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Overhead lines

From Academic Kids

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Tile_Hill_train_550.jpg
An electric multiple unit pulling into Tile Hill station; Coventry, England. Note the overhead line!

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 (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.

Contents

Construction

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 called pantographs, bow collectors, or trolley poles. The collectors are electrically conductive, and allow current to flow towards earth. This path takes the current through 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. This 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. 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 equipment in the UK the maximum tension length for 25 kV OHL 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, 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

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Tramtrol_029148ed.jpg
Looking directly upward at a crossing in Bahnhofplatz, Bern, Switzerland in 2002 September.
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Tramtrol_029148an.jpg
An annotated version of the above.
Blue = tram conductor
Green = trolley bus wires
Yellow = insulated trough

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-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 - they need to anyway because of the trolley wheel pickup method. 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 will easily bridge over between these different conductor sections 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 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 ever touching the trolleybus wires.

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.

See also

de:Oberleitung fa:سیم بالاسر fr:Catnaire nl:Bovenleiding ja:架線 zh:高架電纜 pl:Przewody trakcyjne

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