/rail/ - 1chan

1=Mono Chan= Chan
Password (post and file deletion)

File: 1550847620136.jpg (890.36 KB, 2272x1704, dscn2748.jpg)


So I've got a bit of a dumb question: why do urban rail systems still tend to stick to 600/750v DC rather than using AC power for transmission? Is stepping down power from 15/25kv to whatever voltage the traction motors require still take up too much space to make sense on something like an LRV or subway car?


File: 1550864631728.jpg (484.17 KB, 1161x800, 1054242.jpg)

I think this is rather an electrical safety issue rather than that of efficiency. Nobody wants multi-kilovolt naked wires hanging all over the city.
Also possible that many of the tram and even metro systems were founded in times when AC was nothing more than just an interesting concept (or at least no one knew how to efficiently harness it for use on vehicles, we don't count those wacky triple-wire setups), so it became kind of a standard.
Nowadays with modern inverters it's actually more reasonable to use DC power to avoid double AC-AC conversion.

I think there will be no picture more fitting than this one in this thread.


File: 1550865558601.jpg (695.16 KB, 1200x800, 1132838.jpg)

Another picture illustrating the dilemma of this thread (this one from Stariy Oskol, Russia). Actually this one might be even better.


I mean, would 25kv be any more dangerous than 750v DC? Either way, you touch one of those cables while grounded and you'll get fired. And I mean suburban "heavy gauge" networks tend to use high voltage and those still cross urban environments and don't seem to pose any more of a threat than lower voltage lines.

>Nowadays with modern inverters it's actually more reasonable to use DC power to avoid double AC-AC conversion.

That's the part I was most interested in. I've dabbled in electronics a bit (never anything this big of course) but I'm curious to know how much more involved an AC to AC converter would be involved compared to the DC to AC most modern vehicles use.

Loving those pics BTW!


File: 1550876418880.jpg (485.83 KB, 1250x800, 1023414.jpg)

What, you don't know what additional danger and problems do higher voltages pose? Do you see those giant insulators on the powerlines above? You know what insulators do, right? Effectively, they help create a safe zone around a wire, where there should be no grounded objects, and for 25 kV this zone is significant. Even cost and bulkiness of the insulators aside, there might be not enough space in the city with other infrastructure incredible tightly packed to fit this safe zone in.
And what about EM induction? 25 kV creates considerable currents on any metal surfaces within a few meters. I even experienced it myself, touching a handrail (which should've been even grounded BTW) within, like, three meters from the 25 kV AC contact wire. It hurts! So yeah, the pedestrians would be thrilled to walk on an old low bridge with 25 kv AC wires directly under it biting them with the induction. And all the sensitive equipment? Road radars/cameras, traffic lights, they all should have been well shielded. That's an urban planning havoc to think about it all even if everything goes right.

And if it does go wrong, say, a ripped wire (happens all the time on trams!) or a short circuit in a crowded space? Have you ever heard [s]of the tragedy of Darth Plagueis the Wise[/s] about the stray voltage?..

The railways always have (or at least should have) an exclusion zone around them, even in the city. What exclusion zone are we talking about when speaking about the tram, especially not the fully isolated LRT type?..

And then again, all this mess for the benefit of having less power substations in the urban area where you have power literally everywhere.
Sorry if that sounded a bit snobby, but I assume you get my point.


No that's entirely fair, I didn't realize the inductive effect with 25kv was so severe. Thanks for he insight.


Hmmm useing 600vac you could use a variac to controll speed. That seems simple.


File: 1551057053490.jpg (58.22 KB, 800x555, STES-AEG_Versuchstriebwage….jpg)

The transformers are pretty heavy, ABB's site says 2.5-4.5 tonnes for 1-2MVA, so for trams/subways etc where having substations every few km the weight saving and more simple equipment on the train is worth it.

The AC line to AC motor conversion in modern trains is big transformer + rectifier from line AC to a DC link bus usually ~700-1500V, then inverted to variable frequency variable voltage AC to the motors.

Capacitive coupling is more of an issue than inductive coupling. 25kV isn't *that* high to be a major concern, look at the top level of power poles in most cities and you'll see ~12-33kV distribution with no special protection around it. Most of the clearance area required is because the wire could move.
Snapped wire is arguably more dangerous on tram ~600-750V systems since with a 25kV system it's much easier to detect the fault and shut off since the "normal" train current is much lower.

The traditional variac with sliding contact doesn't really scale up for train size power level. Tap changing is similar but in discrete steps, used to be common on AC locomotives until power electronics to act like a big ass dimmer became practical in the 70s.

Have some 1900s German experimental 3 phase.


Incidentally, one Norwegian teen was killed and two severely wounded yesterday (2019-02-24) after touching something inside a stabling tunnel fed with 15kV AC.



I'm more surprised that there's not a single example of higher level of DC used anywhere that I know of, not even historically. Using say, 1500 V would allow 5 km substation spacing, which does seem more optimal than the 2-3 we get with 600 V.

>That's the part I was most interested in. I've dabbled in electronics a bit (never anything this big of course) but I'm curious to know how much more involved an AC to AC converter would be involved compared to the DC to AC most modern vehicles use.

Here's my 2cents:

In the past, before variable frequency drives became good enough, a "chopper" type of drive that limited the current by switching the power on and off in quick succession was used, I believe with both traditional commutated as well as brushless DC motors.

It seems that the superior longevity and efficiency of induction motor won in the end, after all, in a brushless DC motor there is also the feedback loop and semiconductors to replace the commutator.

(hum, maybe clearing 1chan cookies helped)


File: 1551175670654.jpg (71.42 KB, 718x405, MATA Melbourne Class W2 tr….JPG)

There are lots of mainline 1500 and 3000V (and a bit higher) systems, a few "tram-trains" that run higher outside the "tram" section and some sections of trains street running at >750, RhB in Switzerland for example has sections on streets running under 11kV 16.7Hz and 1kV DC. Doesn't look like it's ever been done for a proper "tram" system.
A few reasons I can think of are for mostly on street running there's a higher risk of a broken wire falling on something like a car/truck before hitting the ground and having a chance to trip the breaker, so keeping it low is safer there.
Short distance between substations isn't as much of a big deal in a city where you're likely to have a decent medium voltage supply nearby to hook up each substation, and it doesn't have to be as much power delivered per substation.
Trams are also much more of a commodity product than trains, you wouldn't want to be that one weird city that needs special trams for your different voltage. They also get sent around the world more, Melbourne's W class trams are in operation all around the world on heritage lines/museums, pic is one running in Memphis TN.


File: 1551175837437.jpg (130.45 KB, 1024x410, 32562985231_0cd55131cf_b.jpg)

Don't think chopper control with traditional brushless DC motors were ever used for rail. Induction motors need the feedback loops and semiconductor inverters too unless you're using the old school 3 phase electrification systems like >>6890 and don't care about shit efficiency except for at a few specific operating speeds.
The Frenchies had choppers + slow commutation with synchronous AC motors that sort of behave like brushless DC, but with a the field energised by AC through slip rings and brushes instead of permanent magnets. Pic related SYBIC locomotive, same system was used on some of the TGVs.
Permanent Magnet Synchronous Motors which are pretty much Brushless DC but optimised for sine waves instead of square waves are also getting popular in modern EMUs since they're generally lighter than induction motors, use almost the same variable frequency drives.


File: 1551204856565.jpg (132.23 KB, 2000x1189, shackcelerator-schematic-0….jpg)

I built a small chopper drive for a modle layout at the trolley museume. Managed to fit it into a very smsll project box and used a three position seitch for reverseing it works well and is tiney compaired to the origenal controller. Thinking of putting a capasetor bypass switch so the full voltage goes to the unit to over come resestence instead if avargeing out.


The Copenhagen S-trains use 1500 DC with average 4 kilometers between substations.


File: 1551410322974.jpg (492.87 KB, 1250x834, 1157969.jpg)

And on trams you could use (and they do) a rheostat which is even simpler. It's not quite the normal rheostat, it has positions on it, and on more heavy-duty drives you still need to use full-on mechanical group switch, but don't forget that for AC you would still need additionally a rectifier unit, so overall this would be at least bulkier and heavier.

For modern PWM drives the DC-input drive is still much simpler and more efficient as it has only single conversion, from DC to 3-phase (or from 3-phase to DC upon e-braking). The only downside is that to use regenerative braking efficiently you need high-output inverters on your substations but arguably it is simpler to have a few inverters on substations than dozens if not hundreds of smaller ones (being rectifier-inverter units, really) on the vehicle, in case of AC traction.

>I'm curious to know how much more involved an AC to AC converter would be

This was an answer to your question (if I understood it right) too. To feed 3-phase motors adequately you need a DC input for your inverters, the one which is created by a rectifier, which works as a single-phase inverter when upon regenerative braking. Which is, naturally, more heat losses (though unclear would it be compensated by the lack of a rectifier unit on the substation) and more complexity and points of failure in the vehicle.

>Snapped wire is arguably more dangerous on tram ~600-750V

In urban environment you have pedestrians, drivers of other trams who will surely notice the snapped wire. BUT when 600 V hit the ground, no one cares, yet if there will be people nearby when 25 kV wire touches the ground, people will get hurt.

Another glorious piece of Russian tram engineering. Sorry, have a soft spot for those Vityaz-Ms.


Does regenerative braking on trams actually bother feeding power back over the wire? I thought most trams with it stored the power in large capacitors or batteries.


The treshold of not caring goes indeed somewhere around 1kV be the general public, 20kV by the electrical engineers and not even then the funky sources of losses or long arch through distances play a significant role. The arch distance is like only few centimeters for 20kV, and in enclosed space and with proper spacers one can work as close as some 20 odd centimeters (22cm?) from exposed 20kV. As you said, most of the safety enclosure of open lines is due cable swing.

>Does regenerative braking on trams actually bother feeding power back over the wire?
Artic doesn't. It has reservation for supercap.

Things got more complicated with semiconductors, I understand the old DC trams actually dumped energy back to network when coasting down.


File: 1551495121830.jpg (489.48 KB, 1200x800, 1061654.jpg)

>I thought most trams with it stored the power in large capacitors or batteries.

Never heard of this. I'd say that, save for some specific cases, no one would bother with it. Though I can see if many tram systems won't be bothered with the whole regen concept either (despite it offers HUGE savings!), that's why most modern trams and in general DC-input electric vehicles still have braking resistors on them, in case regen is not available. Those grey boxes on pic related are resistor aerodynamic enclosures.

I know for sure some metro systems do bother with proper regenerative braking. Like, Moscow metro calculated that after the modernization of power infrastructure on the most busy lines the power savings on traction reached 30% thanks to regenerative braking!

>As you said, most of the safety enclosure of open lines is due cable swing.

You also have rain or fog which can greatly increase air conductivity. BTW this is why the high voltage insulators have this weird christmas tree shape, to not let water flow directly onto the wire.

Also the danger of snapped wire is not about the arch but about the stray voltage. Like, there is a reason why in worker's safety manual (on Russian railway at least) there is much greater safe standing distance for the electrified lines over diesel ones.

[Return][Go to top] [Post a Reply]
Delete Post [ ]