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description of AC networks

advantages of 15kV 16 2/3Hz and 25kV 50Hz
  • The use of a relatively high voltage allows high power to be delivered through a light catenary over several tens of kilometres from the feeder point with little loss, and little risk of interference to signalling and railside telecoms.
  • 25kV 50Hz:
    the need for a separate high-voltage distribution network (or frequency converter stations) is avoided and smaller transformers to be used on motive power are possible.


disadvantages of 15kV 16 2/3Hz and 25kV 50Hz
  • In order to obtain the advantages mentioned above, AC electrification must be at a relatively high voltage and it may be necessary to provided increased clearance to ensure adequate insulation, eg by lowering the track in old tunnels.
  • Whereas only a transformer is needed on the motive power to convert 15kV 16 2/3Hz down to a voltage suitable for specially-designed traction motors, 25kV 50Hz must also be converted to DC, or variable-frequency AC, before input to the traction motors.
  • 25kV 50Hz:
    Compared with the 15kV 16 2/3Hz system care must be taken to avoid unbalanced single-phase loads on the three-phase public network.


extension of AC networks
  • 15kV 16 2/3Hz:
    Germany, Switzerland, Austria, Norway, Sweden
  • 25kV 50Hz:
    Great Britain, France, Belgium, Portugal, Finland, Denmark, Czech, Slovakia, Hungary, Romania, Bulgaria, Greece and former Yugoslavia

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