"A system for emulating track circuits in railway lines"

The invention concerns a system for emulating track circuits in railway lines, i.e. a system for emulating the conditions prevailing in the track circuit when a train passes along a railway section, in order to test the reliability of the response of the apparatus which is to signal when the track is occupied.

As it is known, in order to detect the progress of trains along a railway line, an alternating voltage is separately applied to each of the track sections or blocks composing a line, and the voltages are then detected by means of specific signaling circuits. Such systems are substantially characterized by a.c. supply, at a frequency larger than o Hz, which is connected across the rails of a track section, by a signaling device, and by an inductive box. Both the supply and the signaling device are galvanically separated from the associated track by respective inductive couplings. The generated current is inductively coupled to an electromagnetic relay in order to keep it normally energized. When a train enters the track block being considered, it will dump a large d.c. current from the overhead trolley line to ground via its own traction motors, and, on the other hand, its axle will short the rails with each other. The short will cancel the signal generated by the supply and inductively coupled to the track by the inductive box; the relay will be drop out and will cause the signaling device to turn on, thereby signaling that a train has engaged that track block.

The above signaling system based on inductive boxes was originally implemented for operating with railway locomotives having rheostat-regulated traction motors, which dump a substantially continuous current to ground. On the other hand, in modern locomotives the traction current is regulated by means of triacs or other switching devices. These devices feed into the motors not only a d.c. component, but also a.c. current components accompanied by an array of harmonics, which flow into the rails through the axle of the locomotive and are superposed to the current generated by the supply, thereby causing distorsions which can be of a considerable level. If such harmonics exceed certain predefined levels, can some- times affect the signaling device and de-energize its relay, thereby causing a false alarm.

It is now the main object of the invention to provide a system for testing track circuits in railway lines, which is capable of replicating and controlling the distortion injected by the train into the track, so that level masks determining the malfunctioning can then be easily evaluated.

Another object is to provide the above testing system so that it will account for all the factors at play, by emulating both the distorted current generated by the train in the track and the behaviors of the track and of the signaling cable.

Another object of the invention is to provide the above testing system so that the test parameters can be readily and accurately determined.

By the testing system of the invention it is possible to exactly define which signal reaches the relay of the signaling device, thereby exactly determining the circumstances of malfunctioning and subsequently finding solutions for removing any malfunctioning.

The above objects, together with other objects and advantages, such as they will appear from the following disclosure, are attained by the invention by providing a system for testing railway track circuits, having the features recited in claim 1.

Further advantageous features of the invention are set forth in the dependent claims.

Preferred embodiments of the invention are described below in more detail, with reference to the attached drawings, wherein:

Fig. 1 is a circuit diagram of a section of a railway line comprising a number of track blocks having inductive boxes (inductances 17-20 and 18-19);

Fig. 2 is similar to Fig. 1, but modified to account for a train in transit;

Fig. 3 is a block diagram of an emulation system emulating track circuits according to the invention;

Fig. 4 is a preferred circuit diagram of a waveshape generator belonging to the system of Fig. 3;

Fig. 5 is a circuit diagram of a track emulation module according to the invention;

Fig. 6 is a cross-section profile of a typical rail used in forming a track;

Fig. 7 is a diagram useful to explain how the parameters of the track emulation module of Fig. 6 may be computed; and

Fig. 8 is a circuit diagram of a cable emulation module according to the invention.

With reference to Fig. i, a section of a railway line, shown schematically, comprises a number of successive track blocks, respectively comprising pairs of rails io, 12, 14. The rails in each pair are electrically insulated from each other and from the rails of adjacent track blocks, but are connected to ground. In a way known per se, an aerial electric power line 16 extends above track 10, 12, 14, and is supplied with d.c. current, typically at 3kV.

The primary inductance 18 of an inductive box placed between rails 12 and 14 is connected across rails 12, and the secondary inductance 20 of the inductive box placed between rails 12 and 10 is also so connected. A supply 22 of alternating voltage at a predetermined, suitable frequency inductively couples its signal to winding 18, from which the signal will be coupled to winding 24 via winding 20, to be finally applied across relay 26 of the signaling circuit. The inductive box is connected between each pair of adjacent track blocks 10, 12, 14, ...

When a track block is free, i.e. no train is in transit on it, the current generated by the supply 22 flows in winding 24, thereby keeping relay 26 on and the signaling off. The path of the signaling current is shown on Fig. 1 in dotted line with reference Is.

Fig. 2 shows the situation arising when a train enters a track block 10. Axle 28 of the locomotive in transit (not shown) short-circuits the rails with each other, the path of the signaling current I _s being modified as shown in dotted lines. At the same time, a d.c. traction current It flows from the aerial line 16 to the track, via the engine of the train, which is here represented by equivalent reistances 30 e 31. In Fig. 2, Iti and It≥ indicate the paths (drawn in dotted lines) of the two partial currents flowing to the two rails, and whose total constitutes the overeall traciont current It.

- A -

D.c. current does not interfere, per se, with the operation of the signaling circuit, in which the useful signal is generated by the supply, and the axle, by shorting both rails 10 together, cuts to zero the input signal to the mutual inductance 20. Relay 26 no longer receives any signal ad drops out, thereby switching a signal to indicate that the track block under consideration is occupied by a train.

However, as set forth in the introduction, in modern locomotives the current fed to the traction motors is regulated by high-frequency clipping through electronic switches such as triacs or controlled diodes. Clipping the current creates an alternate current component, with a swarm of harmonics, at a frequency not too distant from the frequency of the signaling current. The currents generated by clipping, although mainly flowing into the the rails, in practice also give rise to noise currents in the signaling circuit. Such noise can cause malfunctioning as mentioned above.

In order to solve this problem, the emulation system of the invention, with refer- ence to Fig. 3, comprises a waveshape generator 40, described below, supplied from the mains 41, which generates a current having a predetermined waveshape.

The current is applied to the input terminals 42 of a quadrupole 44 which acts as a track emulator, also described below. Output terminals 46 from the quadrupole 44 are then connected, via a cable emulator 49, to a signaling circuit 48, usually comprising, as known per se, a relay (not shown) which, when dropped, causes a switch to close and thereby light up a signaling device (also not shown).

Generator 40, as described below in more detail, is set to replicate the current that was actually measured by an oscilloscope on the motor of the locomottive during a measurement campaign, but could also be programmed to generate a waveshape on the basis of a mask or spectrum of the frequencies arising in the operation of the locomotive.

Waveshape generator 40 must be able to deliver a high-power current waveshape with a wide dynamic range, in order to be able to generate the distorted currents foreseeably required for railway traction and signaling, in a frequency band broad enough for replicating the desired waveshape with reasonable accuracy. Although it is always possible, in principle, to amplify an electric signal up to a required

level, starting from any waveshape whatever, in practice this approach would involve a number of problems, because it is very difficult, if not unfeasible in practice, to implement amplifiers with the required power and dynamic range by conventional transistor-based technologies. According to the invention, such an apparatus is implemented as shown in the circuit diagram of Fig. 4.

As shown on Fig. 4, generator 40 comprises a rectifier stage comprising a diode bridge 50 which is suppliable (as stated above with reference to Fig. 3) from the electric mains with a.c. current at 50 Hz, and a resistive-capacitive filter 52, which delivers a current that has been rectified and roughly smoothed. The rectified current is applied across a modulator bridge formed of four IGBTs 54, having respective free-wheeling diodes 56 in parallel. The nodes between the respective pairs of IGBTs 54 in the two branches of the bridge are connected to the output terminals 42 of the current generator, which deliver the current to the track emulator of Fig. 3.

The respective control gates of the four IGBTs 54 are driven by the outputs of a controller 60, which issues a sequence of switching commands according to prescriptions derived from the current analyzed by an oscilloscope or waveshape analyzer. Moreover, controller 60 receives a feedback signal from an ammeter 64, which reads the current flowing from output terminals 42 of the current generator.

The feedback signal enables the controller 60 to compensate for the a.c. noise components contained in the current fed to modulator bridge 54, 56 by diode bridge 50 and by filter 52, and only roughly filtered. By using this feedback, it is not necessary for the rectified current to undergo an expensive smoothing in order to deliver a clean d.c. current.

Quadrupole 44 has the task of emulating the track block 10, 12 or 14, respectively. With reference to Fig. 5, each of both rails is equivalent to a resistance Ri and an inductance Li in series, as well as a series-connected resistance-inductance L2-R2 which accounts for the difference in behavior at high and low frequency, as better explained below. Moreover, both rails are coupled together by a mutual inductance M and by a loss capacity across the rails, which is shown split into two capacities Cl to account for a distributed capacity.

In more detail, resistance Ri accounts for the track resistance in d.c, and is easy to be measured individually by the usual resistance measurement techniques; mutual inductance M and transverse loss capacity C (split into two) are easily measured by conventional measures of voltage and current.

As far as the unit L1-L2-R2 is concerned, the introduction of these parameters depends on the fact that a rail cannot be assimilated to a wirelike conductor, but rather is a massive conductor (i.e. having a large cross-section), with relatively high resistivity, and substantially subject to the skin effect, due to which the contribution of the surface to conduction increases with increasing frequency, to the detriment of the iternal portion of the conductor, which takes a progressively smaller part in the conduction of the current. Accordingly, parameter R2 increases with increasing frequency, along a curve depending, among other things, on the shape of the rail, which typically has a cross-section profile is exemplified in Fig. 6.

The separation of parameters L1-L2-R2 can be be tackled by the procedure described in the article of A. Geri, M. La Rosa e G. M. Veca "Modeling and analysis of electric and magnetic coupled problems under nonlinear conditions", in J. Appl.

Phys. 75 (10), 15 May 1994. The articole describes a numeric method for solving nonlinear electric and magnetic problems in busbar systems. Reference is made to that article for a more detailed description of the procedure for analysis, and only a summary description is given here.

According to the above approach, the problem of separating the different contributions is tackled by an iterative procedure by which the field equations and the circuit equations are separately solved. The rail is represented as a bunch of wire- like conductors, with a generator of an ideal time-changing voltage, dependent on the resistance of the conductor. The generator accounts for all induced electromagnetic effects.

The field equations used derive from Maxwell's equations for a homogeneous medium. In the case of a bi-dimensional system, and neglecting any phenomena of electromagnetic propagation, the following equations are developed

Vχ(v Vx A) = J (1)

By applying Coulomb's gauge (V«A = o), equation (l) becomes:

- (V«vV)A = J (2)

where

J = _σ VV + σ(dA/δt).

Equation (2) therefore becomes:

-(V.vV)A + σV«V + σ(5A/at) = O (3)

By introducing the estimated values of the voltage gradient, equation (3) is solved as known per se, using Finite Element Analysis. Subsequently, by means of an appropriate post-processing, the induced electromotive force (EMF) on the k-Xh conductor is evaluated by the following equation:

h f dA , _< -, e' ⁼ -T _k {Jϊ ^δs

where l _k and S _k are the length and the cross-section of the conductor, respectively.

The voltage gradient is derived from these two values by successive processes, using circuit analysis. Therefore, for an assigned value of the voltage drop, the corresponding current is evaluated, partly real and partly imaginary, by estimating the contribution of the self-inductance and the losses. The field analysis refers to a rail with a steel structure. The cross-section of this system is shown in Fig. 7. The line of the electromagnetic field is solved by a symetric bidimensional problem qith open-ended limit. In order to do this, the bidimensional domain corresponding to the cross-section of the rail an the surrounding area is split into two sub-domains (see Fig. 7): a central sub-domain, characterized by ordinary finite elements, and a peripheral sub-domain, characterized by infinite boundary elements (IBEs).

By the above described procedure, an optimum approximation of the electric parameters of the emulator can be derived by direct computing, but a rougher estimate of thos parameters can obviously be derived by other means, including an empirical procedure of tests and successive refinements, as will be obvious for a person skilled in the art.

Fig. 8 illustrates the cable emulator from the actual inductive box to the track rails. The cable is an approximately wirelike conductor, and is therefore emulated as a balanced quadrupole having a series resistance R3 and a series inductance L3 in the two longitudinal branches, respectively, a mutual inductance M2 which emulates the coupling between the different wires, and a transverse loss capacity C2 (split into two capacitors).

As with the quadrupole of Fig. 5, it is easy, by conventional measuring techniques, to measure the values of the four electrical parameters playing a role in Fig. 8. With this model, determining the parameters of the cable only requires one test with shorted terminals and one test with open-ended terminals.

As a person skilled in the art will readily appreciate, it is possible, by connecting the system of the invention according to Fig. 3, to check the behavior of a signaling device 48 in actual practice (i.e. with a given track and a given locomotive), without having to make surveys in the field, by using a track emulator 44 that has been preliminarily designed for the specific track under study, and by programming the waveshape generator 40 on the basis of the electrical performance of the locomotive, as measured or as defined in the data supplied by the manufacturer.

Moreover, where the same track structure (rail type, gauge, etc.) is used with multiple lengths of the blocks, the multiple lengths can be easily emulated by cascading several emulation modules, each parameterized for the basic length.

Preferred embodiments of the invention have been described, but changes and modifications are possible within the inventive concept as defined in the attached claims. In particular, current generator 40 might be implemented differently from the embodiment shown, e.g. by means of other circuit schemes or other solid-state switching devices.