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FEATURE ELECTRICAL & ELECTRONICS Simone Bruckner,


managing director at power resistor


manufacturer Cressall


Resistors, looks at why the electrification of


Electrification: the benefits


E


uropean countries are increasingly making moves to power trains with electricity


instead of diesel. According to the Institution of Mechanical Engineers (IMechE), however, the UK’s share of electrified railways comes in at just 42%, while other comparable European countries are over 60%. In 2018, the government announced proposals to phase out all diesel trains by 2040, but for this to become reality we must act on the benefits of electrification.


BI-MODE TRAINS One reason for recent electrification cutbacks is due to the potential of bi-mode trains, which use both electric and diesel to run on electrified sections of the network before switching over to non-electrified sections when the former aren’t available. While this means that passengers can benefit


from electric rail much sooner (as less disruptive electrification works such as overhead wiring and masts are required), uncertainty continues to arise over their performance compared to totally electric locomotives, as well as the implications of continuing to use diesel power. With diesel engines, fuel is significantly more expensive than electric traction, they are





rail transport should be prioritised


expensive to buy and maintain, and their heavier weight requires additional track maintenance. As electric trains are increasingly powered by renewables, their fuel source is not affected by fluctuating oil prices and shortages. Furthermore, the power output of a diesel


engine is limited by its rating, and its traction power is further reduced as the engine also has to supply the train’s hotel load. Electric traction power, however, is limited by its thermal loading so can operate for short periods at peak power. Partly for this reason, an electric multiple unit has typically twice the acceleration of a diesel multiple unit. In diesel mode, bi-mode trains suffer from


the same cost and carbon drawbacks as diesel-only trains. When in this mode, they only have the traction power of around 60% of their electric mode, lacking the speed and acceleration needed to improve services.


REGENERATIVE BRAKING When braking, a train’s kinetic energy cannot be stored on-board. On a diesel train, this energy is dissipated as heat using brake discs or from a roof-mounted rheostat that regulates the current flowing through it by changing the resistance. On electric trains, this braking energy can be fed back into the grid,


  


offering energy savings of up to 20%. During rheostatic braking, electric energy is


dissipated by a bank of onboard resistors, often referred to as the braking grid. In regenerative braking, the electricity is immediately reused by other locomotives or stored for later use. This electricity can be transmitted through overhead wires or an electrified third rail. Alternatively, it can be stored onboard using a flywheel, battery or other energy storage system. As such, regenerative braking benefits both the


environment, by reducing the demand of energy from the public grid, and the economy for the rail operator, by prolonging maintenance intervals thanks to reduced wear of the mechanical brakes.


EXPANDED MESH RESISTORS The ability to return electricity is only available for trains that are electrically powered and in constant contact with their power sources, such as subways and other underground rail systems. In cases where there are no other trains on


the track, or the distances between trains is too great, the regenerated energy is dissipated in brake resistors, mounted either on the trains or at fixed locations alongside the track. Cressall’s expanded mesh resistors are


particularly suitable for this application, being convection cooled and therefore silent, with no moving or wearing parts, and capable of dissipating very high powers in a compact space. While there is some reluctance in switching


to electric power, it is difficult to ignore the benefits. For routes with dense traffic and lines that need electric traction to obtain high speeds, electrifying the rail is the most effective and sustainable means of network upgrade. As part of a fully electrified rail system,


dynamic braking resistors are critical in ensuring excess traction can be dissipated safely.


Cressall Resistors www.cressall.com





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