TEXT OF DESCRIPTION

Technical field

The description relates to a method for detecting an arc occurring between a railway overhead contact line carrying DC voltage and a pantograph of a railway traction unit collecting the current absorbed or injected by the railway traction unit and to a corresponding detection arrangement.

One or more embodiments may be applied to applications for collecting data related to the energy consumption and send them to on-ground data collecting system.

Technological Background In the majority of the electric railway systems, the power is transferred from the supply to the traction unit (locomotive or electrotrain) through a sliding contact between the overhead contact line (OHL) and the pantograph. The latter is a servo- actuated current collector placed on the train roof, which applies and controls the force of the contact strip against the contact line. The large power (up to about 8 MW) to be transferred from the OHL to the carbon blade of the pantograph through a contact section of a few square mm makes the current collection as one of the most critical elements of the electrical railway power chain.

An accurate analysis of the electrical quantities, voltage and current, detected at a pantograph that collects the current absorbed by a railway traction unit such as a locomotive or electrotrain can provide interesting information about the functioning state of both the supply system and the rolling stock. A tool that involves the detection of electrical events, which occurs at pantograph, and methodologies that perform their reliable cataloging can be exploited for predictive maintenance. One of such events is the arcing phenomena which can occur at a pantograph collecting the current absorbed/injected by the locomotive, because of its detachment from the overhead contact line (OHCL). The reasons that can lead to a detachment between the two electrodes of the sliding contact can be summarized in: track irregularities, irregularities of catenary, high speed and weather conditions (snow, ice, frost). A consistent number of arc event can produce: fast degradation of the collector strip, increase of arc events, decrease of power quality and accidents.

It is therefore desirable to count the arc events to monitor the state of both the pantograph and the overhead contact line. A wide dissemination of such tool on-board every train associated with a data collection system can provide a valuable contribute in the predictive maintenance of devices involved in the sliding contact.

In particular, in a future scenario in which all the trains in services have a device able to detect the arc events and can send the number of arc events to an on-ground data collector system, a predictive maintenance of the quality of the contact between pantograph and OHL can be implemented. Also in the next future, all the trains in service in all Europe must collected data related to the energy consumption and send them to on-ground data collecting system. This obligation is imposed by the Technical Specification for Interoperability TSI.

Current systems perform the monitoring of the conditions of the pantograph shoe (the element in contact with the OHCL) by video system registration, which is requires installation of video cameras and a difficult identification of arc events from the acquired images.

Object and summary

On the basis of the foregoing description, the need is felt for solutions which overcome one or more of the previously outlined drawbacks.

According to one or more embodiments, such an object is achieved through methods having the features specifically set forth in the claims that follow. Embodiments moreover concerns a related detection arrangement.

The claims are an integral part of the technical teaching of the disclosure provided herein.

As mentioned in the foregoing, the present disclosure provides solutions regarding a method for detecting an arc occurring between a railway overhead contact line carrying DC voltage and a pantograph of a railway traction unit collecting the current absorbed or injected by the railway traction unit, said pantograph comprising an input low pass filter operating at a given natural frequency receiving at its input a voltage between the pantograph, in particular the pantograph shoe, and a reference node and said current absorbed or injected by the railway traction unit and supplying at its output a filtered voltage to the railway traction unit, said method comprising measuring said voltage between the pantograph and the reference node and said current absorbed or injected by the railway traction unit obtaining a measured voltage and a measured current respectively, extracting from said measured voltage an extracted voltage signal with a delimited spectral portion around a given detection frequency, extracting from said measured current absorbed or injected by the locomotive an extracted current signal with a delimited spectral portion around the given natural frequency, comparing an amplitude of said extracted voltage signal with a delimited spectral portion and an amplitude of said extracted current signal with a delimited spectral portion with a respective threshold, verifying if said comparison operation indicates that both the amplitude of the extracted voltage signal with a delimited spectral portion and the amplitude of the extracted current signal with a delimited spectral portion are above the respective threshold, and in the affirmative signaling detection of an arc.

In variant embodiments, said extracting from said measured voltage an extracted voltage signal with a delimited spectral portion around a given detection frequency includes performing an analog Butterworth filtering or a special resonant filtering around said given detection frequency and said extracting from said measured current absorbed or injected by the locomotive an extracted current signal with a delimited spectral portion around the given natural frequency includes performing an analog pass-band Butterworth filtering, in particular second order, centered at said natural frequency of said input low-pass filter, said comparing operation includes comparing the extracted voltage and the extracted current by a respective threshold comparator.

In variant embodiments, said detection frequency is in the range from 1 kHz to 3 kHz, and the bandwidth of said analog Butterworth filtering in particular is 143 Hz.

In variant embodiments, said natural frequency is in the range from 15 Hz to 30 Hz , and the bandwidth of said analog pass-band Butterworth filtering in particular is 11.2 Hz.

In variant embodiments, said comparing operation includes delaying the output of the comparison of the extracted voltage portion.

In variant embodiments, said measuring a voltage includes analog pre-filtering said measured voltage by an analog filter having a resonant frequency at said detection frequency obtaining an analog filtered voltage then digitally acquiring said analog filtered voltage, said measuring a current includes acquiring digitally the measured current synchronously with the acquisition of the analog filtered voltage, said extracting from said measured voltage an extracted voltage signal with a delimited spectral portion around a given detection frequency includes performing a Fast Fourier Transform of said measured voltage, in particular analog filtered voltage, extracting a tone corresponding to said detection frequency, said extracting from said measured current absorbed or injected by the locomotive an extracted current signal with a delimited spectral portion around the given natural frequency includes performing a Fast Fourier Transform of said measured current, extracting a tone corresponding to said natural frequency, comparing said voltage tone and said current tone with a respective single threshold value.

In variant embodiments, said detection frequency of the analog pre-filtering is in the range from 1 kHz to 2 kHz.

In variant embodiments, said Fast Fourier Transform of said measured voltage is performed with an analysis time in range between 4 and 10 ms and said Fast Fourier Transform of said measured current is performed with an analysis time of in the range between 100 and 300 ms. The present disclosure provides also solutions regarding an arrangement to detect an arc occurring between a railway overhead contact line carrying DC voltage and a pantograph of a railway traction unit collecting the current absorbed or injected by the railway traction unit, said pantograph comprising an input low pass filter operating at a given natural frequency, receiving at its input a voltage between the pantograph, in particular the pantograph shoe, and a reference node and said current absorbed or injected by the railway traction unit and supplying at its output a filtered voltage to the railway traction unit, characterized in that said arrangement comprises a sensor module configured to measure said voltage between the pantograph and the reference node and said current absorbed or injected by the railway traction unit obtaining a measured voltage and a measured current respectively, said arrangement being configured to perform the steps of the method according to any of the above embodiments. The claims are an integral part of the technical teaching provided herein with reference to the embodiments.

Brief description of the several views of the drawings

One or more embodiments will now be described, by way of non-limiting example only, with reference to the annexed Figures, wherein:

Figure 1 is a schematic diagram providing a circuital description for arc events _/

Figure 2A is a time diagram of the behaviour of the voltage and current at pantograph in presence of an arc event;

Figure 2B is a zoomed time diagram of the diagram of figure 2A.

Figure 3 is a flow diagram representing a general embodiment of the method here described;

Figure 4 is a schematic diagram of an arrangement implementing a first embodiment of the method here described;

Figure 5 is a time diagram of voltage quantities of the embodiment of Figure 4;

Figure 6 is a time diagram of current quantities of the embodiment of Figure 4;

Figure 7 is a time diagram of voltage and current quantities of the embodiment of Figure 4; Figure 8 is a schematic diagram of an arrangement implementing a second embodiment of the method here described,

Figure 9 is a time diagram of voltage quantities of the embodiment of Figure 8;

Figure 10 is a time diagram of current quantities of the embodiment of Figure 8;

Figure 11 is a time diagram of voltage and current quantities of the embodiment of Figure 8.

Detailed description of exemplary embodiments

In the ensuing description, one or more specific details are illustrated, aimed at providing an in-depth understanding of examples of embodiments of this description. The embodiments may be obtained without one or more of the specific details, or with other methods, components, materials, etc. In other cases, known structures, materials, or operations are not illustrated or described in detail so that certain aspects of embodiments will not be obscured.

Reference to "an embodiment" or "one embodiment" in the framework of the present description is intended to indicate that a particular configuration, structure, or characteristic described in relation to the embodiment is comprised in at least one embodiment. Hence, phrases such as "in an embodiment" or "in one embodiment" that may be present in one or more points of the present description do not necessarily refer to one and the same embodiment.

Moreover, particular conformations, structures, or characteristics may be combined in any adequate way in one or more embodiments.

The references used herein are provided merely for convenience and hence do not define the extent of protection or the scope of the embodiments.

Throughout the figures annexed herein, like parts or elements are indicated with like references/numerals and a corresponding description will not be repeated for brevity.

The drawings are in simplified form and are not to precise scale.

When a detachment between the pantograph shoe and the overhead contact line occurs in presence of a current flux, an electric arc arises between the two electrodes of the sliding contact. Despite the complexity in the physical description of an electric arc, for our purpose, such event can be easily described, in a circuital model as an arbitrary voltage generator that imposes an arbitrary pulsed voltage waveform between pantograph shoe and OHCL. A more detailed explanation can be found in the publication G. Crotti et al., "Pantograph-to-OHL Arc: Conducted Effects in DC Railway Supply System, " in IEEE Transactions on Instrumentation and Measurement, vol. 68, no. 10, pp. 3861-3870, Oct. 2019. Figure 1 shows a schematic providing a simple circuital description for arc events during the absorption (positive) in the traction unit or generation (negative) of current by the traction unit, the latter possibly occurring during a braking stage.

In figure 1 is therefore shown a traction unit 11, e.g. a locomotive, which is coupled to an overhead line 12 which carries a DC supply voltage V _line supplied by an electrical supply system 13 by means of a pantograph 11a.

The pantograph 11a by its nature is usually in contact with the overhead line 12 receiving the DC supply voltage V _line , however a voltage generator represents the forming of an arc A with arc voltage V _arc for one of the reasons summarized above, between the overhead line 12 and the pantograph 14, while with I _p is indicated the current flowing between the overhead line 12 and the pantograph 11a.

In figure 1 this is also represented by an equivalent circuit of the coupling between the overhead line 12 and the pantograph 11a, where an input filter 15 represents the input stage of the pantograph 14. As shown the DC supply voltage V _line is coupled to a terminal which is separated by the input terminal of the input filter 15 by the equivalent voltage generator representing the arc voltage V _arc . Thus, the input voltage, a pantograph voltage V _p , of the input filter 15 is V _line - V _arc , corresponding to the DC supply voltage V _line when there is no arc. A pantograph current Ip at the input node of the filter 15 is the current absorbed or injected with respect to the overhead line 12.

As shown in figure 1 the pantograph voltage V _p forms between the pantograph 11a and a reference node, e.g. a ground reference. Usually the reference node is zero voltage or at a lower potential with respect to the pantograph voltage V _p or the DC supply voltage V _line . In embodiments the reference node can be represented by the rail.

The input filter 15 is a low pass input stage filter of the traction unit 11, i.e. locomotive, which is commonly present in DC locomotives. This filter plays an important role in the understanding of the electrical conducted effects triggered by the arc. In figure 1 it is schematized as a LC filter with a series inductor L _if and an output capacitor C _if in parallel. At the terminals of the output capacitor C _if , which are the series inductor L _if and the reference node, a filtered DC voltage V _dsf to supply the electric motors of the traction unit 12 is formed. As shown in figure 1, thus also the filtered DC voltage V _dsf is referred to the reference node, e.g. ground reference, as the pantograph voltage V _p . More in general, all the potentials of the input filter 15, including the input node on which the pantograph voltage V _p forms are referred to such reference node, which as mentioned can be a ground reference or in embodiments is represented by the rail. The rail may be not necessarily at zero voltage, only at a lower potential with respect to V _line , V _p , V _dsf nodes.

Figure 2A is a time diagram of the behaviour of the voltage V _p and current I _p at pantograph in presence of an arc event A detected when the train 11 is absorbing current.

Figure 2B is a zoom in the time region of the arc A of the diagram of figure 2A, showing the same electrical quantities.

The electric arc A provokes a voltage dip in the pantograph voltage V _p if the traction unit 11 is absorbing current and a swell if the traction unit 11 is injecting current into the overhead line 12. Such fast variation induces an oscillation in the second- order low-pass filter 15 that reflects on the pantograph current I _p .

Waveforms such as the one shown in figure 2A are described for instance in Crotti, G.; Giordano, D.; Signorino, D.; Femine, A. Delle; Gallo, D.; Landi, C.; Luiso, M.; Biancucci, A.; Donadio, L.: "Monitoring Energy and Power Quality On Board Train". Proceedings of the 2019 IEEE 10th International Workshop on Applied Measurements for Power Systems (AMPS), Aachen (Germany), September 25-27, 2019.].

The solution here described exploits the conducted electrical effects produced by the arc, by a method which in general comprises, as shown in the flow diagram of figure 3, which shows an embodiment 100 of the method, a step 110 of measuring the pantograph voltage V _p between the pantograph 11a and the reference node and the current I _p absorbed or injected by the railway traction unit 11, obtaining a measured voltage V _p,out and a measured current I _p,out respectively, then extracting in a step 120 from such measured voltage I _p,out an extracted voltage signal with a delimited spectral portion V _bp around a given detection frequency f _det , i.e. a signal with a spectrum over a limited frequency range or band around the given detection frequency f _det , e.g. such as after a band pass filtering, extracting 130 from said measured current I _p,out absorbed or injected by the locomotive an extracted current signal with a delimited spectral portion I _bp around the given natural frequency f _N of the input filter, comparing 140 an amplitude of said extracted voltage signal with a delimited spectral portion V _bp and an amplitude of said extracted current signal with a delimited spectral portion I _bp with a respective threshold, voltage threshold t _hv or current threshold t _hv , verifying in a step 150 if said comparison operation 140 indicates that both the amplitudes of the extracted voltage signal with a delimited spectral portion V _bP and of the extracted current signal with a delimited spectral portion I _bp are above the respective threshold t _hv, t _hv, in the affirmative a detection AD of an arc A is signaled in a step 160.

With reference to figure 4 it is shown a detection arrangement 200 which represent a first embodiment of the method here described which is more analog oriented.

As shown schematically in figure 4, the step 110 of measuring the pantograph voltage V _p between the pantograph 11a and the reference node and the current I _p absorbed or injected by the railway traction unit 11, obtaining a measured voltage V _p , _out and a measured current I _p,out respectively is performed by a combined current voltage transducer lib. Preferably, such combined current voltage transducer lib operates on a range of frequency which is compatible, e.g. wider or equal in bandwidth, with the band of the filters downstream in the measurement chain shown in the following with reference to fig. 4, e.g. filter 220, 230. The sensor module corresponding to transducer lib may be embodied also by separate voltage and current sensors.

The step of extracting 120 from such measured voltage V _p,out an extracted voltage signal V _bp with a delimited spectral portion V _filt around a given detection frequency f _det is embodied by filtering with a pass-band second order Butterworth filter 220 centered at an arbitrary, i.e. given, detecting frequency f _det · The value of the detecting frequency f _det in embodiments can be selected in a range from 1 kHz to 3 kHz, while the bandwidth may be for instance 143 Hz. This is an example value, different values of the bandwidth may be possible, selected in order to balance the need to have a bandwidth sufficiently narrow to exalt the peak and the costs of the circuits increasing with the narrowness of such bandwidth.

The extracted voltage signal with a delimited spectral portion V _bp in this embodiment is represented by filtered analog voltage V _filt .

The step of extracting 130 from said measured current I _p,out absorbed or injected by the locomotive an extracted current signal with a delimited spectral portion I _bp around the given natural frequency f _N of the input filter is embodied by filtering with a pass- band second-order Butterworth filter 230 centered at the natural frequency f _N of the low-pass filter 15 of the input stage of the locomotive 11. The value of the natural frequency f _N depends on the resonance frequency of the input filter stage 15 of the locomotive, e.g. the resonance frequency of the LC resonant circuits forming the pass-band second-order Butterworth filter 230. The value of the natural frequency f _N in embodiments can be selected in a range from 15 Hz to 30 Hz, while the bandwidth may be for instance of 11.2 Hz. The same considerations for the value of bandwidth of filter 220 apply here. The extracted current signal with a delimited spectral portion I _bp is embodied in this case by a filtered current V _filt .

To perform the comparing 140 the amplitude of such extracted voltage signal with a delimited spectral portion V _filt and the amplitude of said extracted current signal with a delimited spectral portion I _filt with a respective threshold t _hv ,t _hi , then each filtered signal V _filt , I _filt is processed by a respective threshold comparator 240v, 240i, in particular operating between a positive and negative threshold, respectively +t _hv , ^_ t _hv and +thi, ^_ thi. If the filtered signal V _filt , I _filt is higher than the threshold, t _hv for the voltage and t _hi for the current, the output signal t _hv _ _out , t _hi _ _out of the respective threshold comparator 240v, 240i is a logic 1 otherwise is logic 0. The ranges of variation for the thresholds t _hv ,t _hi are summarized in Table 1, herebelow.

An output signal t _hv _ _out of the voltage comparator 240v is delayed of a delay D in a delay block of a time delay 245, in the example shown delay D is 5 ms, supplying a delayed output signal t _hv-out-D · The value of the delay D is chosen so to compensate the different time delays introduced by the filters 220, 230.

The output signals th _V _out_D,thi_ _ou t from the comparators 240v, 240i are sent to an AND logic gate 250 which embodies verifying 150 if the comparison operation 140 indicates that both the amplitudes of the extracted voltage signal with a delimited spectral portion V _filt and of the extracted current signal with a delimited spectral portion V _filt are above the respective threshold t _hv ,t _hi .

The output signal of the AND logic gate 250 is a flag AD equal to 1, only if both the output signals t _hv-out , t _hi-out are one, i.e. above the respective threshold t _hv ,t _hi . This represents the affirmative or positive answer to the verification step 150, thus a detection of an arc is signaled in a step 160.

Otherwise, the flag AD is zero, if either one of the output signals t _hv _ _out , t _hi _ _out are zero, i.e. below the respective threshold t _hv ,t _hi , meaning that an arc does not occur.

In figure 5 are shown the measured voltage V _p , _out , the filtered voltage V _filt and the delayed output signal t _hv _ _out _D as a function of time t. It is indicated the arc event A associated with a spike of the measured voltage V _p , _out · The delay D in the delayed output signal t _hv _ _out _D is also indicated, with reference to a previous oscillation above threshold. Such delay D is of course present in the following oscillations determined by the arc A.

In figure 6 are shown the measured current I _p,out , the filtered current I _filt and the current output signal t _hi _ _out as a function of time t. It is indicated the arc event A associated with a spike of the measured current I _p , _out ·

In figure 7 are shown the measured voltage V _p,out and the measured current I _p,out on superimposed on a same graph as a function of time t and the resulting flag AD. When the flag AD reaches 1 the arc has been detected.

With reference to figure 8 it is shown a second embodiment of the method here described which is more digital oriented.

As shown schematically in figure 8, the step 110 of measuring the pantograph voltage V _p between the pantograph 11a and the reference node and the current Ip absorbed or injected by the railway traction unit 11, obtaining a measured voltage V _p,out and a measured current I _p,out respectively is performed by a combined current voltage transducer lib.

For what regards the measured voltage V _p,out it is however also performed, prior the step 120, a pre- filtering by an analog filter 312 which is basically low pass band filter, which frequency response is flat at low frequencies and has a resonance at a given detection frequency, f _det , which in this embodiment is preferably in the range [1÷2] kHz, so that only fast transients exploit the filtering. The filter response to the voltage of filter 312 to a real arc event is reported in Figure 9, which shows time diagrams of the measured voltage V _p,out , and of the amplitude V _pf @f _det at the detection frequency f _det of an analog filtered voltage V _pf outputted by the analog filter 312, in addition to an output t _hv _ _out of a comparator 340v better described in the following. Thus, substantially the measuring step 110 for the voltage is followed by such analog pre-filtering of the measured voltage by an analog filter 312 having a resonant frequency at said detection frequency, then is performed digitally acquiring said analog filtered voltage V _pf by an analog to digital voltage conversion module 314v, producing a digital measured voltage V _p,d .

The measured current I _p,out is acquired synchronously with the voltage measured signal in a respective analog to digital current conversion module 314i, producing a digital measured current I _p,d .

Then the digital measured voltage V _p,d is subjected to the step of extracting from such digital measured voltage V _p,d an extracted voltage signal with a delimited spectral portion V _bp , which is in this case a tone VT, around a given detection frequency f _det by applying a Fast Fourier Transform (FFT) in a FFT block 320 for the detection of the resonance frequency f _det of the upstream special filter 312. In the same way, the digital measured current I _p,d is subjected to the FFT in a FFT block 330 for the detection of the natural frequency f _N of the low-pass filter 15 of the input stage of the locomotive 11, as shown in figure 11, extracting as extracted portion, a current tone IT _t .

The tones VT, IT are then compared with respective thresholds t _hv ,t _hi in respective threshold comparatore 340v, 340i. If the amplitude of the tone

VT, IT is higher than the threshold, t _hv for the voltage and t _hi for the current signal, the output signals t _hv-out , t _hi-out of the respective comparator 340v, 340i is 1 otherwise is 0. Figure 10 shows as a function of time the measured current and the tone IT at 15 Hz, together with the current comparator 340i output t _hi _ _out .

Also in this case, the output signals t _hv-out , t _hi-out from the comparators 340v, 340i are sent to an

AND logic gate 250 which embodies verifying 150 if the comparison operation 140 indicates that both the amplitude of the extracted voltage spectral portion, in this case voltage tone VT, and of the extracted current spectral portion, in this case current tone IT, are above the respective threshold t _hv ,t _hi .

The output signal of the AND logic gate 250 is the flag AD equal to 1, only if both the output signals t _hv-out , t _hi-out are one, i.e. above the respective threshold t _hv ,t _hi . This represents the affirmative or positive answer to the verification step 150, thus a detection of an arc is signaled in a step 160. Otherwise, the flag AD is at zero, if either one of the output signals t _hv-out , t _hi-out are zero, i.e. below the respective threshold t _hv ,t _hi , means that an arc does not occur, as shown in the diagram of figure 11, which shows the measured voltage V _p,out and the measured current I _p,out on superimposed on a same graph as a function of time t and the resulting flag AD. When the flag AD reaches 1 the arc has been detected.

For this digital approach, the selection time intervals on which the FFT transforms are computed in blocks 340v, 340i may be relevant for the detection of the researched tones. The operating conditions are usually very far from being stationary so, in order to have a good approximation of a stationary conditions, the time intervals for the FFT operations should be selected as small as possible. In addition to this, a good spectral resolution is also required and, therefore, a sufficiently long analysis time for detecting the tone. For these reasons, the detection of the resonance frequency of the upstream special filter is obtained preferably with an analysis time of [4÷10] ms, meanwhile the detection of the natural frequency of the low-pass filter of the input stage of the locomotive is in range of [100÷300] ms.

The choice of the threshold values to be applied to the filtered voltage and current signals is obviously linked to the type of railway system on which the electric arc detection device is operating. The values are summarized in Table 2 herebelow: The described solution allows to count the arc events to monitor the state of both the pantograph and the overhead contact line without installing any other device on the train roof, apart from the already installed voltage and current transducers. This allows of a wide dissemination of arrangements implementing the solution on-board every train associated with a data collection system, which can thus provide a valuable contribute in the predictive maintenance of devices involved in the sliding contact, in particular for a predictive maintenance of the quality of the contact between pantograph and OHL.

Also the solution here described allows collection of data related to the energy consumption and send them to on-ground data collecting system, such as the one imposed by the Technical Specification for Interoperability TSI. The communication system from train and ground arranged for the energy billing can be used for the collection of arc events of each train. Suitable algorithms that manage such bug-data can be used to understand if problems can occur to the pantograph or to the OHCL. Indeed, if for a specific section all the trains running on that section detect an abnormal number of arc event, the problem most likely is due to the OHCL. On the contrary, if only one train record an abnormal number of events for several sections it means that the problem is related with the pantograph.

It will be otherwise understood that the various individual implementing options exemplified throughout the figures accompanying this description are not necessarily intended to be adopted in the same combinations exemplified in the figures. One or more embodiments may thus adopt these (otherwise non mandatory) options individually and/or in different combinations with respect to the combination exemplified in the accompanying figures.

Without prejudice to the underlying principles, the details and embodiments may vary, even significantly, with respect to what has been described by way of example only, without departing from the extent of protection. The extent of protection is defined by the annexed claims.