Railway electrification system
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A railway electrification system supplies electrical energy to railway traction units for propulsion purposes. In more simple terms, it supplies electricity to trains so they can move. There are many systems for doing this, each with distinctive advantages and disadvantages.
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[edit] Comparison of Electric Railways against Diesel/Steam Railways
[edit] Advantages
The main advantage is the superior acceleration of electrically powered trains. Other advantages include the lack of exhaust fumes at point of use, less noise and lower maintenance requirements of the traction units. In countries where electricity comes primarily from non-fossil sources, such as Austria and France, electric trains also produce fewer carbon emissions than diesel trains.
[edit] Disadvantages
The main disadvantage is the capital cost of the electrification equipment, which tends to militate against electrification of long distance lines that do not carry a heavy service. Suburban railways with closely-spaced stations and high traffic density are the most likely to be electrified, but main lines carrying heavy and frequent traffic are also electrified in many countries.
[edit] Classification
Electrification systems are classified by three main parameters:
- The common voltages are simple multiples of each other:
- 1200 V DC = 2 x 600 V DC
- 1500 V DC = 2 x 750 V DC
- 3000 V DC = 2 x 1500 V DC
- This can be direct current (DC) or alternating current (AC). For AC systems, the frequency (Hertz) is also given. In general, it is assumed that AC systems are single-phase, but if they are not (e.g. three-phase), then this should also be stated.
- Contact System
- This refers to the means by which traction current is collected by the traction unit. There are two main types: (Conductor Rail and Overhead Line).
[edit] Direct current
Early electric systems used relatively 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 voltages are 600 V and 750 V for trams and metros, and 1500 V and 3000 V for railways. The lower voltages are often used with third or fourth rail systems, but voltages above 1000 V are generally limited to overhead wiring for safety reasons.
Through the middle 20th century, rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage at the feeder stations. Today, this is usually done by semiconductor rectifiers after transforming the voltage down from the utility supply.
The DC system is quite simple, but it requires thick cables and short distances between feeder stations because of the heavy currents required; additionally, there are significant resistive losses. The feeder stations require constant monitoring, and on many systems, only one train or locomotive is allowed per section. The distance between two feeder stations at 3000 V system is about 25 km (15 miles).
If auxiliary machinery, such as fans and compressors, are 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 was required anyway for incandescent lights with their narrow voltage limits. Now solid-state converters (SIVs) and fluorescent lights can be used.
1500 V DC is used in The Netherlands, Japan, Ireland, parts of Australia and France, and in Wellington New Zealand. In the United States, 1500 V DC is used in the Chicago area on the Metra Electric district and the South Shore Line interurban streetcar line. In Slovakia, there are two narrow gauge lines in the High-Tatras (one a cog railway)
In the United Kingdom, 1500 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. The only UK system now using this voltage is the Tyne and Wear Metro.
3000 V DC is used in Belgium, Italy, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia, South Africa and in the former Soviet Union. It was also formerly used by the Milwaukee Road's extensive electrification across the Continental Divide, and by the Delaware, Lackawanna & Western Railroad (now NJ Transit, converted to 25 kV AC).
The permissible range of voltages allowed are as stated in standards BS EN 50163 and IEC 60850. 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 | 1000 V |
| 1500 V, DC | 1000 V | 1000 V | 1500 V | 1800 V | 1950 V |
| 3000 V, DC | 2000 V | 2000 V | 3000 V | 3600 V | 3900 V |
| 15000 V, AC, 16⅔ Hz | 11000 V | 12000 V | 15000 V | 17250 V | 18000 V |
| 25000 V, AC, 50 Hz | 17500 V | 19000 V | 25000 V | 27500 V | 29000 V |
[edit] Third rail
Most electrification systems use overhead wires, but third rail is an option up to about 1200 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 of some sort 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 are limited to relatively low voltages, and this can limit the size and speed of trains and the amount of air-conditioning 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. See also third rail.
Some road operating trams (streetcars) also used third rail current collection schemes. In these cases, the third rail was located below street level; the tram picked up the current via a collector accessed through a narrow slot in the road. In the United States, the former trolley system in Washington, D.C. was operated in this manner to avoid the unsightly wires and poles associated with electric traction. The evidence of this mode of running can still be seen on the track that runs down the slope on the Northern access to the abandoned Kingsway Tramway Subway (in central London). The slot between the running rails is clearly visible. The slot can easily be confused with the similar looking slot that allows access to a cable hauled tram system (indeed, in at least some cases, the third rail slot was originally a cable slot).
[edit] Fourth rail
The London Underground is one of the few networks in the world 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 placed beside the track, energised 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.
On lines where London Underground trains run over Network Rail owned lines, sharing the track with third-rail stock, the centre 'negative' rail is directly connected to the return running rail, allowing both types of train to operate.
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 volts and the other at -750 volts. This would have facilitated conversion to 750 V DC three-rail at a later date.
A few lines of the Paris Metro also operate on a '4 rail' power scheme, but for a very different reason. It is not strictly a 4-rail scheme as they run on rubber tyres running on a pair of narrow roadways made of steel, and in some places, concrete. Since the tyres do not conduct the return current, two conductor rails are provided outside of the running 'roadways', so at least electrically, it fits as a 4-rail scheme. 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. Rubber tyres also run against the side contact conductor rails to guide the train on its track. Conventional rails are provided inside the 'roadways' to facilitate the operation of maintenance equipment, and movement of conventional rail stock. The rubber tyres were intended to provide a smoother ride and less vibration to surrounding buildings. They succeed in doing this, but at the expense of considerable running noise inside the trains and very short tyre life. Due to frictional losses, the energy consumption is significantly higher than that of similar trains equipped with steel wheels, running on steel rails.
[edit] Low-frequency alternating current
Common commutating electric motors can also be fed AC (universal motor), because reversing current in both stator and rotor does not change the direction of torque. However, inductance of the windings makes large motors impractical at standard AC distribution frequencies. Many European countries, including Germany, Austria, Switzerland, Norway, and Sweden have standardised on 15 kV 16⅔ Hz (Germany, Austria and Switzerland are now on 16,7 Hz since 1995) (one-third the normal mains frequency) single-phase AC (earlier, 6 kV and 7.5 kV systems were in use). 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, DC and New York City. 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 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 low voltage commutating motors, powered from a separate winding of the main transformer, and are reasonably small.
The unusual frequency means that electricity has to be converted from utility power by motor-generators or static inverters at the feeding substations, or generated at altogether separate electric power stations.
[edit] Standard frequency alternating current
Only in the 1950s after development in France 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 in the 1930s, 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 1100 volts. The number of poles on the 2500 hp motor could be changed using slip rings to run at one of four synchronous speeds.
Today, some locomotives in this system use a transformer and rectifier that provide low-voltage pulsating DC current to motors. Speed is controlled by switching winding taps on the transformer. More sophisticated locomotives use thyristor or IGBT transistor circuitry to generate chopped or even variable-frequency AC that is then directly consumed by AC 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, not to mention acoustic noise.
A list of the countries using the 25 kV, 50 Hz single-phase AC 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 60 Hz. 25 kV AC is the preferred system for 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 station 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 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 safety 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 they can reclose 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) is in use which 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.
