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description of DC networks

advantages of 1500V and 3000V DC
  • especially the lack of the large clearances and extensive insulation needed with high voltage systems
  • simple technology in the vehicle: series-connected DC motors (which can themselves be connected in series, series-parallel and parallel for operation at different speeds)
  • no need to develop robust rectifiers for use on the motive power
  • particularly suitable for lines with relatively short trains and relatively low maximum speeds (eg in the Netherlands)

disadvantages of 1500V and 3000V DC
  • because the voltage in the catenary cannot exceed 3000V with this technology, the current drawn by heavy fast trains is relatively high
  • the feeder stations must be located relatively close together, the catenary must be thick and heavy and the pantograph(s) must be pressed hard against it to minimise the voltage drop between the nearest feeder station and the motive power
  • For example, a line 80km long with a speed limit of 200km/h needs 6 feeder stations if the power is supplied at 1500V DC. That is why TGV trains have only one pantograph raised when running at high speed under catenary at 25kV 50Hz, but need to have both pantographs raised when running on older lines electrified at 1500V DC like between Tours and Bordeaux, where double-units of TGV-Atlantique have raised 4 pantographs at a speed of 220km/h - the limit of this system.
  • Furthermore, the power rating of the line's feeder stations sets a limit to the amount of electrically powered traffic the line can handle (ie the total number, speed and/or loading of the trains), which cannot be adjusted quickly whenever the pattern of traffic changes. Of course, an extra feeder station can be installed wherever the demand for power is particularly high, but this is expensive and takes time.
  • Many of these problems arise because the only simple way to reduce the voltage of DC power is by inserting a resistance in series with the motor, which is inevitably wasteful.

extension of AC networks
  • 750V: South-England, metro networks
  • 1500V: Netherlands, Southern part of France
  • 3000V: Belgium, Italy, Poland, Czech and Slovakia

top of page page last updated: 26. March 2018 ©1998-2013 Thorsten Büker top of page

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