Lesson 2 Electric Traction

2.0 ELECTRICTRACTION In this lesson, we shall talk about electric locomotives and motor coaches and the electric traction distribution system.

2.1 Traction Systems Since 1957, the standard traction supply in India has been 25 KV, 50hertz, 1-phase a.c. However, sections in and around Mumbai, electrified in 1920’s and 1930’s large sections continue to be supplied at 1500Vd.c. In either case, the supply to the locomotive/motor coach is through a contact wire hung from a catenary wire with the help of of droppers, and the return path for the current is provided by the rail. A new system of electric traction supply has been commissioned on Bina-Katni-Anuppur-Chirmirei sections in Central India, which carry heavy coal traffic. Besides the two conductor wires as above, there is another called the negative feeder wire at a voltage of 25KV a.c. it is at a negative potential with respect to the rail, and provides the return path for the current.

2.2 Electric Locomotives Electric Locomotives, both D.C. and a.c., can be broadly classified into three classes based on applications namely Passenger, Goods, and Mixed i.e. both passenger and goods. For a.c. these are codified as WAP, WAG and WAM respectively. Were W stands for broad gauge, A for a.c., and P, G and M stand for Passenger, Goods and Mixed respectively. With C standing for D.C. the three classes are WCP, WCG and WCM respectively for D.C. locos. Those designed to work on both a.c. and D.C. supplies area coded as WCAM. On a.c. sections, we have WAG and WAM class locomotives in large numbers, while some WAP class locomotives have been added in recent years to haul high speed trains. The horse-power and maximum speed of these locomotives working on a.c. sections are typically as below: WAG4 - 3150 h.p. 80 Kmph; WAG 9 - 6120 h.p. 100 Kmph; WAG5 - 3850 h.p. 80 Kmph; WAM4 - 3640 h.p. 120 Kmph; WAP3 - 3900 h.p. 140 Kmph; WAP4 -9 - 5000 h.p. 160 Kmph; Of the above, WAG 5 and WAM 4 class locomotives are the present work-horses of Indian Railways. These are described in detail in the following paragraphs. Some 6000h.p. locomotive have been imported from Japan, Sweden, and recently Switzerland. Trials are on with the Swiss locomotives which have 3-phase a.c. traction motors. Important equipment on the locomotive or the motor coach include the following: (i) Pantograph - There is a pair of pantographs, one working and the other standby, mounted on the roof to

collect current from the contact wire. (ii) Circuit breaker – It is mounted on the under-side of the roof and provides protection in case of a fault in any

equipment or cable in the locomotive (a slant live ) motor coach. (iii) Transformer - It is only in a.c. stock to step down the high voltage of 25KV a.c. to about 1.5 KV a.c. There

are taps on the primary winding, to vary the voltage and thus the speed of traction motors. (v) Rectifier – There are two blocks one for each group of D.C. traction motors. It is obviously required in a.c.

stock only. (vi) Traction motor – There is generally one D.C. series wound motor mounted on each axle/bogie. (vii) Resistances-These are for voltage regulation and speed control in a D.C. Locomotive (a slant live) motor

coach. (viii) Bogie and wheel sets – Generally there are three wheel sets supporting each bogie, but some older classes

have two per bogie. (ix) Braking system – The stock acquired in recent years is air-braked while earlier stock is vacuum braked. 2.3 MAIN ELECTRICAL EQUIPEMENTS Pantograph – It is raised to touch the contact wire through a pair of mild-steel or carbon-fibre strips fitted on a pan. The pan is attached to light aluminum castings which would break if hit by a fouling object, and thus save damage to the overhead line. The locomotive can continue its journey using the second pantograph after securing the damaged one. Compressed air is used to raise the pantograph mounted on porcelain insulators. Circuit breaker – The D.C. Locomotive has a 1.5 KV high speed circuit breaker, while a minimum-oil type 36KV circuit breaker is provided on the a.c. locomotive. In the last decade, vacuum type circuit breakers have been tried successfully and are now in use in several a.c. locomotives. These can clear a fault in 20 mili sec. and require practically no maintenance. The D.C. circuit breaker can also clear a fault as fast, since it incorporates a powerful magnetic blow-out with narrow arc-chute. Transformer – The a.c. locomotive has an oil-cooled transformer. The oil is in turn cooled by an air-blower. The transformer is essentially an auto-transformer with several tapings, the number going as high as 32 in some

cases, on the high-voltage winding. Since tap-positions can be changed while the current is being drawn, this is called an on-load tap-changer. Transformer ratings in WAG 5 and WAM4 classes are 3900 KVA and 3460 KVA respectively. Rectifier - While earlier locomotive purchased for a.c. traction had mercury are rectifiers, silicon rectifiers employing solid state devices have been in use for the lasts 3 decades. The output current of the single phase rectifier is not a smooth one. The resultant pulsation of the field in the traction motor would cause its overheating with bad commutation and sparking. The output current of the rectifier is therefore passed to the traction motor through a smoothing reactor which is similar to a choke in construction. Ratings of each of the two air-cooled rectifiers in WAG5 and WAM 4 classes respectively are 2700 A/ 1050V and 1000A/1207V. Traction motor – The D.C. series motor is the most suitable one for the traction application since it has a high starting torque and a high free-running speed, as well as an inherent protection against excessive overloading as its speed decreases automatically with an increase in the torque. The Present-day series motors have been designed to withstand sudden rush of current due to a sudden voltage rises or a temporary interruption of supply. Special features of a traction motor as compared to an ordinary motor include minimum weight combined with robustness, smaller dimensions due to rail gauge and wheel diameter restrictions, and capability to withstand higher speeds. Cooling of the motor is provided by air blown into it by a blower. For a practical wheel speed which gives the train speed, reduction gearing consisting of a pinion on the motor shaft and a gear wheel on at the axle is provided. The speed of the series motor is varied by varying the voltage applied and/or by varying the field current. The voltage can be varied by a series rheostat as in D.C. locomotives, or by tapings on the transformer as in a.c. locomotives. The field current can be varied by tapings on the field winding or by a shunting rheostat across the field. This is called field-weakening. In recent years, electronic control has been developed which is certainly more efficient. Both WAG5 and WAM 4 class locomotives have 6 traction motors each. In the case of WAG 5, all the six motors are in series at the time of starting. As the speed picks up, 3 of them form a parallel path ie.3, 3 motors are grouped in two parallel paths. Eventually 2, 2 motors are grouped in three parallel paths. In the case of WA M4, all the 6motors are connected permanently in parallel. 2.4 Main Mechanical Equipments Bogies – There are two types of bogies in use - two-axle four wheel bogie with a central bolster, and three-axle six wheel bogie. Both WAG5 and WAM4 class locomotives have the latter type. A nose suspended traction motor is hung from each axile.The axle is driven through a gear pinion set which is the simplest drive arrangement. The earlier locomotive of WAG4 class has the motor mounted in a frame supported by springs. There is only one motor on each bogie which has two axles. There is a flexible coupling between the armature shaft and the axles to take up the play in distance and alignment between the shaft and the axles. This arrangement is cumbersome to maintain but has the advantage of smaller unsprung load on the axles. The bogie frame is supported on side exentions of axle boxes by spiral springs. Inside the springs are oil-filled cylindrical guides which also act as dash-pots to dampen vertical oscillation. The bogies under a motor coach are two-axle type with a central bolster. These are broadly of two designs: (i) the bogie frame is supported on axle boxes through laminated springs; (ii) the bogie frame is supported on spiral springs carried on equalizing bars whose ends are supported directly on axle boxes. The second design is now predominately in use. Brake system – There are two basic types of braking systems: (i) the friction braking where the kinetic energy is dissipated as heat on a brake shoe rubbing on a wheel; (ii) the electric braking where the kinetic energy is converted into electricity which his consumed in resistances or returned to the overhead line. The friction braking can be applied either through compressed air pressure or through destruction of vacuum. A compressor on the loco compresses the air which is pushed in-to brake cylinders at the time of braking, causing brakes blocks to clamp on the loco wheels. High speed coaches also have air brakes activated by the compressed air from the loco. Other coaches have vacuum brakes activated by destroying the vaccum. It takes a longer time to stop the train due to limited differential of pressure. While the friction braking is invariably provided, the trend is to provide the electric braking as well. The electric braking has the advantage of reducing the wear of brake shoes, as also the consumption of electricity if it is returned to the overhead line to be used in other trains in the section. The electric braking in this manner is called regenerative braking which is particularly useful in section with heavy gradients. 2.5 MODERN TECHNOLOGY Thyristor locomotives - Eighteen 6000h.p. locomotives fitted with thyristor control of speed were imported in the late eighties.12 of them were manufactured by Hitachi and 6 by Asia (now A.B.B.). Now working on Waltair-Kirandaul section of South-Eastern Railway, these are hauling heavy iron ore trains over steep gradients. Thyristor is a solid state device which can regulate the voltage applied to traction motors, without any moving parts. The thyristor power convertor equipment is cooled by forced air or kept in Freon, a refrigerant. Traction motors are compound wound i.e. these have both series and shunt windings.

` AC TM VVVF CONVERTER DC LINK PWM CONVERTER TFP

MODERN 3-PHASE PROPULSION SYSTEM

These motors are forced air-cooled. One type has 2 bogies, each 3-axled, while the other type has 3 bogies, each 2axled. The former has bogie-mounted traction motors, while the latter has nose-suspended motors. Braking is through rheostats as well as compressed air. Their layout are shown in figures 3 & 4.

3- Phase induction motor locomotives – Recently 30 more locomotives have been procured from A.B.B. these are fitted with induction motors controlled by variable- voltage, variable-frequency inverters which receive supply from pulse-width modulation control converters performing a.c.-d.c. conversion. In this system, high harmonics in the current which are harmful for traction motors are reduced, and the power factor is also maintained near unity. High harmonics i.e. multiples of 50 Hz. And low power factors are undesirable as they lead to considerable wastage of power. The passenger version classified as WAP 5 is capable of 160 Kmph speed and has started hauling New Delhi – Howrah Rajdhani Express. Chopper control for D.C. stock – Essentially a.d.c. chopper is a power switch which can be closed and opened at a high repetition rate.There is a device for varying the ratio of the switch-closed

time to the total cycle time, which in turn varies the mean output voltage which is applied to traction motors. Thus the chopper replaces

resistances required for starting and accelerating. The same chopper is useful for

regenerative braking when it transforms the voltage generated by traction motors to match that of

the Overhead line so that the current can be returned over a wide speed range. The chopper also blends the resistance braking with the regenerative braking to suit the voltage and load condition of the overhead line.

Pantograph

Circuit Breaker

Aux. Wdg .

380 V AC

Sec. Wdg .I750 V AC

Sec. Wdg .II750 V AC

ARNO Converter

OHE 25 KV Single Phase AC

2 1 3

M ain Transformerwith Tap changer

54 6

Compressor

TM Blower

RSI Blower

SL Blower

Blower for X’mer Oil

Oil Pump

Traction Motors

Rectifier Block - 1

Out Put 750 V DC Smoothing Reactor - 1 Filtered DC

3

380 Volt AC Supply

BLOCK DIAGRAM OF LOCOM OTIVE POW ER CIRCUIT BLOCK DIAGRAM OF LOCOM OTIVE POW ER CIRCUIT

RectifierBlock -2

Out Put 750 V DC

SmoothingReactor -2Filtered DC

2.6 Traction Power Supply Three types of traction power supply are in use on Indian Railways; (a) 25 KV a.c. single phase. (b) 2 x 25 KV a.c. single phase. (c) 750 D.C. with third rail provided as the conductor supplying power to the motor coach.

The source of power supply in every case is the grid substation of the state electric supply company The power is purchased at 220 KV or 132 KV or 66KV ac 3 phase and is stepped down to 25 KV 1-phase in case of a.c. traction of 22KV in case of d.c. traction. The primary of the transformer is connected to 2 of the 3 phases while the secondary winding is connected to the OHE and the earth respectively. In case of d.c. traction, the power at 22KV is distributed to various traction sub-stations where it is further stepped down and then rectified to d.c. at 1500V. AC Traction- The a.c. traction has the advantage of a higher voltage and hence a lower current. The overhead line is therefore lighter, with structures also lighter, thus resulting in much economy, as the number of feeding sub-stations is also much less. The midpoint between the 2 OHEs fed respectively from 2 consecutive traction substations, is provided with an neutral sections which separates the supply from the 2 consecutive sub-stations as there would be a phase difference between them, being connected to different phases on the high voltage side. A.C. traction suffers from the disadvantage of its electromagnetic field causing interference with telecommunication circuits vicinity, to a lesser extent if underground. This can be overcome with the help of conductor with booster transformers connected to it at an interval of 2.66 Km. This of course adds to the cost considerably. The new a.c. system at 2 x 25 KV provided on Bina-Katni-Anuppur-Chirmiri sections does not suffer from this disadvantage since the two currents in the contact wire and the negative feeder respectively, flow in opposite directions. Their electromagnetic fields therefore cancel each other. (See Figure 5) Faults and Isolation :- For isolating the section developing a fault on the O.H.E. (i.e. Overhead equipment or line), such as an accidental connection to the earth, the circuit breaker on the 25KV side of the transformer is tripped of switched off automatically on account of the action of a protection relay. Similarly, a faulty transformer is isolated by the tripping of the circuit breakers on either side. The fault may be a short circuit in its windings or a leakage current through

its oil. An incipient fault in the winding may not give rise to a high current to operate the relay to cause the tripping of the circuit breaker. It is, therefore, necessary to periodically analyses a sample of oil for dissolved gases which result due to the incipient fault. If one of the sub-stations stops supply due to any problem or work on an equipment, the feed zone of the adjacent substation is extended to cover the feed zone of the former substation also. In such a situation, there is a risk of overloading equipment of the adjacent sub-station. An overload relay is also provided to trip the circuit breaker if the current exceeds a pre-set safe value. Computerized control of power supply – A central control is provided to operate various circuit breakers and other switchgear installed in the section to divide it into several smaller electrical sections. This remote control is carried out with the help of a system of computers which also enables monitoring of the condition of equipment as well as of system parameters like power, energy, power factor, voltage and current. This makes the isolation of a faulty section faster enabling the restoration of power supply and traffic on good sections. This also helps in optimization of the parameters, resulting in economy in operation. Computers also enable record-keeping and analysis of a fault. 2.7 OVERHEAD EQUIPMENT (O.H.E) & THIRD RAIL SYSTEM Overhead equipment – The main items are (i) support structure (ii) insulator (iii) bracket assembly (iv) catenary wire (v) droppers (vi) contact wire. The structure is generally a simple steel mast. In yards, portals with two legs and a boom are also used. Two insulators connect the bracket assembly to the structure. A clamp on the top of the bracket tube holds the catenary wire from which the contact wire is suspended by means of droppers. The catenary wire is make of cadmium copper alloy and consists of 19 strands. Its cross-sectional area is 65 sq mm. The contact wire is made of hard drawn copper with two grooves where it is held by dropper clips to maintain it horizontal. Its cross sectional area is 107 sq. mm with 12.24mm diameter. The contact wire is normally kept at a constant height or 4.80m. Above the rail level (See figure5) High speed OHE – With the above-described single catenary system, the increased uplift of contact wire at high speeds causes interruptions of flow of power supply. A compound catenary system (see figure5) is therefore provided on high speed tracks it facilitates uninterrupted current collection. Third rail system – To keep the cost of tunnel for an underground railway low, the over-head line is replaced by a rail which is laid on the ground supported by insulators by the side of one of the two running rails. A collector brush is fitted on the motor coach, which slides on the third rail to collect current during the run. Since the headway between trains is very small, being 1 ½ to 3minutes, the power demand is very high during peak traffic hours. This calls for a very small spacing of 2 to 3 kms. Between consecutive sub-stations. 2.8 Chittaranjan Locomotive Works This production unit was set up in 1948 for manufacturing steam locomotives. With the advent of a.c. electrification in 1959-60, this unit started manufacturing electric locomotives in 1961. Well over 2 thousand electric locomotives have been produced so far. Among important equipment of the electric locomotive, traction motors, silicon rectifiers, smoothing reactors, contactors of various types and bogies are manufactured in this unit itself. Among the important items purchased from the Trade are transformers, circuit breakers, pantographs, blowers of various types, Arno convertors, compressors, exhausters, brake items, relays of various types, gears, wheels and axles. Both WAG5 and WAM 4 class of locomotives are manufactured here. These are called Co-Co type where C stands for 3-axled bogie and O for independent axles.Co-Co indicates two such bogies. Earlier, WAG4 class locomotive of B-B type was manufactured here; B stands for 2-axled bogie with both axles coupled. Braking system incorporates air brake for the locomotive itself while vacuum brake can be applied for the train. Later locomotives have resistance braking as well.