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HISTORY OF ELECTRIC TRACTION IN INDIA:-
Railway Electrification made a modest beginning with the inauguration of first electric
train between Bombay VT and Kurla Harbour on 3rd February 1925 on then existing GIP
Railway system at 1500 V DC. Heavy gradients on the Western Ghats necessitated
introduction of electric traction on Central Railway upto Igatpuri on North East line and
Pune on South East line. 1500 Volt DC traction was introduced on suburban section of
Western Railway between Colaba and Borivili on 05.01.1928 and Madras Beach and
Tambram of Southern Railway on 11.05.1931. This was primarily to meet the growing
traffic in these metros. Thus before the dawn of Independence, India had 388 KM of
electrification on DC traction.
In the post-Independence era, electrification of Howrah-section of Eastern Railway was
taken up on 3000 Volt DC during the First Five Year Plan period and completed in 1958.
The first EMU services were inaugurated in Howrah-Sheoraphulli section by
PanditJawaharLalNehru, first Prime Minister of India on 14.12.1957.
25 kV AC system of traction emerged as an economical system of electrification as a
result of extensive research and trials in Europe, particularly on French Railways (SNCF).
Indian Railways decided in 1957 to adopt 25 kV AC system of electrification as a
standard, with SNCF as their consultant in the initial stages.
In the wake of industrial development, in the Eastern region, due to the setting up of steel
plants, large scale movement of Iron and Coal, substantial growth in freight traffic, which
could not be managed by steam traction, electrification and dieselisation had to be
introduced in early sixties to cope up with the growing traffic.
The first section electrified on 25 kV AC system was Raj Kharswan-Dongoaposi of South
Eastern Railway in the year 1960. With a view to provide continuity of traction system,
Howrah-Burdwan section of Eastern Railway and Madras Beach Tambaram section of
Southern Railway were converted to 25 kV AC system by 1968.The manufacture of
Electric Multiple Units (EMUs) required for Kolkata suburban services was taken up
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indigenously at Integral Coach Factory (ICF), Chennai and first EMU rolled out during
September 1962.
Considering the various limitations in existing 1500 V DC traction systems in Central
Railway and Western Railway, a decision was taken to convert to 25 kV AC traction
during 1996-97. Conversion from DC traction to AC traction is under progress.
TRACTION SYSTEMS:-
(i) Non Electrical Traction: Traction system which does not uses the electrical
power to drive, are called Non Electrical Traction system.
There are various types of non-electrical systems:-
Steam Engine
Diesel Engine
(ii) Electrical Traction: Traction system which uses the electrical power to drive, are
called Electrical Traction system.
There are 4 types of electrical systems:-
D.C. System
Single Phase A.C. System
Three Phase A.C. System
Composite System
D.C. system:-
D.C. system contains D.C. supply of rating 600V, 750V, 1500V, 3000V.
dc system uses the low voltage ,series wound D.C. Series Motor to drive the train. dc
motor is well suited to railroad traction, being simple to construct and easy to control.
Until the late 20th century it was universally employed in traction system.
Single Phase A.C. System:-
Alternating current system was used due to incapability of dc system i.e. high voltage.
The higher voltage limit for the dc system was 3000 Volts. With alternating current,
especially with relatively high voltage over-head wire, fewer substations are required, and
the lighter overhead current supply wire that can be used correspondingly reduces the
weight of structure needed to support it to the further benefit of capital cost of
electrification.
Single phase 16 *(2/3) Hz and 25 Hz frequency of supply is used in single phase ac
system.
A.C. series motor is used due to high starting torque. Substations are used in distance of
30-40 Km to maintain the voltage level along the track. Voltage rating is 15-25 KV for
this system.
Three Phase A.C. System:-
This system is used for high voltage ratings. In three phase system we can transmit high
voltage level which is for efficient transmission is required.
This system uses 3 phase induction motor, 3 phase supply, 50-60 Hz supply. Voltage
rating of motor is 3.3-3.5 KV.
Composite System:-
There are two types of composite systems:-
single phase to dc system
single phase to 3 phase system
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The first one single phase to dc system is used where voltage level is high for
transmission and dc machine is used in the locomotive.
Single phase to 3 phase system is used where 3 phase machine is used in the locomotive
and single phase track we have.
1.3 REQUREMENTS OF AN IDEAL TRACTION SYSTEM:-
The following properties of an ideal traction system:-
High Starting Torque:- An ideal traction system must have high starting torque. It
means it should develop high starting force as a train have high load during starting
period.
Ease of Operating:- The equipment used in the system must have ability to run without
any change on different routes.
Overloading capabilities:- The equipment must have abilities to run with overload for a
short duration as traction load is not very certain.
Minimum Track Distortion:- The traction system should not affect the track where it is
running. It should have as small as possible distortion on the track.
Low Cost:- The cost of the device should be minimum.
Efficient Braking:- The braking arrangement should be efficient for proper control and
easy operation.
The locomotive should be self-contained and able to run on any route braking should be
such that minimum wear is caused on the brake shoes if possible the braking energy
should be regenerated and returned to the supply.Speed control should be easy.The wear
on the track should be the minimum.The starting tractive effort should be high so as to
have rapid acceleration.There should be no interference to the communication lines
running along the lines.It should be Pollution free.Initial and maintenance cost should be
low.
1.4 ADVANTAGE AND DISADVANTAGE OF ELECTRIC TRACTION
SYSTEM:-
Advantages:-
Energy Conservation through Railway Electrification:-
Railway transport is far more energy efficient as compared to road transport.
Railways are:
Six times more energy efficient as compared to road
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Four times more economical in land use
Six times more cost effective vis-à-vis road in construction costs for comparable
levels of traffic.
Among the modes of rail transport, electric traction is the most energy efficient. This can
be seen in the light of the fact that every 100 route kilometres electrified section results in
saving of annual consumption of more than four million litres of diesel oil, which saves
Rs. 2500 Crores worth foreign exchange annually.
Role of Electric Traction in Suburban Transport:-
Electric Multiple Units (EMUs) are ideal for suburban services with higher acceleration
and braking features required for frequent starts and stops. EMU services form the
backbone of suburban transportation in the metropolitan cities of Mumbai, Kolkata and
Chennai. In Mumbai area alone, about 1950 EMU trains are running daily on the
suburban sections of Central and Western Railways which cater for about 5 million
passengers every day.
Electrification has made possible the introduction of EMU services in many suburban like
main line sections. These services have become extremely popular.
Haulage of Heavier Freight Trains and Longer Passenger Trains under
Electric Traction:-
Electrification is making possible running of heavier freight trains. With the imported
6000 HP thermistor locomotives, a consist of two of these locomotives, presently handles
4500 tonne trains in gradients up to 1 in 60 as against three numbers of 4000 HP earlier
locomotives. Running of 9000 tonne trains is also possible with such locomotives, as has
already been proved in tests conducted successfully in Ghaziabad - Mughal Sarai section.
With the introduction of electrification, 21 passenger coaches are hauled by a single
locomotive in most of the sections. A single electric locomotive is now hauling Prayagraj
Express with 24 coaches between New Delhi and Allahabad. Introduction of Shatabdi
Express services operating at 130 KMPH under electric traction have been achieved
purely through indigenous efforts of Indian Railways.
Benefits of clearer environment:-
One of the major advantages of electric traction is pollution free atmosphere not only to
the travellers but also to the surrounding environment. Electric traction is proven to be
less pollutant than the existing diesel mode and thus could be more eco-friendly to an area
having delicate flora and fauna. Electric traction could reduce noise and air pollution and
result in lesser disturbance to wild life habitat of the region.
Coefficient of adhesion is better.
Cheapest method of traction.
Less vibration.
Less maintenance cost.
Rapid acceleration and braking.
High starting torque.
It has great passenger carrying capacity at higher speed.Free from smoke and few
gases, so used for underground and tubular railways.
Disadvantages:-
Electrically operated vehicles have to move on guided track only.
Problem of supply failure.
The leakage of current from the distribution mains and drop of volts in the track
are to be kept within the prescribed limits.
Additional equipment is required for achieving electric braking and control.
High capital cost.
POWER SUPPLY IN TRACTION SYSTEN
2.1 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.
There are three main stages in power circuit of three phases locomotive:-
Input converter:-
A transformer section step-down the voltage from the 25 KV input.This converter
rectifies AC from catenary to as specified dc voltage using GTO thyristors.It has filter
and circuitry to provide a fairly smooth and stable dc output, at the same time attempting
to ensure good power factor.The input converter can be configured to present different
power factor to power supply
DC link:-
This is essentially a bank of capacitor and inductor or active filter circuitry to further
smooth.Also to trap harmonics generated by drive converter and traction motors. The
capacitor bank in this section can also provide a small amount of reserve power in
transient situations (e.g., pantograph bounce) if needed by the traction motors.
Drive converter:-
This is basically an inverter which consist of three thyristors based components that
switch on and off at precise times under the control of a microprocessor. The three
components produce three phase of A.C. The microprocessor controller can vary the
switching of thyristors and thereby produce A.C. of wide range of frequency and voltage.
Third rail and uses:-
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.
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.
Shoes and Shoegear:-
The diagram above shows a top contact third rail system but there are other types as
shown in this diagram. 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.
Being the simplest, it has drawbacks, not the least of which is that it is exposed to anyone
or anything 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. Side contact is not much better 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 DC 3rd rail Top Contact Collector Shoe (London Underground - Central Line) has
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 (photo left), needed radially mounted shoes anyway to allow them to fit
under the cover.
Side and bottom contact shoes are spring loaded to provide the necessary contact force.
An example of a bottom contact shoe as used on a German metro line is shown in the
photo (left). 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.
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
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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.
Current rail gaps are also provided where the substations feed the line (diagram, left).
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. These substation
gaps are usually marked by a sign or a light which indicates if the current is on in the
section ahead. A train must stop before entering the dead section. 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 bus-line to bridge the gap. Some of the more sophisticated systems in use
today now link the traction current status to the signalling so that a train will not be
allowed to proceed onto a dead section.
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
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 metallic. 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 earthings 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 (diagram, left).
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. 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.
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, booster transformers are provided. 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.
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 speed, more difficult of maintenance of good contact.
We have already mentioned the problem 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. 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.
Dual 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, 15kV 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