Overhead line
An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, electric multiple units, trolleybuses or trams. The generic term used by the International Union of Railways for the technology is overhead line. It is known variously as overhead catenary, overhead contact line, overhead contact system, overhead equipment, overhead line equipment, overhead lines, overhead wiring, traction wire, and trolley wire.
An overhead line consists of one or more wires situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regularly spaced intervals along the track. The feeder stations are usually fed from a high-voltage electrical grid.
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 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.Vehicles like buses that have rubber tyres cannot provide a return path for the current through their wheels, and must instead use a pair of overhead wires to provide both the current and its return path.
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 or catenary. 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 mechanical tension.As the pantograph moves along under the contact wire, the carbon insert on top of the pantograph becomes worn with time. On straight track, the contact wire is zigzagged slightly to the left and right of the centre from each support to the next so that the insert wears evenly, thus preventing any notches. On curves, the "straight" wire between the supports causes the contact point to cross over the surface of the pantograph as the train travels around the curve. 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. For tramways, a contact wire without a messenger wire is used.
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, while additional droppers support the contact wire from the auxiliary. The auxiliary wire can be of a more conductive but less wear-resistant metal, increasing transmission efficiency.
The present transmission system originated about 100 years ago. A simpler system was proposed in the 1970s by the Pirelli Construction Company, consisting of a single wire embedded at each support for of its length in a clipped, extruded aluminum beam with the wire contact face exposed. A somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire that could be easily handled at by a pneumatic servo pantograph with only 3 g acceleration.
Parallel overhead lines
An electrical circuit requires at least two conductors. Trams and railways use the overhead line as the positive terminal of the circuit and the steel rails as the negative terminal of the circuit. For a trolleybus or a trolleytruck, no rails are available for the return current, as the vehicles use rubber tyres on the 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 the rare railways with three-phase AC railway electrification.Types of wires
In the Soviet Union the following types of wires/cables were used. For the contact wire, cold drawn solid copper was used to ensure good conductivity. The wire is not round but has grooves at the sides to allow the hangers to attach to it. Sizes were 85, 100, or 150 mm2. To make the wire stronger, 0.04% tin might be added. The wire must resist the heat generated by arcing and thus such wires should never be spliced by thermal means.The messenger wire needs to be both strong and have good conductivity. They used multi-strand wires with 19 strands in each cable. Copper, aluminum, and/or steel were used for the strands. All 19 strands could be made of the same metal or a mix of metals based on the required properties. For example, steel wires were used for strength, while aluminium or copper wires were used for conductivity. Another type looked like it had all copper wires but inside each wire was a steel core for strength. The steel strands were galvanized but for better corrosion protection they could be coated with an anti-corrosion substance.
In Slovenia, where 3 kV system is in use, standard sizes for contact wire are 100 and 150 mm2. The catenary wire is made of copper or copper alloys of 70, 120 or 150 mm2. The smaller cross sections are made of 19 strands, whereas the bigger has 37 strands.
Two standard configurations for main lines consist of two contact wires of 100 mm2 and one or two catenary wires of 120 mm2, totaling 320 or 440 mm2. Only one contact wire is often used for side tracks.
In the UK and EU countries, the contact wire is typically made from copper alloyed with other metals. Sizes include cross-sectional areas of 80, 100, 107, 120, and 150 mm2. Common materials include normal and high strength copper, copper-silver, copper-cadmium, copper-magnesium, and copper-tin, with each being identifiable by distinct identification grooves along the upper lobe of the contact wire. These grooves vary in number and location on the arc of the upper section. Copper is chosen for its excellent conductivity, with other metals added to increase tensile strength. The choice of material is chosen based on the needs of the particular system, balancing the need for conductivity and tensile strength.
Tensioning
Catenary wires are kept in mechanical tension because the pantograph causes mechanical oscillations in the wire. The waves must travel faster than the train to avoid producing standing waves, which could break the wire. Tensioning the line makes waves travel faster, and also reduces sag from gravity.For medium and high speeds, the wires are generally tensioned by weights or occasionally by hydraulic tensioners. Either method is known as "auto-tensioning" or "constant tension" and ensures that the tension is virtually independent of temperature. Tensions are typically between 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. Recently, spring tensioners have started to be used. These devices contain a torsional spring with a cam arrangement to ensure a constant applied tension. Some devices also include mechanisms for adjusting the stiffness of the spring for ease of maintenance.
For low speeds and in tunnels where temperatures are constant, fixed termination equipment may be used, with the wires terminated directly on structures at each end of the overhead line. The tension is generally about. This type of equipment sags in hot conditions and is taut in cold conditions.
With AT, the continuous length of the overhead line is limited due to the change in the height of the weights as the overhead line expands and contracts with temperature changes. This movement is proportional to the distance between anchors. Tension length has a maximum. For most 25 kV OHL equipment in the UK, the maximum tension length is.
An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length is free to move along the track. To avoid this a midpoint anchor, 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, or 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 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 lift 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.