Abstract
This chapter describes the different electric railway power supply systems and their main characteristics from the point of view of the power flow modeling and simulation. It considers the DC traction systems and also the AC ones, explaining the different elements embedded in the network and proposing steady state electrical models of each element. The basic methods for modeling the train behavior in terms of demanded/regenerated power are also detailed. Finally, the procedures to simulate the trains motion into the electrical network and how their motion and power demand affect to the electrical variables will be unraveled, explaining how to merge all the models in a system of equations.
TopIntroduction
An electric locomotive can be defined as locomotive powered by electricity. This electricity can be obtained from a railway power supply system (RPSS) or from an on-board accumulator like a battery, fuel cell or ultracapacitor. Robert Davidson built the first electric locomotive in 1837 and it used the second option, galvanic type batteries powered it. However, the first electric locomotive using a RPSS date from 1879.Werner von Siemens built it and, it was fed by a 150V DC RPSS connected to the train through a contact roller (Day & McNeil, 1966). Most of the RPSS are based in overhead lines or third rail systems; in this case, the third rail was the chosen option. The traction system employed by Siemens locomotive was based in a series-wound motor of 2.2kW.
The fact that electric locomotives were much cleaner than steam ones launched the development of this kind of systems for solving the ventilation problems in tunnels applications. In 1895 a four-mile stretch with multiple tunnels of the Baltimore & Ohio Rail Road was electrified using an overhead distribution system of 675V DC and an electric locomotive built by General Electric with a power output of 4270kW. In 1898, the first electric AC locomotive was inaugurated, it was supplied by a 3000V and 15Hz three-phase RPSS.
The electrification of the railway tracks increased rapidly and by the year 2006, the 25% of all the railway power supply systems were electrified, a total of 240000km and approximately the 50% of all rail transport (Frey, 2012). In present days, countries like India are electrifying lines at a pace of 2000km/year. The drastic increase of the electrified railways was driven basically by the lower running and maintenance cost of electrical locomotives compared with diesel ones and the higher power/weight ratio of the electric locomotives, besides the reduction of noise, pollution and energetic dependence.
To date, many different railway power supply systems with different voltages, frequencies and constructive characteristics have been developed. In Table 1, the main electrification systems are represented.
Table 1. Rated, highest and lowest voltages in most common railway power supply systems
| Rated
Voltage | Lowest Non-Permanent Voltage | Lowest Permanent Voltage | Highest Permanent Voltage | Highest Non-Permanent Voltage |
| DC | 750 | 500V | 500V | 900V | 1kV |
| 1500 | 1000V | 1000V | 1800V | 1950V |
| 3000 | 2000V | 2000V | 3600V | 4000V |
| AC | 15kV, 16.7Hz | 11kV | 12kV | 17.25kV | 18kV |
| 25kV, 50Hz | 17.5kV | 19kV | 27.5kV | 29kV |
| 2x25kV, 50Hz | 42kV | 45kV | 55kV | 58kV |
Key Terms in this Chapter
Overhead Contact Line: It is an electrical line situated over the tracks fed by an electrical substation. The overhead contact line is one of the systems employed to provide electrical power to electric locomotives that are connected to the overhead contact line using a structure called pantograph.
Double-End Feeding: It is a traction network electrical scheme where two adjacent electrical sections are connected, so the two substations at the same time feed any train in whichever section. This scheme produce lower voltage drops that the single-end feeding scheme but the voltage between the overhead conductors and the rails in the two sections must be in phase.
Cross Coupling Feeding: When a traction system has more than one parallel track connecting two nodes, there is one overhead contact line for each track. Usually all these overheads lines are connected to the same substation. When more connections between the overhead contact lines of the two tracks are added at regular intervals, it can be stated that the overhead lines are cross-coupled.
Longitudinal Coupling: This kind of coupling is the connection between two overhead lines of the same track that belongs to different electrical sections. It must be used when the feeding scheme is the double-end feeding. In such cases, even when during the normal operation of the system, the two sections are connected, the longitudinal coupling system must be able to disconnect the sections for isolating possible faults in the network.
Electric Locomotive: It is the vehicle that transforms electrical power into mechanical power for moving a train. The electrical power can be obtained from an overhead wire, a third rail or a storage system. The locomotives that use a combustion engine coupled with an electrical generator to produce their own electric power are also considered electric locomotives.
Single-End Feeding: It is a traction network electrical scheme where each electrical section is fed by one substation; two electrical sections fed by different substations cannot be electrically connected. This scheme is usually employed when the two adjacent substations are connected to different phases of the distribution system.
Third Rail: It is a rigid conductor parallel to the tracks used to supply electric energy to the electric locomotives; this method is only employed in DC traction systems like DC metro systems. Ceramic electrical insulators placed over sleepers usually support the third rail. The train is connected to the third rail through current collectors called contact shoes.
Catenary: It is the wire that supports the overhead contact line; it is called catenary because its shape is similar to this geometrical curve. The catenary is linked to the structures by means of clamps. At regular intervals, wires called droppers connect the overhead contact line with the catenary. The droppers have different length to keep constant the height of the overhead conductor. In some countries the catenary is also called the messenger wire.
Electrical Section: The whole traction network can be divided into different electrical sections. A substation or a feeding branch can supply energy to one or more electrical sections. Depending on the adopted distribution scheme, the electrical sections can be insulated or electrically connected.
Neutral Section: In some occasions, electrical substations connected to different phases feed two adjacent electrical sections. In such cases, the electrical sections cannot be connected and a section that is not electrically supplied separates them. This dead zone is called neutral section. Depending on the kind of trains and their current collector systems, these neutral sections can have different lengths and they must be placed in a location where the train can go drifting (not in traction or braking mode). A safety system must be able to feed the neutral section in case a train is stuck in it.