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Post by Deleted on Jun 13, 2007 23:40:19 GMT
Something that has been puzzling me this week is the traction supply current. If power is generated at 25KVAC 3-phase and transmitted at up to 400KVAC 3-phase how do we end-up with 630VDC? I would imagine that these days one would use a big fat 3-phase transformer and some sort of silicone rectifiers. However, this could not have been the case for the original electrification. Lastly, is the supply smoothed or just raw full-wave rectified? Any ideas anyone?
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Post by Deleted on Jun 14, 2007 1:39:33 GMT
I'm not very knowledgeable about high-voltage electrical plant, but I can say that mercury arc rectifiers were used up to the 1970s/1980s to modify the 3-phase AC current (of which one or two phases was used) into DC current. Somebody like Richard Catlow of uk.railway could tell you the most about it.
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Post by abe on Jun 14, 2007 7:29:44 GMT
I can't speak for the C&SLR (but I'm sure that another member can!), but the Yerkes tubes used rotary converters - basically big AC motors that drive DC generator sets. Hence 11KVAC goes in (via big cables from the power station), and each substation produces line voltage. Substations typically had two or three rotary converters (1.2KW and 1.5KW were typical sizes).
Mercury arc rectifiers were introduced in the late 1920s - Balham was one of the first installations on the Underground, and the Piccadilly extension to Cockfosters used them in place of rotary converters.
Solid-state silicon (not silicone!) rectifiers came in later (probably 1960s, but not able to check this at present). I presume that this technology has continued to evolve for the latest substations, but again, I'll need to check this.
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Post by Deleted on Jun 14, 2007 9:35:37 GMT
These days it's ridiculous. First we have to generate the AC electricity, then change it to DC to put into the juice rails, then turn it back to AC in the train motors, and then we turn it into DC again to put into the negative juice rail. Every step causes a loss of energy.
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Post by Deleted on Jun 14, 2007 9:53:06 GMT
The DC received by the train is across the two conductor rails: it has to be converted to AC only once.
The AC used on modern trains has a much higher frequency than the 50 Hz used for power supplies (which is why the trains "sing"). A 50 Hz supply to the trains would have to be converted to the higher frequency, so there would be no advantage over a DC supply.
The AC traction controls on modern trains are very much more efficient than the old resistor-based DC controls, giving a saving in energy use.
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Post by Deleted on Jun 14, 2007 11:30:05 GMT
Oops, perhaps it’s a new sort of self-adhesive rectifier!  abe do you mean 1.5KA? 1.5KW doesn’t sound anything like big enough. I assume that the rotary converters would have been replaced over time with MARs to improve reliability? I was surprised to learn (from googling) that MARs could be constructed to handle up to 2KA. I would assume that these have now all be removed and replaced with silicon based units…? From memory, (I’m sure I’ll be corrected!) the motors on modern stock are three phase, controlled by what amounts to a variable three phase inverter. As these inverters supply three phase at high frequency, a smoothed DC supply would be the ideal starting point. |
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Post by compsci on Jun 14, 2007 15:08:41 GMT
There is a metal trough Mercury Arc Rectifier next to the Morninton Crescent lift motors in Acton Depot, on the right as visitors enter (not that they will until next March). From memory you could fit 3 or 4 telephone boxes in it. You'd probably overlook it if you didn't have at least some idea of what it is.
I suspect that the box marked "Danger Mercury" next to the "escalator" contains a glass bulb type. I haven't felt a compelling need to open the box for some reason.
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Post by Dmitri on Jun 14, 2007 20:19:15 GMT
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Post by 100andthirty on Jun 14, 2007 20:46:15 GMT
And if you go far enough back to the early days, power was originally denerated as DC
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Post by Oracle on Jun 14, 2007 20:47:10 GMT
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Post by Deleted on Jun 14, 2007 23:47:00 GMT
Oooh  . I don’t think I’d like to find one buzzing away at the end of a dark, disused tunnel. Interesting article though. Just out of interest, how much current is available on each track section and how much does each train pull? According to TfL the whole system uses 1.091 TW/h (1.091 x 10^9 KW/hours)… that’s a big bill! |
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Post by mrfs42 on Jun 15, 2007 16:10:41 GMT
Oooh . I don’t think I’d like to find one buzzing away at the end of a dark, disused tunnel. Interesting article though. Until relatively recently the Manx Electric used these to rectify out the supply to the 600v overhead: I've seen footage of one rectifier fizzing, bubbling and going crazy as a full tram with trailer climbs the 1 in 28 Groudle Bank and then easing off as the trams reaches the crest. Apparently the practised substation attendants could tell how far away their tram home was according to a mixture of mercury bubble and the brightness of the bulbs in the substation. |
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Post by loughtonsiding on Jun 16, 2007 11:24:14 GMT
I can't speak for the C&SLR (but I'm sure that another member can!), but the Yerkes tubes used rotary converters - basically big AC motors that drive DC generator sets. What you are describing is a motor-generator set. That doesn't do justice to the rotary converter, which is actually a very elegant and ingenious bit of kit - one set of windings, with slip rings receiving AC, and a commutator delivering DC. There's a good description at: www.nycsubway.org/tech/power/rotary.htmlAs for mercury-arc rectifiers, they weren't just stationary plant - all the early LMR AC electric locos from classes AL1 to AL4 had mercury-arc rectifiers on board when originally built (later converted to solid-state). |
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Post by Deleted on Jun 17, 2007 20:01:58 GMT
This is all starting to make sense. Lots Road (at least in the early years) had generator sets with a 33 1/3 Hz output rather than the normal 50Hz. I would assume the lower frequency was chosen as they where using rotary converters in the substations (slower speed = longer life).
Moving MARs while in use sounds rather hazardous on two counts. Firstly, would the momentum of the mercury crack/shatter the glass if jolted? Secondly, surely the mercury sloshing around increases the risk of a phase-phase backfire across the input transformer?
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Post by loughtonsiding on Jun 18, 2007 0:59:25 GMT
Moving MARs while in use sounds rather hazardous on two counts. Firstly, would the momentum of the mercury crack/shatter the glass if jolted? Secondly, surely the mercury sloshing around increases the risk of a phase-phase backfire across the input transformer? They were steel tanks. Backfiring was a problem, and led to the conversion of all mercury-arc locos and EMUs to solid-state rectification. The problems that led to the temporary withdrawal of Lea Valley and Glasgow suburban EMU services were largely to do with the mercury rectifiers (exacerbated by issues with the 6.25/25kV changeover gear). The locos with multiple anode rectifiers (AL1 and AL2) had rather fewer problems than the AL3 and AL4 locos, which used single anode rectifiers. |
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. I don’t think I’d like to find one buzzing away at the end of a dark, disused tunnel. Interesting article though.
