Railway circuit
Feeder
The accompanying feeder is an electrical cable that accompanies the track and feeds the catenary sections, which are isolated from each other so that, in case of maintenance, an area of the track can be isolated without affecting the entire track.
The feeders are the cables or conductors that connect the substation with the catenary. They must have mechanical (stress) and electrical (resistivity, conductivity) characteristics.
The most used materials are: Cu, Cu-Ag, Cu-Mg and they usually use different sections 107 – 150 – 250 mm². There are three types of feeder for railway installations.
The positive feeder is the main conductor that supplies electrical power to the train. It is located at the bottom of the catenary and will make contact with the pantograph located on the train. It is also known as LAC (airline contact). The main function is to conduct electrical energy by the conductor to power the train through the pantograph. This feeder is found in railway facilities with direct current and alternating current power.
It is a secondary conductor that is installed parallel to the positive feeder. It is normally located at the top of the structure that supports the catenary and is connected to the positive feeder every certain distance. The main function is to feed the main conductor to feed demand peaks as well as guarantee and provide stability to the electrical supply of the railway installation. This feeder can also be found in railway installations with direct current and alternating current power.
It is the conductor that is responsible for collecting the return current from the installation to the substation. The main objective is to reduce return energy losses. This is because the section of the negative feeder is smaller than if the energy returned through the rail and ground. Note that this feeder is only found in alternating current installations where the powers supplied are higher and therefore return losses are more affected.
airline
To ensure good electricity capture at high speeds, flexible catenaries are constructed so that the geometry of the contact wire remains within certain limits, that is, that it remains straight, parallel to the track and in tension. To achieve this, a “sustaining” cable is hung from the track posts. Said cable describes a catenary, that is, a hyperbolic cosine. The contact wire is required to be straight, therefore, “hangers” that hold the contact wire in a horizontal position are hung from the support.
The catenary is supported by suspension insulators, which electrically isolate the catenary from the bracket.
The stay assembly serves to fix the direction of the contact wire in order to describe a "zigzag" to improve the wear of the pantograph wiper "Pantograph (railway)"). Thus the contact wire does not rub on a single point of the ruber. Said stay assembly also has an electrical insulator.
The pole is connected to the ground using the ground wire that also accompanies the track.
The mechanical tension of the catenary must be maintained since the passage of the pantograph produces mechanical oscillations in the contact wire. These oscillations must propagate faster than the train's running speed to avoid the appearance of standing waves that can damage the cables and cause the pantograph to take off. By tensioning the cable, the waves propagate at a higher speed.
On some occasions (such as in the Malaga metro) you can dispense with the support and the hangers and let the contact wire describe a catenary. This system presents some limitations such as the limitation of the speed of the railway and the need for a shorter distance between posts than in other cases.
This system arose as a solution to the problems of the third lane, that is, the danger of a pedestrian stepping on the lane and being electrocuted. In 2009, for example, 49 people died in the United Kingdom electrocuted by the third rail. What was done initially was simply to move the third rail to the roof. Subsequently, lighter weight and higher performance rails were developed for the collection of electrical energy.
In this case, the conductive element is not a contact wire, but a rigid rail. To maintain the rigidity of the rail, the distance between the supports should not be very large to prevent it from deforming as its mass per unit length is much greater than that of the conductive wire. The spans are usually 10 to 12 m (50 to 60 m for conventional catenaries). Given this limitation, its use is restricted to places with little space such as tunnels or places where a special configuration is required (for example a retractable or folding system using a pivot, with the metropolitan areas of Barcelona and Madrid being the ones that have opted for it the most).
They are compatible with flexible catenary systems; That is, the same train with the same pantograph can circulate on the flexible and rigid catenary without equipping different current collection devices.
The rail currently used consists of an extruded aluminum profile, which has a copper contact wire embedded in its lower part. The transmission of electrical energy is carried out through both aluminum and copper, although only the copper wire must come into contact with the pantograph. The rail is formed by joining bars using flanges with physical characteristics similar to the profiles and maintaining the electrical properties. Each continuous section of rigidly joined bars is called a block and its length usually does not exceed 500 meters. Continuity between one corner and the next is achieved by overlapping their end bars, so that the assembly can expand freely.
Due to the greater weight of this system compared to catenaries that use cables, a greater number of supports are needed to keep it parallel to the track. The distance between supports (span) is usually about 10 to 12 meters, compared to the 50 to 60 meters that occurs in flexible catenaries.
Among its advantages, it is worth highlighting:.
Third rail
This power supply system for railways arises from the need to electrify vehicles that circulate in metropolitan environments, especially in those sections where there are limitations imposed by the gauge of the lines.
As a solution, it was considered to place the feeder rails on the sides of the rolling tracks at a sufficient height to isolate the installation from the terrain and avoid diversions, as well as to allow detours and crossings of the railways.
In the event that the installation runs through accessible areas, it must be correctly marked and closed along the entire length of the line, since it implies serious safety problems.
Power system for railway vehicles, especially those that circulate through historic areas, where the implementation of an overhead line implies an undesirable visual impact.
The system is based on the installation of a third rail located between the traffic lanes, which powers the vehicle through an electric skate.
Unlike traditional third rail power systems, only the sections of track located under the vehicle would have voltage. This advantage is essential for implementation in urban centers where the conventional third rail presents clear safety problems.
As disadvantages compared to traditional systems, APS systems require higher expenses in both maintenance and operation. They also have relatively low availability rates as it is a technology that is in the development phase.
This system is currently used commercially on the Bordeaux tram. (Developed by Innorail, a subsidiary of Alstom).
Fourth lane
It is a rail located between the raceways and, unlike the feeding function performed by the third rail, the fourth rail's objective is to conduct the return current. While the power rail is powered at +420 V DC, the fourth rail is powered at –210 V DC, which would make a total voltage of 630 V DC. In this way we prevent the passage of current through the raceways, extending their useful life and avoiding corrosion problems. This system is present in the London Metro, Milan Metro and Paris Metro, among others.
guide bars
This system consists of two steel profiles placed on both sides of the railway track, which have the function of providing electrical energy to the train, which is captured by it through devices that slide on them, called 'brushes'. The train also has four rubber wheels that rest laterally on these bars, thus helping to reduce the "swaying" movements of the car, keeping it on the center of the track and guiding it during its journey. This type of rail power is the most commonly used in the Mexico City Metro.
Tram suspension
In this system, the contact line is a conductive wire (Cu) laid parallel to the plane of the track, and supported by support brackets using lateral suspension cables called tie rods. Its use is restricted to trams and trolleybuses and service tracks in stations because:
In the case of trams, the current is captured by means of a pantograph and in the case of trolleybuses and certain old trams, the collection system is usually using a trolley (a rod that has a metal wheel at the end that rolls under the cable from which it takes the current).
Other collection systems
These systems that have been analyzed present a main problem, the mechanical-electrical contact of two parts with significant relative movement and that must be strongly united in their movement. This requires significant maintenance costs: inspections of pantographs, catenaries and third rails, replacement of elements, etc., which railway administrations aim to optimize.
To this problem, especially when it comes to trams or metropolitan trains, we could add a negative visual impact when they run through historic centers or parks. For this reason, manufacturers study collection systems with lower maintenance costs, and that eliminate the problem of visual impact, such as those based on collection from the ground:
Supply voltage
The railway was powered by direct current with a voltage between 600 and 1,500 V due to the ease of using direct current motors. But with the increase in speeds, it was necessary to switch to alternating current, thus increasing voltages, lower voltage drops, reducing losses and using lighter catenaries.
On the other hand, the supply voltage is standardized under IEC 60850 (Railway applications – Supply voltages of traction systems) with the following values.
Using the current from the AC distribution lines, direct current traction substations are used to transform the current and feed the DC railway networks. They mainly feed conventional commuter, metro and tram networks since alternating electrification is used for high speed. To do this, they basically use a power transformer and a bridge rectifier based on power diodes.
The voltage values used are significantly lower than those used in AC lines. Therefore, important conversions and adaptations must be made before energy can be captured by rolling stock. Basically, the electric current must be transformed to the service voltage and subsequently rectified to convert it into direct current.
The type of current that powers high-speed lines in Spain is single-phase alternating current. This type of current is very widespread due to the possibility it offers of transporting high voltages, then high speeds.
The current is obtained from transformer electric traction substations that have a three-phase alternating current network as input. The output of the transformer has a voltage range between 15,000 and 25,000 V nominal. In Spain, two electrification systems are used: 3 kV dc and 25 kV ac at a frequency of 50 Hz.
Pantograph
The pantograph is a device found on the roof of a train. Its main function is to make electrical contact with the catenary. It is important that the pantograph can follow the unevenness of the catenary without losing contact with it, which is why, especially at high speeds, the system has to meet demanding requirements. Therefore, the maximum speed at which a train can run without problems with electrical contact is 400 km/h from the current perspective.[10].
There are different makes of the pantograph. For example, the Trolley Arm or Lyre Shape, these are commonly used in low-speed trains such as the urban train or the subway. However, these systems are outdated and are mostly found on older trains.
Nowadays, suspended systems such as the Articulated Arm pantograph are mainly used. This pantograph is asymmetrical and can be used with the ball joint in the direction of travel or against the direction of travel. For an optimal power outlet, a pressure force on the cable should be 50 to 120 N[10] and it is important that the force does not depend on the altitude at which the pantograph is located. It must also be taken into account that the speed of the air influences the pressure, especially at high speed. In these cases the force is regulated by compressed air to obtain a constant value.
For the pantograph material there are several possibilities and the use depends on the requirements of the system. The most used materials are:
To reduce wear the same material should always be used for a catenary whenever possible. However, operation with different materials, for example Carbon and Aluminum, results in great wear as a consequence.
Disconnectors
Disconnectors are mechanisms for opening and closing circuits or electrical installations. They are security and control elements. Their main function is to cut off or isolate the passage of electrical current, either to protect the installation or to carry out maintenance operations. They can be single-phase or three-phase.
It is not a surge protection device "Overvoltage (electricity)") or overcurrents, but it is a very useful element for maintenance tasks as well as for workers, because it can be visually detected if the disconnector is open or closed.
There are three types of disconnectors.