GB2127195A - Impedance bond - Google Patents

Abstract

The bond is for use in a track signalling circuit on electrified railways having traction return current flow in the track rails and has an inductive component which comprises a flat looped conductor (11) partially enclosed in magnetically permeable ferrite core elements (12) preconsisting of annular or part-annular ferrite segments, so distributed that traction return current can flow in the conductor with low electrical losses. The core elements may alternatively be iron dust cores shaped in the form of elliptical rings and placed over the looped conductor. <IMAGE>

Description

SPECIFICATION Track signalling circuit for use in electrical railways This invention relates to impedance bonds for use in track signalling circuits in electrified railways. The invention is particularly applicable to electrified railways where traction return current flows in the running rails of the track.

A problem which arises in such railways is that of avoiding dangerously high voltages arising between the rails due to small differences in the impedance of the rails when very high traction return currents are flowing. Such voltage imbalance is commonly prevented from reaching dangerous levels by the provision of impedance bonds interconnecting the rails at intervals. The impedance bonds are resonated by means of associated capacitive elements so as to present a high impedance at the track signalling frequency pass band and a low impedance outside this band, the track signalling system being rendered "traction-immune" by utilising power of different regimes (AC or DC) for the signalling and traction respectively.

In a typical audio frequency track circuit it is essential to ensure that traction return connections to the track do not impair the ability of the track circuit to detect a shunt across the rails, even in the presence of a broken rail.

Using an audio frequency track circuit a resonated impedance bond has been used having a branch impedance less than 0.5 ohm, and an impedance at resonance of 1 5 ohms with a Q factor of the order of 30.

A typical impedance bond would have an inductive component comprising an air cored coil of heavy duty conductor, for example five turns of a 1 86 mm2 cross section aluminium conductor, resonated with a capacitor connected independently between the track rails.

Being air cored, the inductive component of the impedance bond projects a substantial magnetic field some distance above the track when passing traction return current. This could lead to spurious triggering of train-borne signalling equipment and to guard against such an event it is generally stipulated that the magnetic field of the impedance bond must not exceed 10 gauss at a height of 5 inches above the rail level.

With a view to avoiding the undesirable effects of magnetic fields generated by traction return currents in impedance bonds the present invention provides an impedance bond for use in a track signalling circuit having a signal frequency pass band, for use in an electrified railway in which traction return current flows in the track rails, the impedance bond comprising an inductive component and a capacitive component adapted for separate connection to the track rails and having values such that the bond affords a low impedance between the track rails for traction currents outside the pass band of the signalling circuit, and such that disconnection of either component from the track results in shunting of the rails by the other component, detectable by the signalling circuit, characterised in that the inductive component comprises a looped conductor partially enclosed in magnetically permeable core elements so distributed that traction return current can flow in the conductor with low electrical losses.

The essence of the invention is the effective control of the electric loss of the impedance bond by careful control of the distribution of the permeable core elements and the shielding afforded thereby. Thus the core elements are preferably provided with air gaps or their equivalent to control the electric losses. For example the core elements may comprise annular or part-annular elements of ferrite material, which may be encapsulated in insulating material.

The conductor need not necessarily be completely enclosed in ferrite material. Sufficient containment of the magnetic field to avoid undesirable interference with train-borne equipment may be effected using iron dust core elements fitted over the conductor as annular or toroidal elements. The iron dust is retained in such elements by an encapsulating resin such as an epoxy resin, which is in practice equivalent to an air gap.

To enable the traction return current in the conductor to have a self-annuling effect in the impedance bond the conductor is preferaby formed into two loops connected together at a centre-tap and having opposite ends for connection to the track rails, such that traction current entering or leaving the conductor through the centre tap flows in opposite directions in the two loops.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of part of an electrified railway track provided with traction-immune track signalling and utilising impedance bonds in accordance with the invention; Figure 2 is a diagrammatic plan view of the inductive component of an impedance bond according to one embodiment of the invention, and Figure 3 is a diagrammatic cross section, on line Ill-Ill in Fig. 2, of part of the inductive component shown in Fig. 2.

Fig. 1 illustrates part of an electrified railway track 1 having running rails 2, 3 which carry traction return current, the traction current being supplied by means of an overhead catenary or an additional rail (not shown). The rails 2, 3 are also used for track signalling utilizing audio frequency electrical signals transmitted along defined track sections between tuned transmitter and receiver circuits 4, 5 which are coupled inductively or directcoupled to a track side transmission 6 and a trackside receiver 7 respectively.

In the absence of a train short circuiting the rails 2, 3 of the track section between the circuits 4 and 5, signals transmitted along the track section from the transmitter 6 will be received at the receiver 7. When the track rails 2, 3 are short circuited by the presence of a train on this section of track, the transmitted signals fail to reach the receiver 7.

This enables the presence of the train to be detected, for example by de-energisation of a relay associated with the receiver 7.

Neighbouring sections of the track 1 adjoining the illustrated section may be tuned to a different frequency from the audio frequency of the illustrated track circuit, by means of further tuned circuits (not shown), the circuits 4, 5 having a low impedance at the frequency of the neighbouring track circuits. In this case the neighbouring track circuits are in effect isolated from each other without the necessity for insulating joints between the rails 2, 3 in adjacent track circuits, which would clearly be incompatible with the use of the rails 2, 3 as a traction current return path.

When an electric locomotive is running on the track 1 the traction return current flows to the power supply sub-station through the running rails 2, 3. When there is relatively little leakage to earth of the traction return current, for example in dry weather conditions, small differences in the impedance of the rails 2, 3 can give rise to substantial imbalance between the currents flowing in the two rails 2, 3 which in turn can give rise to unacceptably high voltages between the two running rails 2, 3 under conditions of heavy traction current flow. In order to equalise the traction return current flowing in the two running rails 2, 3 impedance bonds 8 are provided at regular intervals along the track 1. Typically, the impedance bonds 8 would be provided at intervals of 500-900 metres along the track 1.

Each impedance bond 8 comprises an inductive component 9 connected across the two rails 2, 3 and a capacitive component 10 which forms with the inductive component 9 a resonant circuit at the relevant track circuit frequency such that, at this frequency, the impedance bond 8 presents maximum impedance, so that it does not short circuit the audio frequency track signals transmitted along the rails 2, 3 between the tuned circuits 4, 5.

Each inductive component 9 has an impedance at the fundamental frequency of the traction return current (typical 50 Hz) which is so low that it is a virtual short-circuit between the rails 2, 3 for this return current, thereby tending to equalise the return current flowing in the rails 2, 3.

The inductive component 9 and capacitive component 10 of each impedance bond 8 have separate connections to the respective running rails 2, 3. This is an important safety feature, since it ensures that, in the event of a disconnection of either one of the components 9, 10 from the rails 2, 3 the remaining component will present a low impedance at the track circuit frequency, simulating the effect of a train on the relevant track section, and providing, therefore, a "fail-safe" indication at the associated receiver 7. Thus failure of a connection of any one of the components 9, 10 to the track rails 2, 3, which could give rise to unacceptably high imbalance voltages between the rails 2, 3, will be detected at the receiver 7, enabling maintenance staff to make a physical check of the impedance bonds 8 over the track section concerned.

The resonant impedance bonds 8 should in practice have a Q-factor in excess of 20 in order to present a high impedance to track signals while being effective as a traction current bond. This has in practice been achieved using air cored heavy duty aluminium conductors to form the inductive components 9 of the impedance bonds 8. In order to provide an effective impedance at the track signalling frequency, the inductive component should have an inductance of the order of 30 microhenries at a frequency of, for example, 2600 Hz. This necessitates a considerable length of cable in the conductor, which in turn results in a considerable heat dissipation problem. Moreover, being air cored, the inductive component gives rise to a magnetic field which projects above the track rails 2, 3, with the risk of causing spurious triggering of train-borne signalling equipment.

To reduce the upward projection of stray magnetic fields from the impedance bonds, iron cored bonds can be used in certain track circuits, for example where a traction supply of 750 volts d.c. is used. In this case, however, the resulting resonant bond has a Qfactor of the order of 4, and could not be employed as a "detectable" impedance bond in the manner previously described.

The present invention avoids the problem associated with air cored inductive components in tuned impedance bonds by utilising inductive components provided with ferrite or other core elements which partially enclose a looped conductor to contain the magnetic field.

Referring to Figs. 2 and 3, the inductive component 9 of an impedance bond is shown in which a looped conductor 11, in this case copper cable, is enclosed in a number of circumferentially adjacent ferrite core elements 1 2 which partially enclose the conductor 11.

Each ferrite core element 1 2 (Fig. 3) is made up of two segments, 13, 14, separated by air gaps 15, 1 6. The ferrite core elements 2 are encapsulated in an epoxy resin 1 7 which fills the air gaps 15, 16, and which is contained within a protective cover 1 8.

A first sight the use of ferrite material as core elements appears to be unsuitable for an inductive component which has to pass considerable traction currents, due to the low saturation density of ferrite material (typically 0.4 tesla). By careful control of the size of the air gaps 15, 16, however, it is possible to produce an inductive component 9 of the configuration illustrated with an inductance of 30 microhenries and with very low loss, so that the component is able to withstand a traction current of 600 amps between the rails 2, 3.

The illustrated inductive component 9 is designed with a centre tapping 1 9 (Fig. 1) which is connected by a conductor 20 to a corresponding centre tapping of an impedance bond 8 between the rails 22, 23 of a neighbouring track 21, so that the four rails 2, 3, 22, 23 are used for the balanced return flow of traction current. In the case of an electrified railway with overhead A.C. traction current lines (for example 25KV at 50Hz) and return current conductors for each line, the centre tap of the impedance bond would be connected to the return connectors at intervals.

In the embodiment illustrated in Figs. 2 and 3, the looped conductor 11 has two concentric turns which are interconnected by a terminal block forming the centre tapping 19, the opposite ends of the looped conductor having respective terminals 24, 25 for attachment to the respective running rails 2, 3. With this arrangement, traction return current entering or leaving the looped conductor 11 through the centre tapping 1 9 flows in the two turns of the conductor 11 in opposite directions, resulting in a nett zero magnetic field in the ferrite core elements 1 2.

In practice the looped conductor 11 need not necessarily be completely enclosed in ferrite material. In an alternative embodiment of the invention, iron dust cores shaped in the form of elliptical rings may be placed over the looped conductor 11 and retained in position by an encapsulating resin such as an epoxy resin. The encapsulating resin in this case would act, in effect, like a distributed air gap.

Although described in relation to a "jointless" track circuit it will be appreciated that the impedance bond according to the invention is equally applicable to jointed track circuits, that is, track circuits where insulating joints are provided between the rails of neigh bouring track sections.

Claims (1)

1. An impedance bond for use in a track signalling circuit having a signal frequency pass band, for use in an electrified railway in which traction return current flows in the track rails, the impedance bond comprising and inductive component and a capacitive component adapted for separate connection to the track rails and having values such that the bond affords a low impedance between the track rails for traction currents outside the pass band of the signalling circuit, and such that disconnection of either component from the track results in shunting of the rails by the other component, detectable by the signalling circuit, characterised in that the inductive component comprises a looped conductor partially enclosed in magnetically permeable core elements so distributed that traction return current can flow in the conductor with low electrical losses.

2. An impedance bond according to Claim 1, in which the core elements comprise annular or part-annular core elements of ferrite material.

3. An impedance bond according to Claim 2, in which the conductor is enclosed in a number of circumferentially adjacent core elements.

4. An impedance bond according to any one of Claims 1 to 3, in which the core elements are encapsulated in insulating material.

5. An impedance bond according to any one of the preceding claims, in which the conductor is formed into two loops connected together at a centre-tap and having opposite ends for connection to the track rails, such that traction current entering or leaving the conductor through the centre tap flows in opposite directions in the two loops.

6. An impedance bond substantially as herein described with reference to and as shown in the accompanying drawings.

CLAIM (28 Sep 1983)

1. An impedance bond for use in a track signalling circuit having a signal frequency pass band, for use in an electrified railway in which traction return current flows in the track rails, the impedance bond comprising and inductive component and a capacitive component adapted for separate connection to the track rails and having values such that the bond affords a low impedance between the track rails for traction currents outside the pass band of the signalling circuit, and such that disconnection of either component from the track results in shunting of the rails by the other component, detectable by the signalling circuit, characterised in that the inductive component comprises a looped conductor partially enclosed in magnetically permeable core elements of ferrite material so distributed that traction return current can flow in the conductor with low electrical losses.