Electric traction in India uses two main systems - 25kV AC overhead lines and 1.5kV DC third rail or overhead lines. The history of electric traction in India began in 1925 with a 9.5 mile line in Mumbai using 1.5kV DC. Electrification expanded gradually across different regions until the 1950s when India began converting lines to 25kV AC. Today most major lines use 25kV AC while some commuter lines around Mumbai still use 1.5kV DC and the Kolkata metro uses 750V DC third rail.

1. Electric Traction
Q. What are the voltages used for electric traction in India?
1.5kV DC and 25kV AC.
Calcutta had a 3kV DC system until the '60s.
The 1.5kV DC system is used around Bombay (This includes Mumbai CST - Kalyan,
Kalyan - Pune, Kalyan - Igatpuri, Mumbai CST - Belapur - Panvel, and Churchgate -
Virar). There are plans [5/99] to change this to 25kV AC in the next few years. In
preparation for this, BHEL has been retrofitting some EMUs with AC drives to allow
them to operate with both DC and AC traction as the system conversion proceeds.
The Madras suburban routes (Madras-Tambaram in the '60s, extended later to
Villupuram) used to be 1.5kV DC until about 1967, when it was converted to 25kV
AC. (This is where the MG DC locos were used, e.g., the YCG-1 series.)
The 25kV AC system is used throughout the rest of the country. The WCAM series
of locomotives are designed to operate with both DC and AC traction as they move
towards or away from the Bombay DC section.
The Calcutta Metro uses 750V DC traction with a third-rail mechanism for delivering
the electricity to the EMUs.
Q. What's the history of electric traction in India?
The first electric train ran between Bombay's Victoria Terminus and Kurla on
February 3, 1925, a distance of 9.5 miles. In 1926, Thana and Mahim were
connected. In 1927, electrification was complete up to Kalyan. In 1928, Borivili in the
north was connected. In 1929, Kalyan - Igatpuri section was commissioned. In 1930,
the Kalyan - Poona tracks were opened to electric trains. In 1931, meter gauge
track between Madras Beach and Tambaram was electrified (1.5kV DC). After that
the only electrification project undertaken was Borivili - Virar, finished in 1936. For
mainline traffic, GIPR undertook electrification of the Karjat-Pune and
Kasara-Igatpuri sections because it was realized that the heavy traffic to and from
Bombay would be suitable for electric haulage. Following this there was a long gap,
and the next electrification project started only 1953 or 1954, in the Calcutta area
(Howrah-Burdwan via Bandel, Sheoraphulli-Tarakeshwar), using 3kV DC traction. At
this time, the idea of mainline electrification (Howrah-Mughalsarai) was seriously
mooted. Support for 25kV AC traction was also growing at about this time,
especially after some trials of AC locos from SNCF, and studies that concluded that
the single-phase load from electric traction would not seriously unbalance the
3-phase regional grids. So the Calcutta area electrification was done keeping in mind
the eventual migration to 25kV AC system, in terms of the technical requirements
(insulator specifications, etc.). The first 25kV AC electrified section was
Burdwan-Mughalsarai, completed in 1957, followed by the Tatanagar-Rourkela

2. section on the Howrah-Bombay route. The first actual train run (apart from trial runs)
using 25kV AC was on December 15, 1959, on the Kendposi-Rajkharswan section
(SER). Howrah-Gaya was electrified by about 1960. Electrification till Kanpur on the
Howrah-Delhi route was done by about 1972. Through the 1960s and early 1970s
numerous studies were commissioned to investigate the question of which of diesel
or electric traction was really more economical and better in the long run for IR. Most
of these leaned towards electrification, especially for high-traffic sections. The rise in
oil prices in the mid-1970s tilted the argument further in favour of electric traction as
electricity generation in most of India is hydroelectric or coal-based.
India took the plunge from DC to AC electric traction in the mid-1950s, as mentioned
above. Since French developments led the field, the AC locomotives supplied at first
(from SNCF) followed that country's practice, whether built in India or France. These
were the eight-wheeled WAM-1 locomotives that are still in operation in some
places.
The first train to be hauled by an electric locomotive from Delhi Jn. was the Assam
Mail.
Bombay-Delhi (WR) route was fully electrified by Dec. 1987. The CR route was fully
electrified by June 1990, when the Bhusaval - Itarsi section was electrified.
The 2 * 25kV AC system (see below) began to be put in place in the 1990s; the first
regular service using this system was between Bina and Katni (CR) on January 16,
1995. This was later extended to Bilaspur.
The Madras Beach -- Villupuram section remains the only MG electrified section in
the country.
Q. How does electric traction work?
In OHE, or overhead electrification systems, the supply of electricity is through an
overhead system of suspended cables known as the catenary. A contact wire or
contact cable actually carries the electricity; it is suspended from or attached to
other cables above it which ensure that the contact cable is at a uniform height and in
the right position. In the following the term catenary is loosely used even when talking
about the contact wire.
The loco uses a pantograph, a metal structure which can be raised or lowered, to
make contact with the overhead contact cable and draw electricity from it to power
its motors. (Usually it goes first through a transformer and not directly to the motors.)
The return path for the electricity is through the body of the loco and the wheels to
the tracks, which are electrically grounded. Modern electric locos have some fairly
sophisticated electronic circuitry to control the motors depending on the speed, load,
etc., often after first converting the incoming 25kV AC supply to an internal AC
supply with more precisely controlled frequency and phase characteristics, to drive

3. AC motors. Some AC locos (WAG-4, WAM-4) have DC motors, instead. Some AC
locos (WAP-5 and WAG-9, both designs from ABB) generate 3-phase AC internally
using a thyristor converter system; this 3-phase supply is then used to power
asynchronous AC motors. (3-phase AC motors are somewhat more efficient, and
can generate higher starting torque.)
In 3rd-rail systems, electricity is supplied through a thick conductor (the third rail)
running along the track; the loco has a shoe which maintains sliding contact with it
while the train is in motion, to draw current from it. Third-rail traction is seen only in
the Calcutta Metro in India. 3rd-rail systems are usually DC systems at much lower
voltages (500V-750V or so).
Single system (AC): The overhead catenary is fed electricity at 25kV AC
(single-phase) from feeding posts which are positioned at frequent intervals
alongside the track. The feeding posts themselves are supplied single-phase power
from substations placed 35-60km apart along the route. The substations are spaced
closer (down to 10-20km) in areas where there is high load / high traffic. (These
substations in turn are fed electricity at 132kV AC or so from the regional grids
operated by state electricity authorities.) A Remote Control Centre, usually close to
the divisional traffic control office, has facilities for controlling the power supply to
different sections of the catenaries fed by several substations in the area.
DC System: In DC systems with overhead catenary, the basic principle is the same,
with the catenary being supplied electricity at 1.5kV DC. Usually (especially for
EMUs) the current from the catenary goes directly to the motors. A DC loco may
however convert the DC supply to AC internally using inverters or a motor-generator
combination which then drives AC motors.
The Calcutta Metro uses a 3rd-rail system at 750V DC.
Variations: There are variations on the basic AC scheme described above. For
instance, with the basic system described above, the 25kV AC supply to the
energized catenary can cause severe interference in telecom and other circuits which
are near enough, by induction -- the induced voltages can be quite high, in the
hundreds of volts. To mitigate this problem, a return conductor is provided parallel to
the catenary (usually a little above and to the side of the catenary). There are also
booster transformers provided at intervals. The primary winding of such a booster
transformer is in series with the catenary and the secondary winding in series with
the return conductor. This forces a voltage in the return conductor which is always
opposing the voltage in the catenary, thereby reducing the induction effects in
electronic circuits nearby. This method usually reduces the induced voltages in
telephone and other circuits to below 60V (although under fault conditions
400V-500V or more of induced voltage is not unusual). Sometimes the return
conductor is also connected to the rails at some points, to ensure that more of the
return current flowing through the rails stays in them rather than going through the
surrounding ground, which can also cause interference and other problems, although

4. not as bad as those seen in DC systems.
Dual system (AC): In the dual AC system (or 2 * 25kV AC system), the supplied
electricity from the substations is actually at 50kV AC. An autotransformer connected
across the 50kV supply has a centre tap connected to the rails, whereas connections
from either end of the transformer winding go to the catenary and the return
conductor. Thus, there is a 25kV differential between the catenary and the rails as
before, but now there is a 50kV difference between the catenary and the return
conductor. (Note, however, that the centre tap of the autotransformer, and hence the
rails, are grounded to avoid hazards to people or animals walking across the tracks.)
Also, at points further away from the substation, the centre tap of the
autotransformer can be adjusted so that the voltage between catenary and rails is
always 25kV, even as the total voltage across the catenary and the return conductor
drops due to resistive or impedance losses. So this allows substations to be spaced
further apart (50km or more). Thus far [1999] the 2 * 25kV system is in use in only
about 10% of all of IR's electrified routes. The important coal-hauling route Bina -
Katni - Anuppur - Bishrampur / Chirimiri of CR/SER has this system.
The dual system corresponds to what is called the 3-wire system (with transmission
line) in other railways.
Transmission voltage: Power is transmitted to the electrical substations at 750kV,
220kV or 132kV and then stepped down as required to 25kV or 50kV, etc.
Catenary voltage: In practice, the catenary voltage in the 25kV AC system can vary
from something like 18kV to over 30kV because of poor regulation at the substation
or incorrect configurations of the transformers, etc. Most locos are designed to
handle a certain range of catenary voltages, although of course the operation may be
less than optimal at voltages far from the norm.
Catenaries: IR uses catenaries of the constant-tension type. At one end of each
section of the catenary the cable goes over a pulley and is terminated by a hanging
weight. The pulley and weight combination ensures that the catenary cable maintains
the same tension regardless of the ambient temperature and the consequent
expansion or contraction of the cable. This avoids problems with the catenary
sagging too much in hot weather, or, if the tension is too high, snapping in cold
weather. Further, it is important that the tension be within certain precise bounds for
mechanical reasons: the moving pantograph creates a shock wave in the catenary
that travels along the cable; its speed is determined by the tension in the catenary,
and if it is less than the speed of the pantograph, the cable will be prone to buckling
and snapping. The equivalent copper cross-section of the catenary cable is usually
about 157 sq. mm. On a single-track section, this allows a current of up to 600A to
be drawn from the catenary without raising its temperature to more than about 85C,
which is the safe upper limit to avoid risks of fire, equipment failure, etc., and to
maintain the physical properties of the catenary within acceptable bounds. On the
Waltair-Kirandul section which sees extremely heavy ore traffic, catenary

5. cross-sections are up to 200 sq. mm, whereas loops, spurs, sidings, etc. often have
lower-capacity catenaries with equivalent copper cross-sections of 107 sq. mm.
At the end of each section of catenary, a new section begins, with the old and new
catenaries running in parallel for a short distance. On BG routes, this switch from
one catenary to another usually happens over a length corresponding to 4 catenary
masts, with the old and new catenaries overlapping (running parallel) for about 50m.
On MG, this is usually accomplished over a length corresponding to 3 catenary
masts, with one catenary taking off immediately after the point where the other
stops.
When successive sections of the AC catenary are supplied by different phases from
the 3-phase power grid, there is a short, electrically neutral (un-energized) section
(dead zone or neutral section) of catenary that comes between them. The loco has
to coast through this "phase break" with a brief interruption in the supply of power.
Sometimes different sections of the catenary are connected to different phases at
different times and the neutral sections may be a switched neutral section. (The
term also refers to neutral sections at AC-DC switchover points where the neutral
section can be switched to either the AC or the DC supply.) In DC catenaries, there
are similar breaks (power gaps) with neutral sections at points where adjacent
sections of catenary are supplied by different substations.
Control and Monitoring: A Remote Control Center (RCC) is located at or near the
divisional traffic control centre. The RCC has the control and monitoring equipment
for the electric traction in the areas controlled by the traffic control centre. Prior to
1980, IR used an electromechanical control system, Frequency Modulated Voice
Frequency Telegraph (FMVFT). These are still in use in some places. Since 1980, IR
has been installing a microprocessor-based system called "SCADA" (Supervisory
Remote Control and Data Acquisition System) for remote control of electric
substations and switchgear. A central SCADA facility (the division control centre) can
control a region extending to about 200-300km around it. SCADA allows remote
monitoring of electrical parameters (voltage, current, power factor, etc.) and remote
operation of switchgear, as well as automatic fault detection and isolation. SCADA
replaces an older system that used electromechanical remote control apparatus.
Q. How do the new 3-phase AC locos (WAP-5, etc.) work, and how do they
compare with the earlier locos?
Three-phase AC locos such as WAP-5 use some fairly new technology as compared
to the earlier generations of diesel-electrics and electrics. In most of the earlier
locos, the traction motors driving the axles are DC motors. DC motors were used
because they afforded (in those days) far superior speed and torque control
compared to AC motors -- the latter require variation of input frequency for effective
control, which was not an easy matter earlier.
Modern microprocessor technology and the availability of efficient and compact

6. power components have changed that picture. In 3-phase AC locos, the input
(single-phase AC) from the OHE is rectified and then 3-phase AC is generated from
it, whose voltage, phase, and frequency can be manipulated widely, without regard
to the voltage, phase, frequency of the input power from the OHE. AC traction
motors can thus be driven with a great degree of control over a wide range of speed
and torque.
Details There are 3 main stages in the power circuit of a 3-phase AC loco.
Input Converter This rectifies the AC from the catenary to a specified DC
voltage using silicon rectifiers. It has filters and circuitry to provide a fairly
smooth (ripple-free) and stable DC output, at the same time attempting to
ensure that a good power factor presented to the electric supply. There may
also be additional mechanisms such as transformers, inductors, or capacitor
assemblies to improve the power factor further.
DC Link This is essentially a bank of capacitors and inductors, or active filter
circuitry, to further smooth the DC from the previous stage, and also to trap
harmonics generated by the drive converter and traction motors. Since the
traction motors and drive converters present non-linear loads, they generate
reactive power in the form of undesirable harmonics; the DC link acts as a
reservoir for the reactive power so that the OHE supply itself is not affected by
it. During regenerative braking this section also has to transfer power back to
the input converter to be fed back to the catenary. The capacitor bank in this
section can also provide some amount of reserve power if needed by the
traction motors.
Drive Converter This is basically an inverter which consists of three
thyristor-based components that switch on and off at precise times under the
control of a microprocessor (pulse-width modulation). The three components
produce 3 phases of AC (120 degrees out of phase with one another).
Additional circuitry shapes the waveforms so that they are suitable for feeding to
the traction motors. The microprocessor controller can vary the switching of the
thyristors and thereby produce AC of a wide range of frequencies and voltages
and at any phase relationship with respect to the traction motors. Various kinds
of thyristor devices are used to perform the switching; the latest locos use
insulated-gate bipolar thyristors (IGBTs) which offer extremely high switching
speeds allowing for fine control over the waveforms generated.
The 3-phase AC is fed to the AC traction motors, which are induction motors. As the
voltage and the frequency can be modified easily, the motors can be driven with fine
control over their speed and torque. By making the slip frequency of the motors
negative (i.e., generated AC is "behind" the rotors of the motors), the motors act as
generators and feed energy back to the OHE -- this is how regenerative braking is
performed. There are various modes of operation of the motors, including constant
torque and constant power modes, balancing speed mode, etc. depending on
whether their input voltage is changed, or the input frequency, or both.
AC motors have numerous advantages over DC motors. DC motors use

7. 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. 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. In addition, DC
motors tend to draw power from the OHE poorly, with a bad power factor and
injecting a lot of undesirable harmonics into the power system. AC motors suffer less
from these problems, and in addition have the advantage of a simpler construction.
Q. How are phase breaks (AC) or power gaps (DC) handled by the
locomotives?
As mentioned above, the catenary has breaks or gaps in its electrical continuity
every once in a while at points where successive sections are connected to different
substations. A neutral section of catenary is usually provided between the two live
sections of different phases or connected to different substations. At such points,
single locomotives do not drop their pantographs, although on-board equipment such
as the traction motors, compressors, blowers, etc. are switched off manually by the
driver before the neutral section is entered. The main circuit breaker (DJ) is also
opened. (Warning boards at 1000m and 500m before the neutral section are
provided for this purpose.) In the case of multiple unit operation, however,
pantographs are usually dropped on all the lashed-up locomotives, to avoid the
possibility of short-circuiting adjacent sections of the catenary.
Q. How are DC locos swapped for AC locos at the point where traction power
changes from DC to AC?
This is what happens at Igatpuri for a DC-AC change. The catenary connected to the
DC power supply does not reach all the way to the catenary connected to the AC
power supply; there is a neutral section between them.
------------||---------|| ||---------||-------------
||---------||-------------||---------||
DC Section <------- Neutral Section ------> AC Section
The catenary spanning the neutral section overlaps with both the DC and AC
catenary sections as indicated above. This cable can be connected to either the DC
or the AC power supply. Before the arrival of the DC loco., this section is connected
to the DC supply. Once the loco. gets detached and goes off the main line (a DC
branch loop is provided for this purpose from the last DC section), the cable
spanning the neutral section is switched over to the AC power supply and energized,
so that the AC loco. can now come in. Before entering neutral sections, electric locos
often switch off power temporarily to the traction motors so as to prevent any

8. transient disturbances and sparking.
Because the neutral section is switched between AC and DC supply, it is also known
as the dynamic neutral section or the switched neutral section.
This is a different arrangement for DC/AC changeover than at a point like Virar (see
below).
Q. How do the AC-DC locos (WCAM series) switch from one power source to
another on the run?
At DC/AC changeover points as on the Virar-Vaitarna, WCAM locos can switch from
one power source to another without stopping.
The WCAM-1 has a selector on the rightmost side of horizontal control panel for
selecting the pantograph. It has four positions, DC, AC, DC-ALT and AC-ALT. In DC
and AC-ALT mode, pantograph with two collector shoes is raised; in AC and
DC-ALT, pantograph with one collector shoe is raised. The DC pantograph has two
shoes and is thicker in its contact area than the AC pantograph because it has to
carry a larger current corresponding to the lower voltage. The ALT positions allow
the DC pantograph to be used for AC traction or the AC pantograph to be used for
DC traction, in case of damage to one or the other pantograph. I.e., the pantograph
itself does not control whether the DC or AC circuitry is in use; the selector switch
controls this. In real life, the driver rarely gets a chance to see which pantograph is
up; all he knows is the position of selector switch. Sometimes when a damaged
pantograph is replaced, a pantograph of a different kind (one shoe instead of two)
may be installed; the loco still works, although perhaps suboptimally.
About the only time the driver must raise or lower the pantographs when the
locomotive is in motion is at the AC-DC changeover point a little north of Virar on the
Virar-Vaitarna section -- at a "dead zone" or neutral section where there is a length
of overhead catenary with no electricity supplied to it, between the AC and DC
catenaries. This usually extends for a length of about two or three catenary sections.
About a kilometer before this dead zone, a sign alerts driver with a "1000 meters"
warning followed by another for "500 meters" and then a sign saying "Dead Zone".
Going from Mumbai towards Dahanu, the driver shuts most of the equipment in the
loco off (air compressor charged, traction motors cut off, motor generator switched
off, etc.), then lowers the DC pantograph and just waits while the loco coasts without
power through the dead zone. until the AC section of the catenary is reached. At this
point, he raises the AC pantograph. After about 30 seconds, the voltmeter shows 25
kV and he restarts the traction and other equipment.
Note that this arrangement of the catenary is different from that at Igatpuri (see
above for DC/AC loco switchover). There, all locos have to stop and wait for the line
voltage to be switched on in the intermediate neutral section.

9. Q. What happens if the wrong selection has been made at the wrong time ?
Not all IR locos have protection against incorrect line supplies, and the loco can be
severely damaged in such cases.
In some cases, this will blow a fusible link located near pantograph, and the driver
will have to raise the appropriate ALT pantograph to continue. No further damage is
possible because the only equipment that is live when pantograph is being raised is
the voltmeter. All others like the compressor, exhauster, and motor-generator have
to be switched on manually after the pantograph is raised and the voltmeter shows
the correct reading.
Q. What happens if the pantograph isn't lowered when the loco enters the
dead zone?
If the live loco enters this section then there is a possibility of sparking or transient
disturbances, which can trip circuit breakers in the loco and bring the train to a halt or
worse trip breakers for the substation and bring all the traffic to a halt. Regardless of
that, once the loco enters the dead zone it loses power and will grind to a halt once it
loses its momentum, if it cannot coast all the way to the next live section.
Q. When does the driver have to lower and raise a pantograph on the run?
Normally, pantographs do not have to be lowered and raised on the run. The
principal exception is the case of the AC-DC switchover by the WCAM series locos
as described above. Other than that, there are a few points where the catenary has
a gap (no cable physically present, not even a neutral or dead section), for instance
at level crossings where there is provision for extra-tall road traffic, in which case the
pantograph has to be lowered as the loco coasts through the gap. Also, pantographs
may be lowered and raised occasionally for troubleshooting if the driver suspects a
problem.
In the AC sections, when the phase of the overhead cable's power supply changes
(at the "phase breaks") the pantograph need not be lowered and raised at the dead
zone. This happens in many places, e.g., a couple of times on the Virar-Surat,
Surat-Baroda routes. Usually the driver will switch off and switch on the equipment in
the loco in order to prevent transient effects from damaging the equipment.
Q. How does regenerative braking work?
Regenerative braking works on the principle of converting the kinetic energy of the
locomotive (and train) back to electricity by using the traction motors in reverse (as
generators) and feeding the electricity back to the OHE. This is somewhat easier
with DC than with AC traction as with the latter the phase and frequency of the
generated electricity have to be matched to that of the OHE. The newer AC locos
have microprocessor control to aid in this. The regenerated voltage is in effect the

10. loco presenting a negative load to the OHE system, which manifests itself as a slight
rise in the system voltage. Controllers at the substations sense this and above a
certain threshold cut out the generators that are feeding power to the catenary, and
instead switch in circuits that distribute the regenerated electricity back to the grid
where it can be used. This is not easy, though, and in practice the regenerated
power is sometimes just dissipated using large resistive loads at the substation or
elsewhere. Conversely when the system voltage starts dropping, it is an indication
that the locomotive(s) on the section is/are not generating power and are instead
consuming power (the normal case) in which case the normal power supply is
switched back in to feed the catenary.
Q. Where can I learn more about electric traction?
More information can be found on the web and elsewhere. For a start, try these
FAQs and glossaries.
Q. What is NETRA?
NETRA (which stands for Network of Electrification -- Testing and Recording
Apparatus) is a special coach designed for inspection of the catenary in electrified
sections. It has an IRY shell mounted on IR-20 bogies, with a rated top speed of
160km/h. (It is an unpowered vehicle and must be hauled by a loco.) It has a
pantograph (a dummy -- does not draw current) which is used to test and record
various mechanical characteristics of the pantograph-catenary interaction while the
coach is in motion.