Overhead line
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An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses or trains. It is known variously as:
- Overhead contact system (OCS)
- Overhead line equipment (OLE or OHLE)
- Overhead equipment (OHE)
- Overhead wiring (OHW) or overhead lines (OHL)
- Catenary
- Trolley wire
In this article the generic term overhead line is used, as used by the International Union of Railways.[1]
Overhead line is designed on the principle of one or more overhead wires (or rails, particularly in tunnels) 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
Overview
Electric trains that collect their current from overhead lines use a device such as a pantograph, bow collector or trolley pole. It presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram and back to the feeder station through the steel wheels on one or both running rails. Non-electric locomotives (such as diesels) may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries, and electromagnetic induction.
This article does not cover regenerative braking, where the traction motors act as generators to retard movement and return power to the overhead.
Construction
To achieve good high-speed current collection, it is necessary to keep the contact wire geometry within defined limits. This is usually achieved by supporting the contact wire from a second wire known as the messenger wire (in the US & Canada) or catenary (in the UK). This wire approximates the natural path of a wire strung between two points, 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". It is supported regularly at structures, by a pulley, link, or clamp. The whole system is then subjected to a mechanical tension.
As the contact wire makes contact with the pantograph, the carbon insert on top of the pantograph is worn down. The "straight" wire between supports will cause the contact wire to cross over the whole surface of the pantograph as the train travels around the curve, causing uniform wear and avoiding any notches. On straight track, the contact wire is zigzagged slightly to the left and right of centre at each successive support so that the pantograph wears evenly. The movement of the contact wire across the head of the pantograph is called the 'sweep'.
The zigzagging of the overhead line is not required for trolley poles.
Depot areas tend to have only a single wire and are known as "simple equipment" or "trolley wire". When overhead line systems were first conceived, good current collection was possible only at low speeds, using a single wire. To enable higher speeds, two additional types of equipment were developed:
- Stitched equipment - uses an additional wire at each support structure, terminated on either side of the messenger/catenary wire.
- Compound equipment - uses a second support wire, known as the "auxiliary", between the messenger/catenary wire and the contact wire. Droppers support the auxiliary from the messenger wire, and additional droppers support the contact wire from the auxiliary. The auxiliary wire can be of a more conductive but less wear-resistant metal, increasing the efficiency of power transmission.
Dropper wires traditionally provide physical support of the contact wire without joining the catenary and contact wires electrically. Contemporary systems use current-carrying droppers, which eliminate the need for separate wires.
The present transmission system originated about 100 years ago. A simpler system was proposed in the 1970s by the Pirelli Construction Co, consisting of a single wire embedded at each support for 2.5 metres (8 ft 2 in) of its length in a clipped extruded aluminum beam with the wire contact face exposed. With a somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire that could be easily handled at 250 miles per hour (400 km/h) by a pneumatic servo pantograph with only 3 G accelerations.
For tramways there is often only a contact wire and no messenger wire.
Parallel overhead lines
An electrical circuit requires at least two conductors. Trams and railways use the overhead line as one side of the circuit and the steel rails as the other side of the circuit. For a trolleybus there are no rails for the return current—the vehicles use rubber tyres and the normal road surface. Trolleybuses use a second parallel overhead line for the return, and two trolley-poles, one contacting each overhead wire. The circuit is completed by using both wires. Parallel overhead wires are also used on railways with three-phase AC railway electrification, which is rare.
Tensioning
Catenary wires are kept at a mechanical tension because the pantograph causes mechanical oscillations in the wire and the wave must travel faster than the train to avoid producing standing waves that would cause wire breakage. Tensioning the line makes waves travel faster.
For medium and high speeds, the wires are generally tensioned by weights or occasionally by hydraulic tensioners. Either method is known as "auto-tensioning" (AT) or "constant tension" and ensures that the tension is virtually independent of temperature. Tensions are typically between 9 and 20 kN (2,000 and 4,500 lbf) per wire. Where weights are used, they slide up and down on a rod or tube attached to the mast, to prevent them from swaying.
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. The tension is generally about 10 kN (2,200 lbf). This type of equipment will sag on hot days and be taut on cold days.
With AT there is a limit on the continuous length of overhead line 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, 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 1970m.[citation needed]
An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length will be free to move along track. To avoid this a midpoint anchor (MPA), close to the centre of the tension length, restricts movement of the messenger/catenary wire by anchoring it; the contact wire and its suspension hangers can move only within the constraints of the MPA. MPAs are sometimes fixed to low bridges, otherwise anchored to vertical catenary poles or portal catenary supports. A tension length can be seen as a fixed centre point, with the two half tension lengths expanding and contracting with temperature.
Most systems include a brake to stop the wires from unravelling completely if a wire breaks or tension is lost. German systems usually use a single large tensioning pulley (basically a ratchet mechanism) with a toothed rim, mounted on an arm hinged to the mast. Normally the downward pull of the weights and the reactive upward pull of the tensioned wires lifts the pulley so its teeth are well clear of a stop on the mast. The pulley can turn freely while the weights move up or down as the wires contract or expand. If tension is lost the pulley falls back toward the mast, and one of its teeth jams against the stop. This stops further rotation, limits the damage, and keeps the undamaged part of the wire intact until it can be repaired. Other systems use various braking mechanisms, usually with multiple smaller pulleys in a block and tackle arrangement.
Breaks
Section break
To allow maintenance to the overhead line without having to turn off the entire system, the line 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 vehicle's pantograph is in continuous contact with the wire.
For bow collectors and pantographs, this is done by having two contact wires run side by side over the length of 3 or 4 wire supports. A new one drops down and the old one rises up, allowing the pantograph to smoothly transfer from one to the other. The two wires do not 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, requiring an insulator. The driver of the tram or trolleybus must turn off the power when the trolley pole passes through, to prevent arc damage to the insulator.
Pantograph-equipped locomotives must not run through a section break when one side is de-energized. The locomotive would become trapped, but as it passes the section break the pantograph will briefly short the two catenary lines together. If the opposite line is de-energized, this voltage transient may trip supply breakers. If the line is under maintenance, injury may occur as the catenary is suddenly energized. Even if the catenary is properly grounded, the arc generated across the pantograph will likely cause damage to the pantograph, the catenary insulator, or both.
Neutral section (phase break)
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. (Sometimes the sections are 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 is highly undesirable to connect two unsynchronised grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.
A neutral section or phase break is used, consisting of two section breaks back-to-back with 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 through it normally. If the two supplies are not synchronised it is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.
The sudden loss and then reconnection to the supply over the neutral section might cause damage to the locomotive if it was drawing power, so signs warn the driver. When synchronisation is lost and the phase break is deenergised, the driver should put the controller (throttle) into neutral and coast through the section.
On the Pennsylvania Railroad, phase breaks were indicated by a position light signal face with all eight radial positions with lenses and no center light. When the phase break was active (the catenary sections out of phase), all lights were lit. The position light signal aspect was originally devised by the Pennsylvania Railroad and was continued by Amtrak and has been adopted by Metro North. Metal signs were hung from the catenary supports with the letters "PB" created by a pattern of drilled holes.
In countries such as France, South Africa and the United Kingdom, permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted transducer on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes them.
Dead section
A special category of phase break was developed in American practice, primarily by the Pennsylvania Railroad. Since its traction power network was centrally supplied and only segmented by abnormal conditions, normal phase breaks were generally not active. Phase breaks that were always activated were known as "Dead Sections": they were often used to separate power systems (for example, the Hell's Gate Bridge boundary between Amtrak and Metro North's electrifications) that would never be in-phase. Since a dead section is always dead, no special signal aspect was developed to warn drivers of its presence, and a metal sign with "DS" in drilled-hole letters was hung from the catenary supports.
Overhead conductor rails
Where there is limited clearance such as in tunnels, the overhead wire may be replaced by rigid overhead rail. This was done when the overhead line was raised in the Simplon Tunnel to accommodate taller rail vehicles. A rigid overhead rail may also be used in places where tensioning the wires is impractical, for example on moveable bridges. Short overhead conductor rails are also installed at tram stops as for the Combino Supra.[2]
Crossings
Trams draw their power from a single overhead wire at about 500 to 750 V. Trolleybuses draw from two overhead wires at a similar voltage, and at least one of the trolleybus wires must be insulated from tram wires. This is usually done by the trolleybus wires running continuously through the crossing, with the tram conductors a few centimetres lower. Close to the junction on each side, the tram wire turns into a solid bar running parallel to the trolleybus wires for about half a metre. Another bar similarly angled at its ends is hung between the trolleybus wires, electrically connected above to the tram wire. 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.
Until 1946, there was a level crossing in Stockholm, Sweden between the railway south of Stockholm Central Station and a tramway. The tramway operated on 600-700 V DC and the railway on 15 kV AC. In the Swiss village of Suhr, the WSB tramway operating at 1,200 V DC crossed the SBB line at 15 kV AC. Some crossings between tramway/light rail and railways are still extant in Germany. In Zürich, Switzerland, VBZ trolleybus line 32 has a level crossing with the 1,200 V DC railway to Mount Uetliberg; at many places, trolleybus lines cross the tramway. In some cities, trolleybuses and trams shared a positive (feed) wire. In such cases, a normal trolleybus frog can be used.
Another system that has been used is to coincide section breaks with the crossing point, so that the crossing is electrically dead.
Australia
Many cities had trams and trolleybuses using trolley poles. 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 put the controller into neutral and coast through section insulators, indicated by insulator markings between the rails.
Melbourne has four level crossings between electrified suburban railways and tram lines. They have mechanical switching arrangements (changeover switch) to switch the 1,500V DC overhead of the railway and the 650V DC of the trams, called a Tram Square.[3] Proposals have been put forward which would see these crossings grade separated or the tram routes diverted.
Greece
In Athens, there are two crossings between tram and trolleybus wires, at Vas. Amalias Avenue and Vas. Olgas Avenue, and at Ardittou Street and Athanasiou Diakou Street. They use the above-mentioned solution.
From the opening of the tram system in the summer of 2004, trams and trolleybuses in the direction of Pagrati shared the same exclusive lane, about 400m long, on the far right side of Vas. Olgas Avenue, with tram and trolleybus wires side by side above a narrow lane of road. The trolleybus wires were on the far right of the lane, away from the trams' (very wide) pantographs. Trolleybus drivers were required to drive very slowly because the trolley poles were extended to their limits. A change of route for trolleybuses was implemented in mid-2005, ending this arrangement.
Italy
In Rome, at the crossing between viale Regina Margherita and via Nomentana, tram and trolleybus lines cross as well: tram on viale Regina Margherita and trolleybus on via Nomentana. The crossing is orthognal, therefore the arrangement seen before was not available.
In Milan, most tram lines cross its circular trolleybus line once or twice. Trolleybus and tram wires run parallel in streets such as viale Stelvio and viale Tibaldi.
Multiple overhead lines
Some railways used two or three overhead lines, usually to carry three-phase current. This is used only on the Gornergrat Railway and Jungfrau Railway in Switzerland, the Petit train de la Rhune in France, and the Corcovado Rack Railway in Brazil; 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 Marienfelde 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 galvanic corrosion of metallic parts near the railway, such as on the Chemin de fer de la Mure.
All systems with multiple overhead lines have high risk of short circuits at switches and therefore tend to be impractical in use, especially when high voltages are used or when trains run through the points at high speed.
The Sihltal Zürich Uetliberg Bahn has two lines with different electrification. To be able to use different electric systems on shared tracks, the Sihltal line has its overhead wire right above the train, whilst the Uetliberg line has its overhead wire off to one side.
Overhead catenary
A catenary is a system of overhead wires used to supply electricity to a locomotive, streetcar, or light rail vehicle which is equipped with a pantograph.
Unlike simple overhead wires, in which the uninsulated wire is attached by clamps to closely spaced crosswires supported by poles, catenary systems use at least two wires. The catenary or messenger wire is hung at a specific tension between line structures, and a second wire is held in tension by the messenger wire, attached to it at frequent intervals by clamps and connecting wires. The second wire is straight and level, parallel to the rail track, suspended over it as the roadway of a suspension bridge is over water.
Simple wire installations are common in light rail, especially on city streets, while more expensive catenary systems are suited to high-speed operations.
The Northeast Corridor in the United States has catenary over the 600 miles (1000 km) between Boston, Massachusetts and Washington, D.C. for Amtrak's high-speed Acela Express and other trains. Commuter rail agencies including MARC, SEPTA, NJ Transit, and Metro-North Railroad utilize the catenary to provide local service.
In Cleveland, Ohio the interurban/light rail lines and the heavy rail line use the same overhead wires, due to a city ordinance intended to limit air pollution from the large number of steam trains passing through Cleveland between the east coast and Chicago. Trains switched from steam to electric locomotives at the Collinwood Rail Yards about 10 miles (16 km) east of Downtown and at Linndale on the west side. When Cleveland constructed its rapid transit (heavy rail) line between the airport, Downtown and beyond it employed similar overhead catenary, using electrification equipment left over after railroads switched from steam to diesel locomotives. Light and heavy rail share trackage for about 3 miles (4.8 km) along the Cleveland Hopkins International Airport Red (heavy rail) line, Blue and Green interurban/light rail lines between Cleveland Union Terminal and just past East 55th Street station, where the lines separate.
Part of the Boston, Massachusetts Blue Line through the northeast suburbs uses overhead lines, as does the Green Line.
Height
The height of overhead wiring can create hazards at level crossings, where it may be struck by road vehicles. Warning signs are placed on the approaches, advising drivers of the maximum safe height.
The wiring in most countries is too low to allow double stack container trains. The Channel Tunnel has an extended height overhead line to accommodate double-height car and truck transporters. India is proposing a network of freight-only lines that would almost certainly be electrified with extra height wiring and pantographs.
Problems with overhead equipment
Overhead line equipment can be adversely affected by strong winds[citation needed], bringing the wires down and stopping all trains. Power storms can knock the power out with lightning strikes on systems[citation needed] with overhead wires, stopping trains if there is a power surge.
During cold or frosty weather, there is a risk of ice build-up on overhead lines. This can result in poor electrical contact between the collector and the overhead line, resulting in electrical arcing and power surges.[4]
Overhead line equipment may require reconstruction of bridges to provide safe electrical clearance.[citation needed]
Overhead equipment, like most electrified systems, requires a greater capital expenditure when building the system than an equivalent non-electric system. While a conventional rail line requires only the grade, ballast, ties and rails, an overhead system also requires a complex system of support structures, lines, insulators, power-control systems, and power lines, all of which require maintenance over time. This makes non-electrical systems more attractive in the short term, although electrical systems can pay for themselves eventually, in most cases. Also, the added construction and maintenance cost-per-mile makes overhead systems less attractive on long-distance railways, such as many of those found in North America, where the distances between cities are typically far greater than in Europe. Such long lines would require enormous investment in overhead line equipment, and there are major difficulties in energizing long portions of overhead wire on a permanent basis, especially in areas where energy demand already far outstrips supply.
Many people also consider overhead lines to be "visual pollution", due to the many support structures and complicated system of wires and cables which fill the air over an overhead-powered rail line or trolley system. It is such considerations that have driven the move towards replacing overhead power and communications lines with buried cables in many areas, and opposition to overhead power is strong in many areas.
History
The first tram with overhead lines was presented by Werner von Siemens at the International Electric Exposition in Paris 1881: the installation was removed after that event. In October 1883, the first permanent tram service with overhead lines was on the Mödling and Hinterbrühl Tram in Austria. The trams had bipolar overhead lines, consisting of two U-pipes, in which the pantographs hung and ran like shuttles. In April to June 1882, Siemens had tested a similar system on his Electromote, an early percursor of the trolleybus.
Much simpler and more functional was an overhead wire in combination with a pantograph borne by the vehicle and pressed at the line from below. This system, for rail traffic with a unipolar line, was invented by Frank J. Sprague in 1888. From 1889 it was used at the Richmond Union Passenger Railway in Richmond, Virginia, pioneering electric traction.
See also
- Electro-diesel locomotive
- Lineman (technician)
- List of current systems for electric rail traction
- Railway electrification system
References
- ↑ UIC English/French/German Thesaurus.
- ↑ Siemens press release
- ↑ TMSV: Tramway level crossings in Victoria
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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