1. Current collection system:
There are two types of current collection systems used mostly
1. Conductor rail system or third rail system.
2. Overhead system.
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2. 1.Conductor rail system or third rail system:
provides electric power to electric locomotive through continuous rigid
conductor placed alongside or between the tracks.
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4. Basically used in mass transit or rapid transit system.
This system is always supplied from the DC power supply.
third rail is placed at a distance of about 0.4 meter away from tracks.
Cheaper to install as compared to overhead contact system.
Drawback is it presents electric shock hazards close to the ground so not
applicable for high voltages above 1500V.
A very high current is therefore used to provide sufficient power, which
in result increases the resistive losses.
Due to this reason it is applicable only for low voltage bellow 1500V.
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5. 2.Overhead system:
overhead system is used where the requirement of high speed
and high power is applicable.
This system is adopted when the trains are to be supplied at high
voltage (1,500 volts or above).
The collection of the current required by a train can be done by
a collector with a sliding contact.
Overhead construction is universal for all ac railways and is also
used with dc tramways, trolley buses and locomotives operating at
voltages of 1,500 volts and above.
In all these cases the running rails are utilized as the return
conductor, therefore, with dc and single phase system only one
overhead wire is required for each track.
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6. Current Collection from Overhead System:
The current in the overhead system is collected with the help of sliding contact
collector mounted on the roof of the vehicle.
Current collector should not leave the contact of overhead equipment in any
circumstances
Contact wire in all practical installations is never perfectly horizontal, it rises
and falls depending upon the weight of the contact wire.
contact wire comes very low under bridges and tunnels and rises high over
public crossings.
The current collector has to rise and come down according to the speed of the
electric vehicle in order to maintain the contact with overhead equipment.
in order that the current collector gear picks up the current without any spark,
it must be designed so as to maintain an even pressure against the trolley wire at
all speeds.
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7. To ensure an even pressure the current collector gear must have a sufficient
flexibility to adjust itself to the variation of the level of the conductor at high
speeds.
Either a spring or compressed air is used to maintain the pressure.
As the tramcars and trolley buses draw their power from overhead conductors
suspended about 6.7 m (from 4.7 m to 7.5 m) above the road surface, the
current collector must, therefore, be mounted on the vehicle.
Overhead collectors type:
1. Trolley Collector
2. Bow Collector
3. Pantograph Collector
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8. 1.Trolley Collector:
consists of a grooved gun metal wheel or grooved slider shoe with carbon
insert carried at the end of a long pole.
The other end of this pole is hinged to a swivelling base fixed to the roof of
the vehicle. Necessary upward pressure for the pole and current collector is
achieved by means of springs.
As two trolley wires are required for a trolley bus a separate trolley collector
is provided for each wire, the bases being mounted side by side. The pressure
for wheel is approximately 10 kg and for a carbon insert slider is
approximately 17 kg.
Main drawback of trolley collector is that it has to be rotated through 180°
for reversing the direction of motion of the vehicle.
The trolley collector is suitable for comparative low speeds (say 24 to 32
kmph) beyond this speed there is every possibility of its jumping off the
trolley wire.
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10. 2.Bow Collector:
The bow collector is also employed for collecting the current with
tramways.
The bow collector consists of light metal strip or bow 0.6 or 0.9 metre wide
pressing against the trolley wire and attached to a framework mounted on
the roof of vehicle.
Provision of metal strip enables vehicle to run at higher speeds without
running the risk of leaving the contact of trolley wire.
Collection strip is made of soft material such as copper, aluminium or
carbon so that it should wear instead of trolley wire as it is easy to replace
worn out collection strip than trolley wire.
Necessary upward pressure is obtained by springs.
requires either provision of duplicate bows or an arrangement of reversing
the bows for motion in the reverse direction.
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11. The bow collector has smaller inertia but is not so readily adoptable for
collection of large currents as is a pantograph collector.
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12. 3.Pantograph Collector:
The pantograph is employed in railways for collection of current where
the operating speed is as high as 100 or 130 kmph and the currents to be
collected are as large as 2,000 or 3,000 amperes.
Pantographs are mounted on the roof of the vehicles and usually carry a
sliding shoe for contact with the overhead trolley wire.
The contact shoes are usually about 1.2 metres long.
The shoes may be straight throughout their lengths or cambered slightly
or may be in the form of an auxiliary bow.
Material used for pantograph is often steel with sometimes, wearing
plates of copper or bronze inserted.
The pressure varies from 5 to 15 kg.
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13. The above arrangement has the following advantages over other types
of collectors:
1. It can operate in either direction of motion.
2. There is no risk of leaving wire junction etc.
3. The erection of the overhead network is very simple due to absence of
points and grooved crossings required for bows.
4. Its height can be varied from the drivers’ cabin by carrying out simple
operations.
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15. 15
The pantograph is raised or lowered from the driver cab by one of
the following methods or with some modification of it:
(i) Air raised, gravity lowered,
(ii) Air raised, spring lowered, and
(iii) Spring raised, air lowered.
16. Overhead Construction for Trams, Trolley Buses and Railways:
For trams, trolley buses and for railways operating at 1,500 V and above
overhead contact wire is used.
The design of overhead system is more complex in comparison to conductor
rail system because of greater difficulty of ensuring that the collector and
contact wire remain in contact at a fairly even pressure.
Tramways and Trolley Buses:
The trolley wire is to be erected at a minimum height of 5.185 metres above
the street surface (except under bridges).
The trolley wire is to be supported at intervals not exceeding 36.6 metres.
Each trolley wire must be divided into sections not exceeding 0.8 km in length
with an emergency switch between every two sections.
The trolley wire should be capable of withstanding stresses due to variation in
temperature.
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17. The uniform hardness gives uniform wearing of conductor, which is highly
desirable in electric traction. The trolley wire is usually of circular grooved cross
section. The cross-sectional area of a standard trolley wire is 80 mm2.
The trolley wire is supported and insulated from a transverse span wire by
means of a steel bolt screwed into a gunmetal ear. The gunmetal ear is attached
to the trolley wire, and the bolt is insulated from and held in a hanger attached
to the span wire.
The span wire is supported from poles erected facing each other on both sides
of the road and insulated there from.
The span wires may be extended over the whole width of the road (in which
case these are attached to poles on each side of the road) or may be of a shorter
length (in which case these are attached to brackets carried from horizontal
arms attached to the poles at one side of the road).
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20. 20
Railways:
Since in railways a sliding current collector (bow or pantograph) is employed,
therefore, the contact wire is required to be suspended with very small sag so
that the contact between the wire and collector may be maintained at high
speeds.
This is achieved without excessive tension in the wire by employing spans of
comparatively shorter length (3 to 4.5 metres).
For such short spans an indirect method of suspension is adopted, in which
the trolley wire is supported by another wire known as the “Catenary” or
“Messenger” and is fixed with considerable sag (2 to 3 per cent of the span)
between supports at suitable distance. The wire is usually made of more than
seven strands of steel.
To avoid the use of insulated droppers the whole of the messenger wire is
usually insulated, and it may therefore be used if required for carrying current.
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The position of contact wire remains practically constant at all temperatures
which may cause large amount of sag in the catenary wire.
Nowadays the catenary construction employed is of either the single catenary
or compound catenary.
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In single catenary construction, a
steel wire with 7 or more strands
supports the trolley wire by means of
droppers clipped to catenary and
trolley wire at equidistant horizontal
intervals.
The span of catenary wire may be from 45 to 90 metres with a sag of from 1 to
2 metres respectively for straight track.
The distance of droppers is between 3 and 5 metres on curved track the spans
of shorter lengths are employed and the trolley wire is kept in position by pull-
off wires.
Such construction provides sufficient flexibility for speeds up to 120 kmph
without any problem of current collection.
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Besides this it is relatively cheaper, easy to erect and does not require heavy
maintenance. Single catenary construction is considered suitable for most light
weight ac systems and for branch and loop lines and secondary routes where
traffic density is low and operating speeds are low.
The compound catenary construction is employed for heavy current i.e., where
traffic density is high. It consists of three wires, all in the same vertical plane.
The upper wire is catenary wire, which is insulated from the supporting
structures. From this wire another wire, known as intermediate catenary, is
supported by droppers clipped to both wires.
The third wire i.e., trolley wire is maintained in definite and constant tension
by means of automatic tightening gear. Both contact and catenary wires are
divided in sections of 150 metres and each section is anchored at the centre. The
purpose of intermediate catenary besides increasing the current carrying
capacity is to provide more uniformity in elasticity.
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SAG AND TENSION CALCULATION FOR A TROLLEY WIRE:
A flexible wire when suspended
horizontally between two supports it
would assume the form of catenary.
But in case of tramways the sag in
the trolley wire rarely exceeds 1 or
15 per cent of the span length and,
therefore, the wire may be
considered to hang in the form of a
parabola.
If 2l is the span length in
metres, w is the weight in kg per
metre length of wire and T is the
tension of wire in kg.
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The change in temperature causes change in in length which will cause change
in sag and tension in a span.
That is, the decrease in temp. Reduces the sag and increases the tension. The
reduction in sag causes stretching which tends to decrease the sag.
The elasticity of conductor material play and important role in determining the
exact values of the sag at different temp.
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Traction Substation:
Traction substation is supplied
by 220/132 kV ac supply either
from two sources of supply or by
double circuit 220/132 kV
transmission line for the sake of
reliability of supply.
For rapid clearance of fault(s)
occurring on transmission line or
substation suitable protective
equipment is provided. Two sets of
220/132 kV bus-bars (one main
bus-bar and other transfer bus-
bar) are provided in each
substation. Both of these bus-bars
are connected through bus coupler
circuit breaker. The incoming and
outgoing transmission lines are
connected to both the bus-bars
through the circuit breakers, as
shown in Fig.
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With such an arrangement of the two bus-bars and isolators on both sides of
circuit breakers continuity of supply can be maintained while doing
maintenance work on any bus-bar or circuit breaker on primary side of
transformer.
Normally two single phase transformers of rating 10 or 12.5 MVA, 132/25 kV
are installed in a traction substation. Of the two transformers only one will be
in operation normally, the other one is acting as standby.
Output voltage at 25kV is maintained within -5% to +10% with the help of
tapings provided on the transformer. One 25 kV transformer circuit breaker
and one 25 kV feeder circuit breaker equipped with an overload relay and an
impedance relay are provided on the output side of transformer for clearance of
all faults on OHE.
One terminal of 25 kV winding of transformer is connected to earth solidly at
substation and return feeders from all the rails of electrified track opposite to
feeding post are also connected to this earthed end solidly.
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Provision of 25 kV bus-bar on the output side facilitates the maintenance of
any transformer or its associated circuit breaker without any interruption of
power supply.
These substations are unattended type and are controlled by remote control
operation .
Spacing of these substations are 50 to 90 km and they are located closed to
feeding posts, maximum distance being 2 km.
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Location of DC Substations :
In case of trolley bus both the conductors are trolley wires, therefore, more
voltage drop takes place in it and the spacing between sub- stations is reduced
in comparison to tramway substations for similar capacity.
Hence the substations required are more in number and of lesser capacity.
Spacing of substations is kept 4.8 km for single track and 3.2 km for double
track (up and down). Each section of 0.8 km length is individually fed, as
shown in Fig.
30. 30
For suburban railways the spacing of substations is usually determined from
consideration of permissible variation of voltage at the trains and economy.
The length of section depends upon the traffic density and substations spacing.
The length of section may be from 3 to 5 km in general and distance between
substations may be from 13 to 16 km. The sections can be isolated by sectional
switches.
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Substation Equipments:
The major equipment provided at traction substations is
(i) Traction transformer
(ii) circuit breakers and
(iii) Interruptor.
1. Traction Transformer :
Transformers are required to step down the grid supply voltage (220 or 132
kV) to 25 kV. These are specially designed so as to withstand severe
operating conditions and have large short-time peak capacity.
Its windings are designed with class A insulation and are specially reinforced
to withstand the stresses of frequent short circuits on 25 kV overhead
equipment.
Transformers are designed to have large per cent impedance (not less than
8.5% in case of 10 MVA and 10% for 12.5 MVA transformers) so as to limit
the effect of short circuit.
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Transformers need not to be provided with on-load tap changer as voltage at
substations is maintained within permissible limits by the supply authorities.
However, off-load tappings are provided on the hv side at - 10%, - 5%,0% and
+10% of rated voltage so as to provide voltage between 27,500 V and 23,750 V.
This has the advantage of less copper in the taps and smaller switches, which
also may be all immersed in the oil used for cooling the transformers.
These transformers are provided with standard accessories such as
conservator tank, Buchholz relay, breather, oil level indicator, drain valve, dial
gauge thermometer, relief pipe and two thermostats to indicate the oil
temperature rise.
One is meant to give an alarm indication while the other is for tripping of
circuit breaker in case of excessive temperature rise. The cooling arrangement is
obviously oil cooling, which is provided by radiators.
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2. Circuit Breakers:
For each single unit substation there are four line circuit breakers and one
transformer circuit breaker.
Line circuit breaker trips in case of fault in the overhead equipment.
Transformer circuit breaker on 25 kV side usually remains closed and is
intended to trip on the occurrence of any internal fault on the transformer.
The circuit breakers provided are of low oil content type and of rating 750 A at
25 kV and rupturing capacity of 500 MVA.
Circuit breakers can be operated electrically by local push button or by remote
control. These can also be operated manually.
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3. Interruptor:
Interruptor is a non-automatic type breaker. It is not provided with
protective relays to enable it to operate automatically in the event of occurrence
of fault in the system.
However, it can be operated for opening and closing of OHE by remote
control from remote control centre. These are provided at feeding, sub
sectioning and paralleling posts and sub sectioning posts. Circuit breakers and
interruptors are provided with the following interlocks.
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INTERFERENCE IN TELECOMMUNICATION CIRCUITS:
In all traction systems track is used as a return path for the current. Some part
of the current flows through the track and the remaining portion of the current
flows through earth, as shown in Fig. Current flowing through earth causes
interference with communication lines.
Now to constrain the return current to flow through restricted path i.e., along
the track some steps are required to be taken.
In ac traction system use of booster transformer, is made while in dc traction
system negative booster, is used to reduce the potential drop across the track.
Path of return current is also constrained by reducing the track resistance. The
resistance per km length of continuous track rail is of order of 0.05ohm but the
resistance of fish-plate joints employed in jointing the commercial length rail
mechanically is more than that of a rail.
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Hence in order to reduce the resistance of the track as a whole the joints are
supplemented with a good conductor or "bond".
Modern bonds are of the flexible type and are designed to be as short as the
conditions of locations permit. The bonds are usually fixed under the fish plates
as they are protected from damage while carrying out the repairs to the paving.
In single phase ac traction system presence of harmonics in supply system also
causes interference in telecommunication systems.
Interference in telecommunication and signalling lines is also caused due to
electromagnetic induction. Current flowing through contact wire sets up a
magnetic field around itself. This magnetic field links with nearby circuits and
induces emf in circuits.
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The interference effects on telecommunication circuits owing to reasons
mentioned above can be reduced by taking following steps.
(i) All telecommunication circuits are installed under ground in cables instead
of in overhead lines.
(ii) Telecommunication or signalling lines are provided with aluminium sheath
instead of lead.
(iii) Use of PVC or rubber covering is made to prevent sheath corrosion.
(iv) Telecommunication circuits may be removed to the distant place, to the
extent practically possible, from the railway track.
(v) Long telecommunication circuits are broken by providing isolating
transformers.
(vi) Booster transformer employing return feeder is also used to avoid magnetic
linking of telecommunication lines with power lines.
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Pantograph
The pantograph is a current collection system from the overhead line. The
locomotive takes current from the overhead line through this pantograph.
Pantographs are held up by compressed air pressure to connect with the
overhead line. It also designed to collapse if it detects an obstruction and can
be manually down to isolate the locomotive from the overhead line.
Circuit Breaker
It is an electrical protective device. It used to disconnect the locomotive from
the main power supply coming through the pantograph when any fault occurs.
Axle Brush
Pantograph helps to connect phase or line conductor whereas axle brush helps
to connect neutral conductor. We can say a locomotive takes current from the
overload line through the pantograph and return that current through the axle
brush.
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Step Down Transformer
In a Kando system, the locomotive is supplied by 16KV or 25KV, 50 Hz single-
phase supply. So the step down transformer is used to step down this high
voltage. Tap changer also used with this transformer.
Main Rectifier
Rectifier is used to convert the single phase AC supply coming from the
transformer into DC supply. Generally SCRs, IGBTs etc. are used for this
rectifier because it carries a very high current and electrical power.
Filter
In the above diagram you can see there are two Filters are used. These are the
DC filter used to filter the DC output of the rectifier.
Motor Controller
Motor Controller is used to control the three phase motor such as speed,
torque, acceleration, etc.
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Three Phase Inverter
This single phase to three phase inverter is used to convert the DC supply
(from the output of the rectifier) into three phase AC supply. This three phase
AC supply is used to operate the three phase induction motor. Generally, high
power carrying SCRs, IGBTs are used for this inverter circuit.
Traction Motor
Generally, the modern locomotive system use three phase asynchronous motor
or induction motor. Induction motors are provides a very good speed
regulation, regenerative breaking, high acceleration, etc.
Single Phase Inverter
In the locomotive system, a single phase inverter is also used which supply
power to Lights, Fans, AC Sockets, cooling fans, battery charger, etc. This
single phase inverter also take power from the main rectifier.
