Overhead lines

From open-encyclopedia.com - the free encyclopedia.

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, trolleybusses, 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, a device on the train roof called the pantograph makes contact with the wire (trams generally use pantographs as well but sometimes use bow collectors, or trolleypoles). The pantograph, bow collector, or trolleypole is electrically conductive, and allows current to flow to earth. This path takes the current through the traction motors of the train/tram, and back to the feeder station via the steel wheels and one or both track running rails. Diesel trains may pass along these tracks without affecting the overhead line, although clearance may be an issue.

To achieve good high speed current collection, it is necessary to keep the contact wire nominally level 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 shape, 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 street tramways there often is just a simple conact wire and no message wire.

Tensioning

For medium and high speeds the wires are generally tensioned by means of weights, or occasionally, by hydraulic tensioners. This 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 25kV OHL is 1970 metres.

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. MPA's 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 continual 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 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 trolleypoles this is done by having a neutral section between the wire, but this requries an insulator. The driver of the tramcar turn off the power when the trolleypole passes through to prevent arching from burning-out the insulator.

Sometimes on a larger electrified railway, tramway, or trollebusway, 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 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 in the overhead with the letters PB on it, created by holes drilled in the metal. When the phase break is "dead", a signal consisting of eight lit lights in a circular pattern indicates this to the crew.

History

The use of overhead cables conducting electricity, invented by Granville T. Woods in 1888, amongst several other improvements by Woods, led to the development of electrified railways, the first of which was operated at Coney Island from 1892.