Electric Traction Power | The Railway Technical Website | PRC Rail Consulting Ltd

Introduction

There is a wide variety of electric traction systems around the world and these have been built according to the type of railway, its location and the technology available at the time of the installation. Many installations seen today were first built more than 100 years ago, some when electric traction was barely out of its diapers, so to speak, and this has had a great influence on what is seen today.

In
the last 20 years there has been a rapid acceleration in railway
traction development. This has run in parallel with the development of
power electronics and microprocessors. What had been the accepted norms
for the industry for sometimes, 80 years, have suddenly been thrown out
and replaced by fundamental changes in design, manufacture and
operation. Many of these developments are highly technical and complex,
the details of which are therefore beyond the scope of these texts.

Because
the changes have been so rapid, there are still plenty of examples of
the original technology around and in regular use, so I have covered
these in my articles. This is useful, since it helps the reader to get
to grips with the modern stuff.

Power Supply

To
begin with, the electric railway needs a power supply that the trains
can access at all times. It must be safe, economical and user friendly.
It can use either DC (direct current) or AC (alternating current), the
former being, for many years, simpler for railway traction purposes, the
latter being better over long distances and cheaper to install but,
until recently, more complicated to control at train level.

Transmission
of power is always along the track by means of an overhead wire or at
ground level, using an extra, third rail laid close to the running
rails. AC systems always use overhead wires, DC can use either an
overhead wire or a third rail; both are common. Both overhead systems
require at least one collector attached to the train so it can always be
in contact with the power. Overhead current collectors use a
“pantograph”, so called because that was the shape of most of them until
about 30 years ago. The return circuit is via the running rails back to
the substation. The running rails are at earth potential and are
connected to the substation.

Figure
1: A section of the Old Dalby test track in England showing both third
rail and overhead electrification. Both systems are provided to allow
train testing from the main line or London Underground. The 3rd rail
system is common around the world but the 4th rail is rare. London
Underground is the largest user of the 4-rail system in the world.
Photo: Author.

Shoes and Shoegear

Third
rail current collection comes in a variety of designs. The simplest is
what is called “top contact” because that’s the part of the rail upon
which the pick-up shoe slides (Figure 2).

Figure
2: A 3rd rail collector shoe on a suburban EMU in London. The shoe is
suspended from an insulated beam hung between the axleboxes. It is a
top contact system. Being the simplest, it has drawbacks, not the least
of which is that it is exposed to anyone or any thing which might come
into contact with it. It also suffers during bad weather, the smallest
amount of ice or snow rendering top contact third rail systems almost
unworkable unless expensive remedies are carried out.

There
is also a side contact system. Side contact is not much better than top
contact but at least it is less exposed. Bottom contact is best – you
can cover effectively most of the rail and it is protected from the
worst of the cold weather. This diagram shows a DC 3-Rail Traction
System with the location of the current rail in relation to the running
rails. The third rail system uses a “shoe” to collect the current on the
train, perhaps because it was first called a “slipper” by the pioneers
of the industry (it slipped along the rail, OK?) but it was not very
pretty to look at, so perhaps someone thought shoe was a better
description. Whatever the origin, shoe has stuck to this day.

Figure
3: Docklands Light Railway train with 3rd rail bottom contact
electrification system. There have to be gaps in the thrid rail where
crossovers or junctions are provided. Photo: tubeuserstravels.

Modern
shoe systems have remote lifting facilities. All shoes need some way of
being moved clear of the current rail, usually for emergency purposes.
The most common reason is when a shoe breaks off and its connecting lead
to the electrical equipment on the train has to be secured safely. The
other shoes on the same circuit must be isolated while this is done,
unless the current is switched off from the whole section – perhaps
disabling several other trains.

Isolation
used to involve inserting a wooden “paddle” between the shoe and the
current rail and then tying the shoe up with a strap or rope. More
recently, mechanical or pneumatic systems have been devised to make it
possible to lift shoes from inside the train remotely from the driving
cab.

Most types of top
contact shoes simply hang from a beam suspended between the axleboxes of
the bogie. The suspension method was originally just a couple of
slotted links to compensate for movement which allowed gravity to
provide the necessary pressure. Later systems had radially mounted shoes
to provide more stable contact through lever action. Top contact
systems with protective covers over them, like the New York Subway
(Figure 4), needed radially mounted shoes anyway to allow them to fit
under the cover.

Figure
4: 3rd rail current collection system on the New York Subway showing
the third rail with a wooden cover fitted to reduce the effects of snow
and ice. Photo: Author.

Side
and bottom contact shoes are spring loaded to provide the necessary
contact force. An example of a bottom contact shoe as used on the
Dockland Light Railway line in London is shown in Figure 3 and in the
video (Figure 5). Some top contact systems have also used spring loading
but they are mechanically more difficult to control because of the
hunting action of the bogie and the risk that the shoes will get trapped
under the head of the rail and turn it over.

Figure
5: Simple diagrams of different types of 3rd rail contact systems. Source: Author.

Gaps

You
will often see trains with only one pantograph but, on trains which use
shoes, there are always several shoes. The contact with the overhead
wire is not normally broken but the third rail must be broken at
junctions to allow for the continuity of running rails. These third rail
breaks, or “gaps”, as they are called, can lead to loss of power on the
train. The power losses can be reduced by locating shoes along the
train and connecting them together by a cable known as a busline. In
spite of this, there can be problems. Woe betide the driver who stops
his train with all the shoes “off juice” or “gapped”. Yes, it happens
more often than you think and yes, before you ask, it’s happened to me.
It is an embarrassing nuisance only solved by being pushed onto the
third rail by another train or by obtaining special long leads with a
plug at one end for the train and shoes at the other end for the third
rail. Of course, it does cause a long delay.

Figure
6: Diagram showing a 3rd rail DC power supply system and how current rail gaps are provided where the
substations feed the line. Normally, each track is fed in each direction
towards the next substation. This allows for some over supply and
provides for continuity if one substation fails. The gaps
are usually marked by a sign or a light which indicates if the current
is on in the section ahead. Since the current may have been switched off
to stop an arc or because of a short circuit, it is important that the
train does not connect the dead section to the live section by passing
over the gap and allowing its busline to bridge the gap. Modern systems
link the traction current status to the signalling so that a train will
not be allowed to proceed onto a dead section. Diagram: Author.

At
various points along the line, there will be places where trains can be
temporarily isolated electrically from the supply system. At such
places, like terminal stations, “section switches” are provided. When
opened, they prevent part of the line for being fed by the substation.
They are used when it is necessary to isolate a train with an electrical
fault in its current collection system.

Although
3rd rail is considered a suburban or metro railway system, 750 volt DC
third rail supply has been used extensively over southern England and
trains using it run regularly up to 145 km/h. This is about its limit
for speed and has only spread over such a large area for historical
reasons.

Return

What
about the electrical return?  There has to be a complete circuit, from
the source of the energy out to the consuming item (light bulb, cooking
stove or train) and back to the source, so a return conductor is needed
for our railway. Simple – use the steel rails the wheels run on.
Provided precautions are taken to prevent the voltage getting too high
above the zero of the ground, it works very well and has done so for the
last century. Of course, as many railways use the running rails for
signalling circuits as well, special precautions have to be taken to
protect them from interference.

The
power circuit on the train is completed by connecting the return to
brushes rubbing on the axle ends. The wheels, being steel, take it to
the running rails. These are wired into the substation supplying the
power and that does the job. The same technique is used for DC or AC
overhead line supplies.

AC or DC traction

It
doesn’t really matter whether you have AC or DC motors, nowadays either
can work with an AC or DC supply. You just need to put the right sort
of control system between the supply and the motor and it will work.
However, the choice of AC or DC power transmission system along the line
is important.  Generally, it’s a question of what sort of railway you
have. It can be summarised simply as AC for long distance and DC for
short distance. Of course there are exceptions and we will see some of
them later.

It is
easier to boost the voltage of AC than that of DC, so it is easier to
send more power over transmission lines with AC.  This is why national
electrical supplies are distributed at up to 765,000 volts AC . As AC is
easier to transmit over long distances, it is an ideal medium for
electric railways. Only the problems of converting it on the train to
run DC motors restricted its widespread adoption until the 1960s.

DC,
on the other hand was the preferred option for shorter lines, urban
systems and tramways. However, it was also used on a number of main line
railway systems, and still is in some parts of continental Europe, for
example. Apart from only requiring a simple control system for the
motors, the smaller size of urban operations meant that trains were
usually lighter and needed less power. Of course, it needed a heavier
transmission medium, a third rail or a thick wire, to carry the power
and it lost a fair amount of voltage as the distance between supply
connections increased. This was overcome by placing substations at close
intervals – every three or four kilometres at first, nowadays two or
three on a 750 volt system – compared with every 20 kilometres or so for
a 25 kV AC line.

It
should be mentioned at this point that corrosion is always a factor to
be considered in electric supply systems, particularly DC systems. The
tendency of return currents to wander away from the running rails into
the ground can set up electrolysis with water pipes and similar
metallics. This was well understood in the late 19th Century and was one
of the reasons why London’s Underground railways adopted a fully
insulated DC system with a separate negative return rail as well as a
positive rail – the four-rail system. Nevertheless, some embarrassing
incidents in Asia with disintegrating manhole covers near a metro line
as recently as the early 1980s means that the problem still exists and
isn’t always properly understood. Careful preparation of earthing
protection in structures and tunnels is an essential part of the railway
design process and is neglected at one’s peril.

Overhead Line (Catenary)

The
mechanics of power supply wiring is not as simple as it looks (Figure
1). Hanging a wire over the track, providing it with current and running
trains under it is not that easy if it is to do the job properly and
last long enough to justify the expense of installing it. The wire must
be able to carry the current (several thousand amps), remain in line
with the route, withstand wind (in Hong Kong typhoon winds can reach 200
km/h), extreme cold and heat and other hostile weather conditions.

Overhead
catenary systems, called “catenary” from the curve formed by the
supporting cable, have a complex geometry, nowadays usually designed by
computer. The contact wire has to be held in tension horizontally and
pulled laterally to negotiate curves in the track. The contact wire
tension will be in the region of 2 tonnes. The wire length is usually
between 1000 and 1500 metres long, depending on the temperature ranges.
The wire is zigzagged relative to the centre line of the track to even
the wear on the train’s pantograph as it runs underneath.

The
contact wire is grooved to allow a clip to be fixed on the top side
(Figure 7). The clip is used to attach the dropper wire.  The tension of
the wire is maintained by weights suspended at each end of its length.
Each length is overlapped by its neighbour to ensure a smooth passage
for the “pan”.  Incorrect tension, combined with the wrong speed of a
train, will cause the pantograph head to start bouncing. An electric arc
occurs with each bounce and a pan and wire will soon both become worn
through under such conditions.

Figure 7: Overhead contact wire showing the grooves added to provide for the dropper clips. Photo: Coptech.

More than one
pantograph on a train can cause a similar problem when the leading
pantograph head sets up a wave in the wire and the rear head can’t stay
in contact. High speeds worsen the problem. The French TGV (High Speed
Train) formation has a power car at each end of the train but only runs
with one pantograph raised under the high speed 25 kV AC lines. The rear
car is supplied through a 25 kV cable running the length of the train.
This would be prohibited in Britain due to the inflexible safety
approach there.

A
waving wire will cause another problem. It can cause the dropper wires,
from which the contact wire is hung, to “kink” and form little loops.
The contact wire then becomes too high and aggravates the poor contact.

AC Sections

Overhead
lines are normally fed in sections like 3rd rail systems, but AC
overhead sections are usually much longer. Each subsection is isolated
from its neighbour by a section insulator in the overhead contact as
shown in this picture below. The subsections can be joined through
special high speed section switches. 

Figure 8: Insulated neutral section in an overhead line. Photo: Author.

Figure
9: To reduce the arcing at a neutral section in the overhead catenary,
some systems use track magnets to automatically switch off the power on
the train on the approach to the neutral section. A second set of
magnets restores the power immediately after the neutral section has
been passed. 

Catenary Suspension Systems

Various
forms of catenary suspension are used depending on
the system, its age, its location and the speed of trains using it.
Broadly speaking, the higher speeds, the more complex the “stitching”,
although a simple catenary will usually suffice if the support posts are
close enough together on a high speed route. Modern installations often
use the simple catenary, slightly sagged to provide a good contact. It
has been found to perform well at speeds up to 125 m/hr (200 km/hr).

At
the other end of the scale, a tram depot may have just a single wire
hung directly from insulated supports. As a pantograph passes along it,
the wire can be seen to rise and fall. This is all that is necessary in a
slow speed depot environment. I haven’t yet mentioned trolley poles as a
method of current collection. These were used for current collection on
low speed overhead systems and were common on trams or streetcars but
they are now obsolete.

Figure 10:
Overhead line suspension system. The weights and pulley system is
designed to maintain contact wire tension. Photo: Author.

DC overhead wires are
usually thicker and, in extreme load cases, double wires are used, as in
Hong Kong Mass Transit’s 1500 v DC supply system. Up to 3000 volts
overhead is used by DC main line systems (e.g., parts of France, Belgium
and Italy) but below 1500 volts, a third rail can be used. In operating
terms, the third rail is awkward because of the greater risk of it being
touched at ground level. It also means that, if trains are stopped and
have to be evacuated, the current has to be turned off before passengers
can be allowed to wander the track. Third rail routes need special
protection to be completely safe. On the other hand, some people
consider the overhead catenary system a visual intrusion. Singapore, for
example, has banned its use outside of tunnels.

Booster Transformers

On
lines equipped with AC overhead wires, special precautions are taken to
reduce interference in communications cables. If a communications cable
is laid alongside rails carrying the return current of the overhead
line supply, it can have unequal voltages induced in it. Over long
distances the unequal voltages can represent a safety hazard. To
overcome this problem, many systems used booster transformers. These are
positioned on masts at intervals along the route. They are connected to
the feeder station by a return conductor cable hung from the masts so
that it is roughly the same distance from the track as the overhead
line. The return conductor is connected to the running rail at intervals
to parallel the return cable and rails. The effect of this arrangement
is to reduce the noise levels in the communications cable and ensure the
voltages remain at a safe level.

Figure
11: A schematic showing the arrangements for 25kV AC electrification
systems using booster transformers (upper drawing) and the auto
transformer system (lower drawing). The auto transformer system allows
substations to be further apart without voltage drop. Drawing: Author.

Auto Transformers

A
more efficient system of AC electrification is known as the auto
transformer system. In effect, it is based on distributing power at 50
kV AC but feeding the power to the trains at 25 kV AC. To achieve this,
the supply sub-station transformer is provided with a centre tap
secondary winding at 50 kV (set to 55 kV for the maximum limit of
contact wire voltage of 27.5 kV). The centre tap is solidly connected to
ground so that one terminal is at +25 kV and another at -25 kV. The two
supplies at a phase difference of 180 degrees. 

With this system,
the contact wire is fed at +25 kV and the feeder wire at -25 kV thus
the voltage in between these circuits is 50 kV but to ground is 25 kV.
The insulation and clearances may still be designed for 25 kV AC only.

Pantographs

Current
is collected from overhead lines by pantographs. Pantographs are easy
in terms of isolation – you just lower the pan to lose the power supply
to the vehicle. However, they do provide some complications in other
ways.

Since the
pantograph is usually the single point power contact for the locomotive
or power car, it must maintain good contact under all running
conditions. The higher the speed, the more difficult the maintenance of
good contact. We have already mentioned the problem (above) of a wave
being formed in the wire by a pantograph moving at high speed.

Pantograph
contact is maintained either by spring or air pressure. Compressed air
pressure is preferred for high speed operation. The pantograph is
connected to a piston in a cylinder and air pressure in the cylinder
maintains the pantograph in the raised condition.

Originally,
pantographs were just that, a diamond-shaped “pantograph” with the
contact head at the top. Two contact faces are normally provided. More
modern systems use a single arm pantograph – really just half of the
original shape – a neater looking design (photo above).

The
contact strips of the pantograph are supported by a lightweight
transverse frame which has “horns” at each end.   These are turned
downwards to reduce the risk of the pantograph being hooked over the top
of the contact wire as the train moves along. This is one of the most
common causes of wires “being down”. A train moving at speed with its
pantograph hooked over the wire can bring down several kilometres of
line before it is detected and the train stopped (Figure 12). The most
sophisticated pantographs have horns which are designed to break off
when struck hard, for example, by a dropper or catenary support arm.
 These special horns have a small air pressure tube attached which, if
the pressure is lost, will cause the pan to lower automatically and so
reduce the possible wire damage.

Figure 12: Russian video of a pantograph being damaged by an overhead wire out of position. Video is 30s long.

Multi-Voltage

Some
train services operate over lines using more than one type of current.
In cities such  as London, New York City and Boston, the same trains run
under overhead wires for part of the journey and use third rail for the
remainder. In Europe, some locomotives are equipped to operate under
four voltages – 25 kV AC, 15 kV AC, 3,000 V DC and 1,500 V DC. Modern
electronics makes this possible with relative ease and cross voltage
travel is now possible without changing locomotives.