East Coast: Electric v Diesel

[kheggie]

Remember that when the 91+Mrk4 was designed, distributed power for High Speed was pretty rare, certainly in Europe. Also, the set as a whole was in part a revival of the ATP-S design. With tilting planned to be retrofitted to the carriages, the power car would have been non-tilting- avoiding the issue of having tilting pantographs.

it is very unlikely that anyone, anywhere, will design "loco hauled" (or seperate power car) high speed trains again. Even the Austrian Railjet was designed purely because of an excess of fast locos sitting around.

I got a Virgin Train from Euston to Birmingham and it was subject to diversion and it was loco hauled! I don't know why!

 

[TGV]

There just might be some truly knowledgeable people around here , but AFAIK, the 2x25 kV system uses the reverse polarity return wire (or rather, feeder wire, the term return wire as such is used in the older 25 kV system) merely to return the current to the feeder station.

The main catenary wire from the "power" station delivers the juice to the section between two transformers that the train is occupying at the moment (5 mi long or something like that), then the power is fed to the pantograph from two directions along the wire, front and behind. That being possible due to the juice passing through the train to the return rail and then divided 50/50 towards the front and behind, to the aforementioned transformers, partly to be again fed to the pantograph from two directions, partly to the feeder wire to be returned to the power station.

This all indeed does double the voltage delivered along the wire system due to reverse polarity, but due to the coil earthing at the power station, in effect the voltage at the pantograph remains the 25 kV. The advantage of using a doubled voltage system is that it allows the doubling of power feeding station distances as the current can be halved to deliver the same electric power (P=U*I).

So, AFAIK, as such no advantage to the power delivery if the power is kept at the same level due to halved current. Wiser men may elaborate.

Are you talking about the autotransformer system on HS1 and other LGV's?

In which case it's fed from a 50kV supply with what amounts to a centre-tapped transformer (providing 2 x 25kV) and these are fed via some switchgear and power electronics - one to the catenary, and the other to the return side of the OLE masts which is in the opposite phase to the traction supply. The rail, as in other such systems is the earth. So it's a 25kV, 0v, -25kV system in effect.

The pantograph and train itself still see 25kV as in any AC OLE setup in the UK. It has the advantage of lower current and losses in the main supply due to it being twice the voltage.

However I think the question here is does it improve the physical performace of the trains using it? No.

If you're not talking about autotransformer 25kV systems, I shall apologise and shut up.

 

[At_traction]

Are you talking about the autotransformer system on HS1 and other LGV's?

If the term means the 2x25 kV system, then yes.

However I think the question here is does it improve the physical performace of the trains using it? No.

The interesting question here is whether an increment in electric current (through closer spacing of feeder stations, if one does not want to increase the heat load in the wire or the wire's diameter to combat this) could add more "juice" to the main transformer and whether the traction motors can benefit from this at all.

 

[TGV]

If the term means the 2x25 kV system, then yes.



The interesting question here is whether an increment in electric current (through closer spacing of feeder stations, if one does not want to increase the heat load in the wire or the wire's diameter to combat this) could add more "juice" to the main transformer and whether the traction motors can benefit from this at all.

I don't think so. The traction motors are only going to see what are fed them by their own power systems - so depending on the train, it'll be a series of GTO's or IGBT's and inverters "downstream" from the transformer and other switchgear.

In many cases the line volts can drop surprisingly lower than the nominal 25kV and the train performs as normal - as low as 19kV before any system intervention. I've been in cabs where the supply has been indicated as ~21kV (the indicator has no accurate scale, but it's clear it was in this range) and there is no notable performance drop according to the driver - clearly though something will be getting a bit hotter in the system, but that is just increasing losses in order to provide the same demand from the train, and I suspect that is what would happen in the case you describe, increased current gives more transmission loss for a given power demand.