A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world. Railway electrification has many advantages but requires significant capital expenditure for installation.
Contents |
The main advantage of electric traction is a higher power-to-weight ratio than traction systems such as diesel or steam that generate power on board, but which must carry their own fuel supply. Electricity enables faster acceleration and higher tractive effort on steep gradients. On locomotives equipped with regenerative brakes, descending gradients require very little use of air brakes as the locomotive's traction motors become generators sending current back into the supply system to be used by other vehicles, and/or on-board resistors, which convert the excess energy to heat.
Other advantages include the lack of exhaust fumes at point of use, less noise and lower maintenance requirements of the traction units. Given sufficient traffic density, electric trains produce less carbon emissions than diesel trains, especially in countries where electricity comes primarily from non-fossil sources.
A fully electrified railway has no need to switch between methods of traction thereby making operations more efficient. Two countries that approach this ideal are Switzerland and Hong Kong, but both use more than one system, so unless multi-system locomotives or other rolling stock is used, a switch of traction method may still be required.
The main disadvantages are the capital cost of the electrification equipment, most significantly for long distance lines which do not generate heavy traffic. Suburban railways with closely spaced stations and high traffic density are the most likely to be electrified and main lines carrying heavy and frequent traffic are also electrified in many countries. Also, if the overhead wiring breaks down in some way, all trains can be brought to a standstill.
Electrification systems are classified by three main parameters:
Six of the most commonly used voltages have been selected for European and international standardisation. These are independent of the contact system used, so that, for example, 750 V DC may be used with either third rail or overhead lines (the latter normally by trams).
There are many other voltage systems used for railway electrification systems around the world, and the list of current systems for electric rail traction covers both standard voltage and non-standard voltage systems.
The permissible range of voltages allowed for the standardised voltages is as stated in standards BS EN 50163[1] and IEC 60850.[2] These take into account the number of trains drawing current and their distance from the substation.
| Electrification system | Lowest non-permanent voltage | Lowest permanent voltage | Nominal voltage | Highest permanent voltage | Highest non-permanent voltage |
|---|---|---|---|---|---|
| 600 V DC | 400 V | 400 V | 600 V | 720 V | 800 V |
| 750 V DC | 500 V | 500 V | 750 V | 900 V | 1 kV |
| 1,500 V DC | 1,000 V | 1,000 V | 1,500 V | 1,800 V | 1,950 V |
| 3 kV DC | 2 kV | 2 kV | 3 kV | 3 kV | 3 kV |
| 15 kV AC, 16.7 Hz | 11 kV | 12 kV | 15 kV | 17.25 kV | 18 kV |
| 25 kV AC, 50 Hz | 17.5 kV | 19 kV | 25 kV | 27.5 kV | 29 kV |
Early electric systems used low-voltage DC. Electric motors were fed directly from the traction supply and were controlled using a combination of resistors and relays that connected the motors in parallel or series.
The most common DC voltages are 600 V and 750 V for trams and metros and 1,500 V, 650/750 V third rail for the former Southern Region of the UK, and 3 kV overhead. The lower voltages are often used with third or fourth rail systems, whereas voltages above 1 kV are normally limited to overhead wiring for safety reasons. Suburban trains (S-Bahn) lines in Hamburg, Germany, operate using a third rail with 1,200 V, the French SNCF Culoz-Modane line in the Alps used 1,500 V and a third rail until 1976, when a catenary was installed and the third rail was removed. In the UK, south of London, 750 V third rail is used while, for inner London, 650 V is used to allow inter-running with London Underground, which uses a 650 V fourth rail system but with the fourth (centre) rail connected to the running rails in inter-running areas.
During the mid-20th century, rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage at feeder stations. Today, this is usually done by semiconductor rectifiers after stepping down the voltage from the utility supply.
The DC system is quite simple but it requires thick cables and short distances between feeder stations because of the high currents required. There are also significant resistive losses. In the United Kingdom, the maximum current that can be drawn by a train is 6,800 A at 750 V . The feeder stations require constant monitoring. The distance between two feeder stations at 750 V on third-rail systems is about 2.5 km (1.6 mi). The distance between two feeder stations at 3 kV is about 25 km (16 mi).
If auxiliary machinery, such as fans and compressors, is powered by motors fed directly from the traction supply, they may be larger because of the extra insulation required for the relatively high operating voltage. Alternatively, they can be powered from a motor-generator set, which offers an alternative way of powering incandescent lights which otherwise would have to be connected as series strings (bulbs designed to operate at traction voltages being particularly inefficient). Now solid-state converters (SIVs) and fluorescent lights can be used.
1,500 V DC is used in the Netherlands, Japan, Hong Kong (parts), Ireland, Australia (parts), India (around the Mumbai area alone,[3] to be converted to 25 kV AC like the rest of the country[4]), France, New Zealand (Wellington) and the United States (Chicago area on the Metra Electric district and the South Shore Line interurban line). In Slovakia, there are two narrow-gauge lines in the High Tatras (one a cog railway). In Portugal, it is used in the Cascais Line and in Denmark on the suburban S-train system.
In the United Kingdom, 1,500 V DC was used in 1954 for the Woodhead trans-Pennine route (now closed); the system used regenerative braking, allowing for transfer of energy between climbing and descending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC.
3 kV DC is used in Belgium, Italy, Spain, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia, South Africa and former Soviet Union countries (also using 25 kV 50 Hz AC). It was formerly used by the Milwaukee Road's extensive electrification across the Continental Divide and by the Delaware, Lackawanna & Western Railroad (now New Jersey Transit, converted to 25 kV AC) in the United States, and the Kolkata suburban railway (Bardhaman Main Line) in India, before it was converted to 25 kV 50 Hz AC.
600 V DC is used by Milan's network of tramways and trolleybuses.
Most electrification systems use overhead wires, but third rail is an option up to about 1,200 V. While use of a third rail does not require the use of DC, in practice all third-rail systems use DC because it can carry 41% more power than an AC system operating at the same peak voltage. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
Third rail systems can be designed to use top contact, side contact or bottom contact. Top contact is less safe, as the live rail is exposed to people treading on the rail unless an insulating hood is provided. Side- and bottom-contact third rail can easily have safety shields incorporated, carried by the rail itself. Uncovered top-contact third rails are vulnerable to disruption caused by ice, snow and fallen leaves.
DC systems (especially third rail systems) are limited to relatively low voltages and this can limit the size and speed of trains and cannot use low-level platform and also limit the amount of air-conditioning that the trains can provide. This may be a factor favouring overhead wires and high voltage AC, even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100 mph (160 km/h) because above that speed reliable contact between the shoe and the rail cannot be maintained.
Some street trams (streetcars) used conduit third-rail current collection. The third rail was below street level. The tram picked up the current through a plough (U.S. "plow") accessed through a narrow slot in the road. In the United States, much (though not all) of the former streetcar system system in Washington, D.C. (discontinued in 1962) was operated in this manner to avoid the unsightly wires and poles associated with electric traction. The same was true with Manhattan's former streetcar system. The evidence of this mode of running can still be seen on the track down the slope on the northern access to the abandoned Kingsway Tramway Subway in central London, United Kingdom, where the slot between the running rails is clearly visible, and on P and Q Streets west of Wisconsin Avenue in the Georgetown neighborhood of Washington DC, where the abandoned tracks have not been paved over. The slot can easily be confused with the similar looking slot for cable trams/cars (in some cases, the conduit slot was originally a cable slot). The disadvantage of conduit collection included much higher initial installation costs, higher maintenance costs, and problems with leaves and snow getting in the slot. For this reason, in Washington, D.C. cars on some lines converted to overhead wire on leaving the city center, a worker in a "plow pit" disconnecting the plow while another raised the trolley pole (hitherto hooked down to the roof) to the overhead wire. In New York City for the same reasons of cost and operating efficiency outside of Manhattan overhead wire was used. A new approach to avoiding overhead wires is taken by the "second generation" tram/streetcar system in Bordeaux, France (entry into service of the first line in December 2003; original system discontinued in 1958) with its APS (alimentation par sol — ground current feed). This involves a third rail which is flush with the surface like the tops of the running rails. The circuit is divided into segments with each segment energized in turn by sensors from the car as it passes over it, the remainder of the third rail remaining "dead". Since each energized segment is completely covered by the lengthy articulated cars, and goes dead before being "uncovered" by the passage of the vehicle, there is no danger to pedestrians. This system has also been adopted in some sections of the new tram systems in Reims, France (opened 2011) and Angers, France (also opened 2011). Proposals are in place for a number of other new services including Dubai, UAE; Barcelona, Spain; Florence, Italy; Marseille, France; Gold Coast, Australia; Washington, D.C., U.S.A.; Brasília, Brazil and Tours, France. At least initially there were teething troubles in terms of maintaining current feed, however, and the fact that the system is used exclusively in the historic center, with the cars on leaving this zone converting to conventional overhead pickup, underlines how, esthetics aside, for streetcars/trams it is hard to beat the overhead wire system in terms of overall efficiency.
The London Underground in England is one of the few networks that uses a four-rail system. The additional rail carries the electrical return that, on third rail and overhead networks, is provided by the running rails. On the London Underground, a top-contact third rail is beside the track, energized at +420 V DC, and a top-contact fourth rail is located centrally between the running rails at −210 V DC, which combine to provide a traction voltage of 630 V DC. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary.
This scheme was introduced because of the problems of return currents, intended to be carried by the earthed (grounded) running rail, flowing through the iron tunnel linings instead. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together. The problem was exacerbated because the return current also had a tendency to flow through nearby iron pipes forming the water and gas mains. Some of these, particularly Victorian mains that predated London's underground railways, were not constructed to carry currents and had no adequate electrical bonding between pipe segments. The four-rail system solves the problem. Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels.
London's sub-surface underground railways also operate on the four-rail scheme since in a number of areas (for example the Piccadilly Line and Metropolitan Line services to Uxbridge) sub-surface and deep-level stock run on the same tracks.
On tracks that London Underground share with National Rail third-rail stock (the Bakerloo and District lines both have such sections), the centre rail is connected to the running rails, allowing both types of train to operate, at a compromise voltage of 660 V. Underground trains pass from one section to the other at speed; lineside electrical connections and resistances separate the two types of supply.
Fourth-rail trains occasionally operate on the National third-rail system. To do so, the centre-rail shoes are bonded to the wheels. This bonding must be removed before operating again on fourth-rail tracks.
A system proposed (but not used) by the South Eastern and Chatham Railway around 1920 was 1,500 V DC four rail. Technical details are scarce but it is likely that it would have been a mid-earth system with one conductor rail at +750 V and the other at −750 V. This would have facilitated conversion to 750 V DC three-rail at a later date.
A few lines of the Paris Métro in France operate on a four-rail power scheme because they run on natural rubber tyres which run on a pair of narrow roadways made of steel and, in some places, concrete. Since the tyres do not conduct the return current, the two guide rails provided outside of the running 'roadways' double up as conductor rails, so at least electrically it fits as a four-rail scheme. One of the guide rails is bonded to the return conventional running rails situated inside the roadway so a single polarity supply is actually required. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey. The loop was originally provided to save the original steam locomotives having to 'run around' the rest of the train saving much time. Today, the driver doesn't have to change ends at termini provided with such a loop, but the time saving is not so significant as it takes almost as long to drive round the loop as it does to change ends. Many of the original loops have been lost as lines were extended.
These are overhead electrification systems. Alternating current can be transformed to lower voltages inside the locomotive. This allows much higher voltages and therefore smaller currents along the line, which means smaller energy losses along long railways.
Common DC commutating electric motors can also be fed with AC (universal motor), because reversing the current in both stator and rotor does not change the direction of torque. However, the inductance of the windings made early designs of large motors impractical at standard AC distribution frequencies. In addition, AC induces eddy currents, particularly in non-laminated field pole pieces, that cause overheating and loss of efficiency. In the previous century, five European countries, including Germany, Austria, Switzerland, Norway and Sweden, standardized on 15 kV 16⅔ Hz (one-third of the normal mains frequency) single-phase AC in an attempt to alleviate such problems. On 16 October 1995, Germany, Austria and Switzerland changed the designation from 16⅔ Hz to a nominal frequency of 16.7 Hz (though the actual frequency has not changed, its designation has). In the United States (with its 60 Hz distribution system), 25 Hz (an older, now-obsolete standard mains frequency) is used at 11 kV between Washington, D.C. and New York City and between Harrisburg, Pennsylvania and Philadelphia. A 12.5 kV 25 Hz section between New York City and New Haven, Connecticut was converted to 60 Hz in the last third of the 20th century.
In the UK, the London, Brighton and South Coast Railway pioneered overhead electrification of its suburban lines in London, London Bridge to Victoria being opened to traffic on 1 December 1909. Victoria to Crystal Palace via Balham and West Norwood opened in May 1911. Peckham Rye to West Norwood opened in June 1912. Further extensions were not made owing to the First World War. Two lines opened in 1925 under the Southern Railway serving Coulsdon North and Sutton railway station.[5][6][7] The lines were electrified at 6.7 kV 25 Hz. It was announced in 1926 that all lines were to be converted to DC third rail and the last overhead electric service ran in September 1929.
In such a system, the traction motors can be fed through a transformer with multiple taps. Changing the taps allows the motor voltage to be changed without requiring power-wasting resistors. Auxiliary machinery is driven by small commutating motors powered from a separate low-voltage winding of the main transformer.
The use of low frequency requires that electricity be converted from utility power by motor-generators or static inverters at the feeding substations, or generated at altogether separate traction powerstations.
Since 1979, the three-phase induction motor has become almost universally used. It is fed by a static four-quadrant converter which supplies a constant voltage to a pulse-width modulator inverter that supplies the three-phase variable frequency to the motors.
Three-phase AC railway electrification was used in Italy, Switzerland and the United States in the early twentieth century. Italy was the major user, for lines in the mountainous regions of northern Italy from 1901 until 1976. The first lines were the Burgdorf-Thun line in Switzerland (1899), and the lines of the Ferrovia Alta Valtellina from Colico to Chiavenna and Tirano in Italy, which were electrified in 1901 and 1902. Other lines where the three-phase system were used were the Simplon Tunnel in Switzerland from 1906 to 1930, and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1939.
The early sysems used a low frequency (16⅔ Hz), and a relatively low voltage (3,000 or 3,600 volts) compared with later AC systems. The system provides regenerative braking with the power fed back to the system, so is particularly suitable for mountain railways (provided another locomotive on the line can use the power). The system has the disadvantage of requiring two (or three) separate overhead conductors plus return through the rails for the three phases. Locomotives operate at one, two or four constant speeds.
The system is still used on four mountain railways, using 725 to 3000 V at 50 or 60 Hz: the (Corcovado Rack Railway in Rio de Janeiro, Brazil, Jungfraubahn and Gornergratbahn in Switzerland and the Petit train de la Rhune in France).
Only in the 1950s after development in France (20 kV then 25 kV) and former Soviet Union countries (25 kV) did the standard-frequency single-phase alternating current system become widespread, despite the simplification of a distribution system which could use the existing power supply network.
The first attempts to use standard-frequency single-phase AC were made in Hungary since 1923, by the Hungarian Kálmán Kandó on the line between Budapest-Nyugati and Alag, using 16 kV at 50 Hz. The locomotives carried a four-pole rotating phase converter feeding a single traction motor of the polyphase induction type at 600 to 1,100 V. The number of poles on the 2,500 hp motor could be changed using slip rings to run at one of four synchronous speeds. The tests were a success so, from 1932 until 1960s, trains on the Budapest-Hegyeshalom line (towards Vienna) regularly used the same system. A few decades after the Second World War, the 16 kV was changed to the Russian and later French 25 kV system.
Today, some locomotives in this system use a transformer and rectifier to provide low-voltage pulsating direct current to motors. Speed is controlled by switching winding taps on the transformer. More sophisticated locomotives use thyristor or IGBT circuitry to generate chopped or even variable-frequency alternating current (AC) that is then supplied to the AC induction traction motors.
This system is quite economical but it has its drawbacks: the phases of the external power system are loaded unequally and there is significant electromagnetic interference generated as well as significant acoustic noise.
A list of the countries using the 25 kV AC 50 Hz single-phase system can be found in the list of current systems for electric rail traction.
The United States commonly uses 12.5 and 25 kV at 25 Hz or 60 Hz. 25 kV, 60 Hz AC is the preferred system for new high-speed and long-distance railways, even if the railway uses a different system for existing trains.
To prevent the risk of out-of-phase supplies mixing, sections of line fed from different feeder stations must be kept strictly isolated. This is achieved by Neutral Sections (also known as Phase Breaks), usually provided at feeder stations and midway between them although, typically, only half are in use at any time, the others being provided to allow a feeder station to be shut down and power provided from adjacent feeder stations. Neutral Sections usually consist of an earthed section of wire which is separated from the live wires on either side by insulating material, typically ceramic beads, designed so that the pantograph will smoothly run from one section to the other. The earthed section prevents an arc being drawn from one live section to the other, as the voltage difference may be higher than the normal system voltage if the live sections are on different phases and the protective circuit breakers may not be able to safely interrupt the considerable current that would flow. To prevent the risk of an arc being drawn across from one section of wire to earth, when passing through the neutral section, the train must be coasting and the circuit breakers must be open. In many cases, this is done manually by the driver. To help them, a warning board is provided just before both the neutral section and an advanced warning some distance before. A further board is then provided after the neutral section to tell the driver to re-close the circuit breaker, although the driver must not do this until the rear pantograph has passed this board. In the UK, a system known as Automatic Power Control (APC) automatically opens and closes the circuit breaker, this being achieved by using sets of permanent magnets alongside the track communicating with a detector on the train. The only action needed by the driver is to shut off power and coast and therefore warning boards are still provided at and on the approach to neutral sections.
On French high-speed rail lines, the UK High Speed 1 Channel Tunnel rail link and in the Channel Tunnel, neutral sections are negotiated automatically.
In Japanese Shinkansen lines, there are ground-operated switched sections installed instead of neutral sections. The sections detect trains running within the section and automatically switch the power supply in 0.3 s,[8] which eliminates needs of shutting off powers at any moment.
In 2006, 240,000 km (25% by length) of the world rail network was electrified and 50% of all rail transport was carried by electric traction.
Advantages include:
Disadvantages include:
Trade-offs include:
Additionally, there are issues of connections between different electrical services, particularly connecting intercity lines with sections electrified for commuter traffic, but also between commuter lines built to different standards. This can cause electrification of certain connections to be very expensive simply because of the implications on the sections it is connecting. Many lines have come to be overlaid with multiple electrification standards for different trains to avoid having to replace the existing rolling stock on those lines. Obviously, this requires that the economics of a particular connection must be more compelling and this has prevented complete electrification of many lines. In a few cases, there are diesel trains running along completely electrified routes and this can be due to incompatibility of electrification standards along the route.
Summary of advantages and disadvantages:
There is a significant amount of published material that concludes that electric trains are more energy efficient than diesel-powered trains and, with suitable energy production, can have a smaller carbon dioxide footprint. Some of the reasons include:
According to widely accepted global energy reserve statistics,[10] the reserves of liquid fuel are much less than gas and coal (at 42, 167 and 416 years respectively). Most countries with large rail networks do not have significant oil reserves and those that did, like the United States and Britain, have exhausted much of their reserves and have suffered declining oil output for decades. Therefore, there is also a strong economic incentive to substitute oil for other fuels. Rail electrification is often considered an important route towards consumption pattern reform.[11][12]
The external cost of railway is lower than other modes of transport but the electrification brings it down further if it is sustainable.
Also, the lower cost of energy from well to wheel and the ability to reduce pollution and greenhouse gas in the atmosphere according to the Kyoto Protocol is an advantage.
It is possible to supply power to an electric train by inductive coupling. This allows the use of a high-voltage, insulated, conductor rail. Such a system was patented in 1894 by Nikola Tesla, US Patent 514972.[13] It requires the use of high-frequency alternating current. Tesla did not specify a frequency but George Trinkaus[14] suggests that around 1,000 Hz would be likely.
Inductive coupling is widely used in low-power applications, such as re-chargeable electric toothbrushes. The contactless technology for rail vehicles is currently being marketed by Bombardier as PRIMOVE.[15]
Another result of electrification is the effect on locomotive and wagon productivity and it is going to be more effective by more railway research in this field. The trend of technology in railway electrification is very important to adopt the efforts for better results, for example the trend from GTO (Gate turn-off thyristor) to IGBT (Insulated-gate bipolar transistor) for more powerful locomotives with higher reliability is one of the elements of Technology roadmap (TRM) and the loop to have a mature system as in Maturity road mapping with the Technology transfer provision.
|
|||||
|
||||||||||||||