge arrester from the point of stroke 21 Ground resistance at the location of stroke 22 Number and locations of flashovers 23 Flashover characteristics of the OCS insulators 24 Arrester discharge voltage 25The following expression [B4] is used for power distribution overhead lines and may be used for 26the OCS system:

27IA = (ES - EA)/ Z (18)

28Where:

29EA = Arrester impulse discharge voltage (kV) for current IA (kA)

30ES = Prospective surge voltage (kV)

31Z = Surge impedance of the conducting path of the surge arrester ()

32IA = Impulse current (kA) associated with impulse voltage 33Energy discharged by the arrester, J, in kilojoules (kJ), may be conservatively estimated by the 34following expression [B18]:

1J = 2 DL EA IA /v (19)

2Where:

3EA = Arrester discharge voltage (kV)

4IA = Switching impulse current (kA)

5DL = Line length (miles) or (km)

6v = the speed of light (190 miles/ms) or (300 km/ms) 7The expression assumes that the entire line is charged to a prospective switching surge voltage 8and is discharged through the arrester during twice the travel time of the line.

9If the surge wave shape is known, then another easier expression for the energy discharged 10through an arrester may be calculated by using the equation (11) described under section 5.1.5.

1117. Lightning Stroke to OCS system 12For analysis purposes, assume maximum distance between the substations to be near 1 1/2 13miles. Assume that there are surge arresters installed only at the feeder poles adjacent to each 14traction power substation, and there are no other poles between the substations that are 15equipped with surge arresters as shown in Fig. 5. A lightning strike hitting the OCS wire midway 16between the two substations will propagate equally with 1/2 the impinging surge current 17magnitude to each substation [B2]. Thus, in this example the surge will travel maximum distance 18of 3/4 mile before reaching a pole with dc surge arresters.

19For a 750V dc nominal LRT system voltage, consider a dc surge arrester rated at 2 kV duty cycle 20with MCOV rating near 1800 V dc with discharge voltage rated at 7.0 kV. This discharge voltage 21is the surge arrester test voltage, which is based upon 20 kA peak current of a standard 8/20s 22wave.

23Time in milliseconds to travel 3/4 mile will be 3/4x1/190 (4s). The energy discharged through the 24surge arrester in kJ using equation (19) may be calculated as shown in equation (21).

25Assume surge wave is magnified to twice its magnitude (2 times 35 kV dry flashover value of 26OCS) due to open circuit condition of a sectionalizing dc disconnect switch. Using OCS surge 27impedance of 40 ohms (surge impedance of OCS wire in parallel with underground 28supplementary cable), and assuming surge arrester discharge voltage (7 kV) to be the test 29voltage at 20 kA peak, the discharge current IA and surge energy discharge will be as follows:

30IA = (2x35 – 7.0)/40 = 1.575 kA (20)

31J = 2 x ¾x(1/190) x 7.0 x 1.575 kJ = 0.088 kJ (21) 32It should be noted that the time to travel 3/4 mile distance by the lightning stroke is very small and 33it is possible that the lightning stroke time may be longer than two times the travel time for 3/4 34mile distance. Under such circumstances, the maximum estimated time for the lightning stroke 35should be used for estimating the energy discharge through the surge arrester. The time 363/4x2/190 ms (8 s) used in calculating energy J in kilojoules should be increased to a 37reasonable value, say 300 s, the maximum estimated time the lightning flash containing more 38than an average of three strokes may exist. This will lead to calculated energy of 3.30 kJ, which 39will still be below 4.4 kJ (2.2 kJ/kV) value for a 2000V dc surge arrester. This estimation of surge 40energy is very conservative as the assumed value of surge time and arrester discharge voltage 41seems to be on the high side. However, dc surge arrester selection based upon such high energy

1discharge requirements will assure that the arrester will not be damaged by high isokeraunic 2lightning flash hitting very close to its location.

3In the above calculation it has been assumed that lightning surge impinging the OCS wire will 4flash over to grounded metallic OCS poles once the surge wave voltage reaches 35 kV peak. 5Time to reach 35 kV peak will depend upon the rate of rise of lightning surge wave. If the rate of 6rise for example is near 200 kV/µs, then the time to reach 35 kV peak will be 35/200 µs, which is 7far less than the travel time of 4µs for 3/4 mile distance. Thus, OCS wire will not charge more 8than 35 kV peak voltage unless the surge comes across an open circuit caused by open position 9of a disconnect switch. However, as the surge propagation time to reach open circuit location is 10quite higher than the time to develop 35 kV peak voltages at lightning flash striking location, the 11OCS pole flash will occur before double peak voltage (2x35 kV) is impressed upon the OCS 12contact wire. Thus, flashover phenomena will reduce the surge energy that will be discharged 13through the dc surge arrester.

14To provide assurance that OCS wire flashing over occurs in case of a direct lightning strike 15impinging the system in high isokeraunic areas horn air gap type arresters should be considered 16midway between adjacent traction power substations.2

173 Underground Supplementary Cable Connections to OCS 18In certain sections of the OCS, it is assumed that there will be underground supplementary cable. 19For discussion purposes, it is assumed that the average distance between the OCS contact wire 20and the underground supplementary cable tap connections are in the order of approximately 400 21feet. For calculation purposes, assume 4000 feet length of such supplementary cable which will 22then require a total of eleven (11) OCS contact wire-to-supplementary cable tap connections. 23This configuration of OCS and supplementary cable connections will have total of nine (9) 24underground cable splices. Such underground supplementary cable installation, electrically in 25parallel to OCS will require cable splice connections to be located in the underground manholes. 26Lightning surge withstand capability of such underground cable splices in the manholes and the 27tap point connection of cable to overhead OCS contact wire are of concern.

28Analysis of the cable splices and cable connections to the OCS contact wire would require a 29derivation of the peak value of the lightning surge voltage expected at these connection points 30and cable splices. Then this surge voltage peak value will be compared to the tolerable values of 31basic surge withstand impulse voltages (BSL) of the cable splices and cable-to-OCS connections.

32The basic switching surge level (BSL) of the 2 kV cable is near 75 kV peak. Underground cable 33splice BSL levels to match with the cable BSL level are also available.

34Assume the following:

35ZOCS = 400 ohms (OCS contact wire surge impedance) [B2]

36ZC = 40 ohms (cable surge impedance) [B2]

37Vi = Voltage magnitude of the incident lightning wave at the impedance junction point 38 (connection of cable to OCS contact wire or at the underground cable splice)

39Ii = Current magnitude of the incident lightning wave at the impedance junction point

22 Such air gap horn type surge arresters do not pose any threat of dc leakage current or uncertainty of their 3damage due to ambient temperature. They shall be bonded to the OCS poles that are grounded with its own 4grounding electrode with low resistance-grounding impedance by appropriately sized wire (not less than #6 5AWG) 600V insulated cable to avoid jeopardizing the OCS double insulators criteria. Another alternate to 6the horn gap is to consider surge arrester with higher duty cycle voltage.

1Vr = Voltage magnitude of the reflected lightning wave at the impedance junction point

2Ir = Current magnitude of the reflected lightning wave at the impedance junction point

3V = Total voltage magnitude (refracted voltage) at the impedance junction point

4I = Total current magnitude (refracted current) at the impedance junction point

5The following expressions are well documented [B1].

6V =Vi + Vr (22)

7I = Ii + Ir (23)

8At the interface of two surge impedances Z1 and Z2, the expressions for the above indicated 9surge voltage and current are related by the following expressions:

10V = [2x Z2 /(Z1 + Z2)] Vi (24)

11I = [2x Z1/(Z1 + Z2)] Ii (25) 12For the sake of completeness, expressions for the surge current as well as the surge voltage 13have been described. However, analysis of the surge wave voltage is more critical for the cable 14insulation protection when compared to surge current. It is well understood that the cable can 15tolerate excessive magnitude of surge current for short duration without appreciable heat rise to 16create damage to cable insulation. Hence, only the surge voltage analysis is presented under 17Sections 7.1.1 and 7.1.2 below. Single Line diagram indicated in Fig.1 shows basic elements of 18the power system.

1918. Lightning Hits OCS ahead of the Supplementary Cable Connections

20The initial and final surge voltages at the junction points of cable to OCS or splice point of 21supplementary cable may be calculated by using expression (24) which requires knowledge 22incident voltage magnitude (Vi) through surge impedance Z1 before hitting the junction point of 23surge impedances Z1 and Z2. In the case of OCS and supplementary cable, the surges 24impedances are as follows.

25Z1 = 400 Ώ, for OCS and Z2 = 40 Ώ for the supplementary cable. 26If all cable to OCS taps is spaced equally and the installation is uniform, then, for practical 27purposes neglecting the effect surge impedances of cable splices and the OCS/cable connection 28tap points, the combined surge impedance (Z) of underground supplementary cable and OCS 29contact wire may be represented by expression (26) shown below.

30Z = Z1 x Z2 / (Z1 + Z2) ≈ Z2 (since Z1 >> Z2) (26)

31If voltages (V1), (V2), (V3) and (V11) are successively represented as the surge voltages at the first, 32second, third and last (eleventh) junction points when the surge voltage travels along the OCS 33section with underground supplementary positive cable, the expressions for these surge voltages 34will be as follows:

35V1 = [2x Z2/(Z1 + Z2)] Vi (27)

36V2 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)] Vi (28)

2 37V3 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)] Vi (29)

9 1V10 = [2x Z2/(Z1 + Z2)][2x Z2/(Z2 + Z2)] Vi (30) 2Using values indicated for the surge impedances in ohms, surge voltages represented by 3expressions (27) through (30) will be practically equal in magnitude, approximately 18% of the 4incident stroke surge voltage magnitude.

5The final (eleventh) point will be end of the supplementary cable where the surge impedance will 6become again Z1 and the surge voltage will be escalated as follows:

7V11 = [2x40/(400+40)].[2x400/(400+40)]Vi (31)

8This final voltage appears to be approximately 33% of the initial surge voltage that appeared 9when the surge entered parallel combination of OCS and supplementary cable. 10If installation of the underground supplementary cable riser feeders and OCS connections is 11uniform, then the surge impedance will be practically the same, slightly less than 40 ohms. The 12above calculations indicate that voltage will never be more than the striking voltage unless there 13is a switch that may be in an open position to make this voltage two times the initial surge 14voltage. This twice the initial surge voltage can be derived by using the expression (24) as shown 15below.

16V = [2x Z2 /(Z1 + Z2)] Vi = [2/(Z1/Z2+ 1)] Vi (32)

17Since Z2 at open switch will be infinite, thus Z1/Z2 will become zero in expression (32) making 18surge voltage V as two times the initial voltage Vi. These surge voltage calculations do not take 19into account the effect of surge attenuation due to cable inductance and capacitance effects.

20If we assume that the striking voltage is limited to 35 kV by the flashover phenomenon, then it 21appears that the underground supplementary cable splices and the OCS-to-supplementary 22feeder cable tap connections may not require surge protection, except at the first and the last 23connection points at the OCS.

24 18.1.1 Lightning Hits OCS within Supplementary Cable Connections Zone 25The surge voltage analysis for this case will be identical to the analysis presented under Section 267.1.1 above, with the exception that the incident surge in the air at the OCS will propagate to 27each side traversing the OCS/cable tap connections and underground supplementary cable 28splices. From a theoretical point of view, the current surge that propagates in each direction will 29practically be half the magnitude of the incident surge stroke current. The final maximum surge 30voltage will be at the outermost cable-to-OCS connection tap points, and it will practically become 31double the traveling surge voltage as indicated by the following calculation.

32V = [2x 400 /(400 + 40)] Vi = 1.82x Vi (33)

33This voltage V will be equal to the initial lightning stroke surge voltage, which initially split into half 34the magnitude at the strike locations. All intermediate tap points will see a lesser surge voltage in 35the order of 9% (from analysis in 7.1.1) magnitude of the initial lightning stroke before it splits into 36half the magnitude.

37Thus, if the OCS flashover voltage is near 35 kV without the application of the dc surge arresters, 38then the maximum surge voltage will be near 35 kV peak or 70 kV if the design installation 39involves dc disconnect switch which is open.

119. Lightning Surge Propagation Discussion for Supplementary Cable

2From a theoretical point of view, more surge current may tend to propagate through the 3underground supplementary cable as compared to the OCS wire due to difference in the values 4of their respective surge impedances. However, the speed of surge propagation through OCS 5wire is two times the speed of surge through supplementary cable and feeder taps [B2]. This may 6lead to balancing out the surge energy propagation through OCS and underground 7supplementary cable and feeder taps.

8It should also be mentioned that the underground supplementary cable switching surge peak 9impulse voltage withstand level far exceeds 35 kV peak surge wave that can be expected without 10considering the doubling effect. Thus it is not necessary that dc surge arresters be applied at 11every 400 feet at supplementary cable-to-OCS connection locations if there is no dc disconnect 12connection.

13However, doubling peak voltage effect cannot be avoided at the dc disconnect switches when 14they are in open position. Thus at disconnect switch locations, dc surge arresters are required on 15each side of the switch.

16In addition, it is prudent to perform surge analysis based upon actual configuration of the 17OCS/supplementary cable to optimize the design of surge arrester applications to the dc system.

18In specific configurations of OCS under the high voltage utility overhead lines, consideration shall 19be made to prevent live ac wire contacting the OCS by use of guard wire or some other means. 20Additional surge arrester at the OCS in the vicinity of OCS/high voltage utility lines should not be 21used due to its misapplication for such a hazard. If the high voltage line touches the OCS, then 22the dc system will be exposed to a sustained over voltage condition equal to dc voltage plus the 23peak ac system voltage leading to damage of the surge arresters and other dc system 24components until the utility fault is cleared

25It is a general opinion that guesswork and overconcern of lightning protection without performing 26surge analysis indicated in this standard has led to a design of applying surge arresters at each 27OCS-to-supplementary cable taps. 3

2820. Grounding

29OCS Pole Grounding 308.1.1 OCS support poles with surge arresters shall have dedicated ground rod (ground electrode) 31for connecting the surge arrester to ground by use of exothermic weld. Measured grounding 32resistance of surge arrester ground electrode should not exceed 5 ohms to avoid excessive surge 33arrester residual voltage at OCS when a heavy lightning surge current discharge current takes 34place either through flash over across the arrester or by current discharge through the arrester.

35Surge arrester ground rod should not be bonded to OCS pole foundation re-bars to avoid the 36uncertainly of damage to the footing due to energy dissipation of a lightning surge at a rapid 37speed. Bonding the pole with surge arrester grounding system may also transfer surge hazard to 38the pole which is not desirable. In addition, separate dedicated grounding rod is needed to ground 39OCS pole. All OCS poles with surge arresters shall be grounded individually by a maximum of 25 40ohms grounding system. For those poles that are accessible to public the grounding resistance

23 Such a design should be avoided based upon the surge voltage analysis presented in this standard. 3Addition of such excessive number of dc surge arresters to the OCS is an application concern, especially 4when such surge arresters do not have any indication that the arrester is in a degraded mode and may be 5injecting undesirable dc stray current to ground.

1shall be 5 ohms or less. [B27] Following the installation of the pole mounted surge arrester its 2grounding system resistance shall be measured and recorded for future reference.

321. DC Surge Arresters

44 Application

5 21.1.1 Surge arresters shall be provided and shall be connected to the OCS as a minimum 6 at the feeder poles. Two (2) parallel surge arresters with separate ground conductor and 7 separate ground rods may be needed at feeder poles for high intensity lightning area. [B27]

8 a) Considering the low profile of OCS, proximity of all components, inherently grounded 9 poles, and major portion of the dc rail transit system close to high-rise structures and 10 trees, the probability of lightning striking the OCS is very low. With this configuration, 11 application of the ground shield wire above the messenger and contact wire does not 12 appear to provide any greater degree of protection, especially when the lightning strike 13 tends to flashover the grounded structures. 14MOV surge arresters are sensitive to ambient temperature. In the summer when ambient 15temperature is high, metallic tip of the MOV dc surge arrester may become hot leading to transfer 16of heat to the surge arrester material. This may cause premature surge arrester failures. In 17addition, the surge arrester provides leakage current under TOV conditions leading to heat 18dissipation as well degradation of its internal MOV elements. Thus the installation should consider 19excessive temperature effect on performance and selection of MOV surge arresters.

20 MOV dc surge arresters should be installed at the following locations: 21  At feeder poles, close to pole-mounted or pad-mounted dc disconnect switches on load 22 side of the switches.

23  At pole-mounted or pad-mounted OCS sectioning switches. Arresters shall be installed 24 on both sides of the switch.

25  The application of dc surge arrester for dc switchgear is not covered in this standard; 26 however, review of surge arrester at the OCS and dc switchgear should be coordinated.

27  The application of dc surge arrester for vehicle is not covered in this standard; however, 28 review of surge arrester at the OCS and vehicle should be coordinated.

29  For OCS system within high isokeraunic areas, consider installing surge arresters at the 30 negative bus to protect the equipment under rare circumstances of lightning surge 31 reaching the negative bus via running rails and dc negative feeder cables.4 32  Install dc surge arrester at the first and last OCS to underground positive supplementary 33 feeder cable tap location. This is a minimum requirement.

34  Surge arresters shall be considered at locations where surge impedance changes due to 35 OCS configuration, such as bridges and end points of the tunnels.

36  Surge arresters shall be considered for high isokeraunic areas, at the midway between 37 the adjacent substations.

24 Surge arrester at appropriate locations on running rail may be applied in elevated guideways in areas of 3high isokeraunic levels 4

1 b) An engineering analysis should be performed to determine appropriate voltage and 2 energy capability rating of the dc surge arresters. The analysis should take into 3 consideration the OCS location, ambient environment and operating voltage 4 characteristics.

5 c) MOV dc surge arresters continuously conduct milli-amperes level of current to ground. 6 This current may increase if the internal material becomes defective. 5

7 21.1.2 Name Plate Information

8The following information shall be provided:

9  DC Surge Arrester

10  Rated voltages, duty cycle voltage and MCOV

11  Nominal peak discharge current with current waves of 8/20 s, and 100/1000 s

12  Surge discharge capability kA peak

13  Manufacturer’s name

14  Year of manufacture

15  Serial number

165 Surge arrester Rating

17 21.1.3 Surge arresters discharge voltage shall be no more than 80% of the BIL of the 18 equipment that is being protected.

19 21.1.4 MCOV voltage rating of surge arresters shall be greater than Temporary Overvoltage 20 TOV of the OCS system. For dc surge arrester ratings refer to Tables 1, 2 and 3 under 21 Section 5 and Table 5 below.

22 Table 5: OCS System Surge Arrester Voltage Levels

System Nominal Voltage System Max. Temporary (Volts) dc Continuous Voltage overvoltage (Volts) dc voltage (ETOV ) (Volts) dc

600 to 850 1,020 1,150

1,500 1,800 2,000

3,000 3,600 4,000

25 Future development of dc surge arresters should provide visual indication when the surge 3arrester becomes defective or fails so that it can be removed to avoid the uncertainty of draining 4continuous low-level dc stray current to ground. 5

1 21.1.5 DC Traction Power System Components’ Voltage Withstand Capability. DC Traction 2 Power System components’ including OCS and dc switchgear voltage withstand capability 3 listed in Table 6 is the per IEEE draft Standard P1626 [B22] and IEEE STD C37.14 [B7].

4 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

5* Per IEEE STD C37.14 [B7] 6** Per IEEE Draft STD P1626 [B22]

722. DC Surge Arrester Service Requirements

8Operating ambient temperature shall be within the range of the operating environment between 9-40 degrees Fahrenheit and + 104 degrees Fahrenheit. System OCS to ground voltage shall be 10within the rating of the arrester under all system operating conditions. For OCS systems above 111,800 feet consult the surge arrester manufacturer for revised ratings.

1223. DC Surge Arrester Testing

136 Design Tests

14Surge arresters shall be subjected to the following design tests:

15  Insulation withstand test between the terminals 16  DC dry and wet spark over test

17  Discharge voltage test

18  Impulse protective level voltage time characteristics

19  Accelerated aging procedure

20  Pressure relief test

21  Leakage current at MCOV and 5% voltage in access of MCOV, leakage current v/s 22 voltage

23  Effect of higher temperature on leakage current, current v/s temperature graph

1  Peak discharge voltage under 8/20 µs lightning surge, with peak current of 200 A, 500 2 A, 5 kA and 10 kA

3  Peak discharge voltages under 45/1000 µs switching surge, with peak currents of 100 4 A, 200 A and 500 A. 5  Energy absorbing capability for the 45/1000 µs switching surge with 100A, 200A and 6 500A peak current

7  Energy absorbing capability for the 8/20 µs lightning surge with 200A, 500A , 5 kA and 8 10 kA peak

9New and clean arresters shall be used for each design test. For additional test requirements, see 10IEC Draft Standard NE 50562-1. 11The arrester shall be mounted in the position(s) in which it is designed to be used.

12Ambient temperature for test shall be -40 to +104 degrees Fahrenheit.

137 In Service (Field) Tests

14All surge arresters shall be visually inspected per IEEE Std. P1628 recommendations.

15Surge arresters shall be subjected to electrical tests per manufacturer’s recommendations.

16Apply dc test voltage in very slow increments of 20V dc each till the voltage reaches MCOV rating 17of the surge arrester. At each voltage step, measure leakage current through the arrester as 18excessive leakage current will result if arrester is defective. Do not apply voltage above MCOV 19rating for longer than manufacture’s recommended time to avoid damaging the surge arrester 20during testing. Voltage above TOV rating of OCS system shall only be surge waves and not the 21dc voltage. Surge wave shall be 8/20s with peak current magnitudes of 0.5kA, 1.0kA and 1.5 22kA. Such surge waves if applied should be for a very short duration less than 25s. Review such 23testing requirement with the manufacturer of the surge arrester under test before application of 24the suggested surge current for testing.

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