Functionality in the Design of OCS Sectionalizing

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1 Functionality in the Design of OCS Sectionalizing John Schlick Hatch Mott MacDonald Pleasanton, CA ABSTRACT In order to maximize the functionality of a light rail system during maintenance periods or emergency shut downs, the proper design of sectionalization throughout the line segment and near crossovers is important. The use of various sectionalization methods, placement of sectionalizing and crossover placement within the traction power system must be taken into consideration when planning a new extension or retrofitting an existing system. Proper planning of the overhead contact system (OCS) and traction power system can lead to a more userfriendly system without sacrificing large costs in construction or maintenance. This paper will look into the various types of sectionalizing for both mainline and crossover catenaries. Analysis will be shown as to which methods are more cost effective based on construction costs and which result in the most efficient product in terms of operations and maintenance. Various sectionalization schemes will be presented that yield various levels of functionality and ease of operation during maintenance periods or emergency shut downs. Examples of various sectionalization methods and schemes used by authorities will also be presented. TYPES OF SECTIONALIZATION Light Rail Transit systems throughout North America use various types of sectionalizing. Based on when the system was first constructed it may have been the best sectionalization that was available at the time. It may have been the designer s preference or the preference of the authority based on what sectionalization was more preferable to maintain. Whatever the case the type of sectionalization was probably what was installed the time before and it was best to keep the existing design going. This would allow for the whole line to be consistent based on what the maintainers were familiar with and what spare parts the authority had on hand. If a new light rail system was to be introduced in a new area and design was to start from initial concepts, what would those initial concepts be? What methods of sectionalization would be chosen and why? With a blank canvas to start a new system and develop a clean sectionalization plan, what types of sectionalization would be used? Here are a few types of sectionalization for the overhead contact system with their pros and cons. Insulated Overlaps The most common method of sectionalization is the insulated overlap. An insulated overlap is the point where two OCS tension sections cross along track. These two sections are not linked electrically, because a continuity jumper is not installed. An air gap exists between the two systems, so power fed from one side can be separate from power fed from the other side. An insulated overlap normally does not cost anymore to construct or maintain when used as a sectionalizing point than an un-insulated overlap, which is not used as a sectionalizing point. Normally separated by under a mile overlaps occur naturally in the design of the OCS; they are a common way to connect two adjacent tension sections and allow the light rail vehicles to operate smoothly. It s easy to see that using an existing feature of the OCS as a sectionalizing point would yield little to no additional cost to construct or maintain. Overlaps need to be in the OCS in order to properly tension the system. However insulated overlaps are large in size. In order to properly design an overlap three spans of OCS are needed. The first span is the termination span of one section, the second span is the transition span for both sections and the third span is the termination span for the other section. When considering a double track system with close track centers not allowing for poles to be installed side by side, a staggered overlap would take four spans for two tracks. These four spans can be designed to take up anywhere from 600 to 800 feet along track. Track 1

2 geometry, in-street running conditions and other obstructions and design variables may not make it feasible to have sectionalizing occupy that much track space. Although if space is not a concern, the insulated overlap is clearly the choice that has the least increased cost to construction and maintenance. Airbreaks Airbreaks are another way to sectionalize the OCS. Airbreaks are the presence of an air gap with additional insulators that can be installed anywhere on the segment that space permits. They take up far less space than an insulated overlap, because they can be achieved in just over a span length (as little as 160 feet). The normal catenary OCS is installed as usual. An additional contact wire is installed alongside the mainline contact wire. The two contact wires are usually staggered by 12 inches across track. Vertically at each support the out-ofrunning contact wire is 6 to 9 inches above the in running contact wire. The contact wires transition from in running to out-of-running in the span length with a transition area in the middle. The additional cost to install an airbreak would be the cost of the additional contact wire, some additional hardware and an installation crew for two days. Airbreaks are tricky, because to some it s a new form of sectionalizing they have not dealt with in the past. From an engineering standpoint the section of the airbreak will be at half tension. For example, if the normal contact wire tension is 3000 lbs, two contact wires running parallel and connected at each end with a parallel clamp will only have 1500 lbs of tension. This introduces a softer system susceptible to more sag in the contact wire. In addition to all the normally occurring overlaps throughout the system, additional airbreaks introduce more locations where added maintenance is necessary. Due to the increased amount of maintenance and softer section, many authorities have decided not to install anymore airbreaks in their systems. While there is an advantage to airbreaks in their smaller size, there is an additional cost in construction and maintenance associated with them over the insulated overlaps. An example of an airbreak is shown in Figure A. Insulated Midpoints Insulated midpoints are similar to insulated overlaps; however they occur at the midpoint of a tension section as opposed to the ends of two tension sections. Insulated midpoints were developed to help avoid designing a full overlap where sectionalizing was needed. At an insulated midpoint the messenger wire is continuous just like a normal midpoint. The contact wire, however, terminates with a fix termination assembly on either side of the messenger wire midpoint similar to an overlap. The two contact wires are positioned similar to an insulated overlap over three spans with a transition in the middle span. The insulated midpoint has advantages over the typical insulated overlap, because it only requires the contact wire to form an insulated overlap rather than both the messenger wire and contact wire. While it does take up more space and cost more to install in construction costs than an airbreak, it allows the contact wire to be at full tension instead of half tension. It is also easier to maintain than an airbreak. The advantages and disadvantages to the insulated midpoint need to be considered along with the physical make up of the system and the parameters being dealt with. Sketches of an insulted midpoint are shown in Figure B. 2

3 Section Insulators Section insulators provide electrical separation at any given point where sectionalization is desired. It consists of an assembly installed in place of the contact wire that provides a smooth passage of the pantograph and a cut in insulator in the messenger wire. The contact wire insulator can be either bridging or non-bridging. Bridging type section insulators permit continuous current collection when the pantograph passes through. Nonbridging type section insulators ensure that adjacent sections of the OCS will remain electrically isolated when a pantograph passes through. Section insulators require much less space than an insulated overlap, insulated midpoint or airbreak. When space is a limitation, the section insulator can be installed wherever it is needed. They typically are only about six to eight feet long. Provisions in the layout and design of the OCS do not have to be considered; the section insulator can simply be installed immediately adjacent to the substation feed points or wherever sectionalizing is desired. A disadvantage to section insulators is their heavy and rather bulky stature, so they require support. In a catenary system, the messenger wire will support the section insulator. In a single wire system the section insulator must be supported by an aerial guy or some other overhead hanger. Section insulators are also prone to cause more wear to pantograph carbon strips than insulated overlaps, insulated midpoints or airbreaks. Because of this speed restrictions of 15 mph are placed on vehicles traveling through section insulators. Routes that are more travelled and travelled at higher speeds are not candidates for installation of a section insulator. Maintenance of not only the pantograph carbon strips but of the section insulator itself is greater than with other forms of sectionalization. The section insulator has construction costs comparable to an airbreak, but takes much less time to install. service. Proper sectionalizing of the OCS will make this possible. Proper sectionalizing of the OCS starts with placement near the substations. Insulated overlaps or insulated midpoints give you the best options for sectionalizing on the mainline. Planning the OCS with insulated overlaps or insulated midpoints near the substations also allows for the minimum length of feeder cables required to carry power to the OCS. At crossovers sectionalizing is critical to assure the functionality of the system during power outages. There are different schemes that can be used to effectively sectionalize the overhead system. One-Point Mainline Sectionalization When one sectionalizing point is introduced on each mainline track, the most efficient place to install it would be between the two turnouts of a double crossover on a given track. With sectionalization present there, the crossover wires still act as a jumper between the two mainline tracks, so section insulators are required to isolate the two mainline tracks from each other. Two examples of this sectionalizing approach are shown below. In Figure C that shows two section insulators in the crossover, all necessary isolation between mainline tracks is achieved, but the most efficient use of the system is not achieved. In fact the crossover wires serve as a jumper crossing the mainline sectionalizing point. If power is cut to one of the wires that feed the center of the crossover, then the crossover is rendered powerless and not able to power trains through it. As is illustrated in Figure X the magenta section represents a de-energized section of OCS. As can be seen, the crossover is not able to power trains from one mainline track to the other in this situation. PROPER DESIGN OF SECTIONALIZATION In order to maximize the functionality of your light rail system placement of traction power substations, feed points and OCS sectionalizing must be planned properly. Based on the desired locations of your station platforms, crossovers are located where they will best facilitate the standard routes of the passenger vehicles for revenue service and the use of these vehicles during power outages for maintenance or unexpected emergencies. During power outages it is important to make sure the light rail system is functional with certain segments out of 3

4 The sectionalizing scheme shown in Figure D using four section insulators and two by pass switches will allow the full use of the crossover if any of the four mainline sections are de-energized. This allows the crossover to be utilized in any circumstance involving the shut down of one section of OCS. Of course if the two sections to the right were both de-energized, the crossover would not be able to power trains. The ideal case would be four by-pass switches. located near one of these insulated overlaps. Replacing an insulated overlap with an insulated midpoint at one of the locations would save on the additional cost of a messenger wire overlap. Of course mainline section insulators are another option with several drawbacks. One overlap properly designed could give you sectionalization on both sides of a double crossover. An arrangement such as the one shown in Figure F can give you two points of sectionalization with only one overlap. Two-Point Mainline Sectionalization Designing two points of sectionalization on a given mainline track can reduce the number of section insulators needed to always keep the crossover fully functional when one OCS section is de-energized. In fact, using a special type of crossover arrangement can eliminate the need for section insulators all together. From Figure E we can see an arrangement that places sectionalization points outside each turnout creating a fifth OCS section about the crossover. This section can be powered from a fifth substation breaker or from a bypass switch from any other four OCS sections. The one out-of-running wire to the left reaches the track a span before the turnout. This span is one of your insulated overlap spans. Contact wires alternate from in running to out-of-running. They do the same for the span in between the turnouts and again to the right of the second turnout before the OCS to the left finally terminates. The central overlap does not have to be an insulated overlap. Jumpers can be installed to keep the entire central section as one feeding the crossover tracks. No Mainline Sectionalization A system with no sectionalizing on the mainline can be seen in Figure G. When no sectionalizing is present on the mainline, de-energizing of either section would make a powered move to the other track impossible. There are several ways of achieving two points of sectionalization on one track. Designing two insulated overlaps 2000 to 2500 feet apart would work. This arrangement would be a good option, if a substation were 4

5 Extreme Cases of Power Shutdowns What happens when power needs to be removed from two adjacent sections of track at the same time. This situation could arise when work is done on a center station platform or on an overcrossing above the whole alignment. There would be no power on the track on the opposite side of the crossover. This situation would prevent a train from returning on the other track in a normal direction of travel. With a two-point mainline sectionalization such as the one shown in Figure F, it would not be possible for a train to clear the switch on one track without going past the point where the sectionalization is installed. With two overlaps spaced far apart similar to the arrangement in Figure H, a train consist can clear the switch and travel in the opposite direction to return through the powered section. crossover track would not be possible. Another method of sectionalizing would need to be implemented. Four section insulators By placing four section insulators in a crossover along with one point of sectionalization on each mainline track, the crossover is fully functional during a power outage on one leg of the mainline track. There is one catch to using four section insulators. The four section insulators form a dead spot about the crossover frog. Power must be jumped from one of the powered sections to this dead spot. This can be done several ways. A simple jumper can be installed between a mainline powered section and the central OCS. Instead of jumping one of the insulators, only three could be installed. This arrangement can be seen in Figure I. This would be the only case where the crossover could be fully functional in the event of two mainline tracks being de-energized at a given time. SECTION INSULATORS IN CROSSOVERS There is really only one option for sectionalizing on a single crossover. A section insulator is placed at the midpoint of the crossover span resulting in separation between the two mainline circuits. For double crossover (diamond crossovers), there are a few options for installing section insulators on crossovers. Two section insulators When placing two section insulators in a double crossover separation between mainline track circuits must be achieved. Section insulators are placed at the midpoint between one of the mainline tracks and the central frog of the crossover. Figure G shows a typical installation. With this method of sectionalizing, if one of the mainline tracks were de-energized, a powered move along a However, this mainline section would always need to be energized in order to use the crossover. If this leg of the mainline track were de-energized, the crossover would not be powered. The option to energize the central OCS from multiple sections would be more ideal. The use of by-pass switches to power the central OCS is a better method. If the normally feeding section is de-energized, the by-pass switches can be adjusted to allow the central section to be fed from another section. This arrangement can be seen in Figure D. While this method of sectionalizing a double crossover gives the most operational functionality, it comes at the cost of extra installation and maintenance of more section insulators and by-pass switches. This, however, has become a standard for sectionalizing in certain cases. However, there is always the extreme case of two adjacent tracks being de-energized. If those two tracks are the two with the by-pass switches, the crossover can not be used. The most functional case would then be an arrangement with four by-pass switches. 5

6 INSULATED KNUCKLE CROSSOVER Historically, in sectionalizing tracks at a double crossover the practice has been to cut in at least two section insulators on the crossover catenary system and tie each side of the sectionalization to its corresponding mainline section. This practice has been used in the past in order to keep two separate mainline sections isolated from each other. Because of the section insulators disadvantages, a system that does not utilize section insulators provides many benefits. Improvements in the design of crossover catenary systems are quickly making this type of installation into a non-standard practice. Standard installation has been to install catenary through the crossover from one mainline track to the other. This essentially provides a link or a jumper between two separate sections. The installation of the section insulators allows this jumper to be split. If the overhead wires did not cross over to the other mainline section, then a section insulator would not be needed. A crossover catenary arrangement using insulated knuckles would allow this to happen. In an insulated knuckle crossover arrangement, overhead wires for the crossover tracks are installed from one turnout to the center of the crossover and back to the turnout further up the mainline on the same track it started. The same installation is done for the other section. Both overhead wire systems for the crossover tracks are pulled together by insulated knuckles of varying lengths. This allows overhead wires to only come in contact with mainline wires that are potentially equal to them. The limits of each crossover track are fully covered by overhead wires above them. The insulated knuckle double crossover arrangement can be seen in Figure J. knuckles are light in comparison to section insulators. The assemblies and parts are- less expensive and easier to procure and easier to change out with other similar parts that may be found elsewhere on the system. No additional maintenance is required of the crossover overhead due to additional parts. Contact wire and pantograph carbon strips will not have excessive wear as is present with section insulators. Installation of overhead wires with an insulated knuckle crossover arrangement is no harder or longer than the standard arrangement; however the installation of the knuckles takes far less time than installing two or more section insulators. Furthermore, standard crossover arrangements require support at each section insulator, while arrangements with insulated knuckles do not require as much support. The insulated knuckle crossover arrangement can work with any type of overhead system such as single wire, catenary or double messenger. It can work with any type of tensioning such as fixed, balance weight or spring. This arrangement is most useful with sectionalization that has the OCS at both turnouts on a given mainline track in the same OCS section such as an arrangement with no insulation on the mainline or a two-point sectionalization. The one-point sectionalization arrangement with the sectionalization between the two turnouts will not work with the insulated knuckle crossover arrangement unless section insulators are installed. This arrangement can also only be done at a double crossover. At a single crossover, there is no other OCS to pull off from. The use of an insulated knuckle crossover arrangement provides the same sectionalization benefits of a standard crossover arrangement without the added weight, expense and maintenance while increasing the useful life of the system and its parts. Clearly the insulated knuckle crossover arrangement offers a viable alternative to section insulators on the crossover catenary system and they are expected to become the new standard of crossover sectionalizing. AUTHORITY S OPINION Several authorities have their opinions on sectionalizing and how they are moving forward with extensions and refurbishments. Others do not have an opinion and receive their extensions with sectionalizing they inherit based on project management s decisions. The advantages of an insulated knuckle crossover arrangement occur as a result of no longer having the requirements of section insulators. The insulated In Calgary, Alberta, Canada, Calgary Transit (CT) is involved in two major light rail projects, one extension and one refurbishment. Their design-build contract for an extension west of their downtown involves some interesting sectionalization schemes. Calgary s West 6

7 LRT Project is planning the use of insulated knuckle double crossover arrangements with two-point sectionalization. Designs are in progress for an arrangement as seen in Figure F. RFP documents were developed using this arrangement and the design-build contractor has enhanced this concept. As Calgary Transit is a separate entity from the West LRT project, CT did not have the chance to weigh in on the sectionalizing chosen for the extension they would soon operate and maintain. In order to prepare for the overhead arrangements they would soon accept, CT decided to test out the insulated knuckle double crossover arrangement in their 7 th Avenue Refurbishment Project. A new double crossover was planned for the eastern end of their downtown corridor. CT agreed with the suggestions of their designer to use this crossover arrangement as a test run in operating and maintaining this type of overhead arrangement. benefits authorities covet. Those that have implemented this arrangement are pleased with the results. CONCLUSION While different authorities have their own views of how they would like sectionalizing to be placed on their systems, there seems to be a trend to stick with what they know best based on what methods are currently on their systems. New light rail systems are moving toward sectionalizing methods that are easier to maintain and provide more functionality for revenue service during power outages. The more functional methods of sectionalizing are created in the planning and design of the system from inception. With proper planning Completed in 2005, San Jose, California s Valley Transportation Authority (VTA) added two extensions to their system. The Tasman East/Capitol Extension and the Vasona LRT Project were constructed with four insulated knuckle double crossovers. Design had been developed on the Capitol Expressway Extension for an additional insulated knuckle double crossover. Likewise in Salt Lake City, Utah, Utah Transit Authority accepted a design using the same insulated knuckle double crossovers. After installing this arrangement on their University Line, two insulated knuckle double crossover arrangements are in progress for their Airport Extension. Travis Baxter of UTA commented that they decided the continued use of the insulated knuckle double crossover arrangement due to the traditional methods presence of hard spots, excessive arching and added maintenance. This arrangement is especially used in their double crossovers that have a higher frequency is normal or special operations. He also added that two individual wires of the traditional method tend to move independently and create additional hard spots. The insulated knuckle double crossover arrangement provides a smooth transition with no deviation in contact wire elevation. Baxter encourages the use of insulated knuckle double crossover arrangements whenever possible. The use of the insulated knuckle double crossover arrangement is catching on in light rail transit. Other authorities such as Los Angeles County s Metro light rail are considering this arrangement. This overhead sectionalizing scheme has the cost and maintenance 7