Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook (2020) / Chapter Skim
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From page 8... ...
8 Literature Review A literature review was conducted to identify the body of knowledge, both domestic and international, which pertains to principles, procedures, methods, and criteria for achieving, documenting, and maintaining acceptable levels of stray current. This review will help the reader understand stray current, the process of its evolution, the associated corrosion, and the mitigation methods from the early days of the introduction of electrified rail systems to present-day design.
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Literature Review 9 produced a manual on corrosion control (9) and the first reference book (10)
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10 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook as those for galvanic corrosion, the metal loss is much faster due to the large amount of stray current leakage (14)
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Literature Review 11 geomagnetic activity. The influence of telluric currents on structures is for a limited duration due to nonlocalized discharge areas and thus is rare to find.
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12 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook To reduce this stray current, the rail-to-earth potential should be as uniform as possible over the entire length of the utility pipeline and the utility line needs to be electrically continuous (8)
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Literature Review 13 to DC-powered transit agencies and detrimental effects to surrounding infrastructure and utilities. A recent TCRP study in 2007 on "rail base corrosion detection and prevention," suggests that the steel used in the fabrication of rails can hold up to the effects of the environment (galvanic corrosion)
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14 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook high-speed lines, primarily due to much higher reliability and reduced maintenance requirements of AC traction motors (21)
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Literature Review 15 salts found on the track. For most soils, the pH value falls within the range of 5 to 8 and is generally not considered to be the dominant variable affecting corrosion rates (though higher acidic soils present serious corrosion risks)
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16 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook potential will have a direct link to the stray current effect on buried utilities. The earth potential gradient is calculated by using Equation 9 (17)
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Literature Review 17 the running rails is an optional characteristic. This system permits the unregulated flow of stray current where stray currents will leave the running rails along the entire length and will return at the substation ground grid using paths other than the running rails.
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18 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook currents to return to the substation via the diode path, which can potentially increase stray current corrosion. Diode systems provide a unidirectional flow.
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Literature Review 19 safe limits is yet to be discovered. Numerous studies highlight the advantages and disadvantages of each scheme over the other; however, the effectiveness of grounding systems varies for different systems.
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20 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook and animals. To avoid impermissible effective touch voltage, in certain cases, voltage-limiting devices are installed (e.g., at passenger stations)
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Literature Review 21 were recommended by the corrosion committee to reduce the leakage and the severity of stray current corrosion. Some of the measures that were successfully developed to control stray current leakage and corrosion follow: • Use of properly bonded joints (welded joints)
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22 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Non-insulated negative return feeders were widely used in early constructions, especially where track bonds could not be well maintained. Supplementary conductors were installed in parallel with the track and connected to the track at frequent intervals to carry the current to the negative feeders and to ensure continuity of the return circuit.
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Literature Review 23 e = potential difference between tracks and ground at any point distance x from the end of line, i1 = total leakage current up to any point, r = leakage resistance between tacks and remote earth per unit length of line, d = resistance of track per unit length of line, x = distance from outer end of line of any point under consideration, and L = total length of line. Making use of the above equations, Figure 8 defines the stray current curves.
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24 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook Equation 17 shows the maximum stray current for a grounded system.
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Literature Review 25 whereas the actual curves will be less since a portion of the current will leak to the earth (1)
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26 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook all stray currents from the affected structure, because the interfering structure does not have a sufficiently negative potential. The technique is used where the stray current originates from a DC traction system.
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Literature Review 27 • Maintaining an ungrounded or diode-grounded negative circuit, though some researchers have observed that the rail life on diode-grounded transit system reduces to 20% of the normal life (10)
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28 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook cell-to-cell potential measurements are recommended by some transit agencies in their design criteria manuals. Various isolation techniques have been implemented by DC-powered rail transit agencies for the control of track-to-earth resistance in embedded tracks.
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Literature Review 29 It is also evident that most of the mitigation methods and principles suggested by the corrosion committee that originated from the 1920s are still in use. Technical advancements have been made in mitigation methods, and new methods have been embraced by newer rail transit systems.
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30 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook • Use of high resistivity concrete mix (chloride free)
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Literature Review 31 Special attention is also warranted for the proper placing of the rail boot and the subsequent concrete pour around it during construction. When the proper placing of the rail boot and the subsequent concrete pour around it during construction had not been done meticulously, there were instances where the track failed the safety test and the boot had to be reinstalled.
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32 Stray Current Control of Direct Current-Powered Rail Transit Systems: A Guidebook mentioned the origin or the basis of the limiting values for the return voltage or the stray current. In some cases, it was not clear if the transit agencies conducted any initial baseline surveys to come up with these limiting values.
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Literature Review 33 2.10 Chapter Summary With light rail transit (LRT) systems typically operating on embedded tracks in city streets, stray current corrosion is a major concern for track, utility, and other infrastructure owners near the DC-powered transit system.
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