GB2058421A - Track Circuits for Use in Electrified Railways - Google Patents

Abstract

In an electrified railway in which traction return current flows along the track rails (1, 2) bonding units bridging the rails are provided at intervals to afford low impedance paths between the rails for all traction currents outside the pass band of a track circuit. Each bonding unit (8) which resonates at the track circuit frequency has an inductive component (9) and a capacitive component (10) with separate connections to the two rails and with values such that disconnection of either component from the track results in a sufficient shunting of the rails by the other component to be detectable by the track circuit. A fault in any bonding unit along a section of track controlled by the track circuit is thus detectable automatically. <IMAGE>

Description

SPECIFICATION Track Signalling Circuits For Use In Electrified Railways This invention relates to track signalling circuits for use in electrified railways.

The invention is particularly applicable to electrified railways in which traction return current flows along a pair of rails, usually the running rails of the track. Thus the invention is applicable to electrified railways having direct or alternating current traction supply, and particularly, but not exclusively, to electrified railways in which an overhead catenary supply delivers the traction current, whether D.C. or A.C., and return current flows through the running rails of the track to the power generating sub-station.

Typical of such systems are those with an overhead catenary supply at 1500 or 3000 volts D.C., or a high voltage overhead catenary supply at 25 kilovolts A.C.

It will be appreciated that under conditions of heavy use the supply line and the return line can carry considerable currents. For example, a 4000 H.P. electrical locomotive working at full power will give rise to a return current of the order of 120 amps in the running rails when using a 25 KV alternating current supply. In some countries where tandem working of a number of high power locomotives is common the return currents in the track would be considerably higher than this.

The existence of very heavy traction return currents in the running rails of a track presents a considerable problem when the rails also form part of track signalling circuits and have to carry signal currents of the order of milliamps. To avoid the possibility of false signals being received due to the traction return currents in the rails track circuits which are "traction immune" are employed in which the power used for signalling along the track is of a different regime from that used for traction. Thus in a D.C. traction railway it is common to employ A.C. track circuits, neighbouring track circuits being isolated from each other by block joints in the track at which the traction current flows through balanced impedance bonds.In A.C. traction systems it is not uncommon to employ D.C. track circuits with joints in only one of the running rails, special protection being provided to prevent the traction return currents in the rails from increasing the voltage in the D.C. track circuit.

A more recent development is the "jointless" track circuit which uses audio frequency signal currents and employs tuned circuits to define the limits of neighbouring track circuits. By appropriate selection of the modulation coding of the track circuits it is possible to avoid interference with harmonics of the traction current in the rails, remembering that even in the case of D.C. traction the traction current is usually derived by rectification of an alternating current supply and therefore inevitably includes residual harmonics. One solution of this problem is to employ frequency modulation for the track signalling in such jointless track circuits.

In electrified railways where traction return current flows through the running rail or through return conductors it is sometimes necessary to take precautions so as to ensure that return current flows predominately through the rails and not through the surrounding earth, since when very heavy traction currents are in use undesirable interference with telecommunications and electricity supplies can arise if heavy return currents are permitted to flow through the earth.

To ensure that the return current flows predominately in the rails and not through the earth return conductors may be employed having frequent connections to both running rails of the track, and current transformers may also be used to "pull" current into the rails and the return conductor.

Where heavy return currents flow through the rails of a track even small differences in the impedance of the rails of the track can give rise to considerable voltages between the rails. Track signalling circuits are normally designed to operate satisfactorily even where considerable voltages exist across the rails. There is, however, a danger to railway operating staff if the voltage between the running rails caused by imbalance in the traction return currents reaches high values, particularly at frequencies outside the pass band to filters employed in the track signalling circuit.

To avoid such imbalanced voltages impedance bonds are used to interconnect the running rails of the track at intervals. Such impedance bonds are tuned to resonance at the track circuit frequency at each track location so that they present a high impedance at the track circuit frequency, but a low impedance outside this frequency, affording a virtual short circuit to traction return current.

From the safety standpoint it is clearly desirable to have some means of monitoring the integrity of impedance bonds across the track.

Since, however, the impedance bonds are designed to present a high impedance at track circuit frequency, the track circuits themselves would not in the ordinary way provide any indication if, for example, an impedance bond were to become open-circuited, or were to be disconnected from the track, giving rise to a potentially dangerous situation due to increased traction current imbalance between the running rails.

An object of the present invention is to provide a track signalling circuit which affords, for most practical purposes, an indication of the integrity of impedance bonds across the track.

According to the invention there is provided a track signalling circuit for an electrified railway having a signal frequency pass band, in which traction return current flows along a pair of rails which are bridged at intervals by bonding units which afford low impedance connections between the rails for all traction current outside the pass band of the track signalling circuit, each bonding unit comprising a capacitive component and an inductive component having separate connections to the two rails and having values such that disconnection of either component from the track results in a sufficient shunting of the rails by the other component to be detectable by the signalling circuit.The bonding units afford low impedance shunts between the rails at traction current frequency and significant harmonics thereof in the case of A.C. traction, and for direct current components and residual harmonic frequencies present in the case of D.C. traction.

By providing separate connections to the two rails of the track from the capacitive and inductive components of each bonding unit a measure of additional safety is introduced, since loss of any one connection will result only in the disconnection of one of the components of the bonding unit from the track, leaving the other component connected across the track so as to afford a sufficient shunting of the rails to be detectable by the signalling circuit. The invention therefore affords a simple means of detecting loss of integrity of any bonding unit in the section of track monitored by the signalling circuit.

Simultaneous failure of both connections to any one rail of the track from a bonding unit would not, of course, be detectable as described above, since this would result in disconnection of both the capacitive and inductive components of the bonding unit from the track. The occurrence of a simultaneous disconnection of both components of the bonding unit is, however, a very remote possibility.

The inductive component of each bonding unit may comprise a lumped inductance physically connected to directly opposite points on the two rails. Such a lumped inductance may, for example, consist of a coil enclosed in an oil-filled housing.

The inductive component of each bonding unit may alternatively comprise a distributed inductance, which may take the form of a single length of wire of suitable gauge. Thus the inductive component of each bonding unit may comprise a length of wire having portions located adjacent and substantially parallel to the running rails of the track and connected at opposite ends to the respective rails. The said respective portions may be located outside or inside the respective rails, and in one embodiment each said portion is connected to the opposite rail to that which is next to the said portion, so that the current flow in said portions is in the same direction as the traction return current in the rails adjacent said portions.

The invention is applicable to all audio frequency track circuits, whether jointed or jointless.

Where the invention is applied to a railway having more than one track further equalisation of return current in the tracks may be ensured by interconnecting the inductive components of bonding units in adjacent tracks. Such interconnections would preferably take the form of centre tap interconnections between the inductive components, effectively interconnecting the rails of the adjacent tracks and increasing the track current return carrying capacity.

The invention is illustrated, by way of example, in the accompanying purely diagrammatic drawings, in which: Figure 1 shows schematically a section of electrified railway track provided with a track signalling circuit according to one embodiment of the invention; Figure 2 is a schematic plan view illustrating the electrical connection of a bonding unit having a distributed inductance for use in another embodiment of a track circuit according to the invention, and Figures 3, 4 and 5 are schematic plan views showing further alternative forms of bonding unit for use in a track signalling circuit according to the invention.

The section of electrified railway track illustrated schematically in Figure 1 has a pair of running rails 1, 2 and an overhead catenary line 3 supplied with high voltage alternating current, in this case at 25 KV and 50 Hz.

The illustrated section of track is provided with track signalling using frequency-modulated audio frequency signal at a base frequency F1 transmitted along a section of the track 1, 2 defined between two tuned circuits 4, 5 which are tuned to the frequency F, and which are coupled inductively to a trackside transmitter Tx and a trackside receiver Rx respectively. In the absence of a train on the section of track between the circuits 4 and 5 signals transmitted along the track 1, 2 from the transmitter Tx are received at the receiver Rx; when a train is present on this section of track, however, the rails 1, 2 are shortcircuited and the transmitted signals fail to reach the receiver Rx, resulting in detection of the train, for example by the de-energisation of a relay associated with the receiver Rx.

The sections of track 1, 2 adjoining the illustrated section are tuned to a different frequency, F2, by means of further circuits 6, 7, the circuits 4, 5 having a low impedance at the frequency F2 so that for signalling purposes the neighbouring track sections are effectively isolated from each other, without the provision of insulating joints between the adjacent track signalling circuits.

An electric locomotive running on the track draws traction current from the overhead catenary line 3, return current flowing to the power supply substation through the running rails 1,-2. Under certain conditions, particularly dry weather, there is relatively little leakage to earth of the return traction current flowing in the rails 1, 2, and this can give rise to substantial imbalance between the currents flowing in the two rails 1, 2 as a result of the inevitable difference between the impedance of the two rails. Even where this impedance difference is very small, a substantial voltage can in consequence be developed between the two running rails 1, 2, particularly under conditions of heavy traction current flow.

For the purpose of equalising the traction return current in the running rails 1, 2 bonding units 8, one of which is shown diagramatically in Figure 1, are provided at regular intervals along the track. In a typical installation there would, for example, be one such bonding unit every 1000 meters along the track.

Each bonding unit 8 comprises an inductor 9 connected across the two rails 1, 2 and having an impedance at the fundamental frequency of the traction return current which is so low that it affords a virtual short-circuit between the rails 1, 2 for the traction current, thereby tending to equalise the traction return current flowing in the rails 1, 2. In a typical bonding unit the inductor 9 would be rated for continuous operation at up to 200 amps, with a short-circuit rating of 2000 amps for a time (typically 100 milliseconds) long enough to allow for circuit breakers in the traction current supply sub-station to blow in the event of short-circuiting of the traction current supply. The inductor 9 should also be designed to withstand the effects of auto-reclosure of the traction supply circuit breakers.The mechanical connections of the inductor 9 to the rails 1, 2 and the physical construction of the inductor itself, are such as to withstand the thermal and magnetic stresses resulting from such current overloads. Typically the inductor 9 would comprise a coil of appropriate dimensions enclosed in an oilfilled housing. The inductor would be air cored to avoid the effects of magnetic saturation, and would typically have an inductance of 30-60 ,uH, giving an impedance at 50 Hz of approximately 0.015 ohm which, assuming a D.C. resistance of, for example, 0.002 ohm would result in a total impedance to traction current at the fundamental frequency of the order of 0.01 5 ohm.Under conditions of imbalance between the traction return currents in the rails 1, 2 a current of 200 amps flowing through the inductor 9 of the bonding unit would result in a voltage of only 3 volts across the inductor 9, that is, between the rails 1,2.

The bonding unit 8 further includes a capacitor 10 which is selected so as to form with the inductor 9 a resonant circuit at the relevant track circuit frequency such that, at this frequency, the bonding unit 8 presents a maximum impedance.

The bonding unit 8 does not therefore interfere with the normal train detection function of the track circuit. For a typical track circuit audio frequency (1 699 Hz) the capacitor 10 would have a capacitance in the range 1 50-300 ,uF.

It will be noted that the inductor 9 and capacitor 10 of the bonding unit 8 have separate connections to the respective running rails 1, 2. In the illustrated examples the inductor 9 is connected to the inside faces of the rails 1, 2, while the capacitor 10 has connections to the outside faces of the rails 1, 2. The loss of any one of the four connections of the bonding unit 8 to the rails 1 , 2 will not result in complete electrical disconnection of the bonding unit, but will leave one component of the bonding unit in place.

Whether the component remaining is the inductor 9 or the capacitor 10 the result will be a low impedance at the track circuit frequency, simulating the short-circuiting effect of a train on the monitored section of track, and resulting in an appropriate detection function at the associated receiver Rx. Such an occurrence in the absence of a train on the relevant section of the track provides an immediate indication of the loss of integrity of one of the bonding units 8 in the monitored section of the track, signalling a potentially dangerous situation which could give rise to undesirably high imbalance voltage between the running rails 1,2. In response to such an indication maintenance staff can make a rapid physical check of the bonding units 8 in the track section in question.

The resonant circuit formed by inductor 9 and capacitor 10 should in practice have a O of the order of 20 to be effective as a traction current bond while at the same time presenting a high impedance to track signals.

Figure 2 illustrates diagrammatically an alternative form of bonding unit 1 8 which may be employed in place of the bonding unit 8 shown in Figure 1. The bonding unit 1 8 has a distributed inductance formed by a length of wire 1 9 of appropriate gauge connected at opposite ends to respective points 20, 21 on the running rails 1,2 which are spaced from each other in the longitudinal direction of the track. This form of distributed inductance does not present the problems of mechanical stress during periods of traction current overload which arise when using a lumped inductor 9 of the kind illustrated in Figure 1.

Figure 3 shows a further alternative form of bonding unit in which the inductance comprises a single loop 28 located adjacent the track and connected at opposite ends to the respective running rails 1,2. The inductive loop 28 has respective portions 29, 30 which are parallel to and located outside the rails 1,2, each portion being connected to the opposite rail 2, 1 to that which is next to the said portion 29, 30, so that the traction return current flows in the portions 29, 30 in the same direction as the traction return current in the adjacent rails 1,2 as indicated by the arrows in Figure 3. Alternatively, the portions 29,30 of the loop 28 could be located inside the rails 1, 2 of the track. This gives rise to a high inductance using a minimum quantity of wire, by taking advantage of the mutual inductive coupling between the portions 29, 30 and the respective rails 1,2.

Figure 4 shows another alternative form of bonding unit having distributed inductance in the form of a loop 38 connected to the rails 1,2 in a similar manner to the loop 28 in Figure 3, but consisting in this case of two turns of wire. This configuration has the advantage that the centre of the loop 38 is close to the capacitor 10, so that centre-connections to the loop 38 of two adjacent bonding units on a track, for example, for the purpose of making a current return connection to the track, can be made with a minimal amount of wire.

Figure 5 illustrates a simple alternative to the bonding units shown in Figures 2 and 3, having a distributed inductance in the form of a wire 48 having connections at its opposite ends to directly opposite points on the rails 1,2 adjacent the capacitor 10, the wire having two portions 29, 30 which are parallel to the respective rails 1, 2. The portions 29, 30 are shown inside the track but one or both portions may alternatively be located outside the track.

In each of the embodiments shown in Figures 2 to 5 the capacitor 10 is shown located between the rails of the track, but may alternatively be positioned beside the track.

Claims (7)

Claims

1. A track signalling circuit for an eiectrified railway having a signal frequency pass band in which traction return current flows along a pair of rails which are bridged at intervals by bonding units which afford low impedance connections between the rails for all traction currents outside the pass band of the track signalling circuit, each bonding unit comprising a capacitive component and an inductive component having separate connections to the two rails and having values such that disconnection of either component from the track results in a sufficient shunting of the rails by the other component to be detectable by the signalling circuit.

2. A track signalling circuit according to Claim 1, in which the inductive component of each bonding unit comprises a lumped inductance physically connected to directly opposite points on the two rails.

3. A track signalling circuit according to Claim 2, in which the lumped inductance comprises a coil enclosed in an oil-filled housing.

4. A track signalling circuit according to Claim 1, in which the inductive component of each bonding unit comprises a distributed inductance formed by a single length of wire of suitable gauge.

5. A track signalling circuit according to Claim 4, in which the inductive component of each bonding unit comprises a length of wire having portions located adjacent and substantially parallel to the running rails of the track and connected at opposite ends to the respective rails.

6. A track signalling circuit according to Claim 5, in which the inductive loop has respective portions which are generally parallel to and located outside or inside the respective rails, each portion being connected to the opposite rail to that which is next to the said portion so that the current flow in said portions is in the same direction as the traction return current in the rails adjacent said portions.

7. A track signalling circuit substantially as herein described with reference to and as shown in the accompanying drawings.