VISHVESHWARA INSTITUTE OF ENGG & TECH S UMMER TRAINING REPORT ON

INDIAN RAILWAY SUBMITTED TO - H.C.SHARMA

HOD,EN DEPARTMENT SUBMITTED BY-

AVANISH KUMAR VERMA

EN-VII A Generation alternators

 

The KEL 4.5KW Train Lighting System consists of a three-phase homopolar inductor type alternator and a static Regulator-cum-Rectifier Unit. Such brushless alternators render a trouble free long service without practically any maintenance, as it is completely free for many moving contacts or winding on rotor. The regulator has been designed for a reliable performance in any operational conditions by eliminating transistors and thyristors, which are comparatively less reliable.

Brushless Alternator type KELA 45135-D is of totally enclosed construction capable of developing a constant voltage of 120V at a load current of 37.5A from minimum speed for full output to maximum speed. The machines are used for:

1. Charging the coach battery

2. Operation of fans, lights etc. in the coach

 

Principle of operation

The alternator consists of two sets of winding namely AC winding and field winding, both accommodated in the stator. The AC windings are distributed in the small slots and field windings are concentrated into two slots. Each field coil spans half the total number of stator slots. AC coils are connected in star and field coils are connected in series.

The rotor consisting of stacked stampings resembles a cogged wheel having eight sets of teeth and slots, uniformly distributed on the rotor surface skewing the rotor axis.

The core of the stator, which is completely enclosed by field coils, will retain a residual magnetism if excited by battery once. The flux produced by the field coils find its path through the rotor. When the rotor is rotated, the passage of the rotor teeth and teeth alternately under the field offers a varying reluctance path for the flux produced by the field coils. This flux, which varies periodically links with the AC, coils and induces an alternating voltage in the AC coil. The frequency of the induced voltage depends upon the speed of the rotor. The magnitude depends on the speed of rotation and level of excitation. The field is strengthened by a positive feedback system in the regulator to attain the desired outputvoltage.

    

Regulator-rectifier unit  The regulator rectifier unit has mainly following functions:

Rectifying the three phase AC output of the alternator to DC using full wave rectifier bridge

Regulating the voltage generated by the alternator at the set value.

Regulating the output current at the set value.

 

Power-rectifier    

This consists of six numbers of silicon diodes, connected in three-phase full wave bridge. The three phase dC output of the alternator is rectified by these diodes to obtain DC output terminals +DC and –DC of the regulator-rectifier Unit. Each diode is protected against transient surge voltage by capacitor C1. The whole bridge is protected against high frequency surges by capacitor C3 and DC output is filtered by capacitor C2.

 

Voltage-regulation  

The voltage induced in the AC winding of the alternator is dependent on the speed of the alternator, the excitation current and the load current. In the absence of a voltage regulator, the output voltage will rise indefinitely due to positive feedback to the field. The voltage regulator monitors the voltage and passes a command to the control circuit to reduce the excitation current to reduce the excitation current as soon as the output voltage reaches the set value.

The control circuit is wired in a rack consisting of the following parts:

 

Excitation transformer  

This is a double winding transformer with tappings for input and output. The transformer steps down the voltage for the field coils. The transformer has five sets of terminals brought to a terminal strip.

Terminal 14 and 15 – Input from the two phases of Alternator

Terminal 19 – Center tapping of the transformer going to the negative terminal of field supply

Terminal 18 and 161 – Output of the transformer going to the respective terminals of Magnetic Amplifier.

 

Voltage detector – DT     

The voltage detector serves the function of providing necessary ‘error signal’ for voltage regulation. It consists of a network of zener diode, potential divider and rheostat. When the output voltage of the alternator exceeds the set value, the voltage drop across the resistance R1 reaches sufficient value to cause the zener break breakdown and this will sent a current through the control winding of Magnetic Amplifier, which causes in the impedance of load winding decreasing the field current, maintaining the output voltage of alternator at set value. The zener diode starts conducting only at a designated voltage (zener voltage). The voltage across zener will be maintained even if the voltage input to the circuit is increased. Thus it serves as a base for comparison. A rotary switch is provided for setting the voltage at 120, 122 and 124V.

Magnetic amplifier – MA  The Magnetic Amplifier forms the nucleus of the regulator circuit. It works on the principle of saturation of Magnetic core. It has 6 sets of winding designated as

follows:

Load windings – Two sets (18-162 and 17-161)

Control windings – Four sets (10-11, 26-27, 29-30 and 20-40)

Of these four sets of control windings only three sets are used in the circuit for voltage control, current control and gain control.

The load winding is connected in the field circuit and the field current passes through this winding. Subject to the command from the voltage and current sensing circuits, manifested through control windings, load winding offers a variable impedence to the field thereby regulating the voltage and current at set value.

 

Field rectifier unit – D2-D3  

Silicon diodes D2 and D3 act as a full wave rectifier for the field supply. These diodes conduct alternately as the terminals 18 and 161 become positive with respective to the center tapping. The rectified current from the diodes is taken through the feedback winding of Magnetic Amplifier to the terminals, 20 and 19 the positive and negative terminals for the field supply.

 

Free wheeling diode – D4  

In case there is a voltage surge from the field circuit, which will have a polarity opposite to that of excitation, the free wheeling diode will conduct avoiding creepage of surge voltage to important components like Magnetic Amplifier.

 

Rectifier bridge – RT  

It consists of three silicon diodes connected for three phase full wave rectification with the negative terminal taken from the Power Rectifier Bridge PR.It rectify the AC output of the alternator and supplies DC voltage to the voltage detector for voltage sensing.

Current regulation  

Current regulation circuit consists of a variable shunt (SHR) connected in series with the load circuit and a diode D1.When the load current exceeds the set value, the drop across the shunt will be sufficient to

drive the diode D1 into conduction and pass a current through the control winding of Magnetic Amplifier. The effect of this control current is to retain the current at the limited value reducing the output voltage on further loading. The current limiting circuit prevents the alternator from over loading.

 

Alternator  

The alternator is provided with two cast nylon suspension bushes to accommodate a suspension pin of 31.75 + 0.1mm dia as per the RSDO specification. The tension mechanism should also be mounted accordingly.

Rectifier – Regulator  

The rectifier – regulator box is designed for under frame mounting. It can also be mounted inside the coach, provided, sufficient ventilation is provided. Care must be taken to fix the box firmly to the underframe. It should be ensured that all enclosures are water tight, before mounting.

Air Conditioning UnitAir conditioning unit classified on the basis of1.AC GENERATION

2.TYPE OF AC PLANT

According to generation, it is again classified as

1.End on generation

2.Self-generation

 

End on generation  

As the name suggests, in this type generators are at the end of the coaches. There are two generators of rating 250kvA, 750V, 3phase.

In RAJADHANI EXPRESS&SHATABTI EXPRESS, they are using this type of AC unit. In these trains, the provision of dedicated rakes llows the use of a separate ‘power-car’ to supply electricity for all the coaches. There are usually 2 generators in each power car; each generator (an ‘end-on generator’ (EOG)) generates 3-phase 750V AC power, which is then distributed across the train, and stepped down to 415V AC (3-phase) for the air-conditioning, or 110V (single-phase) for other appliances. The elimination of generation equipment also allows the coach bogies to be designed with higher speeds in mind. The power car capacity is 250kVA.

These 250kVA power cars were introduced in 1992. Before that the power cars in use had a capacity of 125kVA and used 440V as the AC distribution voltage. With these, most Rajdhanis and Shatabdis needed three power cars — one at either end, and one in the middle of the rake, which split the rake into two portions (termed ‘Unit I’ and ‘Unit II’). As the power cars are (were) not equipped for anyone to walk through, there was no way to get from one portion of the rake to the other while the train was in motion.

A very small number of other trains also use such EOG cars for power; these EOG cars tend to be different from the ones used for Rajdhani and Shatabdi trains. 

Self generation 

Here there are three power supply arrangements.

1. Axle driven brushless alternator

2. Battery

3. Precooling Arrangement

In axle driven brushless alternator, practically 98-103V is generated. The capacity of this alternator is 25KW or 18KW.There is one Regulation cum rectifier unit. Here also there is magnetic amplifier, which senses the variation in voltage and current and accordingly regulates the field excitation. There is also an Over

voltage protection circuit to prevent the damage due to over voltages.

Battery unit: Some years back, they were using Lead acid cell batteries of 800AH capacity. But due to its severe drawbacks like frequent addition of distilled water, sulphonation etc now they are using Valve regulated lead acid battery popularly known as Maintenance free battery of capacity 1100AH.Its lifetime is around 4years.

Pre cooling Arrangement: This is essential because when the train happens to halt in a station for more than say half an hour it is not economical to switch on the air conditioning unit by the battery. So in this condition, we tap the 440V ac supply from the station itself using an arrangement. Then there is a step down transformer, which step down 440V to 110V.

According to type of plant they are again classified into

1. Under Slung

2. Roof Mounted Ac Package Unit

 

Roof mounted ac package unit  The roof mounted package unit is factory assembled, gas charged and Hermetically sealed refrigerant system. It has two Hermetically sealed, 3phase reciprocating compressors, two vertical flow condenser fans and one blower motor with two forward curved blowers. Condensor fan

motors are 3 phase motors whereas the blower motor is double shaft 3-phase motor.

There are two independent refrigerant circuits in each roof mounted package units having a compressor, condensor coil and evaporator (cooling) coil .The condenser fans and evaporator blower motor are common to both the circuits. Each circuit has its own fresh air and return air filters, condensate drain pan and drain outlet.

    The condenser fans and compressors can be accessed from the top of the unit.whereas the return air filters can be pulled out from under the unit by opening the access door, in the corridor of the coach .The blower, heaters and safety controls can be accessed by opening the access door at the center of corridor of the coach .The blower motor can be opened without disconnection, by opening the central access door .The blower assembly is hinged and opens to hang vertically down. This provide access to heaters, blower motors and blowers .To open this access this access door remove all bolts while supporting the motor disengage the safety leych and gradually lower the central access door. Care should be taken while doing this, as the weight of blower motor and blower are supported on the central access door.The terminals for connecting cables are located below the evaporator area and are accessible directly once the coach access door in the false ceiling is opened.

As the roof mounted package unit is modular Hermetically sealed type, it requires minimum maintenance .The refrigerant system is Hermetically sealed and has no fittings or gauge ports .The safety devices are provided to protect against abnormal operating refrigerant pressures, loss of air and over heating of heating elements, low voltage

and high voltage.

        

Air-conditioning unit specifications

1. Unit type             : Roof Mounted Package

2. Power Supply         : 415V±5%AC, 3Ø, 50Hz

3. Power Consumption     : 13.5kW

4. Current             : 22 Amps

5. Conditioned Air flow rate : 4000M³ /Hr at 20mm 6. Refrigerant         : R-22, 2.4 kg per circuit

7. Dimensions         : Length * Width * Height              2150mm 2250mm 600mm 8. Weight             : Approx.580kg

9. Outline Drawing No.     : CC-90012 (alt d)

10. Compressor         : Hermetic reciprocating type       Kirloskar MG52/Maneurop MT57HL4

11. Condenser Fan         : Direct drive propeller fan 2 Nos. per unit

12. Condenser Fan Motor     : 6 pole (2 Nos. per unit) 0.55kW 415 VAC 3Ø,50Hz

13. Evaporator Fan         : Direct driven centrifugal fan 0.75kW 2 Nos. per unit

14. Evaporator Fan Motor     : 4 pole, 415 VAC, 3Ø, 50 Hz, 1 No. per unit

15. Heaters             : 3kW 2 Nos. per unit 415 VAC 3Ø, 50Hz.

Control and Power Circuitry  

    The electrical circuit of ac package unit is divided into two sections:

1. Three phase ac power circuit at 415 V, 50Hz

2. Single phase control circuit at 110 V, 50 Hz

 

Power Circuit-  

The 3 phase 415 V, 50 Hz power supply is used to operate two Hermetically Sealed Compressors, One double shaft blower motor , two heaters and two condensor motor through 6 contactors .The equipments are protected by using MCBs and Overload relays.

                                    

Control Circuit 

    A step down transformer of 415/110 V is used to provide 110 V ,single phase to control circuit. The thermostats ,PCBs and other interlocking devices operate on control voltage for protection of various equipments .However ,the Crank Case Heaters of both compressors are operated on 415 V and are kept energized all the time and are not switched off even when it is switched off .This is to ensure that the crank case lubricating of the compressor is kept warm during Off cycle and machine shut down so that refrigerant does not migrate into the compressor crank case .This will ensure that during compressor start up ,no liquid refrigerant is present in the compressor crank case to cause foaming of the oil and consequent compressor damage due to absence of lubricant .

     Three stage thermostats are provided for cooling and heating to maintain 24 to 26 deg.C during summer (for cooling) and 19 to 21 deg.C during winter(for heating) at Low ,Medium and High positions of rotary switch respectively .

    The control circuit is designed such that in case of power failure the machine shuts down and when the power is restored the machine restarts automatically .

     The incoming power is indicated by three lights ,Red ,Yellow ,and Green .A normally ON status of each equipment is indicated by Green lights . Red lights indicate overload tripping in case of the heaters .In addition for the compressors Red

indicators are provided for low and high pressure trip. Green lights indicate the manual/auto and heating/cooling status of the unit.

 

Rotary switch

There are mainly five types of rotary switches.

RSW1 to switch on/off mains supply.

RSW2 to switch on blower .

RSW3 to select auto / manual heat /manual cool /vent mode as per requirement of inside condition.

RSW4 to select the three stage of temperature low/ medium/ high by operating thermostat .

RSW5 to select the working of compressors on/ bypass.

 

Control/protection components in electrical power and control circuit.

 

1. Overload protection used for blower motor ,condenser motor and compressor.

2. Time delay relay (TDR) used for compressor motor.

3. Contactors used for blower motor ,condenser motor , compressor and heater.

4. Overheat protection used for heater.

5. Cooling relay

6. Heating relay

7. PCB with LEDs for status indication.

8. Vane relay used for air proving SPDT switches.

9. Transformer for control circuit.

Functioning of various equipments

Power supply 1.The Supply Authorities supply power at 220/132/110/66 kV EHV at each traction sub-station, which is under the control of the Railway. 2.The Railways receives three phase power supply from Supply Authority at a single point near the grid sub-station from where the Railway runs its own

transmission lines providing its own traction sub-station.

3. All EHV and 25kV equipment is under the control of the Supply Authority except 25Kv feeder circuit breaker which are the under the control of the Railway.

4. All EHV and 25kV equipment is under the control of the Supply Authority but 25kV feeder circuit breakers alone are operated on remote control by the Traction Power Controller (TPC).

Duplicate supply    To ensure continuity of supply under all condition, the high voltage feed to the traction sub-station is invariably arranged either from two sources of power or by a double circuit transmission line, so that even if one source fails, the other remains in service. Suitable protective equipment is installed at the sub-stations to ensure rapid isolation of any fault in transmission lines and sub-station equipment, so that the power supply for electric traction is maintained under all conditions. At each traction sub-station, normally 2 single-phase transformers are installed one of which is in service and the other is 100% stand by. The present standard capacity is 21.6MVA. These transformers step down the grid voltage to 25kV for feeding the traction OHE. 25kV feeders carry the power from sub-stations to feeding posts located near the tracks. Each feeder is controlled

by a single pole circuit breaker equipped with protective devices.

Voltage regulation   The permissible variation of the bus bar voltage on the bus bars at the grid sub-stations is +10% and-5% i.e. between 27500V and 23750V. The tappings on the transformer are on the secondary winding and are set to ensure that the voltage is maintained as high as possible but not exceeding 27.5kv at the feeding post at any time.

OVERHEAD EQUIPMENT The fundamental aim of design overhead equipment is to install the contact wire at the requisite height and to keep it within the working range of the pantograph under all circumstances.

1. The catenary and contact wire together have an equivalent copper section of 157 mm². The current normally permissible on single track is 600 A approximately, because of cross-sectional area of OHE.

2. For loop lines, sidings, yards and spur lines excluding the main running lines and first loop or lines taking off from main running line, tramway type OHE having only grooved hard drawn copper contact

wire of 107 mm² section is provided.

Height of contact wire      The normal height of contact wire for regulated OHE is 5.60 m (with 10 cm presag for 72 m span) above rail level. For unregulated OHE in areas with a temperature range of 4ºC to 65ºC, this figure is 5.75 m and in areas with a temperature range of 15ºC to 65ºC, it is 5.65 m. in certain cases, such as under over-line structures, the height may be as low as 4.65 m on BG and 4.02 m on MG. For passing oversize consignments on such lines, special precautions have to be taken.

span of supportingmast/structures

The span normally used for supporting the OHE from masts/structure using the cantilever type bracket assembly varies from maximum 72 m on straight track to 27 m on curved track, the spans depending upon the degree of curvature. The catenary system is normally supported on straight tracks at maximum intervals of 72 m (63 m on MG) by cantilever type arms fixed to galvanized broad flange or I section steel masts or fabricated steel structures. On curves the catenary is supported at closer intervals, the spans adopted depending upon the degree of curvature.

Stagger

 The contact wire is staggered so that as the pantograph glides along, the contact wire sweeps across the current collecting strips of the pantograph up to a distance of 200 mm on either side of the centerline on straight runs and 300 mm on one side on curves. This ensures a uniform wear of the current collecting strips of the pantographs.

Overlaps     The OHE conductors are terminated at intervals of about 1.5 km with an overlap, the conductor height being so adjusted that the pantograph glides from one conductor to the other smoothly.

    There are two types of overlap spans as under:

1.Uninsulated overlap spans where the distance of separation between two contact wires is 200 mm

2.Insulated overlaps, where the two OHE systems are kept apart at a distance of 500mm.Normally the electrical discontinuity at insulated overlaps is bridged by interruptors or isolator except at neutral sections.

Mechanical independence of OHE track-structures

By providing independent structures for supporting the OHE of each track, complete mechanical independence of each OHE is secured. Any irregularity or damage or maladjustment of the OHE of one track will not, therefore, affect the performance of the other.

Flexible head-span and rigid portals In large yards, where difficulty is experienced in locating individual supporting structures between the tracks, a catenary wire system called flexible head-span is provided to maintain two or more catenaries and their contact wires at the appropriate heights and locations. Where the OHE has to be regulated, rigid portal structures are used.

Maximum speed  

The OHE with maximum span of 72m and with presag of that span of 10mm and with tension of 1000kgf in contact and catenary wire is designed for a speed potential of up to 160kmph.The existing system is generally at for 140kmph with AM-12 pantographs now in use on ac locomotives.

Droppers  

The contact or trolley wire is supported from the catenary or messenger wire by means of droppers. The droppers are made of solid copper of usual cross section of 5 sq mm and are spaced closely along the contact wire at 9m distance from one another. The lengths are so adjusted so that although there is a sag in the messenger wire, contact wire is practically level.

Booster transformer (BT) system

: In the simple AC system, there can be severe inductive interference in telecom lines and other equipment because of the large loop area between the catenary and the rails which carry the return current. Some of the return current also flows in the earth causing conductive interference and corrosion problems in buried cables, pipes, etc. Such earth currents are higher if the conductive path in the rails is degraded because of rail joint problems.

In booster transformer (BT) feeding system there is now a return conductor, a wire that is close to and parallel to the catenary wire. The return conductor is connected to the rails and earthed. Periodically, there are breaks in the catenary where the supply current is forced to flow through one winding of a booster transformer ; the other winding is in series with the return conductor. The 1:1 turns ratio of the BT means that the current in the catenary will be very nearly the same as the current in the return conductor . The current that flows through the loco goes to the rails but then up through a connecting wire to the return

conductor, and through it back to the substation.

Insulated rail joints are also provided — this ensures that current flows in the rails only in the particular section where the loco is present. At all other places, the inductive interference from the catenary current is nearly cancelled by that from thereturn current, thus minimizing the interference effects. The problem of stray earth currents is also reduced.

One disadvantages in this system is that as a loco passes a booster transformer, there is a momentary interruption in the supply (because of the break in the catenary) with the attendant problems of arcing and transients on the line, as well as radio frequency interference.

IMPORTANT EQUIPMENTS OF ELECTRIC LOCO / EMU 

1. Pantograph For collecting power from 25kv ac contact wire pantographs are mounted on the roof of the traction vehicles. AM12 pantograph of Faively design has been adopted by Indian Railways for 25kv ac electric locomotives and EMUs. These pantographs are provided with steel strips for current collection. The raising and lowering of a pantograph is by means of a pneumatically operated servo motor. This pantograph is a single pan design having two o-springs mounted on it. For keeping the pantograph in the lowered condition, main springs have been used. The suspension of pan is on plungers.

This pantograph is suitable for operation up to 140km/h. For increasing the speed potential, improved pantograph with lower dynamic mass and independent pan heads have been used. Further, in order to increase the life of contact wire, use of carbon strips have also been tried.

 

1. Transformer  

Power to traction vehicles is available at 25kv ac single phase from the contact wire. In order to step down the voltage as well as to control the same for feeding to the traction motors, the traction power transformers are provided on the traction vehicles.

These transformers generally have a primary winding, a regulating winding, traction secondary windings and auxiliary windings. The regulating winding is designed for choosing appropriate voltage for the traction motors. The auxiliary winding is required for feeding the auxiliary motors on the locomotive.

With the introduction of thyristorised convertors, the design of the traction transformer has undergone simplification with the deletion of regulating winding. The transformer for thyristorised convertor becomes a two limb construction and a traction secondary winding split

into 4 windings for two step sequence control.

    The traction transformer necessarily has to have forced oil circulation and forced air cooling. For this purpose oil pump, oil cooler and blower form an integral part of the traction transformer.

 

1. Traction-motor

 

Traction motors power the driving wheels which actually move the locomotive. In case of traction motors great emphasis is being given to improve power to weight ratio, keeping in view the limited space available on locomotive for mounting the same. There is a continuous effort to improve the performance of traction motors by making them lighter/compact, at the same time more reliable.

Improvements in the basic design of traction motors have become possible due to availability of new insulating materials with high thermal margins. Instead of dealing with individual insulating material, the specification now covers the combination and system as a whole. The new feature is added because of thermal

endurance of the system which may not be directly related to the thermal capability of individual materials.

PROTECTIVE RELAYSA protective scheme includes circuit breakers and protective relays to isolate the faulty sections of the system from the healthy sections. A circuit breaker can disconnect the faulty element of the system when it is called upon to do so by the protective relay. The function of the protective relay is to detect and locate the fault and issue a command to the circuit breaker to disconnect the faulty element.

Protective relays can be classified into the following categories depending on the duty they are required to perform.

1. Over current relays

2. Undervoltage relays

3. Impedance relays

4. Underfrequency relays

5. Directional relays

These are some important relays. Many other relays specifying their duty they perform can be put under this type of classification. The duty which a relay performs is evident from its name. For example, an overcurrent relay operates when the current exceeds a certain limit, an impedance relay measures the line impedance between the relay location and the point of fault and operates if the point of fault lies within the protected section. Directional relays check whether the point of fault lies in the forward or reverse direction. The above relays may be electromagnetic, static or microprocessor-based relays. 

Remote Control System  To maintain continuity of electric supply to track is of utmost importance. In order to ensure this it becomes necessary to have a comprehensive picture of supply conditions and arrangement to operate switchgear with minimum delay. This is the purpose of a remote control centre (RCC). The in charge of a remote control station is called traction power controller (TPC).The TPC monitors the remote control centre around the clock He is responsible for carrying out all switching operations on the electric supply system. Each remote control centre is connected to grid substations, feeding posts, sectioning and sub sectioning post , station masters offices, important signaling cabins, divisional office and traffic control office. All the interruptors at various control posts and circuit breakers at various substations are controlled by what is called a remote control system .  

1.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. Earlier Indian Railways used an electromechanical control system, Frequency Modulated Voice Frequency Telegraph (FMVFT). These are still in use in some places. Now, 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.) in real time and remote operation of switchgear, as well as automatic fault detection and isolation, allowing better control of maximum demand, trouble- shooting ,etc.

 

SCADA  

The term SCADA is an acronym for Supervisory Control And Data Acquisition. It is a system that provides a real time diagrammatic representation of the entire traction system. Its predecessor was a mimic diagram installed at every remote control system.

A remote terminal unit is installed at every switching station this unit relays data to the remote control centre regarding various station parameters. Data is relayed through a network of optical fibers to a master server placed at the RCC. A front end processor(FEP) analyses the acquired data. The data is not received continuously

but at predetermined intervals from each station. The receipt of valid data is noted on a counter as is the receipt of invalid data. Three consecutive error messages trigger a warning indicating that the system is malfunctioning. The processed data is displayed at a number of computer nodes or workstations.

    The display is in the form of a diagram showing the complete traction system. Different symbols are used to represent equipment such as interrupters, circuit breakers, transformers etc. Colour coding is used to represent the status of the equipment. For e.g. green lines may indicate a healthy section of track while red would indicate a fault. Relevant data such as the voltage and current output of a substation is also displayed alongside. This kind of a display allows easy analysis of the situation at any given instant. Operations such as closing an interrupter or isolating a faulty section can be done by clicking on the concerned symbol. The program is protected by authorisation codes which ensure that only authorized personnel can carry out switching operations or access data. This redundancy minimises the probability of failure.

    The advantages of using the SCADA system are ease of operation, fast response and greater coordination and control over the entire system. If a fault occurs immediate isolation of the faulty section and rerouting of power to healthy sections can be carried out.