Multi-Voltage Trains

[ryan125hst]

How do electric locomotives/multiple units that except multiple voltages work?

I know that, in the UK, DC trains used to use resistance control from 750v DC, with trains that run on 25kv AC stepping the power down using a transformer and rectifying it to DC before the resistors. I also know that modern trains use inverters to power three phase motors.

I know that dual voltage trains in the UK (750v DC and 25kv AC) will step down/rectify the current to 750v DC and then send it to the DC bus that is used when the train is on DC. is this correct?

I wondered how some trains in Europe, such as the TGV, can operate on four voltages: 1.5kv DC, 3kv DC, 15kv AC 16 2/3 hz and 25kv AC 50 hz. AC power can be transformed to a lower voltage and the different frequency problem will be solved with the rectifier. What about the DC voltages? Can the DC bus be at different voltages and the equipment designed to work at any of these, or are they converted to a single voltage? I have seen on the Velaro specifications pages (see Wikipedia) a mention of inductors. What do they do? Are they something to do with this problem?

 

[Peter Mugridge]

I think they have different transformers for each voltage system, and they do a pantograph down / pantograph up at the changeover points.

Some of the newer ones, I think, have different connections from a single pantograph with the connection being changed, but I'm not sure on that.

 

[ryan125hst]

You are right about transformers for different AC voltages, e.g. trains that run on both 25kv 50 hz and 15kv 16 2/3 hz and about the changeover points. The fact that the current then goes through a rectifier solves the problem of the different AC frequencies as the rectifier turns the current into DC. It is then sent to the traction inverter to be turned into three phase AC power and sent to the motors.

It is the DC switching that I am confused about. Some trains can run on more than one DC voltage. For example, as well as 25kv AC, the Eurostar can run on 3kv DC, 1.5kv DC, and before the shoes were removed, 750 DC third rail in the UK. What voltage is the current entering the traction converter and auxiliary converters at?

This page: http://www.trainweb.org/tgvpages/motrice.html shows a TGV atlantique power car. The DC voltage here seems to be 1500v DC, but this is the only DC voltage it can run on.

Are the DC voltages inverted to AC and then stepped up or down to a common voltage before being rectified and inverted again? Surely, this would be too expensive and complicated. Can the traction inverters accept all three DC voltages, with the AC power being stepped down to one of them? What do inductors do?

 

[sdx]

(...)they do a pantograph down / pantograph up at the changeover points.

(...)

On most of "multi-current" engines, all the pantographs are connected together (a matter of electric safety, plus two pantographs use in some specific conditions : starting with DC supply, ice conditions...), and the choice of the pantograph, although related to the characteristics of the electric supply is done regarding the mechanical characteristics of the overhead line.

Some pantographs are wider than others : South of France DC lines (ex "midi" railways) have a longer spacing between posts, and require a wider pantograph, while some networks require vertical pressure to remain wthin certain limits, some DC networks, with a lower voltage, thus a higher intensity, require pantographs with two elements, but some don't. As far as I remember, the old 4 current CC 40100 didn't use the same panto beneath French DC (1500V) and beneath Dutch DC... (1500V)!

There usually is an electrical "sensor" wich prevents from dangerous connections beneath the panto: avoiding sending DC in the primary circuit of the HT transformer for instance...

So there are actually two different operations at the border between two networks... checking the current (type and voltage) and checking the overhead line characteristics, in order to chose the relevant panto.

 

[TGV]

Let's take a multi-current TGV as an example. The 25kV operation is as you would normally expect. The AC pantograph (a Faiveley GPU unit) is raised, and a system known as "palpage" detects if the correct voltage is in fact present as selected before the VCB is closed. Once closed, the main transformer is energised, and steps the 25kV down, before sending it to be rectified to DC. At this point Aux Inverters take what they need for hotel power, and the inverters in the motor blocks take what they need for traction. That is converted into 3 phase AC, fed to VVVF controllers (Variable Voltage, Variable Frequency) and sent to the traction motors to make it move.

When the system is configured for DC running, (it could be 1.5kV or 3kV - it makes only subtle differences in internal configuration), the DC input totally bypasses the transformer as you'd expect and is fed directly in via a different main breaker before being smoothed and then the aux inverters and traction inverters convert what they need respectively. So from there on its as the AC system. Again, though there is monitoring that the voltage present at the pantograph is what is selected by the system. If not, you won't be going anywhere unless you have an emergency mode set. In such a case, you can use the DC pan on the AC wire at reduced speed, but you can't use the AC pan on the DC wire due to excessive current (the DC pans have much heavier duty carbons).

In the case of A TGV Atlantique or Thalys, or Eurostar for that matter, both AC and DC pantographs are physically and permenantly linked at the same potential. There is another contactor on the roof that moves depending on AC or DC operation being set.

I hope that answers some of your questions.

 

[LE Greys]

Weren't there sections of the GEML wired to 6.25 kV? If so, how did they cope with the voltage change on one pantograph?

 

[Nym]

They didn't it changed operation in one go.

Anyway, looking at some of the questions raised...

The AC Traction Motors nowerdays are syncronous machines or induction machines and require to be run at a synconous speed or slip, this means to change the speed of the machine, either the poles need to be switched and/or frequencys changed and/or rotor currents changed (although this will only change torque).

This is generally done by use of inductors and tyistors (in older systems), or posh transistors (in newer systems). Usually working from a rectified DC source with a large series inductance (to reduce ripple for a clean signal).

Adjusting the firing angles and rates, and changing stator and rotor currents is used to alter the speed of an 'AC Motor', it doesn't run directly from the source, the large inductance also means that passing over any neutral zones doesn't effect the systems as much.

I really should be able to answer more on this since I just sat an exam on electric machines...