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Regenerative braking on Locomotives with AC transmission

  1. Mar 6, 2015 #1

    rollingstein

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    Many modern electric train locomotives have the option of regenerative braking. Most of these locos have HVAC power transmission.

    I'm wondering, when they brake in regenerative mode do they actually generate DC (I'm not sure whether modern electric locos prefer DC motors or AC motors or is the choice still open), then invert it to the right AC frequency, then match the frequency / phase on the fly precisely to the grid & then transmit power back?

    Sounds like a lot of work especially if you factor in the large power loads involved e.g. a typical electric locomotive can easily exceed 8000 hP though I'm not sure how large the max regenerative braking load can be. Anyone know?
     
    Last edited: Mar 6, 2015
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  3. Mar 6, 2015 #2

    Averagesupernova

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    I would think some creative google searches could turn up some info. Your suggestion is something I hadn't thought of. Last I knew all the diesel electrics dump the power generated by braking.
     
  4. Mar 6, 2015 #3

    rollingstein

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    Well, the diesels have no choice than to dump power as resistive heating because there's no transmission line they can dump to.

    I think that's the difference between dynamic braking (dump your power as heat to a heavy duty, fan cooled, resistor grid on board the loco) versus regenerative braking (ship power to the grid).

    Even among electric locos I think some use only dynamic braking but I guess that's more wasteful.
     
  5. Mar 6, 2015 #4

    rollingstein

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    Just as a concrete example:

    One of the large electric locos used in China is the HXD2B with a tractive power of ~10 MW & a max allowable regenerative braking load of almost the same amount. (Source Wikipedia)

    This loco seems to have AC motors.
     
  6. Mar 6, 2015 #5
    AC motors, in regenerative breaking, generate AC power (async. generator mode) and feed the network.
     
  7. Mar 6, 2015 #6

    rollingstein

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    Thanks. Would this AC power be always of a fixed frequency irrespective of the speed of rotation of the decelerating, braking wheels?

    I think not. If so how do they get it up to the grid frequency? Do they rectify this variable freq. AC & then re-invert it using a static inverter to the desired grid freq.?

    Also, they must have to match phase too, using separate circuitry?
     
  8. Mar 6, 2015 #7
    AC→DC→AC conversion systems are most frequently used today.
     
  9. Mar 6, 2015 #8

    jim hardy

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    They know wheel speed and apply AC at frequency to make desired torque, be it positive or negative.
    http://www.republiclocomotive.com/ac_traction_vs_dc_traction.html

     
  10. Mar 6, 2015 #9

    rollingstein

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    Thanks for clarifying that.

    I was earlier confused why an AC motor loco using transmitted AC power had in its specs a rectifier & inverter system.

    The AC to DC to AC bit explains that I guess.
     
  11. Mar 6, 2015 #10

    rollingstein

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    Very interesting article, thanks.

    But that explains traction & dynamic braking, right? My core confusion is regarding regenerative braking.

    In dynamic braking what frequency / voltage / phase you generate at doesn't matter much because you are burning away the power in a local resistor grid. But not so in regenerative. There if you must feed into the grid all three (frequency / voltage / phase) will have to be precisely matched.
     
  12. Mar 7, 2015 #11

    jim hardy

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    Diesel electrics aren't connected to the grid .. at least the tracks near my house have no wires overhead....

    I dont know about those fully electric trains with overhead wires, i suppose surely they would return power to the source.
    But that's just a guess.

    An AC induction motor rotating faster than the source supplying it will return power to that source. That's called "negative slip", and is a handy term for electric motor guys..
    but the locomotives i watch go by here are at idle when coasting downhill. I think you'd want to not risk overspeeding the engine when coming down out of the mountains, so dump the energy as heat . There's noplace else to put it for they cant turn it back into diesel oil...
    The key is with electronics they can create whatever frequency they need, per that article slightly above or below wheel speed so as to modulate power flow into or out of wheels by making either slip positive or negative.

    I guess if "regenerative" is defined as making useful work out of the energy that's been re-generated by the traction motors, then it's not "regeneration".
    Eternal Vigilance is the price of precise vocabulary, eh?

    Interesting discussion. Thanks for letting me play !

    old jim
     
  13. Mar 7, 2015 #12

    rollingstein

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    Indeed. Diesel electrics cannot regeneratively brake. They can only dynamically brake i.e. burn up the EMF generated as heat from a resistor grid. (unless the diesel electric was pulling a Passenger car load and used the EMF for hotel power; not sure. ) Yes, regeneratively means using the power generated for something useful. Otherwise it is dynamic braking.

    The question I had was in the context of the true electrics. The one's pulling power from a panto-graph and overhead centenary.

    Interestingly the most powerful locomotives today are almost all true electric and not diesel electric. (I think)
     
  14. Mar 9, 2015 #13
    True Re-gen needs somewhere to put the energy - and in trains it is a lot. So this is in Electric Trains connected to AC or DC supply.

    The technology is pretty common - you can also look up regenerative AC motor drives. These do require an inverter on the supply side - and during re-gen that inverter has to synchronize with the supply to be able to "push" power ( energy) back to the source.

    The secondary advantage is that this configuration usually runs the inverter as an ACTIVE RECTIFIER when it is converting the AC to DC - this makes for a much improved current waveform ( better Power factor and harmonics) than a passive rectifier with just diodes. In large system this actually improves system capacity because the supply would normally have to support the bad PF ( reactive current) or Harmonics.

    The Basic Topo will look like this in IGBT types
    SEMIKRON_sks-b2-120-gdd-6911-a11-ma-pb-08800589_circuit.jpg
     
  15. Mar 9, 2015 #14

    rollingstein

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    Thanks @Windadct!

    Now we are getting somewhere! Thanks for the details.

    So, in the diagram you provided I understand the topology for the AC -to DC conversion. i.e. rectification. But following this DC link there is an inverter right? Generating a variable freq. AC to drive the motors?

    Now in regen do all those blocks get used in reverse? That's the part I'm unclear about. i.e. While doing traction the grid automatically gives you a stable freq. and Voltage whereas in regen the circuitry to match phase, freq. and V will have to be explicitly provided, right?
     
  16. Mar 10, 2015 #15
    I think you have it - the output of this system is variable speed (to the drive motor) - it should be clear cut what is going on, however for the regen - think of a (sine) wave, you fire the IGBTs to just lead the utility 60 Hz - just making 60 Hz is not really ideal ( here in the NE of the USA there is Amtrac at 25 Hz! - must have been a steel barron that made that decision!) . Anyway - you want to be a little ahead of the curve - you then are providing reactive current and power.

    So in Regen - you want higher V and very slightly leading PF ( typically!!! ) -Higher V is actually easy - since the Anti-parallel diodes in the inverter are now just rectifiers - the DC link voltage can get quite high as it absorbs energy from the motor
     
  17. Mar 10, 2015 #16

    rollingstein

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    Thanks!

    V & freq. we took care of. But what about phase? Isn't that critical too? There ought to be no reason why the regen voltage will fundamentally by in sync. with the transmission line, correct? You'd need circuitry to explicitly do that phase matching business, right?

    In the whole rectifier-inverter AC drive system normally phase matching isn't a concern at all, is it? But for regen it should be?
     
  18. Mar 10, 2015 #17
    Huh? ... Certainly? - If I get your post correctly, yes, on the Grid / source connection side you have to match the phase for sure, my point being that the grids V is essentially fixed - to push power back onto the grid you need higher V and generally a slightly leading PF ( once you are synchronized)
     
  19. Mar 10, 2015 #18

    rollingstein

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    No,. sorry. My bad. I should have been clearer.

    I agree with what you wrote. I am only asking if due to regenerative braking you must add special circuitry to the locomotive to perform this extra task of phase matching.

    Normally for traction nor dynamic braking you'd never need this phase matching circuitry (I'm thinking). If so, I'm wondering how does that system look like. With IGBTs / thyristors etc. how does one design a phase matching circuit.
     
  20. Mar 10, 2015 #19

    anorlunda

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    Whoa whoa, you are all ignoring what Jim Hardy said about induction motors (see the quote below) No fancy inverters, or frequency or phase matching needed. But you do need railways with electric power lines. They are quite common in Europe and they have been inherently regenerative for a long time.

     
  21. Mar 11, 2015 #20

    rollingstein

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    Interesting.

    Well, forget regen but with no inverters how do these AC induction motors achieve speed control during traction duty? Can you describe the system in more detail?
     
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