Railway electrification
Railway electrification is the use of electric power for the propulsion of rail transport. Electric railways use either electric locomotives (hauling passengers or freight in separate cars), electric multiple units (passenger cars with their own motors) or both. Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network and distributed to the trains. Some electric railways have their own dedicated generating stations and transmission lines, but most purchase power from an electric utility. The railway usually provides its own distribution lines, switches, and transformers.
Power is supplied to moving trains with a (nearly) continuous conductor running along the track that usually takes one of two forms: an overhead line, suspended from poles or towers along the track or from structure or tunnel ceilings, or a third rail mounted at track level and contacted by a sliding "pickup shoe". Both overhead wire and third-rail systems usually use the running rails as the return conductor, but some systems use a separate fourth rail for this purpose.
In comparison to the principal alternative, the diesel engine, electric railways offer substantially better energy efficiency, lower emissions, and lower operating costs. Electric locomotives are also usually quieter, more powerful, and more responsive and reliable than diesel. They have no local emissions, an important advantage in tunnels and urban areas. Some electric traction systems provide regenerative braking that turns the train's kinetic energy back into electricity and returns it to the supply system to be used by other trains or the general utility grid. While diesel locomotives burn petroleum products, electricity can be generated from diverse sources, including renewable energy.[1] Historically, concerns of resource independence have played a role in the decision to electrify railway lines. The landlocked Swiss confederation which almost completely lacks oil or coal deposits but has plentiful hydropower electrified its network in part in reaction to supply issues during both World Wars.[2][3]
Disadvantages of electric traction include: high capital costs that may be uneconomic on lightly trafficked routes, a relative lack of flexibility (since electric trains need third rails or overhead wires), and a vulnerability to power interruptions.[1] Electro-diesel locomotives and electro-diesel multiple units mitigate these problems somewhat as they are capable of running on diesel power during an outage or on non-electrified routes.
Different regions may use different supply voltages and frequencies, complicating through service and requiring greater complexity of locomotive power. There used to be a historical concern for double-stack rail transport regarding clearances with overhead lines[1] but it is no longer universally true as of 2022, with both Indian Railways[4] and China Railway[5][6][7] regularly operating electric double-stack cargo trains under overhead lines.
Railway electrification has constantly increased in the past decades, and as of 2022, electrified tracks account for nearly one-third of total tracks globally.[8][9] Electrification projects are constantly undertaken to service rapidly growing suburbs formerly served by non-electrified trains which typically have low capacities.
History[edit]
Railway electrification is the development of powering trains and locomotives using electricity instead of diesel or steam power. The history of railway electrification dates back to the late 19th century when the first electric tramways were introduced in cities like Berlin, London, and New York City.
In 1895, the first railway in the world to be electrified was the Gross-Lichterfelde Tramway in Berlin, Germany. It was followed by the electrification of the Baltimore and Ohio Railroad's Baltimore Belt Line in the United States in 1895–96, the first electrified mainline railway.
The early electrification of railways used direct current (DC) power systems, which were limited in terms of the distance they could transmit power. However, in the early 20th century, alternating current (AC) power systems were developed, which allowed for more efficient power transmission over longer distances.
In the 1920s and 1930s, many countries worldwide began to electrify their railways. In Europe, Switzerland, Sweden, France, and Italy were among the early adopters of railway electrification. In the United States, the New York, New Haven and Hartford Railroad was one of the first major railways to be electrified.
Railway electrification continued to expand throughout the 20th century, with technological improvements and the development of high-speed trains and commuters. Today, many countries have extensive electrified railway networks with 375000 km of standard lines in the world, including China, India, Japan, France, Germany, and the United Kingdom. Electrification is seen as a more sustainable and environmentally friendly alternative to diesel or steam power and is an important part of many countries' transportation infrastructure.
Comparisons[edit]
AC versus DC for mainlines[edit]
The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive, and within the locomotive, transformed and rectified to a lower DC voltage in preparation for use by traction motors. These motors may either be DC motors which directly use the DC or they may be three-phase AC motors which require further conversion of the DC to variable frequency three-phase AC (using power electronics). Thus both systems are faced with the same task: converting and transporting high-voltage AC from the power grid to low-voltage DC in the locomotive. The difference between AC and DC electrification systems lies in where the AC is converted to DC: at the substation or on the train. Energy efficiency and infrastructure costs determine which of these is used on a network, although this is often fixed due to pre-existing electrification systems.
Both the transmission and conversion of electric energy involve losses: ohmic losses in wires and power electronics, magnetic field losses in transformers and smoothing reactors (inductors).[25] Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher.[26] However, the higher voltages used in many AC electrification systems reduce transmission losses over longer distances, allowing for fewer substations or more powerful locomotives to be used. Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for.
Standard AC electrification systems use much higher voltages than standard DC systems. One of the advantages of raising the voltage is that, to transmit certain level of power, lower current is necessary (P = V × I). Lowering the current reduces the ohmic losses and allows for less bulky, lighter overhead line equipment and more spacing between traction substations, while maintaining power capacity of the system. On the other hand, the higher voltage requires larger isolation gaps, requiring some elements of infrastructure to be larger. The standard-frequency AC system may introduce imbalance to the supply grid, requiring careful planning and design (as at each substation power is drawn from two out of three phases). The low-frequency AC system may be powered by separate generation and distribution network or a network of converter substations, adding the expense, also low-frequency transformers, used both at the substations and on the rolling stock, are particularly bulky and heavy. The DC system, apart from being limited as to the maximum power that can be transmitted, also can be responsible for electrochemical corrosion due to stray DC currents.[13]: 3
