600V DC ~ 750V DC Interoperability

[Giugiaro]

Good morning everyone!

Does anyone know to which extent can a 600V DC tram operate on a 750V DC system, and vice-versa?

My vague understanding from hearsay is that, due to being DC, a unmodified higher voltage train on a lower voltage track will be under powered, but will still work.
No word if the opposite would be any good.

I'm asking because the Porto Vintage Tram and the Metro have the same gauge but different voltages.
It was thought in the embryonic stage of the Metro that the two systems would be connected, and some vehicles would run on both networks.
But, so far, nothing has been done to link the two systems physically.

Despite this, some pressure is mounting to have both networks connected.
Not just because future expansion plans will bring them to come extremely close to each other at same level...
But also because having both networks connected means that any vintage or modern tram can have its major maintenance at Guifões, without needing full disassembly and shipping to the workshops by road.

 

[plugwash]

I would imagine it would depend on what level of safety margins the designers of the 600V tram left in their designs.

 

[edwin_m]

There's a fairly wide tolerance on the voltage on DC systems, because it fluctuates according to demand, distance from substation and whether something is using regenerative braking. So if a 750V tram went onto a line at 600V it would probably still work. However, by the same token the nominal 600V could actually be quite a bit lower at times. A traditional tram probably doesn't know the difference and would most likely keep going, but a more modern one might well shut down due to detecting the voltage being outside the limits it's designed to work with. It would be necessary to compare the worst-case voltage drop on the 600V system and the design parameters of the 750V tram

Another important question is if the new tram might start using regenerative braking and whether this would push the voltage above what is safe for the old trams or indeed for the substations or anything else on the system. If I recall correctly, 750V systems can go above 900V during regeneration. It's very unlikely that a historic tram would be allowed to be exposed to higher voltages than it has traditionally encountered, as this might lead to arcing and fires.

 

[plugwash]

The Blackpool tramway was apparently raised from 550V to 600V before the new trams came in. Presumablly it was decided that this was within acceptable limits for the historic fleet.

It's not exactly a tramway, but wikipedia claims (unfortunately without a source) that the Watford DC line (which is shared by london Overground and London Underground) used to run at a compromise voltage of 660V, but was raised to 750V when the new aventra trains were brought in.

 

[Mothball]

The excess power would have to be used up, usually burned off as heat via resistance banks, we have had completely hypothetical talks many times about using our 600V vintage trams on the neighbouring 750V system and we always came to the conclusion it would just require additional banks.
Its the same principle with modern trams, with regenerative braking overhead line voltage can be saturated well above the 750V, excess power is then burn off on board as heat.

 

[100andthirty]

I'd imagine that there would be many more problems than just the voltage. The historic Porto network has some incredibly steep gradients and I'd wonder whether the Metro style trams would cope with them.

 

[pdeaves]

I suppose a technological solution could be developed with some new build vehicle specifically for through running. Essentially, a dual voltage unit designed to operate off both (nominal) voltages.

 

[100andthirty]

DC traction supply isn't like 240V mains voltage. a DC voltage tolerance of +/- 25% is allowed from the nominal. This means that for 600V, the minimum is 450V and the maximum is 750V. I'm making this point to show that 600V to 750V is not all that big a deal. I'm confident that a 750V rated vehicle would work happily on 600V. But, as others have said, you probably wouldn't want to run a heritage 600V on the 750V system all day every day, especially if the latter allows regenerative braking with a peak voltage of circa 940V. That said, the 630V rated LU C stock didn't suffer when operating on the sections fed by the Network Rail 750V system even after regenerative braking was enabled circa 15 years ago.

 

[Giugiaro]

Well, as long as we are concerned the heritage trams are probably going to be shunted through the Metro network, only so that they can hypothetically go from Matosinhos Sul to the Guifões Workshops.
The other way around, the Eurotrams and Flexity Outlooks are quite wide, and won't fit anywhere else outside of Route 1.

Now, if Route 1 is extended from Matosinhos Sul to the new Intermodal Terminal at Campanhã (through the old Alfândega Branch Line) Eurotrams could, in the future, be deployed as fast trams to replace/reinforce the 500 Bus Line.

Now, this poses a problem, where we will have modern 750V trams running on the same line as the old 600V trams (and possibly also on part of the Metro Network).
What should give in? Should the Eurotram cope with running under 600V DC and regenerate to batteries/capacitors? Or should heritage trams be adapted to run under 750V DC?

 

[Taunton]

It's not only the voltage; although the gauge is nominally the same the profile of wheels is normally very different between trams and trains, and points on tramway "sleeper track" sections are different to those on railways. Just compare tramway grooved street rail with how rail lines cross roads at level crossings.

 

[edwin_m]

It's not only the voltage; although the gauge is nominally the same the profile of wheels is normally very different between trams and trains, and points on tramway "sleeper track" sections are different to those on railways. Just compare tramway grooved street rail with how rail lines cross roads at level crossings.

This is true, and at least two UK tramways fell into the trap (no pun intended) of buying off-street pointwork with railway-style switch blades, only to find the smaller flanges of tram wheels would derail.

However the Porto Metro, despite its name, uses trams and has some sections shared with at least pedestrians and cyclists, so I would guess it has tram-type wheels and rails. This may still mean it is incompatible, as there are various different wheel and rail profiles even for tramways.

 

[Giugiaro]

For the Eurotram, it should be fine.
It coped with 600V in Melbourne and Line 1 is pretty much smooth.

Source of the image

 

[edwin_m]

For the Eurotram, it should be fine.
It coped with 600V in Melbourne and Line 1 is pretty much smooth.

Source of the image

However, it may be configured differently when operating on a 750V system, for example to regenerate up to a higher voltage. It probably isn't possible for it to detect the lower line voltage and change its behaviour to suit (especially as it may occasionally see 600V on a nominally 750V section).

 

[WatcherZero]

The optimum DC voltage to operate off is primarily determined by the motors. But its usually the modern electronics that determine the operating ranges as they are far more sensitive to input variation than the old analog equipment and can quite easily be damaged by spikes or suffer a brown out corrupting data and requiring a computer restart.

 

[Gerald Fiennes]

The optimum DC voltage to operate off is primarily determined by the motors. But its usually the modern electronics that determine the operating ranges as they are far more sensitive to input variation than the old analog equipment and can quite easily be damaged by spikes or suffer a brown out corrupting data and requiring a computer restart.

Yes, WatcherZero has nailed it. it's not the traction voltage that's the issue but the control equipment. If it's all electro-mechanical, then almost certainly fine but if it's electronic no dice.

The control equipment has to be protected from out of range voltage/frequency/current.... remember what happened when the C700s met a national frequency reduction (of 0.5Hz, I think it was) following a massive grid outage at Hornsea a few years back. The train's controls shut the trains altogether unless the vehicles were lucky enough to have a software upgrade installed to make them slightly less sensitive and the poor Siemens fitters had to travel round in vans to find the stranded trains all over the TSGN network....

Incidentally, the SWML operated at 660v around Wimbledon for many years (the East Putney line being electrically connected, of course...) and I believe that it's only recently been upped to 750V (plus tolerances for regen) to give the SWT fleet a bit more oomph.