Overhead lines
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- This article is about the transmission of electrical power to public transport vehicles. For transmission of bulk electrical power to general consumers, see Electric power transmission. For powerlines mounted on pylons, see Overhead powerline
Image:Tile Hill train 550.jpg See also overhead catenary
Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point. These overhead lines are known variously as
- OCS (overhead contact system – US & Europe)
- OLE or OHLE (overhead line equipment – UK)
- OHW (overhead wiring – AUS) or
- Catenary (somewhat inaccurately).
For the purposes of this article the generic term overhead line has been used.
Overhead line is designed on the principle of one or more overhead wires situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals. The feeder stations are usually fed from a high-voltage electrical grid.
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Construction
Image:Pociąg sieciowy pod Łowiczem.jpeg As an electric train passes under the lowest wire, known as the contact wire, one or more current collection devices on the roof of the train make contact with the wire. The devices may be pantographs, bow collectors, or trolley poles. The collectors are electrically conductive, and allow current to flow through to the traction motors of the train or tram, and back to the feeder station via the steel wheels and one or both running rails of the track. Diesel trains may pass along these tracks without affecting the overhead line, although overhead clearance may be an issue.
(Because they run on rubber wheels running on pavement rather than steel wheels running on steel rails, trolleybuses must have a second trolley pole contacting a second wire to return the current to the power system.)
To achieve good high speed current collection, it is necessary to keep the contact wire geometry within defined limits throughout the length of the overhead line. No additional wires are needed to accomplish this on urban tramways, but on railways and higher-speed tramways it is usually achieved by supporting the contact wire from above by means of a second wire, known variously as the messenger wire (US & Europe) or catenary (UK & Canada). This wire is allowed to follow the natural path of a wire strung between two points, which is known as a catenary curve, thus the use of catenary to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as droppers or drop wires. In this way the contact wire is effectively supported at numerous points. The messenger wire is supported regularly at structures, either by means of a pulley, link, or clamp. The whole system is then subjected to a mechanical tension. The messenger wire is usually pulled slightly to the left and right by successive supports, so that the contact wire slides from side to side on the pantograph as the vehicle moves along (if it did not then it would tend to wear a groove in the pantograph's carbon insert). Such a system, with a single supporting wire, is known as simple equipment.
When overhead line systems were first conceived, good current collection was not possible at high speed using a single supporting wire. Two additional types of equipment were developed to combat this problem. Stitched equipment used an additional wire at each support structure, which was terminated either side to the messenger wire. Compound equipment used a second support wire, known as the auxiliary, running the whole length of the overhead line between the messenger wire and the contact wire. Droppers are provided to support the auxiliary from the messenger wire, and additional droppers support the contact wire from the auxiliary.
The dropper wires usually only provide physical support of the contact wire, and do not join the catenary and contact wires electrically. Separate wires are provided for this function.
Another reason to use an auxiliary wire is that such a wire could be constructed of a more conductive but less wear-resistant metal, increasing the efficiency of power transmission.
For tramways there often is just a simple contact wire and no messenger wire.
Tensioning
For medium and high speeds the wires are generally tensioned by means of weights, or occasionally by hydraulic tensioners. Either method is known as auto-tensioning (AT), and ensures that the tension in the equipment is virtually independent of temperature. Tensions are typically between 9 and 20 kN per wire.
For low speeds and in tunnels where temperatures are constant, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the overhead line. Here the tension is generally about 10 kN. This type of equipment will sag on hot days and hog on cold days.
Where AT is used, there is a limit to the continuous length of overhead line which may be installed. This is due to the change in the position of the weights with temperature as the overhead line expands and contracts. This movement is proportional to the tension length, i.e. the distance between anchors. This leads to the concept of maximum tension length. For most 25 kV OHL equipment in the UK the maximum tension length is 1970 m.
An additional issue with AT equipment is that, if balance weights are attached to each end, the whole tension length will be free to move along track. Therefore, a mid point anchor (MPA) is introduced close to the centre of the tension length to restrict movement. MPAs are often fixed to low bridges.
Therefore a tension length can be seen as a fixed centre point with the two half tension lengths expanding and contracting with temperature.
Breaks
To allow maintenance to sections of the overhead line without having to turn off the entire system, the overhead line system is broken into electrically separated portions known as sections. Sections often correspond with tension lengths as described above. The transition from section to section is known as a section break and is set up so that the locomotive's pantograph is in continuous contact with the wire.
For bow-collectors and pantographs, this is done by having two contact wires run next to each other over a length about four wire supports: a new one dropping down and the old one rising up until the pantograph smoothly transfers from one to the next. The two wires never touch (although the bow-collector/pantograph is briefly in contact with both wires). In normal service the two sections are electrically connected (to different substations if at or near the halfway mark between them), but this can be broken for servicing.
On overhead wires designed for trolley poles this is done by having a neutral section between the wires, but this requires an insulator. The driver of the tram or trolleybus must turn off the power when the trolley pole passes through to prevent arcing from burning-out the insulator.
Sometimes on a larger electrified railway, tramway or trolleybus system it is necessary to power different areas of track from different power grids, the synchronisation of the phases of which cannot be guaranteed. (Indeed, sometimes the sections are even powered with different voltages or frequencies!) There may be mechanisms for having the grids synchronised on a normal basis, but events may cause desynchronisation. This is no problem for DC systems, but for AC systems it would obviously be quite undesirable to connect two unsynchronised grids together, even momentarily. A normal section break is insufficient to guard against this since the pantograph briefly connects both sections.
Instead, a phase break or neutral section is used. This consists of two section breaks back-to-back so that there is a short section of overhead line that belongs to neither grid. If the two grids are synchronised, this stretch of line is energised (by either supply) and trains run over it normally. If the two supplies are not synchronised, the short isolating section is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.
The sudden loss of power over the phase break would jar the train if the locomotive was at full throttle, so special signals are set up to warn the crew. Normal instructions are to put the controller (throttle) into neutral and coast through an isolated phase break section.
On the Pennsylvania Railroad, phase breaks were indicated to train crews by a metal sign hung from the overhead with the letters PB on it, created by holes drilled in the metal. When the phase break was "dead", a signal consisting of eight lit lights in a circular pattern indicated this to the crew.
Crossings
Image:Tramtrol 029148ed.jpg Image:Tramtrol 029148an.jpg Trams draw their power from a single overhead wire at about 500/750 V above earth. Trolleybuses draw their power from two overhead wires with about 500 V between them. So where the two cross, they must not touch.
The first requirement for this to work is that the trams must have pantograph- or bow-collector-type power collection. Where the trams use a trolley pole to collect their power, a different crossing style must be used.
The trolleybus wires run continuously through the crossing. The tram conductors are slung a few centimetres lower than the trolleybus wires. Close to the junction on each side, the wire merges into a solid bar which is angled to run parallel to the trolleybus wires for about half a metre. Another bar similarly angled at its ends is hung between the trolleybus wires. This is electrically connected above to the tram wire's catenary cable. The tram's pantograph bridges the gap between the different conductors, providing it with a continuous pickup.
Where the tram wire crosses, the trolleybus wires are protected by an inverted trough of insulating material extending 20 or 30 mm below the level of the trolleybus wires. The tram pantograph or bow collector raises the conductor wire a little as it passes under. These troughs are presumably to limit how far it can do that and to provide a backstop to prevent the tram pantograph or bow collector ever touching the trolleybus wires.
Another system that has been used is to coincide section breaks with the crossing point so that the crossing is electrically dead.
It can also be noted that in some cities, trolleybuses and trams have shared the same positive (feed) wire. In such cases a normal trolleybus frog can be used. In Stockholm, Sweden there was until 1946 a level crossing between the railway south of Stockholm Central Station and a tramway line. The tramway operated on 600-700 V DC and the railway operated on 15kVAC.
Australia
Many cities, including Adelaide, South Australia, had trams and trolleybuses (Adelaide still has one tramline today) both using trolley pole current collection. They used insulated crossovers which required tram drivers to put the controller into neutral and coast through. Trolleybus drivers had to either lift off the accelerator or switch to auxiliary power.
In Melbourne, Victoria tram drivers are still required to put the controller into neutral and coast through section insulators, this being indicated to drivers by insulator markings between the rails.
Melbourne also has another interesting issue - crossings between electrified suburban railways and tram lines at grade. There are four of these level crossings through the systems and each requires complex switching arrangements to separate the operation of 1500 V DC overhead for the railway and 650 V DC for the trams. This is called an overhead square. Proposals have been put forward which would eventually see most or all of these crossings grade separated or the tram routes diverted.
Germany
In Germany there are four traction current lines with two circuits on four conductor cables, and as a rule the traction current lines have a level arrangement of the conductor cables. A rail current pylon is a pylon with at least one electric circuit for traction current. With traction current lines with four circuits and eight conductor cables frequently a two-level arrangement is used, with four conductor cables per level. With traction current lines for newly developed roads, a three-level arrangement is used; on the lowest cross beam the are four, and on the two highest cross beams two, conductor cables. The three-level arrangement is used also for traction current lines with 6 electric circuits (12 conductor cables). Besides there is, in particular for rapid transit railways operated with alternating current, overhead line masts with cross beams for traction current lines above the contact wire. There are also systems that have an overhead line mast, which carries both electric circuits for traction current, and for three-phase alternating current.
Multiple overhead lines
There are and were some railways which used two and even three overhead lines. These are all railways run with three phase AC-current. Nowadays three phase AC-current is used only on the Gornergrat Railway and Jungfraujoch Railway in Switzerland; until 1976, it was widely used in Italy. On these railways the two conductors of the overhead lines are used for two different phases of the three-phase AC, while the rail was used for the third phase. The neutral was not used. Some three-phase AC railways used three overhead wires. These were an experimental railway line of Siemens in Berlin-Lichtenberg in 1898 (length: 1.8 kilometres), the military railway between Mariuenfelde and Zossen between 1901 and 1904 (length: 23.4 kilometres) and an 800-metre-long section of a coal railway near Cologne, between 1940 and 1949. On DC systems bipolar overhead lines were sometimes used to avoid electrochemical corrosion of metallic parts near the railway. A railway run with DC using two overhead lines was the Chemin de fer de la mure. All systems of multiple overhead lines have the disadvantage of high risk of short circuits at switches and are therefore impractical in use, especially when high voltages are used or when trains run through the points at high speed.
See also
- Bow collector
- Pantograph
- Trolley pole
- Third rail, another method of powering electric vehicles
- Frank J. Sprague
- Lineman
- Metro-North Railroad, a US commuter railway serving New York and Connecticut, in which some parts are powered by overhead wires, some by a third rail, and some by diesel engines.
- Railway electrification system
- List of current systems for electric rail tractionde:Oberleitung
es:Catenaria (ferrocarril) eo:Supertraka kontakta linio fa:سیم بالاسر fr:Caténaire nl:Bovenleiding ja:架線 pl:Przewody trakcyjne sv:Kontaktledning zh:高架電纜