Written by Clive Kessell for the rail engineer
Electrified railways have been around since the 1890s and the predicted advantages of efficiency, speed and cleanliness have largely been realised.
But what has been learned about electric traction and infrastructure since that time that can make today’s electrification schemes easier to implement and better value for money?
The annual IET Railway lecture on 3 Nov was given by Peter Dearman, Head of Network Electrification in Network Rail and a long term railwayman. Peter presented an analysis on how electrification has developed over the years and outlined the critical factors that will impact in the future.
The UK in Retrospect
Britain’s first railway electrification scheme (Volks Railway at Brighton being an exception) was introduced by the London Brighton & South Coast Railway in 1909 using overhead line technology at 6.7kV 25 Hz AC for their London suburban lines.
This should have set the scene for the future. However the rival London & South Western Railway, having seen the Manchester to Bury line of the Lancashire & Yorkshire Railway electrified on the 1200V side contact third rail system and being influenced by the adjacent District Line fourth rail top contact system, opted for a 660v third rail system for its suburban lines out of Waterloo. The first sections opened in 1915.
The First World War stopped both these systems being enlarged although piecemeal expansions continued when peace came. The two railways became part of the Southern Railway under the Grouping of 1923 and the SR took the decision to standardise on the third rail system. The LBSC overhead system was dismantled and the lines converted.
With the benefit of hindsight this was the wrong decision and the railways of Southern England have had to live with this legacy ever since.
Nothing much else happened in the rest of the UK during the depression years, which may have been fortuitous as Britain was spared the multiplicity of different systems that emerged across Europe.
Eventually, the London & North Eastern Railway embarked on the electrification of the Liverpool St to Shenfield and Southend suburban line and planned for its cross Pennine route from Sheffield to Manchester.
These were to employ the 1500V overhead line DC system which was finding favour in France and had previously been used by the North Eastern Railway for a freight line. Again, war intervened but eventually both projects were completed by the early 1950s.
The emergence of 25kV 50Hz overhead systems in the 1950s was to prove a godsend, for the system allowed almost unlimited power as well as being much simpler in terms of distribution and control.
All UK schemes since then have adopted this standard except for some Southern Region expansions using third rail.
The rate of roll out has been poor compared to Europe with only the West Coast, East Coast and some suburban lines being converted.
System Pros and Cons
Whilst the third rail system is supposed to be cheap to install (it not requiring the erection of masts and gantries), the need for frequent substations and sectioning cabins, AC feeder cables and rectifiers to obtain the DC power, and complex control arrangements makes the system far more costly than one would think.
Couple this with the limited amount of power than can be safely extracted from the system and the large losses that are incurred by both infrastructure and traction make it an unattractive proposition. Although visually less intrusive, a power rail at ground level is always a safety hazard.
1500V DC overhead is better but suffers from the same weaknesses of needing rectification and limited power output. All remaining UK lines of this voltage have since been converted to 25kV.
So is 25kV the perfect answer? It has become a world standard but the configuration of the system from the 1960s era is in need of modernisation to a) improve the loss factors, b) to interface the system with modern grid practices and c) to make the mechanical parts of the system more reliable and less susceptible to damage.
25kV Design Today
The railway is now busier than ever with more trains being run, each of them consuming more power than previous designs.
The electrification system needs to supply this power demand but given that the volts and amps cannot be varied, the only means of delivering improved efficiency is to reduce the impedance of the system thereby minimising losses.
In earlier times, booster transformers and return conductors were necessary to minimise interference into adjacent copper telecommunications cables, both railway and third party owned.
This worsened the impedance of the system and the electrification engineer installed these devices somewhat grudgingly. With the widespread use of fibre cables, this requirement has gone away and thus booster transformers no longer form part of a modern 25kV design.
The grid supply system has to be part of the efficiency enhancement challenge. In the 1960s the fault current on an overhead line short circuit was limited by grid power availability and technology.
A new design of 4Ω supply transformer has allowed a significant lowering of impedance with consequential rise in fault current from 6kA to 12kA.
This has necessitated the development of high speed circuit breakers but the energy levels under fault conditions are very high. The single phase railway system has never been popular with the grid supply companies as it tends to unbalance grid conditions.
To minimise this, the power supply points are now normally located at 400kV grid access points rather than 132kV. In turn this allows greater distances between feeder stations, which, whilst fewer of them are needed, the impedance of the system from that supply point is worsened.
It becomes a trade off between less equipment but lower electrical efficiency.
To get more power from the system, the use of auto transformers in a 25-0-25kV configuration (sometimes referred to as 50kV) has some advantages and is almost universally used on today’s high speed lines. However in practice, faults and short circuit conditions are found to be more commonplace and the effects are worse.
The way forward for supplying power is the advent of the ‘smart grid’ plus a much closer relationship with the power supply companies.
The traditional approach of having large generating capacity from a small number of fossil fuelled power stations is on the way out, with only nuclear energy being used for such stations in the future.
The growing use of renewable energy sources, be it wind, solar or tidal, will mean a much more distributed series of supply points.
Coupling this to intelligent network control will lead to cost and reliability benefits. Train energy demand is dependent on the number of trains and the speed at which they are travelling.
Some means of using stored energy for high demand periods makes sense and this is where renewable sources, with their inherent battery storage systems, will come into their own.
Losses and Train Design
The business case for electrification needs a re-think. The price of diesel fuel is now an inhibitor but this in itself is not enough.
Electric traction pricing must become cheaper and, to achieve this, system losses must be reduced. An AC system should be capable of having losses of between 3-5%. The worst situation is with the ex-SR third rail DC network.
This legacy from the past has often been examined for conversion to 25kV but the cost has always been too high. However the energy equation is such that a new study is underway and the results are looking more hopeful.
With the wide availability of dual voltage traction, the changeover can be renewal led, probably working from the extremities inwards so that removal of all the third rail kit can be achieved on a line by line basis. It will take many years to happen but it seems to be feasible
Trains are now heavier and more power hungry, which is the reverse to what has happened in the air and automotive industries.
The analogy is that applying the train design trend to cars, a Ford Focus would weigh 4 tons and have a fuel efficiency of 12.5mpg. Improved passenger facilities such as air conditioning are only part of the problem.
Crashworthiness is a major factor and the standards need to be challenged. The advent of TPWS and the declared future with ERTMS has almost eliminated the risk of collisions, although it is acknowledged that level crossing road vehicle crashes still pose a threat.
Traction drives and air conditioning systems need to be made more efficient. Bogies are still reminiscent of a Sherman tank. Take a look at Shinkansen and see what can be done.
The Future and More Questions
The first thrust must be to get better control and distribution of the electric power transmission network with energy management being done properly. Equally it is recognised that overhead line infrastructure must be made more reliable.
Instances of the wires being brought down are far too numerous and a better design will emerge for the forthcoming GW and NW projects based upon best European practice.
Another challenge will be the raw material to be used. The world’s copper supply is expected to be exhausted in 15 years. Quite what will replace it is an unknown although recycling existing copper infrastructure will become more important.
Convincing the freight operators to use more electric traction may be difficult as wiring sidings is not a practical option. Again some form of on-board energy storage technology will likely be the answer.
The whole industry has to look at electrification on a unified basis. Peter Dearman must be thanked for this fascinating insight into the electrification debate.