ELECTRIC TRACTION SYSTEM
Introduction
Act of drawing or state of being drawn propulsion of vehicle
is called tractions. There are various systems of traction
prevailing in our country such as steam engine drive,
electric drive. These systems of tractions may be classified
broadly into groups namely
1) The traction system which do not involve the use of
electricity at any stage and called as non-electric tractions
system such as steam engine drive, IC engine drive etc.
2) The tractions system which involves the use of electricity
at some stage and called as electric tractions. System such a
diesel electric drive, electric drive etc.
In India electrification in tractions are conducted
with three types of locomotives.
1. Using single phase A.C. series commutator
motor.
2. Using D.C. motor with tapped transformer
and rectifier.
3. Using phase converter and induction motors.
Existing Tractions System
Existing tractions system uses D.C. motors.
a) The 25 KV over head voltage is step down to 2000 V with
the help of step down transformer.
b) Rectifier rectifies this A.C. voltage to D.C. voltage.
c) This rectified D.C. voltage is used to operate the D.C.
motors in existing system engine.
Causes favouring the DC motors
1) D.C. series motors are less costly, however for some H.P
more efficient and requires less maintenance than A.C.
series motor.
2) Rail conductor system of track electrifications which is less
costly with D.C. system than with A.C. system
Future Trends Of Tractions System
There are some disadvantages of D.C. series
motor used in system.
1) D.C. motors commutator which prove to
failure because of vibrations and shock. This
results in lots of sparking and corrosion.
2) It is hard to use a D.C. motor for regenerative
braking and for this purpose extra switchgear
is required, which adds to the bulks and
increases the complexity of the locomotives.
This short coming from this overcome by using three
phase A.C. motor in locomotive. Microprocessor
technology and availability of efficient and compact
power components have given a new technology for A.C.
locomotive.
1. In three phase A.C. locos, the single phase input signal
from overhead equipment is rectified
2. then three phase A.C. is generated with the help of
three phase inverter, whose phase voltage and
frequency can be manipulated widely.
3. The three phase induction motors are simple and
robust in construction and have a high operating
efficiency and properly of automatic regenerative
braking with requiring additional equipment.
There are three main stages in power
circuit of three phases locomotive.
Input converter
1.
2.
3.
4.
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
1. This is essentially a bank of capacitor and inductor
or active filter circuitry to further smooth.
2. Also to trap harmonics generated by drive
converter and traction motors.
3. 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.
Advantages Of AC Motors Over DC Motors
1. DC motors use commutators which are prone to failure
because of vibration and shock, and which also result in
a lot of sparking and corrosion. Induction AC motors do
not use commutators at all.
2. It is hard to use a DC motor for regenerative braking,
and the extra switchgear for this adds to the bulk and
complexity of the loco. AC motors can fairly easily be
used to generate power during regenerative braking.
3. In addition, DC motors tend to draw power with a bad
power factor and injecting a lot of undesirable
harmonics into the power system.
4. AC motors have the advantage of a simpler construction.
System Of Supply
25 kV AC single phase
For traction substation (TSS) the incoming EHV supply is
220/132/110/166KV through protective equipment it
can be transformed by using traction transformer to 25
KV AC single phases.
• Spacing between TSS is 30 KM to 40 KM depending
upon the traffic (load).
• To avoid load on one phase and balancing the incoming
supply grid, the section TSS is divided into sub-sector
through switching posts.
1. S.P:- Sectioning and paralleling Post.
2. S.S.P:-Sub-sectioning and paralleling Post.
Traction transformer
There are two traction transformer connected in
parallel of same rating for the purpose of
reliability the rating of transformer are
Capacity: 20,000 KVA. Frequency: 50 Hz
Full load: 25 KV Preferred: 0.9
Zero load: 27 KV
Power supply for signaling
For the purpose of signaling and reliable
operation the 25 KV is converted to 240 V
through auxiliary transformer by tapping 25 KV
OHE at the places where needed.
Sectioning and paralleling post (SP)
1.
2.
3.
4.
These post are situated approximating midway between feeding posts
making the demarcating points of two zones fed from different phases
from adjacent substations.
At these posts, a natural section is provided to make it impossible for
the pantograph of an electric locomotive or EMU train to bridge the
different phase of 25 KV supply, while passing from the zone fed from
one sub-station to the next one.
Since the natural section remains “dead” warning boards are provided
in advance to warn and provided Remind the driver of an approaching
electric locomotive/EMU to open locomotive circuit breaker (DJ) before
approaching the ‘neutral section’ to coast through it and then switch
‘on’ on the other side.
Special care is taken in fixing the location of natural sections, on level
tangent track far away from signals, level crossing gates etc. to ensure
that the train coasts through the neutral section at a sufficiently high
speed, to obviate the possibility of its stopping and getting stuck within
the neutral section .
Sub-sectioning and Paralleling post (SSP)
One and more SSPs are providing between each
SP and adjacent SP depending upon the distance
between them. In a double track section,
normally three interrupters are provided at each
SSP i.e. two connecting the adjacent sub-sectors
of up and down tracks and one for paralleling
the down tracks.
Equipments at switching sections
Certain equipments are installed at various points to protect the lines, to
monitor the availability of power supply and provide other facilities.
These are generally as under
I. Lighting arrestors are provided to protect every sub-sector against
voltage surges.
II. Auxiliary transformers are provided at all the posts and also at certain
intermediate points to supply AC at 240 V, 50 HZ required for signaling
and operationally essential lighting installations. To ensure fairly steady
voltage, automatic voltage regulators are also there, where required.
III. Potential transformers are provided at the various switching stations for
monitoring supply to each sub-sector.
IV. A small masonry cubicle is provided to accommodate remote control
equipment, control panel, telephone and batteries and battery chargers
required for the control of interrupters and the similar equipments.
Control Equipments
To segregate the faults and to isolate the faulty
section, the circuit breaker at traction substation
and interrupters at SP/SSP’s are being remotely
control through remote control centre. A Remote
Control Centre (RCC) is set up near the traffic
control office on each division having electric
traction to work in close liaison with the traffic
control. The RCC includes the main control room,
equipment room, uninterrupted power supply
(UPS) room, remote control laboratory and battery
room and is the nerve centre of the traction power
control.
The following types of remote control equipment are
mainly in use on Indian Railways at present
I. Frequency modulated voice frequency telegraph (FMVFT):
This equipment was in use for all electrification schemes
prior to 1980. Being mainly all relay system, the
equipment has become outdated although some remote
control centers still continue to operate on this system.
II. Supervisory control and data acquisition (SCADA)
systems with microprocessor: The SCADA equipment
based on state of the art technology has come into use
after 1980. Considering the fast growth and development
of computer based equipment, newer types with
enhanced capabilities are being introduced.
The SCADA equipment at the RCC is called master station
while that of the controlled station is referred as remote
terminal unit (RTU).
Remote Terminal Unit (RTU)
The RTU is microprocessor based and include its
associated digital input/output modules, alarm
input modules, analog input modules, watch
dog transducer memory modules, interposing
relay summation CTs, power supply unit(s) surge
resistor and other items necessary for its proper
working.
Transmission Path
Under ground telecommunication trunk cable is
provided for transmitting the signals from and to
the Remote Control Centre (RCC) and the
controlled Remote Terminal Units (RTU). Three
pairs of conductors (one pair for send one pair
for receive and the third as spare) from this
cable are made available for remote control
operation.
Microwave Communication
In some of the sections on Indian railways dedicated
microwave channel at carrier frequency of 18 GHz has
been provided for the purpose of Communication.
Optical Fiber Cable
Optical fiber cable has also been introduced for
communication in some section of Indian railways, which
is also used for Remote Control equipment. Details of
interface between latest communication system and the
RCC/RTU equipment may be seen in the relevant
technical document.
UPS and Batteries at RCC
1. Dual stand-alone UPS system of adequate capacity to
supply 240V A.C., 50 Hz single phase supply to the
SCADA system at master station is provided. Both the
UPS work in parallel sharing the load.
2. In case of failure of one the entire load is automatically
taken over by the healthy UPS without affecting the
working of the system.
3. In case of outage of both the UPS at the same time, the
load of SCADA equipment is directly connected to input
main though a static switch without any break.
4. A single battery is provided with both the UPS with
adequate capacity to provide the supply to various
equipments is case of failure of input for 415V A.C., 50
Hz supply.
Electric Locomotives
Introduction
Electricity is used to drive the electric locomotives.
Two types of vehicles are used for electric traction.
The first type of vehicle receives power from a
distribution network while the second type of
vehicle generates own power. In the second
category comes a diesel engine electric drive. The
former types of vehicle are used on ac or dc power
from the overhead line.
Main Function Of Various
Components
Catenary
This is an overhead
wire of special cross
section. The catenary
is supplied at 25 KV;
50 Hz supply. AC
voltage is trapped
from catenary by
pantograph.
Pantograph
Pantograph is a current
collecting device which is
mounted on both ends of
locomotive
roof
on
insulator and collect the
current from Over Head
Extension and supply it
to power system of
locomotive at various
speed and different
climate/wind condition
smoothly.
Circuit Breaker
The
high
voltage
circuit
breaker
is
special type of electro
pneumatic contactor
mounted on the roof
of the loco. The
electrical equipment
of
the
loco
is
connected
to
or
disconnected from the
OHE by means of the
circuit breaker.
Loco Transformer
This is a main transformer
of locomotive. The 25 KV
single phase AC power
supply of OHE is fed to
the winding of regulating
transformer
through
main
bushing.
The
winding is equally divided
into 32 taps. These taps
are connected to tap
changer.
Tap Changer (Graduator)
1.
2.
3.
4.
5.
6.
7.
The tap changer is directly built on to the transformer. The tapings
of the transformers are brought out and arranged in circular
fashion on an insulated contact plate.
There are two rows of contact segments which are aligned on
outer and inner circles of the contact plate.
An arm which is known as selector arm is driven by shaft at the
centre of the contact plate.
Two rollers are situated at the edge of the selector arm. These
rings are provided in front of the contact plate.
The centre shaft which extends outside the tap changer casing is
driven by an air servomotor.
The design of the air servo motor is such that once the selector
arm begins its movement, it can be stopped only at the required
tap (not in between two taps).
The connection between the inner or outer ring to the
transformer is being established by means of a contactor.
Rectifier
1.
2.
3.
In electric loco
mainly two silicon
rectifier blocks RSI1
and
RSI2
are
provided
for
converting AC to
DC.
They are bridge
connected.
Continuous current
rating of each block
is 1000 amps.
Arno Converter
1. Arno converter is a device
which convert single phase
AC in to three phase AC.
2. The three phase supply
needed for the three
phase induction motors
which used in blowers,
exhausters an oil pumps.
To supply three phase
power to three phase
induction motors arno
converter is used.
3. Arno converter is rotating
device.
Brief Data Of Arno Converter
Single phase input
KVA 150
Volts 380
Amps 395
Three phase output
KVA 120
Volts 380
Amps 190
Due to the voltage variation the speed of the Arno is also
varies so the three phase output supply is not constant but
varies with the OHE voltage which is not desired. Due to this
reason now a days Arno converter are replaced by solid state
static converter unit which is known as SI unit.
Traction Motors (DC Series Motor)
1. The traction motor is a dc series motor, four pole or
six pole, forced ventilated machine arranged for axle
mounting on sleeve baring and supported on the
opposite by the resilient suspension unit, transverse
movement is limited by the flanges of axle.
2. These motors are axle hung, nose suspended type
and are provided with grease lubricated roller
bearings for armature as well as for suspension.
3. Special provision has been made in design of the
motors to ensure the locomotive operates
satisfactorily on flooded track, to max. Flood level of
20 cm , above rail level.
The main parts of motors are given below
1. Magnet frame armature and shields.
2. Brush holders and brushes.
3. Commutator
4. Armature
5. Stator
6. Armature windings
7. Field windings
8. Inter poles
9. Armature bearings
10. Axle bearing
1. Generally in locos there are six traction motors.
2. Three motors per bogies and each motor driving
one axle directly through gears.
3. The motors M1 to M6 are supplied jointly by the
two silicon rectifiers connected in parallel
through two contactors.
4. Each rectifier units is connected to separate
secondary winding and smoothing the current
thus rectified in carried out by means of two
smoothing reactors.