Electric Traction - I

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Electric Traction Voltages

Q. What are the voltages used for electric traction in India?

The 25kV AC system with overhead supply from a catenary is used throughout all of IR's electrified lines.

The Bombay region had 1.5kV DC until 2016. Calcutta had an overhead 3kV DC system until the 1960s.

The Calcutta and Bangalore Metro use 750V DC traction with a third-rail mechanism for delivering electricity to the EMUs.

The Calcutta trams use 550V DC with an overhead catenary system with underground return conductors. The catenary is at a negative potential.

The Delhi Metro uses 25kV AC overhead traction with a catenary system on the ground-level and elevated routes, and uses a rather unusual 'rigid catenary' or overhead power rail in the underground tunnel sections (Line 2).

The 1.5kV DC overhead system (negative earth, positive catenary) was used around Bombay (This includes Mumbai CST - Kalyan, Kalyan - Pune, Kalyan - Igatpuri, Mumbai CST - Belapur - Panvel, and Churchgate - Virar). Conversion to 25kV AC began in 2003 with stabling lines and yards being the first to change over. Throughout the 2000s and the early 2010s, CR and WR both began converting their lines. First by energizing yards and stabling lines to the newer voltage, then by moving over the mainlines block by block. By 2016, both WR and CR had converted their traction to AC. Aiding these conversions were AC-DC EMUs, both retrofitted and new and the WCAM series of locomotives.

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 (all overhead catenary supply). (This is where the MG DC locos were used, e.g., the YCG-1 series.)

History of Electrification

Q. What's the history of electric traction in India?

The first electric train ran between Bombay's Victoria Terminus and Kurla along the Harbour Line of CR, 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 (Colaba-Borivili of WR being inaugurated on May 1). In 1929, Kalyan - Igatpuri section was commissioned. In 1930, the Kalyan - Poona tracks were opened to electric trains.

On November 15, 1931, electrification of the meter gauge track between Madras Beach and Tambaram was inaugurated (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 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, and the entire Howrah-Delhi route was electrified on August 5, 1976. The Bombay-Delhi route was electrified by February 1, 1988.

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.

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 Bishrampur.

Electrification progressed steadily in the 1990s and 2000s with the pace dictated by budgetary allocations and traffic requirements. In early 2019, IR decided to electrify all of its line, including branch and low traffic spurs. This work is expected to be finished by 2025. Mountain, hill and heritage lines are not expected to be converted.

The only electrified MG line on IR was the Madras Beach - Villupuram stretch on the chord route to Tiruchirapalli. Madras Beach - Tambaram was originally on 1.5kV DC electrification but was converted around 1968 to the 25kV AC system along with the extension of traction to Villupuram. YAM-1 locomotives were the workhorses on this line until their withdrawal in 2004 because of the steady conversion of the MG line to BG.

Was Mumbai's the first electric service in Asia?

It is sometimes stated that the electric train in 1925 was Asia's first electric, or electric suburban, train service. This is however not true, because electric services were running in Japan since Jan. 31, 1895, on the Kyoto Electric Railway (officially listed under 'Exploitation Department', Kyoto Municipality, in the annual reports of the Government of Japan's Department of Railways), but this is sometimes classified as a tramway instead of a light rail system. (Interestingly, this was also the year that the first Japanese steam locomotive was manufactured.) Electric railcar services ran on the Government Railways of Japan from about 1905, and (German-built) electric locomotives were introduced in Japan in 1911. In 1919 the first entirely Japanese electric locomotive was built (a class ED-40). In Indonesia, the first electrified section (1500V DC) of the State Railways opened at Batavia (Jakarta) in 1925, the same year as in Bombay.

See also CORE's chronology of IR electrification (data updated until 2004).

3-phase Locomotives

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 and voltage for effective control, which was not an easy matter earlier.

Modern microprocessor technology and the availability of efficient and compact 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.

AC traction motors are also used on diesel-electrics nowadays. The WDP-4, WDG-4, WDG-4G and WDG-6 and their variants are examples of this.

Details of 3-phase locomotive operation

Input Converter: This rectifies the AC from the catenary to a specified DC voltage using GTO (gate turn-off) thyristors or insulated-gate bipolar transistors (IGBTs). A transformer section steps down the voltage from the 25kV input. 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.

The transformer section is designed with high leakage impedance and other characteristics, which together with the fine control possible with the GTO switching, allow the loco to present nearly unity power factor, a very desirable situation from the point of view of the electricity suppliers (the grid). The main transformer also has some filter windings which are designed to further attenuate harmonics from the loco's traction motors which may pass through the filtering in the DC link.

The input converters can be configured to present different power factors (lagging or leading) to the power supply, as desired. WAP-5, WAP-7 and other 3-phase AC locos are generally configured to present a unity power factor (UPF). (Note: the power factor cannot be changed on the run.)

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.

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 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 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 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 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.

(07/2020) Currently produced modern locomotives use insulated-gate bipolar transistors (IGBTs), which offer extremely high switching speeds allowing for finer control over the waveforms generated. When introduced, the WAP-5, WAP-7, WAG-9 and its variants used GTO thyristors (Gate Turn-Off thyristors). In 2010, IR began trials of WAP-7 and WAG-9 fitted with IGBTs with serial production starting sometime in 2015. While retrofitting of IGBTs on older locomotives has begun, there are still hundreds in active service that use GTOs.

Neutral/Dead Zones

Q. How are phase breaks (AC) or power gaps (DC) handled by the locomotives?

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 500m and 250m before the neutral section are provided for this purpose). (Earlier, locos used to routinely drop their pantographs for all neutral sections; this is no longer standard practice.)

In the case of multiple unit operation, however, pantographs are sometimes dropped on all the lashed-up locomotives, to avoid the possibility of short-circuiting adjacent sections of the catenary. (The possibility is remote, as normally there is no power flow between lashed-up units, hence the pantographs may not always be dropped, depending on the particular operational procedures of a division.)

Q. Why is the neutral section provided with a dummy (neutral or electrically dead) cable? Why can't it be a real gap?

Pantographs of electric locomotives have a spring mechanism or compressed-air assembly that keeps the pantograph pushing up against the contact wire with a certain specific pressure. If the neutral section were not wired and the contact wire simply ceased to exist, then then possibility exists that if the driver has not dropped the pantographs at the time the loco reaches the neutral section, then the pantograph will suddenly rise upwards unchecked; when the loco reaches the other end of the neutral zone, it is then likely to smash into the catenary where the next contact wire section begins. It should be noted that in practice, at neutral sections where it is or was a requirement to drop the pantographs, it has been observed that IR crews almost never forget to do so. But now with more locos and neutral sections coming up which do not require the pantograph to be dropped, this does become a concern.

Q. How did DC locos get swapped for AC locos at the point where traction power changed from DC to AC?

When the Mumbai area was DC, there were a couple of important AC-DC transition points such as Igatpuri and Virar. Both these points had different arrangements for traction changeover.

This is what used to happen at Igatpuri until February 2006 for a DC-AC change. The catenary connected to the DC power supply did not reach all the way to the catenary connected to the AC power supply; there was a neutral section between them.

Diagram of AC/DC neutral section overlap

The catenary spanning the neutral section overlapped 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 was connected to the DC supply. Once the loco gets detached and goes off the main line (a DC branch loop was provided for this purpose from the last DC section), the cable spanning the neutral section was switched over to the AC power supply and energized, so that the AC loco could now come in. Before entering neutral sections, electric locos often switched off power temporarily to the traction motors so as to prevent any 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 was a different arrangement for DC/AC changeover than at a point like Virar (see below).

Q. How did 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 and later on Borivali-Dahisar sections, WCAM locos could switch from one power source to another without stopping.

The WCAM-1 had a selector on the rightmost side of horizontal control panel for selecting the pantograph. It had 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 had two shoes and is thicker in its contact area than the AC pantograph because it had to carry a larger current corresponding to the lower voltage.

The ALT positions allowed 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 did not control whether the DC or AC circuitry was in use; the selector switch controls this. In real life, the driver rarely got a chance to see which pantograph is up; all he knew was the position of selector switch. Sometimes when a damaged pantograph was replaced, a pantograph of a different kind (one shoe instead of two) may have been installed; the loco still worked, although perhaps suboptimally.

About the only time the driver would have raised or lower the pantographs when the locomotive was in motion was 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 was a length of overhead catenary with no electricity supplied to it, between the AC and DC catenaries. This usually extended for a length of about two or three catenary sections. About a kilometer before this dead zone, a sign alerted 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 shut most of the equipment in the loco off (air compressor charged, traction motors cut off, motor generator switched off, etc.), then lowered the DC pantograph and just waited while the loco coasted without power through the dead zone until the AC section of the catenary was reached. At this point, he raised the AC pantograph. After about 30 seconds, the voltmeter showed 25 kV and he restarted the traction and other equipment.

Note that this arrangement of the catenary was different from that at Igatpuri (see above for DC/AC loco switchover). There, all locos had to stop and wait for the line voltage to be switched on in the intermediate neutral section.

Just before the dead zone, there was also a sign, 'Open DS for speeds below 40km/h'. The 'DS' was the main Disconnecting Switch, a manually operated circuit breaker in the DC supply path from the pantograph, that isolated and grounded the 1.5kV DC downstream circuits from the 25kV supply. If the speed was below 40km/h, the driver needed to keep on accelerating until the very last moment and then throw this switch to isolate the DC circuits on the fly.

This tricky manoeuvre was necessary when the speed was that low, because of the danger of losing momentum and stopping in the dead zone without power in case of any adverse conditions like emergency brake application, or brake pipe parting, etc. (The dead zone was one length of catenary and considering the cross-over structures on the DC and AC sides it was nearly two lengths, hence the loco and train had to have enough momentum for the loco to get across this distance).

Q. What happens if the wrong selection had been made at the wrong time?

Not all IR locos had protection against incorrect line supplies, and the loco could have been severely damaged in such cases.

In some cases, this would have blown a fusible link located near pantograph, and the driver would have had to raise the appropriate ALT pantograph to continue. No further damage would have been possible because the only equipment that was live when pantograph was being raised was the voltmeter. All others like the compressor, exhauster, and motor-generator had to be switched on manually after the pantograph was raised and the voltmeter showed the correct reading.

Q. What happens if the pantograph isn't lowered when the loco enters the dead zone?

Usually there is no problem, if the master circuit breaker of the loco has been switched off. In most cases of neutral sections, therefore, the driver does not have to lower the pantograph. If a live loco enters this section without its master circuit breaker turned off, then there is a possibility of sparking or transient disturbances, which can trip protective circuits in the loco and bring the train to a halt. (Rarely, it may trip breakers for the OHE and bring all the traffic to a halt.) Regardless of this, and whether or not the pantograph is lowered, 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 was 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. The catenary may also be missing for short sections above diamond crossings or complex track configurations. 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. 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. Why do locos sometimes use the rear pantograph and sometimes the front pantograph?

There is in principle no difference between using the front and the rear pantographs for most locos as each is fully capable of delivering the required electric current from the catenary to the loco. (The AC-DC locos were special in that each pantograph was intended for a different traction supply.) Locos running under high-rise OHE sections raise a pantograph that is able to reach the height of the catenary. Many locos sport at least one of these high-reach pantographs now.

Generally on IR there is no need for both pantographs to be raised at once since there are usually no unusual situations such as frost on the catenary or increased current collection requirements seen with other countries' railways.

Yet, it is often seen that there are some definite patterns in pantograph usage. For running under high-rise OHE, the front most pantograph in the direction of travel is raised. This is because on the front pantograph, the angle of the knee is trailing (<) rather than leading (>). IR's tests have show that such an arrangement helps the pantograph make better contact with the OHE wire, avoid excessive sparking and loss of power.

For regular height OHE operations, it has been the practice in many areas for locos to always have their rear pantographs up. It is thought that this practice arises from the idea that entanglement of the catenary by the front pantograph may result in damage to the rear pantograph as well as the debris or broken equipment lands on it, and using the rear pantograph lessens the chance of this. However, in recent years, this does not seem to have been adhered to very much.

Another pattern that has been seen, especially in northern India, is for the front pantograph to be used extensively in the winter, but not in the warmer months. As a variation of this, it is also known that in certain divisions or zones, orders have been issued for drivers to use the rear pantographs at night. While the reasons for these usage patterns are not entirely clear, it is thought that there is a concern about condensation and the accumulation of dew on the catenary. An adequate technical explanation for the pantograph usage pattern is not known at this time. (Please note - theories about falling water from dew on the catenary causing short circuits in loco equipment are implausible considering that locos operate just fine in heavy rain.) It has been suggested that front pantograph use may be a historical vestige from British practice carried over from conditions in the UK where sometimes the front pantograph was raised to scrape ice from the catenary and allow the rear pantograph to collect current fully, but this has not been substantiated either.

For more on electric traction systems, regenerative braking etc. proceed to the next page.