REACTIVE POWER CONTROL IN HVDC TRANMISSION SYSTEM

TECHNICAL PAPER PRESENTATION ON

“REACTIVE POWER CONTROL IN HVDC TRANMISSION SYSTEM”

ABSTRACT

            This paper describes about the influence & control of reactivepower in HVDC system. There are 2 kinds of links mentioned Monopolar & Unipolar and the reactive power generation & control & sources of reactive power are described. Though synchronous motors when operates on noload serves as the synchronous condensers supplying reactive power, now-a days static VAR systems are used & operation of those systems is presented.

Introduction:

            It is well known that electric power generated in power plants in transmitted to the load centre on three-phase ac transmission lines. However, for bulk power transmission over long distances, high voltage dc (HVDC) transmission lines are preferred. HVDC transmission possesses the following advantages over AC transmission system:

(i)         In HVDC transmission system, one or iwq conductors and smaller towers are required as against three conductors and tall towers in AC transmission system. HVDC transmission, therefore, costs less.

(ii)        Fault clearance in HVDC is faster, therefore DC transmission system possesses improved transient stability.

(iii)       Size of conductors in DC transmission can be reduced as there is no skin effect.

(iv)       Two AC systems at different frequencies can be interconnected through HVDC transmission lines.

(v)        For power transmission through cables, HVDC is preferred as it requires no charg­ing current and the reactive power.

            The additional cost of converting the inverting equipments makes HVDC transmission uneconomical for low-power supply over short distances. However, for large-power transmission over long distances, HVDC turns out to be economical. As a result, HVDC links are being used worldwide at power levels of several gigawatts with the use of thyristor valve.

            Fig. shows the basic layout of an HVDC transmission system. Two AC systems A and B are interconnected by the DC line. If power flows from A to B, converter A then operates as a rectifier and B as an inverter. Reverse power flow from B to A is also possible with B acting as a rectifier and A as an inverter. AC filters reduce the current harmonics generated by the converters from entering into ac systems. DC filters and smoothing inductors L_d reduce the ripple in the dc voltage. Both converters A and B have 12-pulse configuration. The centre-point of converters A and B is earthed with one line, or pole, at +kV and the other Hue, or pole, at -kV with respect to earth for a ± kV system. With both the ends earthed, the power flow can be maintained with +kV line and the ground or with -kV line and the ground.

Types of HVDC Link:

            There are two basic types HVDC transmission systems. These are monopolar link and bipolar link.

            Monopolar or unipolar link shown in Fig. (a) offers the simplest arrangement. It uses a single conductor which has either positive, or negative, polarity. It is preferred to have negative polarity for the single conductor as it produces less radio interference. The return path is provided by ground or sea. The return current through ground or sea leads to higher conduction losses, electrolytic action and large potential gradients.

(a) Monopolar, (b) bipolar

            In bipolar HVDC transmission, two conductors are used, one is positive and other is negative with respect to the ground as shown in Fig. As stated before, the neutral points are grounded at both the ends. As the positive and negative conductors carry equal currents, there is no earth current. In case one line is opened due to fault, the other conductor and the ground will form unipolar link and half the rated power can be transmitted untill the fault is cleared. It is obvious from above that bipolar system of HVDC is more reliable than the unipolar or monopolar system. As such, HVDC bipolar link is more commonly employed. A typical bipolar HVDC arrangement is described in what follows.

Applications of HVDC Transmission System

            For generation, transmission, distribution, and utilization of electrical  energy, 3-phase AC systems are used universally and have a define superiority over HVDC.

            However in following particular applications. High Voltage, Direct Current Transmission (HVDC) is a strong alternative to EHV-AC trans­mission and HVDC lines are preferred.

·               Long distance high power transmission by overhead lines.

·               Medium high power submarine or underground cables.

·               System interconnection by means of overhead lines, or under­ground/submarine cables, or back-to-back HVDC coupling sta­tions, or Multi-Terminal DC systems (MTDC).

·               Frequency conversion links (e.g. 60 Hz/50Hz)

·               Incoming lines in mega-cities.

            In HVDC link AC power is convened by thyristor - convertor valves at one end. The energy is transmitted in HVDC form to the other end. At the other end, the DC power is inverted to AC and fed into the receiv­ing AC system. Fig. illustrates a typical bipolar HVDC link.

            A 2-Terminal HVDC transmission system has a HVDC convertor sub-station at each end and an HVDC transmission line in between. In case of back-to-back coupling station, the convertor and invertor are at the same place and there, is no HVDC line. Multi-Terminal HVDC inter connects 3 or more AC systems, by HVDC transmission lines.

Choice of HVDC Transmission System

            HVDC system are selected as an alternative to extra high voltage. a.c. transmission systems for any one or more of the following reasons : (Table 47.1 gives the summary).

            1.  For long distance high power transmission lines for economic advantage of HVDC with respect to lesser cost of transmission line and better control of power flow. Though the HVDC link needs additional conversion substation equipment (convertor transformers and convertor etc.) on each side, for long distance high power transmission, the total cost of a d.c. system becomes lower than that of a.c. system. The break­even point is decided by economic studies for each scheme.

            The per km cost of one bipolar single circuit HVDC line is lesser than that of an equivalent 3-phase double circuit AC line. Number of conductors for 3 phase AC line is 6 to 24 as against 2 numbers required for an equivalent bipolar HVDC line. HVDC line does not need inter­mediate sub-station for compensation, whereas for EHV-AC line such a sub-station is required at an interval of 300 km. HVDC becomes favourable above 800 km, 1000 MW when cost of EHV-line/sub-station exceeds that of equivalent HVDC line/sub-station. (Refer Sec. 47.2.8)

            2.  For Interconnection (Tie-lines) between two a.c. systems having their own load frequency control. HVDC links have several advantages over a.c. links. HVDC links form an asynchronous-tie . i.e the two a.c. systems interconnected by HVDC tie-line need not in synchronism with each other.

            HVDC interconnection is superior to EHV-AC interconnection in many respects and is selected due to its technical superiority. With HVDC interconnection, power flow can be controlled, the frequency disturbances are not transferred, short-circuit levels remain unchanged at both ends, transient stability of AC network at both end can be significantly im­proved.

            Power flow through the HVDC line can be quickly modulated reversed, changed to dampen the power swing in connected AC Network. Thereby the system stability can be greatly improved.

            HVDC interconnection can provide a weak tie (of lesser capacity) between strong and a weak AC Network. This is difficult with AC interconnection.

            Most important task-of interconnector is to transfer required amount of power in required direction and to assist the interconnected AC. Net-work to maintain transient .stability. AC interconnectors have severe limitations. HVBC interconnections are without such limitations.

            HVDC system control can be modified to dampen oscillations in load angle d. Thereby the stability of both AC systems is-improved.

            3.  For Back-to-back synchronous tie-stations. Where two a.c. sys­tems are interconnected by a convertor sub-station without any a.c. trans­mission, line inbetween Such a He-link -gives an asynchronous interconnection between two adjacent AC systems. The back-to-back cou­pling stations can be located at any suitable location, where to networks meet geographically and exchange of required amount of power is desired.

            4.  Multi-terminal HVDC Interconnection. This is the new HVDC possibility (1987). Three or more AC networks can be interconnected asynchronously by means of a multi-terminal HVDC network. Power flow from each connected AC Network can be controlled suitably. Large powers can be transferred. Overall stability can be improved. At present only one such scheme is under execution (Hydro Quebec Canada to New England USA). More and more multi-terminal HVDC schemes are likely to be executed.

            5.  For underground or submarine cable transmission. Over medium distance at high voltage. The submarine cables are necessary to transfer power across lakes, oceans, etc. In case of AC cables, the temperature rise due to charging currents forms a limit for loading. For each voltage rating, there is a limit of length beyond which the cable cannot transfer load current due to this limit. In such cases HVDC cables arc essential. HVDC cable has no continuous charging current.

Reactive Power in HVDC System:

            The converters in HVDC stations are line commutated, which implies that the current initiation in the valve can only be delayed with reference to the zero crossing of the converter bus AC voltage. This results in lagging power factor operation of the converters, requiring reactive power sources connected at the converter bus for better voltage control. The reactive sources are required at both the rectifier and inverter stations. While the rectifier station appears as a load in the system, the inverter station can be viewed as a generator consuming reactive power. This characteristic of the inverter is generally not desirable and requires suitable modifications by providing adequate var compensation.

Relation between ignition delay and phase displacement

            The reactive power soruces that are used vary switched capacitors to static var systems. The requirements of voltage control and the costs dictate the choice of the speed of response of the reactive power control under dynamic conditions.

SOURCES OF RECTIVE POWER

            The reactive power requirements of the converter are met by one or more of the following sources:

1)                  AC system

2)                  AC filters

3)                  Shunt capacitors

4)                  Synchronous condensers

5)      Static var system.

These are shown schematically in fig.

            From voltage regulation, losses and stability considerations, it is not desirable to draw rective power from the system except at low loads. Figure. Shows the reactive power drawn by AC system at the inverter bus, as a function of P_d. These are for two cases (i) SCR=2.0 and (ii) SCR=3.0. In both cases, an impedance angle of 84.3⁰ is assumed and V=1.0 p.u.

Reactive power supplied by the AC system

            The voltage regulations the converter, bus is desirable not only from the voltage control view point but also from minimization of loss and stability considerations. This requires adjustable reactive power source which can provide variable reactive power as demanded. For slow variations in the load, switched capacitors or niters can provide some control. However, this is discrete type of control and can result in voltage flicker unless the size of the unit, which is switched, is made sufficiently small. In contrast, the synchronous condensers and static var systems provide continuous control of the reactive power and can follow fast load changes.

            The synchronous condensers are essentially synchronous motors operating at no load, with .excitation control to maintain .the terminal voltage. Their advantages are as" follows:

1.         The availability of voltage source for commutation at the inverter even if the connection to the AC system is temporarily interrupted. This also implies an increase in SCR as the fault level is increased. When the load supplied by the inverter is passive, the synchronous condenser is essential for providing voltage sources for the line commutation at the inverter.

2.         Better voltage regulation during a transient due to the maintenance of flux linkages in the rotor windings. The effect of the armature reaction is counteracted during a transient by induced currents in the field and amortisseur circuits.

            There are also disadvantages of synchronous condensers. These arc (i) high mainte­nance and cost - the former necessitated by slip rings and brushes on the rotor and (ii) possibility of instability due to the machine going out of synchronism.

            The static var systems (SVS) provide the fastest response following a disturbance. The configurations normally used are (i) fixed capacitor (FQ, thyristor controlled reactor (TCR) or (ii) thyristor switched capacitors (TSC) - TCR combination. SVS will be described separately in the next section.

            The passive AC filters that are provided at the converter bus for filtering out AC current harmonics appear as capacitors at the fundamental frequency and thus provide reactive power. These filters and shunt capacitors are mechanically switched- Although these devices are less expensive than SVS or synchronous condensers, they suffer from the inability of continuous control. Also they can cause low order resonances with the network impedance, resulting in harmonic overvoltages.

STATIC VAR SYSTEMS

            The static var systems or compensators were initially used for load compensation where the objective is to dynamically control the reactive power demand of large fluctuating loads such as rolling mills. They were subsequently used for voltage control applications in transmission systems, where, by maintaining voltage support at specified locations, it is possible to provide increased power transfer capability, control of dynamic overvoltages and damping of oscillations. By using auxiliary control signals, it is also possible to damp subsynchronous frequency oscillations.

            In HVDC convener stations, the provision of SVS mainly helps to have fast control of reactive power flow, thereby controlling voltage fluctuations and also to overcome the problem of voltage instability. The first example of the installation of SVS is at chateaguay HVDC link in Canada in 1984.

            In this section, the basic configurations and their characteristics are briefly reviewed. There are basically three types of SVS chemes.'

There are basically three .types of SVS schemes.

1)         Variable impedance type SVS

2)         Current source type SVS

3)         Voltage source type SVS.

            The variable impedance type is most common in power system applications and will be described next.

Thyristor Controlled Reactor (TCR)

            The single phase thyristor controlled reactor is shown in Fig. 7.10. By controlling the firing angle of the back to back connected thyristors, the current in the reactor can be

Single phase thyristor controlled reactor

Controlled. This is shown in Fig. For a = 90⁰, the current is maximum, while for a=180⁰, the current is zero.

           

Current control in TCR

The fundamental component of the inductor current is given by

where V is the rms voltage across the TCR, X_L is the fundamental frequency reactance and s is the conduction angle related to aby the following equation.

s=2(p-a)

Equation (7.15) can be written as

I₁=B(s)V

Where

is the variable susceptance of the reactor resulting from the control action.

            The harmonic component of the current corresponding to harmonic of order ‘h’ is given by

h=3,5,7 ……….

Variation of lower order harmonics with the conduction angle

            The variation of the lower order of the harmonics with conduction angle s is shown in fig. The variation of the fundamental and the total harmonic components is shown in fig.

            The triplen harmonics in the lines are eliminated by the delta connection of the three single phase TCRs.

            The typical control system for a TCR is shown in Fig. Where the control signals are obtained from the voltage and the reactor current. The controller is usually an integral controller

A typical control system for a TCR

With variable gain to avoid the problems of contol istability. The auxiliary signal V may be derived from the bus frequency, line reactive power or other locally measured quantities.

            The TCR is usually operated with fixed capacitor (FC) to provide the variation of reactive power consumption from inductive to capactitive. The schematic of FC-TCR is

The schematic diagram of FC - TCR

Shown in Fg. The steady-state characteristics in the V-I plane is shown in Fig. 7.16. The control range is ‘ab’ which shows a positive slope which can be adjusted from the gain in the current feedback path.

            The harmonics injected by TCR into the system can be considerably reduced either with wolve pulse arrangement or with additional filters tuned to 5^th and 7^th harmonics.

Conclusion:

            Though static VARS systems are relatively expensive, they are employed for reactive power injection & its control. In this way reactive power can be generated & controlled.

A TECHNICAL PAPER PRESENTATION ON ROLE OF FACTS & HVDC IN INTEGRATING INDEPENDENT POWER PRODUCERS TO NATIONAL GRID

3-PHASE ASYNCHRONOUS DRIVE IN TRACTION

3-phase ASYNCHRONOUS DRIVE in traction

                                        By

                                                      S.RAMAKRISHNA   

RAM_SIRAPURAPU@YAHOO.COM

CONTENTS:

                    1. ABSTRACT 

                                                                                         

                   2. INTRODUCTION

                                                  

                   3.POWER CIRCUIT

                                                    

                   4. ADVANTAGE OF 3-PHASE LOCOS OVER 

                            CONVENTIONAL LOCOS 

                                                        

                   5. INDUCTION MOTOR ELECTRICAL

                             REGENERATIVE BRAKING

                             

                    6.CONCLUSION                                                                

                    

ABSTRACT:

This paper deals with the advanced technology of replacing the D.C. Series Motors by the 3-phase Induction Motors in Traction and the Electric Regenerative Braking of the Induction Motor.

Series Motors are widely using in traction because of their fulfillment in the desirable characteristics of the traction motors. But their voltage and power ratings are limited by the fact that the sparking will occur when the voltage per commutator segment exceeds certain limit. The regenerative braking is difficult and also frequent maintenance should be required for the commutator and brushes, that is way these series motors can be replaced with induction motors.

Experiments showed that at low frequencies, induction motor develops high starting torque, drawing low starting current. And they have robust and cheap construction. Due to the advent of power electronic devices, now it is possible to change constant supply frequency to a variable frequency.

The pantograph draws single-phase A.C. supply from OHE and the NSR converts it to D.C. supply. The D.C. supply is then converted to 3-phase A.C. supply by means of ASR. Thus by varying the firing angle of SCRs in the NSR & ASR, Variable Voltage Variable Frequency supply can be fed to 3-phase induction motor.

During the train-braking period, the Kinetic Energy of hauling masses can be converted into electrical energy by the method of pole changing. Thus the supply is fed back to the OHE, the D.C. link supplies the required reactive power.

INTRODUCTION:

In the last one decade the electric traction technology has undergone sea change with 3-phase ac traction motors, gate turn off thyristor (GTO) & variable voltage variable frequency traction system.  This technology offers many advantages in addition to being high energy efficient.  The advent of 3-phase passenger electric locomotives on the Indian Railways is a quantum technological jump.

            The main feature of 3-phase electric locomotives is the use of 3-phase asynchronous motor drive, which is powered by microprocessor controlled variable voltage variable frequency ac power supply from the GTO based power inverter. The 3-phase asynchronous motors are far more reliable, compact & require very little maintenance attention besides this main feature. There are several other advanced features in the 3-phase locos namely regenerative braking and unity power factor, which result in reduced energy bills, higher adhesion, fine step less control on tractive effort, facility of pre-set speed, high tractive effort, reduced harmonics, less unsprung mass resulting in less rail wear & tear and less distortions to the track geometry.  It is crew friendly & maintenance friendly.

POWER CIRCUIT:

         The conventional locomotive was equipped with dc traction motors, while these locomotives run on 3- phase squirrel cage induction motors. Let us understand the power circuit, fig.1.  The pantograph draws power from OHE at 25kv.  The same is stepped down (to 2x2180V) using a fixed ratio transformer, NSR (line side inverter) converts 1-phase AC to DC, ASR (drive side Inverter) converts DC to 3-phase AC, which provides the supply to the induction motors.  In between the two is DC link shown by a capacitor.  DC link reduces the current ripples and provides a fixed voltage source to ASR. 

        This locomotive is capable of working in motoring & regenerating modes. In regenerating mode, power is transferred from locomotive to OHE and this results in brake application on the locomotive.  In regenerative mode, the motor acts as a generator and the power so produced is fed back to OHE.  The 3-phase AC from the motors in converted to DC by ASR.  NSR converts DC to 1-phase, which is stepped up by transformer & fed to OHE. The ASR ensures that the power is drawn at unity power factor by the locomotive.

           

             

                                         There are two power converters in               NSR, ASR & DC link along with its control circuit from a unit referred to as power converter. each locomotive.  One power converter feeds all the induction motors of one bogie.  The NSR & ASR modules are made up of GTO and  other associated devices. These modules are forced oil cooled.

Drive: Before the advent of power electronics, DC series motor was used for traction as it was easy to control and satisfied the requirement of providing high torque during starting.  However now it is becoming possible to control the induction motor characteristics using the above referred power converter in such a way that they provide high starting torque.

            The output from the power converter is variable voltage & variable frequency (popularly known as VVVF).  As the speed increases, the frequency supplied to the induction motor also increases. The induction motor has two regions (1) constant torque zone (ii) constant horsepower zone as shown in fig (2).

            The ratio between the voltage & frequency is kept constant resulting in high starting torque.  This is constant torque zone.

            After reaching the rated value the voltage is maintained constant and only the frequency of supply is increased.  This is the constant power zone.

Auxiliary circuit: The motors used for auxiliary circuits are 3-phase, squirrel cage induction motor and 3-phase power required is supplied by auxiliary converter (BURs).  The auxiliary converter converts 1-phase A.C to 3-phase A. C.

            The auxiliary converter performs similar functions as that of static converters used in some locos.  There are 3 auxiliary converters each of 100KVA.  In case one fails the other one takes over its load automatically, (without driver’s intervention).  The interesting feature of auxiliary converters is that it provides soft start for the compressors.  The system does not maintain the unity power factor and is not capable of regeneration.

            The 1-phase ac is first converted to dc.  There after, dc is converted to 3-phase ac and supplied to the induction motors.  The output is square wave.

  Control Circuit:  The control system is not relay based, instead uses the distributed micro -computer control.  (The system can be thought of as conceptually similar to the SCADA being used in TRD).  The control system used is ADTranz proprietary MICAS-52 (micro -computer-Automation system).

                The control system is user friendly for the drivers as well as the maintenance staff.  In case of any fault, message comes on the display screen of the driver, with necessary instructions to staff to act upon.  The appropriate control like isolation of

                                                     Fig.2

 subsystem also takes place automatically.  All the faults are logged with time stamp along with related locomotive parameters.  At the time of fault in non-volatile memory of computer, which can be down loaded using a PC for off-line analysis in workshops or sheds.  Seven bus stations carry out the control functions of the locomotive.  Each bus station consists of input/output cards, signal-processing cards, and micro -processor cards.  The bus stations communicate with each other, the status of various electrical parameters.  The control system programming is done using high-level graphic compilers (FUPLA/ALS) and also on some occasions, characteristics have been written in C++ or assembly language.

Bogies: Locomotive has bogies with fully suspended traction motors (TMs).  Or it may have bogies with axle hung, nose-suspended TMs.  These are many common features for the two bogies, as discussed below.

            The bogie has two stages of suspension.  The bogie frame rests on the axle box supported by springs.  The loco rests on two pairs of secondary springs located on the long beams of the bogies.  There is no center pivot or side bearer.  The vertical load is transmitted through two stages of suspension hydraulic dampers have been provided at various places.

            The tractive effort is transmitted from the axle to the axle box and then from axle box to bogie frame by guide rod.  Tractive effort from bogie frame of the loco is transmitted through traction bar.

ADVANTAGES OF 3-PHASE LOCOS OVER CONVENTIONAL LOCOS:

1). Squirrel cage induction motors, which have the following advantages, have replaced the dc series motors.

      (a). Less maintenance of induction motors as compared to dc motors.  No carbon         brushes are required to be replaced. Cleaning of commutator is not required.

       (b). Due to precise control of induction motors using power converters, it is possible to obtain improved adhesion (higher tractive effort) in comparison to dc series motors.

        (c). The rated voltage of induction motor is around 2000V, in comparison to 750V in dc series motors.  Hence for the same power the amount of current to be fed is low.  This results in lesser rate of induction motors, which means that lower unsprung mass.  This reduces the unsprung mass.

    2). Regeneration:  This loco is capable of regeneration, which results in direct         saving of 10 to 15% of energy.  Due to electrical braking, the wear of wheel & brake blocks reduces.

    3). Unity power factor:  This loco operates at smaller unity power factor.  The conventional locos operated at power factor around 0.8.  unity power factor results in (a) saving penalty imposed by SEBs (b) improved voltage regulations (c) It enhances the system capacity & (d) reduces the copper losses.

    4). Harmonics: 3-phase locos produce less harmonics, in comparison to    conventional locos.                         

    5). Diagnostics: In conventional locomotives, in case of any failure on line the driver has to do the troubleshooting, which results in considerable wastage of time.  In these 3-phase locos, a driver is provided with menu-based screen, where the fault messages are displayed with necessary instructions to staff to act upon.  The proper control action results in isolation of subsystem.  The fault & loco parameters are logged which can be used for “off line” analysis in workshops or sheds.

    6). Riding comfort:  This loco has two steps of suspension.  WAP5 has fully suspended motors. These features result in better riding comfort. This makes the drivers more comfortable and also reduces failures, which result due to vibrations in the loco.

7). Three-phase locos will require less maintenance & duration between two schedules is much longer than the conventional locos.  The failures in these loco are lower than the conventional locos.  Mean time, between failure/equipment (MTBF) is higher & availability of these locos is high.  

Induction Motor Electrical Regenerative Breaking:

Motor, without disconnecting it form the supply, is made to generate (instead of made to motor) and feed back energy to supply. Magnetic drag, produced on account of generation action, offers the braking torque. This method of braking is most efficient. In many cases, the transition from motoring action to generating action is smooth and without any switching operation. As soon as overhauling load drives the motor, it works as generator.

In case of induction motor Fig. 3, we find that for speeds above synchronous, motor torque becomes negative. Machine is now working as induction generator. Depending upon the speed, motoring or generation action of the machine will be automatic. By this method of braking, overhauling load may be prevented form rising much above synchronous. If the driving torque of load exceeds the maximum braking torque which motor can develop, the motor will cross over to unstable operation. Rise in speed will then decrease braking torque. Motor in that case in heading for run away conditions.

            We can bring the speed below synchronous only where arrangement of pole changing is available. If number of stator poles is increased, its new synchronous speed will be less than the actual running speed. Machine will now work as induction generator and bring the motor below its first synchronous speed till new operating speed will be little less than synchronous speed corresponding to increased number of poles. It will then continue to work as induction motor at this reduced speed. This method of electric braking is applicable to squirrel cage motors because rotor winding of slip ring motor cannot be reconnected for different number of poles

            As soon as motor speed exceeds its synchronous speed, it starts delivering active power P to the 3 - phase line.  However, for creating its own magnetic field, it absorbs reactive power Q from the line to which it is connected.  As seen, Q flows in the opposite direction to P.

            The active power is directly proportional to the slip above the synchronous Speed.  The reactive power required by the motor can also be supplied by a group of capacitors connected across its terminals.  Hence the capacitor bank must be large enough to supply the reactive power normally drawn by the motor.

Fig. 3

CONCLUSION

            We can conclude from this paper that dc series motors can be replaced by 3-phase asynchronous drive (i.e. Squirrel cage induction motor).

The 3-phase induction motor in traction is still in developing stage. The basic idea of operation and regenerative braking of the 3-phase induction motors in electric locomotives have been mentioned. The 3-phase A. C. Technology in traction is still in developing stage and now is running at some places in India. The technology of using linear induction motor is also in the development stage by using the magnetic levitation technique.