Track Design Handbook for Light Rail Transit, Second Edition (2012)

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11-i Chapter 11—Transit Traction Power Table of Contents 11.1 GENERAL 11-1  11.1.1 Traction Power System Components 11-1  11.1.2 Traction Power/Track Interfaces 11-1  11.2 TRACTION POWER SUBSTATIONS 11-2  11.3 WAYSIDE DISTRIBUTION SYSTEM 11-3  11.4 CATENARY SYSTEMS 11-4  11.4.1  Introduction 11-4  11.4.2  Catenary Alternatives 11-4  11.5 CATENARY DESIGN 11-5  11.5.1 Introduction 11-5  11.5.2 Conceptual Stage 11-6  11.5.3 Application of the Catenary System to the Track Layout 11-6  11.5.3.1 Track Centers 11-7  11.5.3.2 Horizontal Curves 11-7  11.5.3.3 Vertical Profile 11-7  11.5.3.4 Vertical Curves 11-7  11.5.3.5 Interlockings 11-8  11.5.3.6 Track Adjacent to Stations 11-8  11.6 TRACTION POWER RETURN SYSTEM 11-8  11.6.1 Territory with Two-Rail Track Circuits for Signaling 11-8  11.6.2 Territory with Single-Rail Track Circuits for Signaling 11-9  11.6.3 Territory without Signaling Track Circuits 11-9  11.6.4 Rail Conductivity 11-9  11.7 CORROSION CONTROL MEASURES 11-9  11.8 MAINTENANCE FACILITY YARD AND SHOP BUILDING 11-10 

11-1 CHAPTER 11—TRANSIT TRACTION POWER 11.1 GENERAL Light rail systems, as defined in Chapter 1, use electrical power from overhead wires to provide traction power to the light rail vehicles. The rails, sometimes in conjunction with supplemental negative return cables, act as the return conductor to the negative terminal of the rectifiers. Therefore, the electrical properties of the rails and tracks require special consideration. To obtain good conductivity for the track as a whole, a rail system must have a low resistance not only for reasons of economy but also for safety. This requires a low voltage drop in the rails over the length of the track structure. 11.1.1 Traction Power System Components The complete traction power system consists of the following: • Traction power substation (TPSS) that converts commercial alternating current electricity into the direct current power used by the light rail vehicles. • Cables connecting this substation to the wayside distribution system. • Wayside distribution system providing adequate current at appropriate voltage levels throughout the alignment. The principal element of the wayside distribution system is the overhead contact system (OCS), more commonly called the “catenary.” In some cases, there will be supplemental cables running parallel to the route to “feed” additional power to the contact wire. • The rails, which carry the negative return current from the LRV back to the vicinity of the substation. In some cases, these will be supplemented by negative return feeder cables. • Return system cables connecting the running rails to the substation. • In some cases, a system of corrosion control drainage cables to collect stray traction power current and take it back to the appropriate substation. These corrosion control drainage cables are separate from and not to be confused with negative return feeder cables. 11.1.2 Traction Power/Track Interfaces There are four elements in the traction power system that affect, or are affected by, track alignment and trackwork design and the construction and maintenance of track systems: • Traction power positive supply system, including substation locations • Wayside catenary distribution positive system, providing power to the vehicles • Traction power negative return through the rails • Corrosion control measures to minimize the level and effects of stray currents on adjacent conduits, pipes, and cables

Track Design Handbook for Light Rail Transit, Second Edition 11-2 11.2 TRACTION POWER SUBSTATIONS Traction power substations take commercial alternating current power from the local utility company and convert it into the direct current required by the LRVs. The optimal locations for the traction power substations are determined using a computer model that simulates proposed LRT operations along an accurate geometrical and geographical depiction of the planned route. The model will include not only the horizontal and vertical alignment of the track, but also the achievable design speed so as to determine the power demand of the LRT system during peak periods. Therefore, in the early stages of any light rail transit project, track and traction power designers must interface to integrate the traction power system into the overall system design. The final selection of substation sites is an iterative process with repeated simulations to confirm the capability of the traction power system to sustain peak-hour operations. The sequence of events to develop substation sites is as follows: • The traction power designer, using the simulation program, selects theoretically ideal TPSS positions along the route, taking into account the distribution system’s voltage drop and the lowest voltage acceptable to the vehicle without degrading performance. The normal, single contingency criterion for determining traction power system sufficiency is to test the system with alternate substations out of operation and verify whether an acceptable level of LRT operations can be sustained. • The designer discusses these proposed locations with the local power utility to determine any impacts of the proposed power demand on their network. The utility then evaluates the availability of power circuits and the potential impacts on its other customers. • An agreement is eventually reached, if necessary, by moving the substation to enable it to be supplied from lightly loaded power circuits or by building spur cables to the substation location. It is also important, for reliability, that the power company avoid supplying two adjacent substations from the same circuit. • It is not always possible to position the traction power substations in the optimal location, particularly in urban areas where available sites may be limited by many issues, including political realities. After an agreement is reached with the power company, the traction power designer can finalize the substation design. While the TPSS can be a constructed building into which equipment is installed, most substations for new and renovated light rail systems are modular, factory- assembled units that are delivered to the site complete. They are erected on a prepared base that incorporates an extensive grounding network below the concrete. These modular units are more economical than constructed buildings. Depending on the neighborhood where they are sited, modular TPSS units are sometimes screened by landscaping or architectural walls. Substations are located along the route as close to the tracks as possible within the constraints of available real estate. However, the final placement must also consider interfaces and underground cable duct routes for the power distribution supply and return systems, access roadways, and security requirements. The impact of this construction on trackwork design is limited to the interfaces with the supply and return power distribution system.

Transit Traction Power 11-3 The electrical sectionalization of the distribution system usually takes place at the substation for all travel directions. Placement of a substation at, or near, a crossover is often desired to sectionalize electrical supply for each travel direction and to optimize the operational flexibility of the track system. 11.3 WAYSIDE DISTRIBUTION SYSTEM Broadly speaking, wayside distribution systems can be subdivided into the overhead contact system, which is discussed in Article 11.4, and supplemental cabling systems to connect the catenary to the traction power substations. The latter is the topic of this article. Each TPSS is typically linked to the trackway by underground conduits. One set of conduits runs to and up one or more catenary system poles to carry the cables that provide power to the catenary. The conduit risers can be located either on the outside surfaces of the OCS poles or within the poles, either of which can require an appreciable foundation at trackside. Once the power supply is terminated to the catenary, the positive side of the traction power supply distribution usually remains on aerial structures and does not interface further with the track. Another set of conduits and cables runs to the track and provides the negative return path for traction power back to the TPSS. The design of the trackbed needs to accommodate these traction power supply system conduits. Adequate space is required beneath the track for the conduit systems (including terminations), conduit risers, and manholes. The track itself must accommodate connections of the negative return cables. If the overhead contact wire system is a single filament trolley wire, it is usually necessary to have supplemental feeder cables as well so that the overall traction power distribution system has sufficient electrical capacity to provide current without unacceptably large voltage drops. In urban areas, these feeder cables are, for aesthetic reasons, most often routed through underground duct banks that run parallel to the tracks. The feeder cable must be periodically connected to the trolley wire, usually at every third to fifth OCS pole. At each such location, a manhole will be located along the trunk duct bank and a branch conduit will run over to the OCS poles. The overall design of the trackway must accommodate these ducts and manholes. A messenger/feeder can be placed above the trolley to obviate the need for parallel feeders. The messenger-to-trolley vertical dimension (construction depth) can be made small (6” to 12”) to reduce visual impact, with no effect on track design. Less often, supplemental feeder cables are carried on the OCS poles rather than being routed in underground conduits. Many legacy streetcar lines used this configuration. This substantially reduces the impacts on the trackway design, but the overhead cables negate some of the visual aesthetic benefits of a single filament trolley wire system as it effectively just moves the catenary messenger cable from a position directly over the track to a location along the poles.

Track Design Handbook for Light Rail Transit, Second Edition 11-4 11.4 CATENARY SYSTEMS 11.4.1 Introduction The OCS on a light rail system usually consists of a simple catenary system that incorporates both a messenger cable from which a contact wire (also known as a trolley wire) is suspended. This configuration is both electrically efficient and economical to construct. The word “catenary” is actually a mathematical term that describes the curve assumed by a flexible cable that is suspended at its ends. It therefore can technically be applied to virtually all types of OCS. However, it is usually intended to mean an OCS where a messenger cable supports a simple trolley wire with both conductors being used to carry the power used by the light rail vehicles. In visually sensitive areas, a single trolley wire may be utilized so as to minimize the number of wires above the tracks. This is a common requirement where the tracks are in city streets and the light rail line has the characteristics of a streetcar. The style of catenary and most of the basic design parameters can be developed prior to finalization of the track configuration. However, the application of a catenary design to suit the track layout can only proceed after the track alignment has been finalized. 11.4.2 Catenary Alternatives The track alignment engineer needs to understand what type of OCS is proposed so that adequate clearances can be provided along the trackway for poles, pole foundations, and associated hardware. There are generally three styles of OCS used on LRT systems: simple catenary, low-profile catenary, and single filament trolley wire system. All types of OCS can have either of the following configurations: • Fixed terminations at the end of each wire run, causing the conductors to either sag or rise as the temperature varies, or • Balanced weights at one or both ends of each wire run so as to maintain constant tension and height regardless of the climatic conditions of the project site. Fixed termination OCS typically requires heavier poles, larger pole foundations, and more robust line hardware than balance weight design because of the higher tensile loads that occur in the wires during cold weather. Therefore, modern, lightweight catenary systems almost always use balance weight tensioning to limit the load applied to the poles. Regardless of whether balance weight or fixed termination design is used, the catenary is typically separated into 1-mile segments. The conductors are overlapped at the ends of these segments so as to provide a smooth passage of the vehicle pantograph from one segment to the other. The track alignment design may need to accommodate additional poles at these overlaps. The details of the OCS will also vary with the type of current collector used on the light rail vehicle. Pantograph current collectors can use either a fixed termination or balance weight OCS as the pantograph head can easily sweep over an overlap between one run of trolley wire and the next. Vehicles equipped with a trolley pole generally require a fixed termination system since the

Transit Traction Power 11-5 trolley wire running surface must be continuous, without any gaps or overlaps. In addition, trolley hardware for pole operation is not normally suited for pantograph operation. A simple catenary system uses a messenger wire to support the horizontal trolley wire. Both conductors are used to transmit power from the substation to the vehicle. In a simple catenary system, the system height at the supporting poles—the distance between the contact or trolley wire and the messenger—is approximately 4 feet [1.2 meters]. In tangent track, this allows spans between poles of up to 240 feet [73 meters]. The low-profile catenary system is similar to the simple catenary design, except that the system height at the support is reduced to approximately 18 inches [457 millimeters] and sometimes less. This style is often applied in aesthetically sensitive areas where a lower profile and simple single- wire cross spans are more desirable. The trade-off, however, is that the span length between supporting poles is reduced to approximately 150 feet [46 meters]. Single filament trolley wire systems, which were traditionally used on legacy streetcar lines, are considered by many persons, and particularly by lay audiences, to be visually less obtrusive in the urban environment. It provides power through a single trolley wire that must be supported at least every 100 feet [30 meters]. The span length is limited by the sag of the unsupported trolley wire, which in high temperatures could otherwise fall below the minimum elevation required by the National Electrical Safety Code. It is also limited by the structural capacity of the supporting hardware to carry the weight of the entire length of a span between supports. Single filament trolley wire also usually requires the wire to be supplemented electrically by parallel feeders that must be frequently connected to the trolley wire to maintain sufficient voltage for LRV operation. These feeders may either run underground through a series of ducts and manholes, which are expensive, or be hung from poles. The latter position can arguably be just as visually intrusive as the messenger wire in a catenary system and merely relocated to a different point in the observer’s line of sight. The overhead wire design to accommodate trolley pole operation requires more support and registration points and can therefore have nearly twice the number of poles than the equivalent simple catenary system. 11.5 CATENARY DESIGN 11.5.1 Introduction Generally, technical papers have not addressed rail/catenary interface issues since transit catenary design has developed from operating railway systems where the track is already in place and the catenary must follow the existing track layout. For new light rail transit lines, while it’s possible to consider OCS design during the early planning stages of the project, unfortunately, such is rarely the case. In many new transit systems, the track alignment has been selected prior to the catenary designer’s involvement in the project. The results of this lack of coordination are chronicled in TCRP Report 7: Reducing the Visual Impact of Overhead Contact Systems. Involving the catenary designer during the route selection and conceptual track alignment design stage can be cost-effective in the long run and reduce the visual impact of the catenary system. Horizontal and vertical track alignment, trackwork, passenger station locations, substation sites, etc., must all be determined before the preliminary catenary engineering can proceed. However,

Track Design Handbook for Light Rail Transit, Second Edition 11-6 the locations and design of these components can greatly influence the catenary design and its visual impact on the environment. 11.5.2 Conceptual Stage The catenary engineer's task is to develop a conductor configuration to supply power to the vehicle from a position over the track that will allow good current collection under all adverse- weather, operating, and maintenance conditions. The engineer must develop the most economic solution considering the aesthetic constraints set by the community. This task involves resolving the number of wires in the air with the number of poles, supports, and foundations to achieve an efficient and environmentally acceptable design. The catenary system is the most conspicuous and arguably the most visually undesirable infrastructure element of a light rail transit system. TCRP Report 7 discusses "visual pollution," to the extent that it cited a case where a community refused to introduce an electric-powered transit system because of the expected visual impact. However, with rare exceptions, overhead wires are needed to distribute power to light rail vehicles. Therefore, poles are needed to support and register them over the pantograph under all adverse conditions. However, if the track alignment designer considers the catenary constraints, then the size and number of poles can be minimized. The catenary distribution system interfaces with trackwork in the following manner: • On single-wire catenary systems, the track designer must accommodate the longitudinal and transverse track feeder conduits that support the electrical distribution system. • The track designer must also provide adequate clearance between tracks for foundations, poles, catenary balance weights, guy anchors, and guy cables. • Track design and maintenance standards must be coordinated so that the vehicle pantograph remains beneath the catenary wires under all adverse operating and climatic conditions. 11.5.3 Application of the Catenary System to the Track Layout Since the wire runs in straight lines between support points and the track is curved, pole layout is a compromise between the number of poles and the requirement that the contact wire remain on the pantograph under all adverse climatic, operating, and maintenance conditions. Even though the pantograph head can be up to 6.5 feet [2 meters] wide, allowances for track alignment, gauge, cross-level tolerances, vehicle displacement, roll, pantograph sway, and pole deflection means that only its central 18 to 24 inches [460 to 610 millimeters] are actually available for the wire to sweep across the pantograph head. At the midpoint between supports, this distance is reduced to near zero due to deflection of the wires under maximum wind and ice loading conditions. The allocation of pole positions must take into account the limitations of the catenary style, the profile of the contact wire necessary to accommodate overhead bridges and grade crossings, track curvature, crossovers and turnouts, underground utilities, etc. Therefore, if the track is designed with the catenary constraints in mind, economies can be achieved. The following paragraphs identify parameters that should be considered by the track designer.

Transit Traction Power 11-7 11.5.3.1 Track Centers The clearance between poles and the track is defined by the system’s dynamic clearance envelope, which comprises three elements: the vehicle dynamic envelope, construction and maintenance tolerances, and running clearances. Therefore, if center poles with supporting cantilevers on each side are desired to reduce cost and visual intrusion, then the distance between tracks should allow for this envelope from each track plus at least 12 inches [305 millimeters] to permit installation of standard-sized poles. 11.5.3.2 Horizontal Curves If the track is tangent, there will be no track-related constraints other than right-of-way boundaries when placing the poles along track. However, as the wire negotiates curves using a series of chords, the number of supports is very dependent on the curvature. Therefore, as with other light rail system components, minimization of curvature and avoidance of extremely tight curves is most desirable in catenary system design. This can be a challenge for in-street LRT where even on a nominally straight street the tracks may need to frequently shift laterally to stay in a consistent traffic lane while dodging around left turn lanes. See Chapter 12 for additional discussion on this issue. 11.5.3.3 Vertical Profile The minimum clearances between the underside of the contact wire and the top of rail are dictated by the National Electrical Safety Code. In exclusive guideways, the usual requirement is a clearance of 16 feet under any condition of loading, including wind, snow, and ice—although lower elevations are possible. Where the OCS passes over a public street, the minimum requirement is typically 18 feet. Whenever the height of the contact wire changes, the gradient of the wire relative to the track profile will be restricted, depending on the desired train speed. The track alignment engineer must therefore closely consider any locations where the track and catenary pass beneath a low-clearance bridge shortly after passing over a public road. If the relative gradient of the trolley wire to the track is too steep or the change in grade too great, it may require a speed restriction, which could then affect other issues on the LRT project. 11.5.3.4 Vertical Curves Vertical curves become critical when in the vicinity of reduced-clearance overhead bridges. The rise and fall (sag) of the catenary messenger is governed by the formula: WL2 2T where W is the weight of the catenary L is the distance between supports T is the tension in the messenger Therefore, if there is an abrupt change in track profile near an overhead bridge, the track designer should consult with the catenary designer to ensure that the wire can negotiate the vertical curvature. This can be an issue when the LRT crosses a street at grade where, per the NESC, the wire must be high and then must immediately pass beneath a low-clearance bridge. Extreme cases can require some ingenious interdisciplinary coordination.

Track Design Handbook for Light Rail Transit, Second Edition 11-8 11.5.3.5 Interlockings The catenary/pantograph interface is a dynamic system. There are certain constraints applied to ensure that the system operates efficiently under all speed and weather conditions. The pole positions at turnouts are tied to the point of intersection (PI) of each turnout. It is desirable for the distance between the inner crossovers of a universal interlocking (i.e., two independent crossovers, one right hand and one left hand) to be approximately the same length as each crossover (PI to PI). Double, or “scissors,” crossovers can be wired; however, they present many difficulties for the catenary designer. Usually, for maintenance purposes, the inbound and outbound tracks are separated into different electrical sections. With tracks crossing within roughly 6 feet [2 meters], very limited space is available to insert a section insulator in the contact wire and still avoid the horns of the pantograph head. This is particularly difficult in higher speed sections using constant tension catenary design since the movement of wires along the track due to temperature change can aggravate the problem. By contrast, a single run of catenary can cover both crossovers placed end to end in a universal interlocking, making for much less costly OCS construction. Therefore, scissors interlockings should be avoided when catenary is employed. 11.5.3.6 Track Adjacent to Stations Architecturally, the introduction of the catenary system is obtrusive. Architectural design tends to dictate the position of poles to suit the architectural theme within the station area. This impacts catenary pole positions adjacent to the station area, requiring close coordination between the architect and track and catenary designers to ensure adequate space for poles at stations and approaches. 11.6 TRACTION POWER RETURN SYSTEM The train control signaling system, which often requires insulated joints in the rails, complicates matters of traction negative return, which wants the rail to be electrically continuous. The paragraphs that follow explain the basics of the track engineer’s role in accommodating the conflicting needs of the signaling and traction power systems. 11.6.1 Territory with Two-Rail Track Circuits for Signaling The traction power return system directly impacts track design. The traction power return system uses the running rails as an electrical conductor to “return” the traction power to the substation from which it originated. Traction power supplied to the train enters the running rail through the vehicle wheels and is extracted from the rail through impedance bonds in cables installed at each substation. Therefore, track designers must allow for impedance bond installation along with the associated conduit stub-ups and negative cabling at each substation. Where there is more than one track, in addition to the impedance bonds at each substation, impedance cross bonds are located along the track every 610 meters (2,000 feet) or less to equalize the traction return currents in the rails. At these locations, conduit stub-ups will be installed beneath the tracks connecting the two track directions. Impedance bonds are also required by the signal system at the end of each signal block.

Transit Traction Power 11-9 11.6.2 Territory with Single-Rail Track Circuits for Signaling Although most track circuits for signaling in new light rail systems are of the two-rail type, single- rail signaling track circuits do exist in older systems. In such systems, one rail is used for traction return and the other is designated the signal rail. This type of installation requires insulated joints separating the track circuits. With single-rail track circuits, the impedance bonds described in Article 11.6.1 are not required. The cross bonding provided between the traction return rails of separate tracks uses cables without impedance bonds for this purpose. Except for these differences, the same cabling is required between the traction return rail and substations as described in Article 11.6.1. 11.6.3 Territory without Signaling Track Circuits The requirements for traction return in this type of territory are similar to those described in Section 11.6.1, except that no impedance bonds are required. Instead, cables are installed directly to the rails for both traction return at the substation and for cross bonding between the rails. 11.6.4 Rail Conductivity Concern is occasionally voiced over whether the rails themselves have sufficient conductivity to carry the return traction power current. Well-meaning persons will sometimes suggest that the chemistry of the rail should be revised so as to enhance its conductivity. Such concerns are generally ill founded since rail of normal size (such as 115 RE) with normal rail chemistry, already has far more current-carrying capacity than the OCS. Moreover, the chemistry of rail, whether it is rolled in accordance with the AREMA specifications or the European Norms, has been carefully determined to produce rail with optimal wear and toughness characteristics. Modifying that rail chemistry in the pursuit of enhanced conductivity is extremely likely to result in rail of inferior mechanical properties. Such rail would likely not be guaranteed by the rolling mill. Further, unless the purchaser is buying several heats of steel rails, the steel companies will be unreceptive to interrupting their normal production methods. 11.7 CORROSION CONTROL MEASURES In designing DC traction power systems, it is common and desirable to isolate and insulate the running rails from ground as much as possible. These issues are discussed at length in Chapters 4 and 8. The traction power return system interfaces with trackwork in the following manner: • The siting of impedance bond positions and cross bonds to adjacent tracks must be coordinated. • The selection of rail insulation for tie plates and fastening clips suitable for track and traction power requirements must be agreed to by all parties. • Continuity bonds on jointed rails must also be coordinated.

Track Design Handbook for Light Rail Transit, Second Edition 11-10 • The track designer and construction inspector should ensure that ballast is clear of rails so that return currents do not stray into the ground and cause corrosion problems in underground pipes and cables. • Special consideration must be given to selecting the insulation of the rails at grade crossing and embedded track sections to ensure minimum leakage to ground. 11.8 MAINTENANCE FACILITY YARD AND SHOP BUILDING The traction power system in the maintenance facility yard and shop area is usually different from and totally isolated from that used on the main line. This is because the yard and shop complex, including all of the infrastructure and underground utilities within it, is typically completely new. Therefore, the adverse effects of stray currents can be accounted for and mitigated in its design. For example, all underground utilities can be constructed using non-conductive or insulated pipes that will not conduct stray currents. Also, the traction power return current can be more easily controlled in a yard by increasing the quantity and locations of return cables. Because of these stray current mitigation measures, the trackwork insulation systems provided for the yard tracks could, in theory, be somewhat less robust than those used along main line track. For this reason, yard tracks on light rail systems constructed up through the 1990s were often constructed with timber ties without insulated rail fastenings. However, since then, the prices of concrete cross ties and timber cross ties have gotten closer. More projects now seem to be using concrete cross ties with insulated rail fastenings in their yards as a matter of course. This trend has seemingly expanded from plain track to special trackwork zones as well, with insulated rail fastenings being applied to switch timbers and the adoption of concrete cross ties for yard tracks as well as main lines. While providing this level of electrical insulation within an insular yard facility is perhaps unnecessary from the perspective of stray current control, the simplicity of using the same track materials systemwide has much to commend it. In addition, yard tracks can be very difficult areas in which to change out defective timber crossties, so the increased service life of concrete cross ties can usually be justified by a life cycle cost analysis. The grounding systems for the yard and main line must be electrically separate. This is achieved by inserting insulated rail joints in the yard entry track at each arrival and departure connection. The track alignment designer must carefully coordinate the locations of these insulated joints so that standing trains waiting to either leave or enter the yard do not straddle the joints for more than a few seconds. This can require an iterative coordination process including the track designer, the operations planners, and the traction power engineers. In the maintenance facility building, the rails are installed directly into the shop floor system and are rigorously electrically grounded for safety of the personnel working on the vehicles. The return system is designed for current to return directly to the substation through cables to ensure that there is no potential difference between the vehicle and the ground. Space for the conduit and cables connecting each track section to the building substation must be coordinated. The shop tracks also contain insulated joints that electrically separate these totally grounded tracks from the yard track system. Usually, those joints are located in the ballasted track immediately beyond a concrete apron that typically is positioned along the face of the shop building so as to provide rubber-tired access to each shop doorway.

Transit Traction Power 11-11 Yard track designers must still consider and account for the many conduit risers necessary to feed the numerous electrical sections in the overhead contact system. Extra coordination in yard areas should take place due to the additional users and electrical connections in the complex track layout.