Catenary (railway)
In railways, the catenary refers to the overhead power cables that transmit electrical energy to the locomotives or other motor equipment.
Some authors prefer to use the term "Airline Contact Line" or L.A.C. for short, which may include systems called "tram line", "trolley line", "flexible catenary" and "rigid catenary". There are other electrical power systems for railways that should not be considered as catenaries; the most important are the third rail and magnetic levitation.
The most common supply voltages range from 600 V to 3 kV in direct current, or between 15 and 25 kV in alternating current. Most of the installations work with single-phase direct or alternating current, although there are some three-phase alternating installations.
In overhead cables, the positive pole of the installation is normally the catenary and the negative pole is the rails on which the train runs. The currents coming from the traction substation (general grid voltage transformer or rectifier) reach the train via the catenary through the pantograph and return to the substation through the rails of the railway.
An exception to this rule are the overhead contact cables for trolleybuses, where, since there are no rails, the return current flows to the substation through a second cable parallel to the first one and in contact with the vehicle through a second trolley.
The catenary name comes from the characteristic geometric shape of the curve formed by a flexible wire subjected to its own weight, a curve that occurs in the case of a tram line made up of a single cable (a clear example of this is conductor cables used in trolleybus lines). However, in cases where a higher speed of rolling stock is required (commuter trains, light metros, suburban and interurban routes and, of course, high-speed railways) it is required that the conductive wire, from which the pantograph takes tension, is affected as little as possible due to its own weight, describing a straight line approximately parallel to the track. Therefore, the solution to this problem is to install a second cable from which it hangs. The curve adopted by this second cable will not be a catenary either, since it supports a variable weight per unit length (by supporting the weight of the contact wire). However, the whole set of feeder cables, supports and traction and suspension elements of the cables that transmit electrical energy are called catenary.
Types of catenary
There are several types of overhead contact lines for railways and other electric traction vehicles, the main ones would be the following:
- Line
- Trolleybus lines
- Flexible air supply
- Rigid air supply
Tram line
The tram line is the simplest of applications of this type. It consists of a contact wire suspended in consecutive supports on the railway. The train takes power from this wire through a pantograph or a trolley.
The difference between a pantograph and a trolley is that the pantograph has a carbon or graphite plate that slides under the wire, while the trolley has a pulley or sheave that rolls under the wire. Both mechanisms have a system (mechanical or pneumatic) that provides the necessary pressure to keep the current-capturing element in contact with the power cables.
The tram line has the disadvantage that the deflection of the thread (vertical distance between the support and the lowest point of the thread) is large (quadratically proportional to the span). The introduction of a supporting cable decreases this deflection through the use of hangers. (See flexible catenary).
The speed that a vehicle fed by an overhead contact line can reach depends on the regularity of the height of the wire and the uniformity in the elasticity of the line, for which reason the tram line is only recommended for low speeds. It is commonly used in trams, light rail, charging stations, depots, etc.
Trolley lines
The trolleybus lines are a derivation of the tram lines, their fundamental difference being that there must be a second wire, parallel to the first, for the return of the current (negative).
Since the vehicles lack guidance devices, the line must be able to absorb large lateral deviations that the trolley sheave can transmit towards it. For this, the suspensions of the line have a flexible system that allows the "balancing" of the contact wire in the transverse direction over a very wide range.
Flexible overhead catenary
The flexible catenary consists of two main cables, the upper one of which is roughly in the shape of the curve known as the catenary and is called the "sustainer"; in some Spanish-speaking countries it is also called "carrier cable" or "messenger cable". Through a series of hanging elements (hangers) it supports another cable, the contact cable, called contact wire, so that it remains in a plane parallel to the plane of the tracks. Sometimes there is a third intermediate cable to improve the trace of the contact, which is usually called "false support" or "secondary breadwinner".
Catenaries with a second support throughout their length are often called "compound" catenaries.
The contact wire is not exactly what is known as a cable, with several threads or wires wound in several layers, but a drawing, that is, a solid wire in one piece.
This cable system has a complex geometry, which varies along the line depending on the requirements that are demanded at each point. The most important geometric parameters that define this geometry are the following:
- Vano.
- Height of the contact thread.
- Height of the catenary.
- Elevation.
- Straight of threads.
- Canton length.
- Decent.
Rigid overhead catenary
The rigid catenary differs from the others in that the element that transmits the electric current is not a cable, but a rigid rail. Logically, to keep this rigid rail parallel to the track, since its weight is very great, it is not enough to tighten it or suspend it from another cable with more sag, but it is necessary to increase the number of supports on which it must be suspended, to reduce the distance between them.
As an example, we will say that to suspend a rigid catenary, spans (distance between supports) of 10 or 12 m are used, while the span for flexible catenaries is around 50 or 60 m. This limitation restricts its use to tunnels, structures or sites with a very low gauge, where other systems are ineffective.
The origin of the system is based on a basic idea, and that is to solve the main drawback of the third lane, which is the danger of direct contacts. The simplest solution to this problem was to move the third contact rail to the top of the train. The solution was initially practiced with the same rail (steel), but soon more advanced rails were developed, with less weight and greater conductivity.
Another advantage of the rigid catenary versus the third rail (below) is its compatibility with the flexible catenary system. The same train with the same pantograph can run on the flexible and rigid catenary without equipping different current collection devices. The transitions between flexible and rigid catenaries have been solved in different railway administrations with the use of bars of variable elasticity or establishing transition sections (separation of systems).
The rail currently used consists of an aluminum bar that has a copper contact section at the bottom. The transmission of energy is carried out by aluminum and copper, although only the copper comes into contact with the pantograph.
Elements of a catenary
- Support structures
- Drivers
- Regulation of mechanical tension
- Protection
- Associated systems
Catenary support structures
Support structures are intended to support the cables (conductors) on the train in the proper way.
 Metallic (left) and reinforced concrete (right). |  Wooden poles and triphase line on the La Rhune train (France). |
The support structure of the catenary consists, in the simplest case, of two parts: the post and the bracket. Obviously the post must be fixed to the ground, whether it is natural or not. In the case of natural terrain, it is usually founded with a concrete footing, which in railway slang is called "massive", "solid foundation" or "foundation". In the case of fixing on structures, there are multiple methods, some of the most common being gewi anchors, express anchors, epoxy resins, etc.
Posts are vertical pillars that rise from ground level to the proper height to support the overhead contact line. There are countless types, the most common being metallic and reinforced concrete. The wooden ones are currently almost forgotten, except in some mining or tourist lines.
The corbels are structural elements, cantilevered from the post, whose function is to hold the overhead contact line in its correct position on the train.
Other support structures can be porticos. These can be divided into flexible and rigid.
Flexible gantries (commonly called "funiculars") are made up of two posts on both sides of the tracks and one or more cables that cross transversally over them, tying themselves to the posts. The catenaries hang from these cables, parallel to the layout of the tracks.
Rigid frames are also made up of two posts and, in this case, a rigid lintel between the two posts, which will be in charge of supporting the catenaries.
Drivers
We call the cables that carry the electric current from the substation to the train conductors. The conductors normally associated with the overhead contact line system, or catenary, can be the following:
- Contact thread
- Sustainer
- Positive Feeder or Substation Feeder
- Feeder of accompaniment
- Feeder negative
Regulation of mechanical tension
The conductive cables that form the catenary (sustainer and contact wire[s]) are subject to variations in length due to thermal expansion produced by changes in temperature. By varying the length of the cables, the geometry of the catenary is modified, increasing the deflection of the cables as the temperature rises.
This effect is undesirable for the quality of the pantograph capture, which is why elements for automatic regulation of the mechanical tension are installed.
Mechanical compensation systems by counterweights
The simplest element to avoid this effect (and the most effective) is the installation of a set of counterweights that pull the cable keeping its mechanical tension constant, which keeps its geometry constant.
In order to reduce the number of necessary counterweights, a device is installed that multiplies the effectiveness of the counterweight. The devices used fundamentally in this method are pulleys with solidary axes (concentric) and pulley systems with parallel axes (hoists).
The principle of action for the pulleys with joint shafts is based on the fact that, being attached to the same shaft, they make up a rigid solid. In order to maintain equilibrium in it, it must be true that the sum of moments with respect to the center of the axis is zero; therefore, the force exerted by the counterweights multiplied by the distance from its line of action to the axis is equal to the (mechanical) stress of the line times the distance from its line of action to the axis. Consequently, the ratio between the forces in both cables is inversely proportional to the radii of the pulleys on which they are wound. This ratio is known as the 'multiplying factor' or 'compensating relationship'. The usual values are between 1:3 and 1:5.
P.R2=T.R1{displaystyle P.R_{2}=T.R_{1}}
PR1=TR2{displaystyle {frac {P}{R_{1}}}}{{frac {T{R_{2}}}}}}}{
The principle of action for hoists is also based on the fact that all the elements that make up the system are rigid solids. Each pulley is attached by its axis to some rigid element, or to another of the cables that make up the hoist, and at the same time one of the cables is wound on it. For each of the pulleys, the forces exerted at the different points of the pulley must comply with Newton's third principle (Action/Reaction), in such a way that the vectorial sum of all these forces must be zero.
In this case there is also the multiplication factor or compensation ratio, although this time it does not have to do with the size of the pulleys, but with the number of these and the way in which they are fixed to each other, and at the same time post or structural element on which the assembly is anchored.
Mechanical compensation systems by springs
Another system used in mechanical compensation are springs (springs). These systems are also known by their trademark 'Tensorex'.
The principle of spring action is Hooke's Law.
F=− − kδ δ {displaystyle F=-kdelta,},
As we can see, the spring provides a variable force depending on its elongation, which is undesirable to maintain a constant tension. The way to convert the tension into a constant value is to insert a cam into the device, that is, a variable radius pulley. Said cam must be perfectly adjusted, in such a way that its angular position for each elongation of the spring compensates the Hooke's constant of said spring.
Protections
We understand by protections the elements of the installation of the overhead contact line not associated with the transmission of the current, but that exercise protection functions of the installation against eventual problems such as: short circuits, derivations, surges, vandalism, etc. The protections installed in overhead contact lines depend to a large extent on whether the current flowing through said line is alternating or continuous and on their voltage.
In direct current the most common protections are:
- Ground cable, also called guard cable
- Overvoltage downloaders (sometimes called pararrayos)
- Earth Takes
- Computer connections between structures and lane (only in some administrations)
- Glasses, screens and mechanical barriers
- Brakes and locks that prevent the fall of the line in the event that the same or counterweight cables are cut.
In alternating current
- Ground cable (in parallel with traction return)
- Earth Takes
- Computer connections between support and lane structures
- Overvoltage downloaders
- Glasses, screens and mechanical barriers
- Brakes and Line Fall Locks
Associated systems
This field includes systems whose main purpose is not to conduct current or protect the installation, but whose function is associated with the management of the overhead contact line or services powered by the voltage of the overhead contact line.
This category could include measurement and monitoring systems, disconnector remote control systems, braking energy reuse systems, power supply from the overhead contact line to systems outside the electrification system (GPRS antennas, mobile telephony, needle heating), etc.
Geometry of the catenary
To have a general idea of the geometry of the catenary we must first define the common terms to refer to its geometric values.
- Vano: Distance between two consecutive supports in the sense of progress of the line.
- Height of the contact thread: Vertical distance between the railing plane, defined by the rails, and the lowest point of the contact thread.
- Height of the catenary: Distance between the contact thread and the supporter (in the catenaries that have this) measure in support.
- Decent: Horizontal distance, measured at the height of the contact thread and in the plane parallel to the bearing, which exists between the axis of the road and the position of the contact thread.
- Arrow of contact threads: Vertical distance measured in the center of a vain between the contact thread booth at that point and in the previous and subsequent supports. If the bearing is different in these, the arrow will be established as the semidifference of both sides.
- Canton: Sequence of vains containing the same common conductor thread. The extreme vains, where the streak of the conductor thread starts and ends, are called drains, and in them two conductor threads coexist, the one that ends a canton and the one that starts the next..