CONTENTS   

1. Introduction

2. A.C Traction Systems

3. D.C. Traction Systems

4. Mixed Traction Systems

5. Protection Against Other Forms of Interference

6. Future Developments and Problems

1.         INTRODUCTION 

Electric traction is being used as an economic means of operating a railway. Electric power is taken from the national or regional supplier. Due to the scale of production this can normally be produced at a lower price than with individual power units on each locomotive. Electric locomotives are much simpler and cheaper to control maintain and operate than their diesel equivalent.

Electric traction brings with it a number of additional problems for the signal engineer. Signaling equipment, in most cases, operates at relatively low voltages and currents compared to locomotives and multiple units. Unavoidably, traction circuits run alongside and even share conductors with signalling circuits. Interference, usually through direct contact or induction must be anticipated and equipment protected from its effects.

Even without electric traction, signalling equipment may still be subject to interference from adjacent power supply lines or lightning storms. Similar techniques are often employed to protect against the likely effects.

The objectives of protection (or immunisation) can be summarised as:-

a) Prevention of failures, particularly wrong-side failures.

b) To ensure the safety of staff working on the equipment under normal operating conditions and, as far as possible, under traction fault conditions also.

c) To prevent the effects of traction currents from damaging or destroying signalling equipment.

1.1.       Electrification Systems

 The effects of electric traction and the precautions which must be taken will depend on the type of traction supply.

The electrification system chosen for a railway will, in tum, depend on a number of factors: -

a) Type and frequency of traffic.

b) Availability of Power Supplies and the locations at which they can conveniently be brought to the lineside.

c) Local geographical and climatic conditions.

d) Structural clearances (for overhead wires  etc)

e) The overall length of the line.

f) Whether the line to be electrified is an extension to an existing electrified line or a completely new traction system

As a result many different systems are in use. Once a system has been selected, it is very expensive to change due to the investment in traction units and power supplies.

The systems can generally be divided into two groups: -

A.C. systems, normally employing a high voltage (typically 25kV) with overhead current collection.

D.C. systems, generally at much lower voltages (600 to 1500 volts). The method of current collection is either overhead (requiring a much heavier conductor than for a.c.) or third rail.

Low voltage systems tend to be d.c. because this makes the traction equipment on the trains much simpler. Early electrification did not have the benefits of modern power semiconductors to control traction motors so the power was at a voltage which could be applied directly to the d.c. traction motors.

The cost penalty is a large number of substations and heavy feeder cables.

The use of a.c. traction permitted substations to be spaced considerably wider apart and the overhead conductor to be used as the main feeder. No separate feeder cable is necessary. The equipment on board the train is generally more complex. The high supply voltage must be transformed to a usable voltage and rectified to d.c. for the traction motors. Modern variable frequency a.c. drives have now made the induction motor a viable alternative to the series wound d.c. motor.

1.2.       Modes of Interference

In most cases one or both running rails are used for the return current path. To some extent earth currents will also be present.

In all types of track circuits there is the risk of interference by direct contact, traction currents passing through the signalling equipment or between the signalling equipment and earth.

There is a high risk that traction currents could cause wrong side failures. The incidence of right-side failures could affect reliability to an unacceptable level.

Other circuits are generally not in direct contact but there is still the possibility of induced interference. insulation could also break down causing direct contact with the traction supply.

Protection must also be provided against dangerously high voltages (either induced or through direct contact) which could injure or kill personnel working on signalling and communications equipment.

2.         A.C. TRACTION SYSTEMS

2.1 Feeder Arrangements 

BR's 25KV 50 Hz AC. traction system is typical of those on many other railways and is usually derived from the 132 KV National Grid system. The supplies are taken as single phase through 132/25 Kv transformers provided by the Supplier. The supplies are taken to feeder stations at usually the same phase.

Adjacent feeder stations are not parallel (i.e. not necessarily from the same phase and usually not from the same source. This enables the Electricity Authority to balance the railway load between the 3 phases of its system.

The distance between feeder stations is dependent on traffic and power requirements. An electric locomotive accelerating with a full load may take a current of the order of min 270 amps. Neutral sections are provided between feeder stations to separate the supplies and track sectioning cabins (TSC) are located at intermediate points and the boundaries of feeding sections to provide alternative feeding and track isolation facilities in the event of failures or isolation for maintenance.

2.2  Return Paths for Traction Current 

2.2.1.Rail Return 

This is the simplest form of traction current return. In most cases, only one running rail is used. Modern Track Circuits allow dual rail traction return. On plain track this is normally the cess rail, but in complex layouts the location of return rail changes in order to meet track circuit requirements by cross-bonding. The problem with rail return is that the rail is not well insulated and return traction current will leak to earth over the length of the return rail.   This produces a large imbalance between the current in the overhead wire and that in the rail, which dramatically increases the level of induced voltages in lineside cables (see later sections for details).

2.2.2 Return Conductor

If the traction return path is well insulated, the current imbalance in the circuit is reduced, hence interference is minimised. A return conductor can therefore be provided to carry the traction return current. The current carried is in anti-phase to the current in the contact and catenary wires, and similar in magnitude, therefore the net electro-magnetic inducing field is reduced. The reduction over rail return, where signalling circuits are placed in the optimum position for minimum interference, is of the order of 45%. 

2.2.3 Booster Transformers

Some of the return current will still flow via earth instead of the return wire. This will be in inverse proportion to their relative impedances between the train and the substation. The cost of a conductor large enough to eliminate the earth currents would be prohibitive. To force the majority of the return current into the return conductor, transformers are provided in the traction circuit as shown on FIGURE1

The return conductors are bonded to the traction return rail midway between booster transformers. These are located at  3.2 Km intervals, the primary winding connected in series with the catenary and the secondary winding in series with the return conductor.

The effect of the booster transformer is to produce a current in the return conductor which approximates to that in the catenary but is 180â—¦ out of phase. Maximum suppression of induction is achieved with the traction load at the mid-point connector (95% reduction) and the minimum when at the booster transformers.

This alone does not immunise signalling and communications circuits. As the two conductors are widely separated, induction will still occur whenever a signalling circuit is nearer to one conductor than the other. The relative positioning of the overhead conductors, the return conductor and the signalling cable route must be such as to place the cable route equidistant from the traction conductors.

2.3 Modes of Interference and Immunisation Techniques 

Of the three possible modes of interference, conduction, electrostatic induction and electromagnetic induction, induction is the dominant mode in a.c. traction  areas.

Conduction, by direct contact between the power line catenary and the rails, equipment or cable conductors happens rarely. If unprotected, it is usually fatal to personnel and equipment. The only safeguard is the provision of contact circuit breakers in the traction supply.

Due to bonding of overhead structures direct to the traction return rail, the risk of a traction fault raising the rail potential, and that of track circuit equipment, to dangerous levels is small. Track circuits will however be subject to false operation due to the multiple return paths created by return conductors and earth bonding and extra precautions must be taken when setting up and adjusting track circuits to ensure safe operation.

Due to the distance between the traction equipment and the signalling equipment and the low supply frequency of 50Hz., electrostatic effect are usually negligible.

Electromagnetic induction is caused in one conductor by current flowing in another. This action is similar to a transformer, the traction conductors being the primary winding and the signalling conductors the secondary. The voltage is induced along the conductor (not between the conductor and earth). The effect is maximised when conductors run parallel to each other. The induced voltage increases with length of conductor and decreases with separation of the conductors.

As signal engineers we are concerned with the following aspects of a.c. induction into our lineside cables

a) voltages produced under normal operating conditions that could affect line circuits and tail cable circuits and result in the malfunction, failure or damage to equipment and/or hazard to staff working on the equipment.

b) Higher levels of AC. voltage induced in our cables as a result of traction system.

c) Any increase of these effects due to disconnection, earthing or other failures within the signalling equipment

Dangerous levels of induced voltage are possible if the traction current rises rapidly (short circuit or traction flash over).

Telecommunications circuits are susceptible to all normal levels of induced voltage on unprotected cables from the traction system. Therefore, special measures have to be adopted to protect these systems from the 50 Hz base and odd harmonics of this frequency.

2.3.1 Positioning of Cable Route

The simplest method to reduce the levels of induced voltage is the physical location of the cable route, as described earlier. The best position for the cable route is approximately equidistant from the two traction conductors (where booster transformers and return conductors are provided).

This distance will obviously vary for different designs of overhead equipment, for multiple track lines where two or more return conductors are mounted together, and for lines without booster transformers and/or return conductors.

On most circuits which form a loop (one conductor out and the other return), this provides an adequate level of protection for normal operation  provided  the signalling equipment  is in full working order. Both conductors are subject to the same electromagnetic fields which tend to oppose each other. Under fault conditions (earth faults and/or disconnections) further protection will be required.

2.3.2 Electro-Magnetic Screening 

With any wire installed parallel to an A.C. electrified line, a reduction in interference is obtained by the screening effect of earthed conductors in its vicinity. This includes cable sheaths, metal pipes and running rails.

All communications cables are provided with a screening sheath (usually aluminium) with up to a maximum of 4 steel tapes around the sheath. These are connected to a good earth of no more than 4 ohms located every 1000 m.

Signalling cables run in the same cable routes as screened telecomms cable and therefore benefit from the mutual screening effect between the cables.

Where existing cables, not to electrification standards are to be retained, it may be possible to immunise them by a separate screening conductor. This is a large cross-section copper wire run along the length of the cable route and earthed as above. Cable sheaths should also be earthed. 

2.3.3 Immunisation of Relays 

Relays for d.c. circuits can be designed to withstand substantial a.c. voltages without energisation.

The relays have copper slugs fitted over the cores,  near the pole pieces  and a magnetic shunt is fitted between the cores above the copper slugs.  When a DC voltage is applied to the winding the DC flux is produced as normal and attracts the armature.   Some of this flux is diverted via the magnetic shunt and therefore greater power has to be supplied. However, when an AC voltage is applied, the AC voltage has difficulty in establishing  itself  across the air gap due to the large copper slugs and it tends to short circuit the air gap via the magnetic shunt, thus the AC flux plays little part in the operation of the mechanism. Refer Figure 2 &3   showing the principle.

 

 

 

2.3.4 Choice of Operating Frequency 

Where d.c. circuits are not practical, equipment should be designed to operate at frequencies other than the mains frequency and its harmonics. Filters can be used to keep signalling and traction currents separate

Although the mains frequency is normally very accurate, fluctuations can occur and these must be allowed for in the design of systems. Fluctuations of ± 0.5% are typical so the design may allow for 1% maximum error to give a degree of margin for error.

It should therefore be evident that the available bandwidth between successive harmonics decreases by 2Hz each time and above 1kHz no bandwidth is available. In practice, harmonics of this order are very small and frequencies above 1.5 kHz are successfully used for track circuits. For additional safety, two or more frequencies are used together so that traction faults could not generate both together.

2.2 Practical Immunisation of Signalling Equipment

2.4.1. Limits 

Appreciable earth currents may still flow for some distance away from electrified lines and protection must be extended sufficiently far for their effects to become negligible. At the limits of electrification, or where a non-electrified line leaves the electrified lines, experience has shown that signalling equipment should be immunised for 800m from the electrified line.

2.4.2. Line Circuits 

These should be DC circuits using AC immunised line relays.

The length of line must be limited to 2km to ensure that the induced voltage from the traction system does not exceed limits for electrical safety (maximum induced voltage of 110v). Circuits required  to cover greater distances must be repeated  by means of a relay and new power supply.   Where circuits run along non-electrified branches, they must be cut 800m from the electrified line.

Where circuits from the same supply feed in opposite directions, care must be taken to ensure that the total length of parallel circuits is less than 2km.

All vital line circuits are double cut to reduce possible false operation and hazard to staff where an earth fault is present.

2.4.3.Track Circuits 

The traction return current can usually be carried satisfactorily by only one of the running rails. Single rail DC track circuits, immune to the highest AC voltage that could occur, are used. It has been found desirable to limit the track circuit length to a lower value than on non-electrified lines to prevent the combined effect of a return conductor and a broken rail providing an alternative path to the train shunt.

The track feed set must be designed to prevent a significant DC voltage being applied to the rails as a result of rectification of AC from the traction current. The track relay must also be immune to AC in the same manner as those used for line circuits.

Reed track circuits operate at frequencies clear of 50Hz harmonics and may also be used in AC electrified areas as may most modulated audio frequency track circuits such as the TI21. When little used and therefore rusty rails require track circuiting there are two solutions - a welded stainless steel strip on the top surface of the rail or a high voltage impulsing type of track circuit.   The Jeumont track circuit is immune to false operation by AC traction current, but its length is severely limited in a.c. traction areas. This may not be a serious disadvantage, as it is mostly used for point and crossings into sidings and loops.

There are few types of track circuits mentioned here, Signal Engineer must check with the product owner and manual for other types of track circuits.

2.4.4 Signals 

Colour light signals use tungsten filament lamps, which operate readily on both AC and DC. Therefore the only method of preventing induced AC lighting the lamp is to limit the length of the circuit between control relays and lamps and employ as high a voltage as practical.

The usual practice is to have a transformer for each aspect in the signal head to reduce the voltage to the 12 volts or less required to operate the lamp and supply 110V AC over the contacts of the controlling relays. In this case the maximum parallelism allowed is 183m, i.e. a signal head must be less than 183m from its controlling relays.   If the overall maximum distance between signals fed from the same location or relay room supply exceeds 183 m, then an isolating transformer is required to feed signals on one side of the location or relay room. By limiting the length of signal lamp circuits, it is unnecessary for them to be double cut.

Shunt signals generally operate on 110-volt lamps so no transformer is necessary.

Searchlight signal operating mechanisms must be immune to false operation. This immunity is  achieved by the use of chokes mounted as close as possible to, and in series with, the d.c. searchlight mechanism. An a.c. searchlight signal could be operated at a different frequency (e.g. 83.3Hz)

For LED Signals (current practice), signal head has AC/DC rectifier unit and few types are mentioned below

1)Aldridge RL 400 ,120V Main Line Signal 212mm /127mm.Cable to signals shall be less than 750m for single cut circuits .Between 750m to 1500 meter for single cut circuits with two bleed resistors fitted in the signal head .Fit a 130VAC ,20mm varistor across each LED unit.

2) Aldridge Tunnel Signal 120V AC 127mm.Cables to Signal less than 1200 mm for    single cut circuits and less than 2000m for double cut circuits .No varistors required for surge protection.

3) Alstom Mark 2 120V AC Mainline out door signals 212mm.Cable to signal less than 750m for single cut circuits ,between 750m and 1500 m for single cut circuits with two 4K7 6W bleed resistors fitted in the signal head .Less than 2000m for double cut circuits .No varistors required for surge protection ,but must have some surge protection to earth on each leg of 120VAC supply at power supply locations .

2.4.5.Points 

Electric point machines are immunised by using permanent magnet machines. These ensure that, even if there is an AC voltage at the terminals, the motor's field will remain uni-directional and although there will be considerable vibration, there is no resultant torque and therefore the motor will not move.

Electro-Pneumatic point machine valves must be immune.  These valves are immunised  in a similar manner to that outlined for relays.

Electro-Hydraulic (clamp lock) point mechanisms were found to have inherent immunity and can be used with no special measures being taken.

Mechanical Points should have insulation inserted in the point rodding at the ends adjacent to the lever frame and the points.   This is to prevent stray voltages causing electric shock to personnel. Detection of all types of points is by polarised relays. These relays are fully immunised in the manner already described.

2.4.6.Level Crossings 

Where lifting barriers are used it is desirable to position the barrier so that, if it is knocked over no part of it shall come closer than 150 mm to the overhead line equipment. If other positioning requirements make this impossible, then the barriers should be made of metal or have a continuous metallic strip of adequate section along its length. The barrier or metallic strip should be bonded to a traction return rail or cable.

Control circuits must be immunised as already described.

Where closed circuit television is used special precautions must be incorporated into the design.

2.4.7. Remote Control 

Immunisation of remote control systems is dealt with in the separate notes covering remote control. This mainly involves line isolation at regular intervals. 

2.4.8.Power Supplies 

Although the safety precautions in force for higher voltage power supplies would cater for the possibility of induced voltages at the levels expected, care must be taken when working on supplies which are switched off to avoid the possibility of high induced voltages. As the cable may not be sectionalised in the same way as vital signalling circuits, dangerous voltages could occur on long power feeders.

3.   D.C. TRACTION SYSTEMS

Because of the low traction voltage, traction currents are high, typically, 2,000 - 3,000 amps per train during acceleration.   Due to the large DC return currents present in the earth near to and in the return rail, there are problems caused by corrosion, and problems caused when insulation, equipment and cables break down.

A d.c. supply will normally produce no inductive interference effects other than from switching transients and ripple at harmonics of the mains frequency from an unsmoothed power supply. The main hazard is from conduction (to which track circuits are particularly exposed) by direct contact or via earth faults.

The main precautions therefore comprise operation of equipment from a.c. supplies, effective insulation arrangements and earth leakage detection.

3.1   Limits 

Earth currents from d.c. supplies are much more troublesome than for a.c. They may propagate over much longer distances and immunisation should be provided for at least 3km from a d.c. electrified line.

 3.2 Line Circuits

DC circuits are used which are not sectionalised other than for volt drop purposes. It might initially seem unwise to. use d.c. but the reasons for accepting d.c. line circuits on B.R. in most situations are as follows:-

a) All cables have non-metallic sheaths and are therefore less likely to pick up DC potentials along their routing.

b) All cables are tested to a rigid specification 

c) Additional insulation is provided by terminating cables on non-metallic materials

d) All line circuits are double cut to ensure that an earth fault in one leg of the circuit cannot cause false operation

e)Additional earth leakage detection equipment is used

f) In most dc. traction supplies there is a significant a.c. ripple from the rectifiers. Use of a.c. circuits would not provide immunity from the effects of this ripple

3.3         Track Circuits 

AC track circuits use vane relays which have self immunity to the effects of DC. Even where the traction supply may contain a.c., the two element vane relay will only operate if the interference signal is at the correct phase relationship. The relay will not respond to higher harmonics unless these are contained in the correct proportions in the supply to both coils.

Reed and TI21 track circuits and several other audio frequency track circuits are also immune to the effects of DC.

3.4  Signals 

No special precautions are taken for signal lighting circuits. These are usually a.c. fed with signal head transformers. Search-light signals are operated by a.c. vane type mechanisms, which, like track relays, have self immunity to DC.

3.5  Points 

No special precautions are taken in the choice of machine for the control of points. Although a.c. machines might be considered to have better immunity, d.c. operation is normally acceptable at higher voltages (110-130 volts) together with adequate earth leakage detection.

Clamp locks, having separate valve and motor circuits can only be falsely operated by two simultaneous faults.

Detection of points may be achieved with a.c. circuits using vane type three position relays. Alternatively, 110 volts a.c. may be used from the detector to the location where each individual circuit is transformed and rectified to operate a 50 volt d.c. line relay.

4.         MIXED TRACTION SYSTEMS

 There are some areas where both traction systems are in use, either using the same tracks or an adjacent track. This means that the signalling system has to be immune to both AC and DC traction supplies. Circuits used must operate on a frequency distinct from the mains frequency and its harmonics. 

4.1.       Limits

Although the d.c. immunity is required for a greater distance, it is normal to dual immunise for approximately 3km.

4.2  Line Circuits 

Circuits are usually d.c. with precautions taken as for a.c. and d.c. lines.

4.3 Track Circuits 

Formerly, the most popular method was to use a.c. two-position vane relays but operated from an independent power supply at a frequency different to the mains supply and its harmonics (normally 83.3 Hz for 50 Hz mains). The additional power supplies were a significant added cost. Modern audio frequency track circuits such as the TI21 are immune to both d.c. and a.c. traction.

4.4  Signals

The circuits are identical to those used in AC traction areas. Searchlight signals are immunised by using an AC vane type mechanism with a maximum length of 55 metres for the feed circuit. If signals are more than 55meters  apart on either side of the relay room or location an isolating transformer must provided in the feed circuits of all signals on one side of the relay room/location.

4.5 Points

The methods of operating points are identical to those used in areas of AC traction.

Detection of all types of point is either by a two element vane relay from an independent supply at a separate frequency, or by a filtered circuit such as the GEC vital Reed system. Frequencies 477.5 Hz and 414.75 Hz (type RR 4000) have been allocated for use with point detection circuits on B.R. and are employed for Normal and Reverse detection respectively.

5.  PROTECTION AGAINST OTHER FORMS OF INTERFERENCE 

Apart from traction, adjacent power supplies and lightning are the main sources of electrical disturbance.

Power transmission lines generate the same type of interference as a.c. traction and protection is therefore similar.

Lightning protection is a very specialized area and will only be covered in outline.

A direct lightning strike produces voltages far higher than the worst traction fault conditions. It is virtually impossible to provide effective protection. When lightning finds a path to earth, it will however raise the voltage at the point at which it enters the earth. Currents will flow which, although of extremely short duration, can induce large voltages in adjacent equipment. The danger is that the voltages may be large enough to find a path through the signalling equipment or break down insulations in cables etc.

Fortunately, protection can be provided against this effect by the prov1s10n of surge arrestors. The most common type is a gas discharge tube which has three electrodes, one connected to each leg of a loop circuit. and the other to earth. For the normal operating voltages of the signalling circuit, it will appear as an open circuit. When the voltages induced by lightning are sufficiently high, an arc will form in the gas discharge tube which will provide a low impedance, high current capacity path to earth for the duration of the lightning strike. When the current is insufficient to sustain the arc, the surge arrester will revert to its open circuit state.

Semiconductor devices are also available to operate in a similar manner. Their switching time is much faster than a gas discharge tube, giving quicker protection, but their current capacity is much lower. They provide better protection against moderate strikes but may not be able to handle the current caused by severe strikes.

Both types of device are also suitable for protecting vulnerable equipment (e.g. electronic track circuits) from traction fault conditions where required.

6.  FUTURE DEVELOPMENTS AND PROBLEMS 

We have concentrated so far on interference caused by the supply. This is always at the mains frequency or harmonics of it and these frequencies can therefore  be filtered or avoided as appropriate to the type of equipment.

Electric traction units have in the past invariably employed d.c. traction motors. These do not produce any different interference to that already produced by the power supply.

Many railways are now employing traction units either with d.c. motors and variable frequency thyristor chopper controllers or more recently, ac. variable frequency induction motors. The control equipment for these types of traction unit can produce interference at many (often continuously variable) frequencies over a broad spectrum.

It is impossible to detail specific problems but the signal engineer must be very careful in the future as new traction units are introduced that he is aware of their possible effects on the signalling equipment. In Part 2 we will cover Electronic Interference