GB2232836A - Traction current monitoring arrangement - Google Patents

Abstract

An arrangement for monitoring the current (I) drawn by a railway traction drive system indicates when interference may occur with a track-based signalling system. The current (I) is measured by a transducer (9) at a point between the supply filter (14) and the power switching means (4) of the drive system. A first signal (20) representing the current (I) at the predominant frequency of the switching means is derived from the transducer output (19). A second signal (18) representing a predicted current at the predominant frequency is derived from currents (17) in the controller (7) of the switching means (4). The first and second signals (20, 18) are continually compared to provide a constant check on the functioning of the transducer (9) and the drive system. The outputs (22) of the transducer (9) in selected frequency bands used for signalling are also monitored by detectors (13) to detect potential interference from harmonic currents. <IMAGE>

Description

Traction Current Monitoring Arrangement This invention relates to railway traction and, in particular, to an arrangement for monitoring the operation of a railway traction drive system on a track that is also used for train signalling.

The power for an electric train is usually supplied from a third rail or, alternatively, an overhead power line, the track rails providing a path for the return current. The track rails may also carry small a.c. currents generated by the train signalling system. The signalling system will typically use track circuits operating at power frequencies in the range 25Hz to 150Hz or audio frequencies in the range 400 to 2000 Hz. The track is divided into a sequence of sections, each section having an associated track circuit including a transmitter at one end and a receiver at the other end, the intervening rails providing a transmission path for the audio frequency signals. The transmitter in each section is arranged to send a signal down the rails to the receiver at the other end of that section.When a train is present in a section it electrically shorts the rails so that the transmitted signal is not received by the receiver in that section. The absence of a received signal can then be used to indicate the presence of a train in a particular track section.

Traction drive systems commonly use semiconductor switching devices to control the power applied to the motors. The switching devices may be configured for operation at a fixed frequency or for variable frequency operation. In either case, in operation the drive system will draw harmonic currents from the power supply, which harmonic currents will then be present in the track rails also carrying the signalling currents. The signalling currents may be of the order of 100mA, whereas the fundamental frequency drive current may typically be of the order of 5000A.If the harmonic currents generated by the traction drive are of the same frequency as the currents used by the signalling system, it will be apparent that there is potential for dangerous interference, which, in the worst case, may cause the signalling system to indicate that a track section is clear when, in fact, it is occupied by a train.

This potential for interference between traction drive systems and signalling systems has led many railway operators to stipulate that trains be fitted with current monitoring units to detect the frequency and/or amplitude of the a.c. harmonic currents generated by the drive system. If the currents detected exceed predetermined safety limits the train is prevented from proceeding.

Traditionally such current monitoring has been done at the voltage supply point for the train, i.e. in the cable from the collector shoe in the case of a third rail system or the pantograph for an overhead line, or in the current return path. The interference currents at these points in the circuit have very low levels since they are deliberately attenuated by the supply filter to reduce interference problems. Consequently the monitoring transducer needs to be very sensitive and to have a large dynamic range so that the monitoring circuits are not saturated by the drive load currents. The difficulty of maintaining a high dynamic range in equipment operating in the high-noise electrical environment of an electrified railway is a severe limitation of existing designs of current monitors.

It is generally required that the interference current monitoring system has a fail-safe design. This is difficult to achieve as the sensing system cannot readily differentiate between a lack of interference because the drive system either is not operating or is operating within specification and an apparent lack of interference because the sensing transducer has failed. This is a further limitation of present designs.

Existing designs of current monitoring unit are subjected both to currents driven by the traction equipment into the source impedance presented by the local substation and to currents caused by ripple voltages on the track which flow into the input impedance of the drive system. The monitor is thus not able to distinguish between currents caused by a fault on the traction unit and currents caused by excessive voltage ripple in the substation output. These difficulties arise because the monitoring transducer is positioned at the supply point, where the voltage ripple has not been reduced by the supply filter, and, in consequence, such monitoring units are frequently subject to spurious tripping which is disruptive of service.

It is an object of the present invention to provide a traction current monitoring arrangement that is less susceptible of the aforementioned difficulties.

According to the present invention there is provided an arrangement for monitoring the current drawn by a railway traction drive system, the drive system including a motor, switching means for supplying power to the motor from a power source and filter means for coupling the switching means to said power source, the arrangement including detector means connected between the filter means and the switching means, means for deriving from the output of the detector means a first signal representing the actual current into the switching means at a predetermined frequency, means for deriving from output currents of the switching means a second signal representing an expected current into the switching means at said predetermined frequency, comparison means for comparing said first and second signals so as to monitor the correct functioning of the detector means and the drive system, and means for deriving from the output of the detector means selected frequency band signals representing the actual current into the switching means in selected frequency bands used for railway signalling so as to monitor interference currents generated by the traction drive system in those frequency bands.

The comparison means is preferably adapted to produce an output signal of the arrangement when said first and second signals differ by a predetermined amount for a predetermined time.

The selected frequency band signals may be respectively fed to level detectors, each level detector having an associated preset level so that when at least one of the selected frequency band signals exceeds the associated preset level a further output signal of the arrangement is produced.

The means for deriving said first and second signals, means for controlling the switching means, the means for deriving said selected frequency band signals and said level detectors may be operated under the control of microprocessing means.

A traction current monitoring arrangement in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing, Figure 1, which is a schematic block diagram of a railway traction drive system incorporating such a monitoring arrangement.

Referring to the drawing, the block diagram shown includes the basic elements of a typical railway traction drive system fed by a d.c. power supply (not shown) and including, by way of example, a three-phase GTO thyristor inverter 4 arranged to drive an induction motor 5. Current from the power supply is fed into the inverter 4 by means of a collector shoe 1 which is in contact with a third rail (not shown), and is returned to the power supply through an axle brush 6 in contact with the track rails. The inverter is isolated from the supply by a low-pass filter 14 consisting of an inductor 2 and a bank of capacitors which is represented by the single capacitor 3 in the figure.The filter 14, which typically has a cut-off frequency of 30 Hz, serves a number of functions: it converts the relatively high impedance of the track into a low impedance suitable for driving the inverter 4; it smooths out line transients and substation-generated voltage ripples; and it provides attenuation (typically up to 20dB) of interference currents generated by the drive system.

The control system 7 for the inverter 4 measures currents 15 in the inverter output and generates switching signals 16 for the thyristors. Within the control system 7 there are currents 17 related to the current I flowing into the inverter. These currents 17, which may be the inverter output currents 15, or the currents drawn by the individual switching devices in the inverter, are processed by a calculating means 8 to give an output signal 18 representing the expected magnitude and frequency of the ripple current I flowing from the filter capacitor 3 to the inverter 4.

Under normal operating conditions, the predominant frequency of the ripple current I will be six times the fundamental frequency of the motor current. In a microprocessor-based system the current calculation process could be performed within the inverter control circuitry.

The actual ripple current I flowing into the inverter is measured at a point between the filter capacitor 3 and the inverter input by a transducer 9, which could be a Rogowski coil or any other type of transducer not subject to saturation by the d.c. current.

The output 19 of the transducer 9 is supplied to a frequency analysing means 10, which gives an output signal 20 representing the frequency and amplitude of the inverter input current I. The signals 18 and 20, representing respectively the expected and the actual measured input currents to the inverter 4, are compared by a comparator 11. If the expected and the actual values of the input current differ by more than a predetermined amount the comparator 11 will produce an output signal 21 which is sent to a timer 12.Lack of correspondence between the signals 18 and 20 is to be expected for a few seconds during line gaps, wheel-spin and other transients, but if the comparator output signal 21 persists beyond a predetermined time, measured by the timer 12, the system will recognise either that there is a major fault in the power system (for example, a failed semiconductor device or open-circuit capacitor) or that the monitoring transducer has failed. In either case the continued operation of the drive system may represent a safety problem and this is indicated by an output signal X generated by the timer 12.

It will be apparent that the system as described above provides a continual self-check on the correct functioning of the monitoring transducer 9 and on the operation of the drive system itself. The operation of the system is based on the assumption that there is an approximately constant relationship between the ripple current levels in the inverter input and those drawn from the supply system. This depends on the characteristics of the filter circuit remaining constant. A major failure in the filter 14, which may affect the attenuation of any interfering currents being generated by the drive, would result in a change in the output impedance of the filter and hence in the measured ripple current being detected by the comparator 11 and the subsequent generation of the signal X by the timer 12.

As well as giving a measure of the drive system input current I at a predetermined frequency, which is preferably the predominant frequency of the inverter, the frequency analysing means 10 is arranged in conventional manner to produce output signals 22 representing the levels of drive current in selected frequency bands used by the signalling system. Each of these output signals 22 is fed to an associated level detector 13, four such level detectors 13a-d being shown in the figure by way of example. Each level detector 13 has a preset threshold level, such that, when the output signal 22 into at least one of the level detectors 13 exceeds its associated threshold level, a signal Y is generated to indicate that interference may be occurring.The threshold level for each frequency band is determined according to what amplitude of interference current is considered acceptable in that frequency band before safety is likely to be compromised.

It is a feature of the above-described monitoring arrangement that the traction current is monitored at a point between the supply filter 14 and the semiconductor switching unit 4 rather than at the collector shoe 1 or pantograph or the current return point 6 as in previously existing designs. As already described, one function of the filter 14 is to attenuate the higher harmonic currents reaching the current return point, i.e. the track rails. By arranging for the current detection to be done on the switching unit side of the filter there is the advantage that the interfering currents here have not been attenuated by the filter.

Thus, the dynamic range required of the monitoring equipment is reduced. A further benefit of the location of the transducer which is now proposed is that it is isolated by the filter from currents resulting from disturbances in the supply voltage; the monitoring system accordingly is less susceptible to spurious tripping.

It is another feature of the monitoring arrangement that its operation is continually checked by predicting the current waveform which it is expected is being drawn by the switching unit and comparing the predicted waveform with the measured waveform to ensure that the monitoring device is operating correctly. By arranging for the measurement of currents at the signalling frequencies to be carried out by the same microelectronic system as is used to measure the predominant frequency waveform, it is possible to achieve a high degree of certainty that the system is functioning normally, obviating the need for special checking systems or duplication to ensure integrity. In a practical arrangement the comparator 11 and the timer 12 could be designed to fail-safe standards and the level detectors 13 could be implemented in a software routine running under the control of the same microprocessing means as the frequency analysing means 10. In this way protection is conferred on the operation of those components of the system not otherwise covered by the aforementioned self-checking feature.

Although the embodiment of the invention described with reference to Figure 1 relates to a traction drive system having an induction motor supplied by a voltage-fed inverter, the invention is not limited to such an application. It may be-applied to any semiconductor-controlled drive system using variable frequency switching elements, such as, for example, current-fed inverters driving induction motors, current-fed inverters driving synchronous motors, and switched variable reluctance drive systems. In each case the input current to the switching circuitry is monitored directly by the transducer 9 and simultaneously output currents of the switching circuitry, which, for example, may be the currents drawn by the individual switching elements or, alternatively, may be the currents drawn directly by the motor, are used to predict the expected input current.The invention may also be applied to fixed frequency drive systems, where the motor speed is governed by the amplitude of a fixed frequency d.c. output from the switching equipment. Fixed frequency drive systems include naturally commutated convertors in which the semiconductor devices are synchronised to an a.c. power supply and fixed frequency chopper systems. The potential for generation of interference currents is naturally much less in fixed frequency drives than in variable frequency drives, and, in the case of chopper systems, monitoring is usually limited to a check of the chopper operating frequency.

Nonetheless, in some situations, the characteristics of the signalling system may be such that the railway operator insists on current monitoring as a precaution, for example, against oscillation at a frequency other than the fundamental chopping frequency. In such situations, the present invention may be applied to provide the necessary current monitoring.

Claims (5)

1. An arrangement for monitoring the current drawn by a railway traction drive system, the drive system including a motor, switching means for supplying power to the motor from a power source and filter means for coupling the switching means to said power source, the arrangement including detector means connected between the filter means and the switching means, means for deriving from the output of the detector means a first signal representing the actual current into the switching means at a predetermined frequency, means for deriving from output currents of the switching means a second signal representing an expected current into the switching means at said predetermined frequency, comparison means for comparing said first and second signals so as to monitor the correct functioning of the detector means and the drive system, and means for deriving from the output of the detector means selected frequency band signals representing the actual current into the switching means in selected frequency bands used for railway signalling so as to monitor interference currents generated by the traction drive system in those frequency bands.

2. An arrangement as claimed in Claim 1, in which the comparison means is adapted to produce an output signal of the arrangement when said first and second signals differ by a predetermined amount for a predetermined time.

3. An arrangement as claimed in Claim 1 or Claim 2, in which said selected frequency band signals are respectively fed to level detectors, each level detector having an associated preset level so that when at least one of said selected frequency band signals exceeds the associated preset level a further output signal of the arrangement is produced.

4. An arrangement as claimed in any one of Claims 1 to 3, in which the means for deriving said first and second signals, means for controlling the switching means, the means for deriving said selected frequency band signals and said level detectors are operated under the control of microprocessing means.

5. An arrangement substantially as hereinbefore described with reference to the accompanying drawing.