Why don't we use HVDC for railway overhead electrification?

Discussion in 'General Electronics Chat' started by AidanCroft, Jan 7, 2015.

  1. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
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    We use, most commonly, 25kV AC electrification on overhead lines. Why can't we do this with DC at a high Voltage on the order of a few ten thousand Volts?

    Any information would be hugely appreciated.

    Kind regards,

    Aidan.
     
  2. MikeML

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    AidanCroft likes this.
  3. alfacliff

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    we used to have electric train lines here in Kansas, but they were taken down years ago. I guess the railroads wanted to have their equipment more universal, and useable on all lines, not just the lines with electrification. dont matter now, most of the rails here have been pulled up.
     
  4. ronv

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    I think it has to do with converting the high voltage down to a more manageable voltage. But it is probably becoming more practical.
     
  5. MaxHeadRoom

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    This has been posted before, the Hydro electricity produced here is transmitted long distance using DC, one less conductor plus the radiation losses are greatly reduced, been that way for some time.
    Max.
     
  6. ronv

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    What do they do at the other end? Big motor generator?
     
  7. BR-549

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    Sep 22, 2013
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    ronv, that's why it wasn't used for many years. But now with solid state devices, it is practical.
    We can now do what we do in variable speed drives, at very high dc voltages.
    In some ways...a hvdc bus is more versatile than an ac main.
     
  8. MaxHeadRoom

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    Early days switching was done with Ignitron & thyratron tubes, now solid state equivalents.
    Max.
     
  9. #12

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    We can. See post #2

     
  10. WBahn

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    Mar 31, 2012
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    Let's bear in mind the title of the thread -- I think the OP is asking specifically about power for electric trains/trams and such.

    I don't know to what degree HVDC is used in such installations. I would imagine it comes down to whether using it onboard the train at the same voltage as the power line is practical. If it needs to be converted down then doing so may not make physical or economic sense.

    Even with long-haul HVDC lines the cost of rectification and inversion at the endpoints compared to using transformers is significant and has to be made up for in other savings, such as reduced line losses due to radiation or, it is my understanding, the biggest one is usually the lower capital cost of the towers and line and, most especially, the physical width of the right-of-ways that have to be purchases/leased. I wouldn't be surprised if a railway installation just isn't in a position to leverage these other savings enough to justify the additional costs of using HVDC.
     
  11. profbuxton

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    Feb 21, 2014
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    We use 25kv AC for rail traction. I am not sure what the advantages/disadvantages might be in regard to DC.
    One problem that immediately comes to mind is electrolysis with DC. Could problems with any metalwork adjacent to the lines since rail generally uses earth return through the rails.
    Also I guess it is easier to convert 25KV AC on board to suitable voltage for on board appliances etc. I must do some more research on this interesting question.
     
  12. DNA Robotics

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    Jun 13, 2014
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    "The power system of the Long Island Railroad consists of three- phase alternating current transmission at 11,000 volts, 25 cycles. Rotary converter substations convert this A.C. power to 700 volt DC third rail power at. 13,000 amps. Because extremely high voltages exist in sub-stations, no entry should be made into the area."
    A friend of mine was a big shot electrical engineer and programmer for the Long Island Railroad. He said a big problem that they have is disconnecting that DC when trains are running. He said the arcing wouldn't quit and would melt the contacts, so they use hydraulic pressure to open the contacts and force dielectric oil between them to extinguish the arc.
    That is some serious electricity!
     
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  13. kubeek

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    In parts of europe they use 3kV DC.
     
  14. takao21203

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    Apr 28, 2012
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    And also much lower frequency. That needs larger transformers!
    At 30 kilovolts you dont have so much corona losses.

    Here in Ireland we actually use DC 1500 volts for the DART trains,
    and 700 VDC for the city trams (LUAS). Some 20km outside Dublin, electrification ends, or even before of that.

    the countryside isnt electrified at all.
     
  15. alfacliff

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    Dec 13, 2013
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    most railroad engines in the us use a diesel motor generator putting out dc for the traction motors. a few ac ones were made for special purposes, like supplying power for cities during blackouts. the newer engines still burn diesel, but go with ac now and use vfd's for the traction engines.
     
  16. ErnieM

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    Apr 24, 2011
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    My Dad worked for the LIRR till the mid 70's and I got to see the rotary converter stations before they were gone. Huge 50 by 150' brick strucures 3 stories tall, with a basement, all open inside with a mobile crane to lift the motor-gen sets in and out ad needed. Staffed 24/7/365 just in case something went wrong.

    The motor gen set was an open frame unit some 20 or so feet in diameter, half into the basement. The one sub station I am most familiar with was set up for 3 sets, though only 2 were installed. Along one side wall were circuit breakers 1 by 4 feet with huge arms to engage them. The place was LOUD and smelled of ozone and oil. Beautiful brick buildings from the time brickwork was as much an art as a trade.

    All sadly gone now, though a shell still exists here and there. Replaced by what look like trailers in gravel and all solid state works inside. All quiet, though the ozone is still there. I saw one of the first when it went into Hicksville.

    The Ronkonkoma branch, electrified in the 80’s I believe, use sub station buildings made of cinder blocks with no are to them beyond a pale pinkish color to the bricks. That was after my Dad’s time so we never got inside one.

    I still have his keys, doubt they still fit anything. I should donate them to a museum.

    The power goes to the track below the train on the “third rail,” literally another rail set off to the side & slightly above the two main rails. It is of lighter rail (running rails say 160 pound, third rail say 60) and the cars get power via an overrunning shoe collector. The third rail is backed either side by an additional metal piece, I believe for lower resistance (citation needed).

    Makes nice sparks at the gaps.
     
  17. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
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    Thanks folks!

    Does anyone know what is preventing the use of HVDC for railways if we had a magic wand and could convert all 25kV AC infrastructure instantly? Why wouldn't we perhaps?

    Kind regards,

    Aidan.
     
  18. kubeek

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    Most likely the price tag of such conversion.
     
  19. profbuxton

    Member

    Feb 21, 2014
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    What would be the advantages/disadvantages of HVDC compared to HVAC power for tail traction? I'm sure you could do a search and let us know.
     
  20. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
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    Remember we have a magic wand to do it instantly, is there anything inherent in HVDC and HVAC that causes one to be preferred over another?

    Kind regards,

    Aidan.
     
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