Electrification
The story behind BR’s landmark decision to adopt 25kV AC 50 cycles as the standard for overhead line eletrification.
Railway electrification in Britain has a diverse history. Its first use dates from the late 19th century and there were a number of applications during the first two decades of the 20th century.
Opened in 1893, the Liverpool Overhead Railway (the so-called dockers’ umbrella) used electric traction from the start, while the Mersey Railway between Liverpool and Birkenhead converted from steam to electric in 1903.
Prior to railway grouping in 1923, several companies tried third or fourth rail direct current (DC) schemes, while there were several experiments with both alternating current (AC) and DC overhead line systems.
Indeed, at the time there was no national power grid or standardisation among electricity producers of voltage or frequency. This restricted electrification schemes to what a single power station could deliver in an economic radius at a voltage and frequency suitable for conversion for use in variable speed traction motors on a train.
After the Grouping, the Southern Railway adopted third rail DC traction power supply at 660-750V DC.
In 1926, 50 cycles (hertz) frequency became the national standard for electricity production. Following this, the Government’s Weir Committee chose 1,500V DC via an overhead line for new railway electrification projects, because it was better suited to heavier train haulage than that used on the Southern Railway.
After nationalisation and the creation of the British Transport Commission (BTC), with British Railways as its largest constituent, the BTC set up the Railway Electrification Committee. The Committee met in 1951 and reaffirmed the Weir Committee standard of 1,500V DC.
Prior to the Second World War, the London & North Eastern Railway had authorised electrification of its routes between Manchester-Sheffield Victoria/ Wath and Liverpool Street-Shenfield. Both schemes were completed post-nationalisation in 1954 and 1949 respectively, using the national standard.
The Railway Electrification Committee did, however, propose the trial of a 50 hertz system, and the former Midland Railway lines between Lancaster, Morecambe and Heysham were chosen. These had been electrified by the Midland Railway in 1908 at 6,600V AC 25 hertz, using a supply from the MR’s power station at Heysham, and so the overhead line equipment was therefore already in place.
The first three-car electric multiple unit started running in August 1953. And while there were some problems with interference to telecommunications, overall the experiment was judged to be a success.
With optimism for the future, BR’s 1953 annual report commented that the BTC’s capital asset needs had been clearly assessed and that several important lines of development were in hand.
Electrification schemes at various stages of consideration included Liverpool Street to Enfield and Chingford, King’s Cross outer suburban, and reactivation of the Southern Railway’s 1946 plans for all routes east of a line drawn between Reading and Portsmouth. Since the 1953 report was written during 1954, the notes of optimism for the future are interesting.
The Modernisation Plan prepared during 1954 and announced in 1955 included £120 million for main line electrification and £ 65m for new and already authorised but not yet announced suburban schemes. The May 1955 announcement of electrification projects included the lines in Essex and the West Coast route from Euston to the West Midlands and the North West by the overhead system, and Kent by the third rail.
At the time, there was disappointment in some quarters at the apparent slow progress with these projects. Part of the reason was that the BTC’s Technical Committee was wrestling with whether to continue with 1,500V DC or whether to emulate developments in France, where the Valenciennes to Thionville route had been wired at 25kV AC.
The Technical Committee first considered the topic on March 11 1954, when SNCF progress was discussed, and it was decided to carry out a technical and engineering appraisal for a typical main line.
On May 13, both the Eastern and London Midland Regions were selected for a cost comparison between 25kV and 1,500V DC, and it was felt that more experimental rolling stock was required.
The implications of 50 hertz power supply were set out in a report to the Technical Committee in September 1954, and this was followed up in January 1955 in a memo saying that investigations were to continue into several technical issues. French Railways was consulted because of the significant progress it had made in this field.
The BTC’s Chief Electrical Engineer S B Warder continued evaluating options, and he reported to the Technical Committee on September 29 1955. His report took account of practice on the continent, where DC overhead line electrification was much more extensive and where there was no logic to changing existing equipment until it became life-expired. Countries that had commenced new schemes had opted for 50 hertz, even if they already had routes which had been electrified at 1,500V DC.
Warder argued that technology and the cost of equipment had moved on from the factors that influenced the Railway Electrification Committee to continue with 1,500V DC, not least because it was now much less attractive economically.
For example, 50 hertz offered a 32% saving in copper and 83% in steel for overhead line structures, because the overhead line could be half as large and this reduced the size of the supports. Warder estimated that an AC locomotive would cost 10% less than its DC equivalent.
Voltage drop between the source and final application of power meant the lower-voltage DC approach needed substations at more frequent intervals, and therefore more highvoltage cables to the substations. On the
Manchester- Sheffield/ Wath system, the interval between substations was every 6.2 miles compared with an estimated 250 miles for 25kV supply.
High-voltage AC power could now be delivered direct to the traction unit, reducing the number of substations. This meant that one set of cables could send power to the train, instead of those from the national grid to the substation and another set (the catenary) from there to the train.
When it came to traction, however, technology had not advanced sufficiently for the use of AC traction motors. In any event, DC motors had evolved to the point where they were relatively lightweight and very rugged, with a traction characteristic wellsuited to rail applications.
The 25kV supplied to the traction unit had to be reduced to around 1,000-1,800V for the traction motors, and this was done by a transformer. The driver selected the power he needed from the transformer, and this was then rectified from AC to DC for the traction motors. This meant that each motor received an independent supply from the transformer (that is, the motors were connected in parallel).
Where 1,500V DC was drawn from the catenary, it passed through resistances in order to control the supply to the motors, and so energy was lost in the form of heat in the resistances. Only when all the resistances had been cut out was the highest economy in running achieved. Additionally, driver control was judged by footplate crew to be easier for AC traction.
Moreover, with DC supply transmission to the motors was sequential (that is, from motor to motor) or in series, and this offered an inferior traction characteristic to connection in parallel.
Demonstrating the difference in traction characteristic between the two systems, in France, using AC power supply and DC traction motors, a 78-ton locomotive had restarted a 2,400-ton train on a 1-in-100 gradient, while two other locomotives had achieved a world record speed of 200mph. For routine service, French Railways permitted a 970-ton load limit to its DC line supply electrics and 1,400 tons to equivalent AC ones.
Counterbalancing these factors, the much higher voltage required significantly larger clearances under overhead structures ( bridges, tunnels and station canopies), and rebuilding these therefore added to the cost.
To minimise this, Warder put forward the option of a dual-voltage arrangement whereby 25kV would be used generally, but with 6.25kV (a quarter of 25kV) used in (for example) the London inner suburbs in order to avoid rebuilding structures. Some of the new Eastern Region schemes had this dual-voltage approach, but research subsequently permitted a smaller clearance and the lower voltage was abandoned.
On the West Coast route from Euston, freight train load limits for a single locomotive were estimated to be 900 tons for DC but 1,250 tons for AC. A maximum axle-load of 20 tons and maximum tractive effort of 60,000 pounds were laid down, and the power output needed for express passenger work was predicted to be around 3,500hp.
It was estimated that for services between Euston, Birmingham, Liverpool and Manchester, a total of 150 passenger and 510 mixed-traffic AC electrics would be needed at a total cost of £ 38.3m. For a DC system the number of mixed-traffic machines was given as 570, but each locomotive cost less and so the overall figure was £ 36.4m. A larger number of DC machines were needed to handle the maximum anticipated trailing loads.
In advance of Warder’s report, cost estimates for the proposed West Coast scheme out of Euston were produced, based on both AC and DC overhead supply. The figure of £118m represented a saving of £ 6m for the former.
Interference with telecommunications and colour-light signalling was also an issue that,
unexpectedly, was found in some places to require complete replacement of existing colour-light equipment. In conclusion, Warder favoured AC supply because of its superiority in the areas of power supply, fixed equipment and rolling stock.
The Technical Committee accepted Warder’s report and, together with that containing cost estimates for the West Coast, it was reviewed by the BTC on October 19 1955. The BTC deferred its agreement until the 27th, when the chairmen of the Area Boards and their Regional Managers were consulted. Future Southern Region schemes would, however, continue with the third rail system.
The Technical Committee was asked to investigate the implications for converting the Eastern Region’s London suburban schemes that had already been authorised at 1,500V DC to 50 hertz. It reported that the lines from Shenfield to Chelmsford and Southend would be electrified as authorised, but converted to the new standard later.
The national standard for main line electrification was a government-directed matter, and accordingly the Commission informed the Ministry of Transport (MoT) of its intention to change to 25kV as the new standard.
A note dated December 3 1955 in the MoT files observes that the creation of the BTC had removed the need for government to opine on this matter. On May 31 1956, the Commission recorded the Minister’s affirmation of the decision.
The West Coast route from London to the North West was chosen to be the first main line for 50 hertz electrification. At the time, the Eastern Region was fully committed with London suburban schemes and lacked the resources to handle the East Coast route as well, which was deferred in consequence.
The BTC also discussed the West Coast scheme on October 27. H P Barker was an industrialist and part-time Commission member, and he proposed that the whole scheme be approved, but the professional railwaymen wanted to deal at first only with the Crewe-Manchester section to prove the 50 hertz concept.
Barker said the system had already been proved overseas and that only the equipment needed to be trialled, but he was overruled. He was, of course, right - and this decision not to sanction and start the whole project nearly caused it not to go ahead. This was because the Minister of Transport and Prime Minister in 1960 were unconvinced about its viability when compared to diesel traction. The go-ahead did come eventually, but at the price of higher costs and delays.
Barker’s proposal to try out new equipment before mass introduction should also have been heeded, because traction components bought without trialling for the Glasgow and Great Eastern schemes gave trouble.
His proposal in 1954 for a trainset that could be serviced and reversed in a terminal station for its next duty was also rejected as impractical by the professional railwayman, only for such a train to appear six years later as the ‘Blue Pullman’ ( RAIL 910).
The most powerful electrics built for the Manchester-Sheffield/ Wath scheme in 1954 weighed 104 tons, were rated at 2,500hp, and were permitted 90mph.
By contrast, the prototype 50 hertz locomotives delivered in 1960 for the West Coast weighed no more than 80 tons, had a continuous rating of 3,000-3,300hp and a top speed of 100mph.
Such was the progress in technology and design philosophy over just six years. Sixtyfive years on, Warder’s decision to opt for 50 hertz has been vindicated.