Electrification: DC or AC?

Even with a 20/20 hindsight mirror, and 100 years later, that's debatable. The LSWR electrified its system much faster than the LBSCR. By the time of the grouping the wires had only reached Crystal Palace, but the live rails had got to Shepperton, Hampton Court and Claygate. Even by 1925 the wires had only reached Coulsdon and Sutton. At that rate, it is most unlikely electric trains would have reached the south coast by 1939, let alone 1932.

And given the glacial pace of electrification since 1948, it is quite likely that most of the former Southern Region would still be diesel hauled now - or, more likely, would have suffered the fate of most lines in the south that had not been electrified by the time the Doctor prescribed major surgery to the network.

It's worth noting that Network Rail's policy now is that when DC equipment comes round for renewal, where possible they will try to start converting, or at the very least doing the basework for conversion to AC OHLE. General feeling is that the BML would be the most sensible option to convert first, because before long, a large amount of the stock working it will be Dual Voltage. There is also a policy of no more third rail installations wherever possible, you can expect lines like Uckfield and Marshlink to be electrified at about the same time that the rest of the network is.

The LB&SCR was not entirely right they chose 6.6 kV AC supply.

The other problem with this system was that it came from Allgemeine Elektricitäts Gesellschaft of Berlin, which was a bit of a problem when the Government decided to go to war with Germany!  I believe this system requires fewer transformers, and so would be quicker and easier to install once a British company would be found to deliver the electrical equipment.

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Smaller transformers, maybe, but you would still need one on board, as 6,600V is still a lot to design traction equipment for. And in any case it needs to be rectified to dc first - even if you use modern variable-ac motors you need to remove the supply frequency.
A DC traction supply puts all the heavy and sensitive equipment on shore, so to speak, instead of lugging it around and shaking it about on board. The problem is that you can't send low voltage DC very far, so you need a lot more substations to cover an area. That is why DC is preferred for operations with a lot of trains in a small area, and ac for fewer trains spread over a wider area.



I can't see the deep tubes ever needing it, and space it as a premium thee anyway, but I don't think anyone in the UK other than LUL has not specified such passive provision since at least the Class 465s, which are now over twenty years old.

NF, the LNER realised that the short distance Shenfield electrification and widening would be better done with 1500v DC OHE than third rail, even though the LMS had electrified its suburban lines to Upminster without overhead. And of course, the LNE was also handing over the Loughton/Fairlop lines to LT DC. By the 30s the SR surely knew it was extending an already outmoded and less advanced system, especially on lines such as the Portsmouth direct.

It is not the voltage that is an issue (6.25KV AC was used by BR in the 60s as a way of minimising the amount of bridge works on some newly electrified lines), its frequency.

The German system of electrification uses a low frequency of 16.75Hz because it was discovered very early on that DC traction motors would work with this power source*. The LBSCR latched onto this because it made long distance AC electrification viable - while it was readily understood that AC transmission was far more efficient - the only way of converting it was large rotary converters (i.e. turning AC into mechanical movement then generating DC) which wouldn't fit into a train. Hence everyone else (most of whom were only interested in urban electrification to compete with trams) going for DC based schemes which required less expensive electrical equipment that could be sourced from the UK.

Yes by the 30s the Southern could have invested in 1500VDC Overheads - but this introduces more complications in having two separate systems to look after. Given the Southern was a private company with shareholders to answer too, introducing a totally new electrification system requiring trains fitted with equipment to cope with both, when the 3rd rail schemes had proved to be very cost effective to date was not going happen.

It took until the late 1950s and the introduction of Mercury Arc rectifiers before AC - DC conversion could be undertaken on the train itself.

As things stand the German system, while being AC is far from perfect due to its use of a non standard frequency - lines built in the past few decades are at 25KV AC @ 50Hz for this very reason, with most traction built in recent years being similarly equipped. However as with our DC network, the cost of conversion and resulting disruption outweighs the benifits

*AC traction motors are difficult beasts to control and they have only become a realistic option thanks to computer control in the past few decades.

Same principle and reasoning be it bridge or tunnel though. In the event later research allowed the clearances to be reduced - put it this way there is no way the Thameslink tunnels under London (even with their lowered trackbeds) would be considered suitable for 25KV in the early 1960s - yet by the early 80s such clearances were considered fine for 25KV.

Chingford/Enfield definitely was 6250v for about 20 years. Don't remember any bridges being raised on conversion. Some of the masts on Shenfield are still the 1949 ones. Big programme currently to replace.

Don't be misled by the ac traction package - everything from Networkers on have had ac traction motors, but the traction system still needs a dc supply - even if the external supply is ac you still need to convert from 50Hz to 0Hz (dc) and back again to variable ac.

It might be possible to convert the 455s - the 317s and 318s are very similar in most respects except for the presence of a pantograph and transformer.

The 465s appear to be very similar to the 365s, which certainly can work on both ac and dc.

Anyway - the 455s won't last for ever - they are already over 30 years old.

Maybe the way to convert 465s to dual voltage is to build extra coaches which have the pantographs and other control gear?

Simon

Likewise the 455s, a possible way of creating ten car trains in both cases (althoiugh SE want to go to twelve which is not so easy, and already makes the future of the five-car 376s uncertain). Any such augmentation would, of course, require extra power (maybe a third power bogie under one of the cars?), and result in units in which one car was very much newer than the rest. This is by no means unheard of on the Southern (indeed the class 455/7s already include one vehicle older than the rest, and the immediate post war period saw many original 3SUBs augmented with a post-war all-steel trailer - see also the Q stock, "58 trailers" and the 1960 stock for LU examples) , but it does leave a problem that the new cars will not have a long life before the rest of the unit is life expired.

All in all, the re-engineering is unlikely to cost much less than building new units - certainly when whole life costs are taken into account. And as SWT is rediscovering at the moment with its 458s, you can't re-engineer your existing fleet without taking it out of service first, whereas with a new fleet you can maintain the service with the old trains until the new ones are ready. This is a lesson they should have learned from the old Southern Region, which had the same problem when building the 442s, using equipment recovered from the REPs.

NF, you are probably right. I remember a BR manager talking about soot deposits (from generations of N7s and B17s) in those tunnels being an inch or so thick!

brigham
The thread opened with "discuss the merits of DC vs AC electrification". Certainly the supply to the traction motor type is a factor, although not as much as you might think except for the rather unusual systems initially used by the Brighton and the Midland.
Indeed, until rectifiers suitable for onboard use were available in the 1950s, delivery to the train was normal, and the main debate was between overhead (as used by the LNER) and live rail (as used by everyone, including the LNER on Tyneside).

As far as I am aware, 2 have been converted, and are currently doing mileage accumulation. None in service yet.