Energy and Power in Electric Traction


The President chaired the meeting. He called on the speaker, who had spent most of his working life on various engineering aspects of electric traction in many countries, to deliver his talk.

Mr. Calverley said railway electrification was basically an exercise in electric power distribution, but it differs from commercial/ domestic distribution in that the loads move about and also in several other ways. The difference is exemplified by the fact that, worldwide, general distribution is by alternating current at 50 or 60Hz whereas railways use direct current or a.c. at 16 2/3Hz, 25Hz or 50/60Hz. This talk aims to explain the reasons for this and to discuss some of the interesting special technical problems.

As the overall energy efficiency, from fuel to wheel, is about the same for electric and diesel railways the principal advantage of electrification energywise is that it allows any type of fuel or power source to be used.

The speaker recounted the historical development from the low voltage (500V) d.c. recreational installations, tramways and city railways of the turn of the century. Equipment was developed to raise the voltage and so facilitate the distribution of higher powers over greater distances to make main line electric railways viable. Thus 750V d.c. third rail became usual for urban railways and 1500V or 3000V d.c. overhead wire for main line. The d.c. motor can produce high torque at standstill and is easily controlled by series resistors.

In the early 1900’s some railways in Switzerland and Germany used a.c. – three phase with two overhead wires and rail return – at about 4000V 15Hz. The motors were of the wound rotor induction type with pole changing windings. Problems with the overhead wires caused this system to be abandoned in favour of the single phase system requiring only one wire. However the motor had now to be of the commutator type and the low frequency had to be retained to ensure reasonable commutation. Control was by on-load tap-changing on the vehicle mounted transformer. The low frequency systems were steadily developed 16 2/3Hz in Europe and 25 Hz in USA but, of course, the need for special low frequency generating stations or expensive converters was a major drawback.

Attempts to use 50/60HZ on the contact wire include the 120km line near Budapest electrified in the 1930’s using the system of Dr. Kando in which a rotating synchronous phase converter on the locomotive fed a single large three phase motor driving the four coupled axles. The German Railways experimental 50Hz Hollenthall line and the later Annecy line of the SNCF (1951) led to the heavy freight line at Valencienne (1955)- In Britain, the experimental Lancaster- Morecambe-Heysham line (1953), London Manchester etc. (1960) . So the 50/60Hz system, using d.c. motors fed via train mounted rectifiers, was accepted world-wide by i960 for new main line electrifications.

Harry Calverley then went to basic kinematic principles relating to energy and showed the advantages of high rates of acceleration, the use of coasting and the value of hump stations.

Using the Hong Kong Mass Transit Railway as an example, the power supply system and substation equipment were described. The degree of security for an underground railway supply, particularly one in the tropics, has to be very high as many lives can be at stake. The presence of two completely separate sources, China Light and Power and Honkong Electric, provides cover against one of them failing. The normal peak load for the first line, about 15km of route, is about 45000kW and all of this appears as heat underground and must be extracted by the ventilation system. This environmental control system uses as much power as do the trains; it cools the air of the stations by means of a closed circuit flow and rejects the heat to the outside air or to the harbour water. Only a minimum of fresh air is admitted. The tunnels are ventilated by the trains pulling station air through them and the trains themselves are air conditioned. If a train should stop in a tunnel for more than 90s the tunnel ventilating fans start automatically. Trains using thyristor choppers are now in use with a reduction in energy and heat.

The problem of electrolytic corrosion by d.c. currents leaking from the running rails was illustrated and its solution discussed. Harmonic currents in the power supply and in the d.c. circuit were referred to.

Turning to 50Hz traction the three basic methods of spreading the railway load between the three phases of the primary power system were described; single phase transformers, Scott connected transformers and the Leblanc transformer. The point of common coupling should be at a position of high short-circuit capacity to reduce the effects on other users of the phase unbalance and harmonics of the railway load. These harmonic currents, all the odd harmonic numbers, also interfere with railway and national telecommunications, and the booster transformer method of reducing this interference was described.

Another important factor is the minimising of the elctrical clearance to earth of the 25kV overhead equipment, to reduce cost of tunnel and bridge alterations. Values as low as 70mm are used.

Substations can be spaced further apart, so reducing costs, by using 50kV on the contact wire and this is done on several mineral railways, often with trains up to 20,000t. A type of three-wire a.c. system is being increasingly adopted and was described.

In conclusion the speaker said he hoped he had shown that the type of system chosen for a railway had considerable impact on several disciplines e.g. civil structures, track, signalling, communications, power supply, railway operating etc. There are advantages in both a.c. and d.c. and the historical “battle of the systems” between them would go on.

In opening the meeting for discussion the Chairman said he had found the talk enjoyable and educational. Mr. L.G. Hill referred to thyristors and to the problems BR had in I960 with their dual voltage system. He said new control systems would bring new problems. The speaker agreed thyristors were revolutionising electric traction, both a.c. and d.c. with choppers, inverters etc. Points made by Messrs. Fife, Bailey, Barwell, Jones, Thomas and others included flicker of lights, maximum demand charging, make-up air for ventilation, fire risks, relative success of BR and SNCF developments, viability of Shinkansen. The speaker dealt with some points and the very lively discussion had to be stopped due to shortage of time.
Don Plyer expressed the thanks of the members present for the interesting and liberally illustrated talk.