SYSTEM FOR MONITORING THE CONDITION OF A CABLE BY DISTRIBUTED REFLECTOMETRY

Description

[0001] The invention relates to the field of monitoring the condition of long cables, for example submarine cables, power cables or communication cables. More generally, the invention applies to any cable having significant attenuation for a signal propagating from one end of the cable to the other.

[0002] A general objective of the invention is to provide a solution for assessing the state of degradation of a very long cable and in particular for detecting electrical faults which may appear when the cable is degraded.

[0003] Among the known solutions for diagnosing the health status of a cable, methods based on reflectometry can detect the appearance of faults on cables or cable networks. These methods are based on injecting a test signal into the cable to be analyzed. The signal is reflected on the impedance discontinuity caused by the fault and then back-propagated to the injection point where it is measured. Analysis of the reflected signal can detect an electrical signature corresponding to the fault. The electrical signature can, for example, take the form of an amplitude peak. Some reflectometry methods can also locate the fault.

[0004] Different methods of reflectometry exist, based on analyses in the time or frequency domain with test signals of various types (impulse, multi-carrier or more complex). The temporary reflectometry system and method disclosed in document US 2014 / 222356 A1 are known. In addition, document US 2010 / 0351092 A1 discloses methods and devices for determining leakage currents in transmission networks.

[0005] A disadvantage of known reflectometry methods is that they are not suitable for very long cables due to the attenuation and dispersion of the signal which is too significant beyond a certain distance.

[0006] There is therefore a need for a system for analyzing the state of degradation of a very long cable.

[0007] The invention proposes a solution that makes it possible to detect defects in a very long cable by means of a monitoring system composed of several sensors arranged along the cable and connected to each other and to a control unit by means of a communication link. The control unit controls the different sensors to carry out tests on each section of the cable using a reflectometry analysis method.

[0008] The invention solves the signal attenuation problems by means of a multi-sensor system that allows tests to be carried out by cable sections. Thus, the path of the signal injected into the cable and then measured is reduced. The invention is defined by the monitoring system according to claim 1. Additional implementations are defined by claims 2-11. Furthermore, the invention relates to an assembly, defined in claim 12, comprising a cable and the monitoring system according to claims 1-11.

[0009] The invention relates to a system for monitoring the condition of a cable, the system comprising a plurality of reflectometry devices each being capable of injecting a test signal at an injection point of the cable and measuring a back-propagated signal in the cable at the injection point of the cable, the reflectometry devices being intended to be positioned along the cable at predetermined positions so as to break down the cable into successive sections, the system comprising a control member capable of communicating with the reflectometry devices and configured so as to carry out at least one reflectometry test consisting of injecting a test signal into the cable by means of a first reflectometry device and measuring the test signal, by means of the first reflectometry device, after its propagation in the cable and its reflection on an impedance discontinuity,the system further comprising a post-processing member capable of communicating with the reflectometry devices and configured to analyze the measurement of the test signal to detect an amplitude peak corresponding to a defect.,

[0010] According to a particular aspect of the invention, the post-processing member is configured to evaluate, from the measurement of the test signal, the distance between the impedance discontinuity and the injection point of the signal into the cable and to conclude on the presence of a fault if the distance is less than the distance between two neighboring reflectometry devices.

[0011] According to a particular aspect of the invention, the control member is configured to control, according to a predefined test procedure, each reflectometry device by activating or deactivating the injection of a test signal into the cable by means of said device and by activating or deactivating the acquisition of a measurement of a signal propagating in the cable by means of said device.

[0012] According to a particular aspect of the invention, the test procedure consists of successively executing the following commands: Activate the injection of a test signal by a first reflectometry device located at the start of a first section of the cable, Activate the acquisition of a signal measurement by the first reflectometry device, After a first predetermined monitoring duration, deactivate the injection of a test signal by the first device, deactivate the acquisition of a measurement by the first device, the procedure also includes the repetition of the previous steps; the repetition can be carried out after a second delay period or simultaneously.

[0013] According to a particular aspect of the invention, the first monitoring duration is predetermined as a function of the propagation speed of the signal in the cable and the distance between two consecutive reflectometry devices.

[0014] According to a particular aspect of the invention, the second delay time is predetermined as a function of the propagation speed of the signal in the cable and a signal attenuation factor.

[0015] According to a particular aspect of the invention, the control member is configured to simultaneously activate several reflectometry tests for several sections of the cable spaced apart by a predetermined distance depending on the speed of propagation of the signal in the cable and the attenuation factor of the signal.

[0016] According to a particular aspect of the invention, the control member is configured to simultaneously activate several reflectometry tests for several sections of the cable by means of test signals orthogonal to each other.

[0017] According to a particular aspect of the invention, each reflectometry device comprises a means of coupling to the cable for injecting and measuring a signal at a point on the cable.

[0018] According to a particular aspect of the invention, the coupling means is an inductive or capacitive contactless coupler.

[0019] According to a particular aspect of the invention, the coupling means is produced by several toroids arranged in parallel in the vicinity of a point on the cable, each toroid being connected to the reflectometry device by a connection wire forming several windings around the toroid.

[0020] According to a particular aspect of the invention, the coupling means is produced by physical contact with the cable.

[0021] The invention also relates to an assembly comprising a cable and a system for monitoring the condition of the cable according to the invention.

[0022] Other features and advantages of the present invention will become more apparent upon reading the following description in relation to the following appended drawings: [ Fig. 1 ] there figure 1 represents a diagram of a system for monitoring the condition of a long cable according to an embodiment of the invention, [ Fig. 2 ] there figure 2 represents an illustration of a reflectometry test on a section of cable, [ Fig. 3 ] there figure 3 represents a time diagram illustrating a reflectometry test applied to a measurement of a reflected signal, [ Fig. 4 ] there figure 4 represents a diagram of an example of implementation of the system according to the invention.

[0023] There figure 1 represents a diagram of a system for monitoring the condition of a cable C according to an embodiment of the invention. The system comprises several sensors or reflectometry devices M 1 , M 2 , M 3 , ... M n-1 , M n arranged along the cable C at selected points which thus delimit cable segments S 1 , S 2 , ... S n-1 .

[0024] Each reflectometry device is configured to perform two distinct functions: a first function of injecting a test signal into the cable C and a second function of measuring a signal propagating in the cable C.

[0025] For this purpose, each reflectometry device comprises means for generating a test signal, for example a signal generator or a memory in which a digital signal is stored. The signal may be analog or digital. In the case where the signal is digital, the device also comprises a digital-to-analog converter. Each device also comprises a coupler for injecting the test signal into the cable C. Advantageously, the coupler also has the function of capturing a signal propagating in the cable. The coupler may be made by physical contact or by capacitive or inductive contactless coupling.

[0026] The captured signal is eventually digitized via an analog-to-digital converter and then transmitted, via an RC communication network, to a PTR post-processing unit which is responsible for analyzing the signal.

[0027] The type of test signal used can be a pulsed signal, for example a square wave or a Gaussian pulse, or a more complex signal, for example a multi-carrier signal of the OMTDR (Orthogonal Multi-Tone Time Domain Reflectometry) type. The type of signal, the power of the signal injected into the cable, its frequency and its sampling frequency can be configured according to the nature of the cable to be monitored, in particular the attenuation characteristics of the cable. These parameters also depend on the nature of the coupler used and the precision desired for the signal measurement.

[0028] The distance between two devices M 1 ,M 2 depends in particular on the attenuation and dispersion of the cable as well as the desired level of precision for the measurements. The distance between two devices M 1 ,M 2 is chosen in particular so as to limit the level of signal attenuation to a threshold value when the signal travels the path between two neighboring devices M 1 ,M 2 . The threshold value is chosen, for example, so as to respect a minimum signal-to-noise ratio previously calculated to respect a chosen link budget. Thus, the positioning of several devices along the cable C makes it possible to monitor each section of cable without being dependent on the signal attenuation phenomenon.

[0029] The communication network RC can be implemented by any means allowing the transmission of the signal measured by each device M 1 , M 2 , M 3 , ... M n-1 , M n to a remote post-processing unit PTR. For example, the communication network RC is a wired network, by optical fiber or other communication cable or even a wireless network. In the case of a wireless network, each device M 1 , M 2 , M 3 , ... M n-1 , M n is equipped with a transmitter capable of transmitting data to the post-processing unit PTR and a receiver capable of receiving control information transmitted by a control unit CTRL.

[0030] The CTRL control unit is responsible for implementing a test procedure by controlling the various devices located along the cable. In particular, the CTRL control unit transmits commands to each device to activate or deactivate the injection of a test signal into the cable and a measurement of this signal back propagated in the cable to the injection point (which is also a measurement point).

[0031] The CTRL control body is responsible for managing the sequencing of activations of the different devices according to the signal propagation time.

[0032] In reference to the figure 1 , an example of a test procedure implemented to successively monitor the state of section S 1 and then the state of section S 2 of cable C is described. Initially, all devices are deactivated, i.e. the signal injection function and the signal measurement function are deactivated.

[0033] Generally speaking, the testing procedure consists of implementing the following successive steps: the monitoring of a first section S 1 is activated for a first monitoring duration, A second time delay duration is then respected, then the monitoring of a second section S 2 is activated for the same first monitoring duration, The steps are iterated for all the sections or a set of chosen sections.

[0034] Without loss of generality, the different sections S 1 , S 2 , S n-1 can be of the same length or of different lengths. In the second case, the monitoring durations of each section can be adapted to the respective lengths of the sections and therefore be different. To simplify the implementation of the system, a common monitoring duration can be chosen by selecting the longest monitoring duration, i.e. the one corresponding to the section of greatest length.

[0035] To carry out the monitoring of the first section S 1 , the control unit CTRL transmits an activation command to the first device M 1 . The activation command of the device activates the injection of a test signal and activates the acquisition of a signal measurement.

[0036] There are two possible scenarios. Either the cable portion S 1 is fault-free, in which case the signal is either reflected on the impedance discontinuity caused by the coupling between the second device M 2 and the cable, or transmitted without reflection beyond M 2 . If, on the contrary, the cable portion S 1 is degraded by a fault, the signal is entirely or partially reflected on the fault.

[0037] The measured signal is transmitted by the device M 1 to the post-processing device PTR. The signal injected by the device M 1 can continue to propagate beyond the device M 2 to the device M 3 .

[0038] The first monitoring duration is determined based on the duration of the measurement that we wish to carry out to analyze the state of a section.

[0039] When the first monitoring time has elapsed, the control unit CTRL transmits a deactivation command to the first device M 1.

[0040] The second delay time is determined as a function of the signal propagation speed in the cable, the length of a cable section and an attenuation factor of the signal in the cable. In particular, it is calculated so as to take into account the travel time of the signal injected by the first device M 1 until it propagates beyond the second device M 2 . Furthermore, possible multiple reflections of this signal on impedance discontinuities must also be taken into account. Thus, to calculate the second delay time, a worst-case situation is taken into account. A worst-case situation is, for example, obtained by considering that reflections of the signal take place just after and just before the second device M 2 .By considering the average signal power and its attenuation factor (which depends on the physical characteristics of the cable), the signal attenuation over time and as multiple reflections occur can be calculated. When the attenuated signal has a power (or amplitude) below a predetermined threshold, its influence can be considered negligible. The second delay time can therefore be taken as the cumulative duration of the signal's multiple paths until the signal's power or amplitude falls below a predetermined threshold.

[0041] More generally, the second delay time can be determined so as to take into account sufficient margins to avoid interference between the signals emitted by two neighboring devices.

[0042] When the second time delay has elapsed, the control device CTRL activates the monitoring of the second section S 2 . To do this, it transmits an activation command to the second device M 2 to repeat the reflectometry analysis for the next cable section S 2 .

[0043] More generally, the CTRL control body takes into account possible interference between signals injected by different devices in order to avoid them.

[0044] The different reflectometry devices do not need to be precisely synchronized with each other.

[0045] The test procedure implemented by the CTRL control body can take different forms. It can consist of a successive test of the condition of each cable section S 1 , S 2 ... S n-1 when the objective is to monitor the entire cable. Alternatively, it can also consist of an independent test of a cable section, for example to monitor the evolution of a previously detected degradation.

[0046] In an alternative embodiment of the invention, the control member CTRL simultaneously activates the monitoring of two or more cable sections when the distance between two sections monitored simultaneously is sufficiently large for the signal propagating over this distance to be sufficiently attenuated. In other words, the signal attenuation factor is taken into account to determine the minimum distance between two devices such that the simultaneous activation of these two devices does not cause interference.

[0047] In yet another embodiment of the invention, orthogonal signals are used, for example signals coded using a CDMA (Code Division Multiple Access) type code. In this case, each device uses a different signal, orthogonal to all the others. Thus, it is possible to carry out simultaneous monitoring of all the sections at the same time. In this embodiment, the CTRL control unit no longer manages the sequencing of the measurements but the distribution of the orthogonal signals between the different devices of the system.

[0048] The measurements taken are transmitted to a PTR post-processing unit which carries out a reflectometry test to detect a fault in a section of cable.

[0049] A reflectometry test consists of identifying in the measurement of the signal obtained, an amplitude peak characteristic of an impedance discontinuity on which the incident signal has been reflected.

[0050] There figure 2 schematizes the operating principle of a reflectometry diagnostic method applied to a section of S1 cable presenting a DNF fault, for example a non-clear fault. The example described below corresponds to a time domain reflectometry method.

[0051] A reference signal S, also called incident signal, is injected into the cable by the device M 1 . This signal propagates in the line and encounters, during its propagation, a first impedance discontinuity at the input of the DNF fault. The signal is reflected on this discontinuity with a reflection coefficient Γ 1 . If the characteristic impedance Z c2 in the area of ​​the non-clear fault DNF is lower than the characteristic impedance Z c 1 before the appearance of the defect, then the reflection coefficient Γ 1 is negative and results in a negative amplitude peak in the reflected signal R. In the opposite case, the reflection coefficient Γ 1 is positive and results in a positive amplitude peak in the reflected signal R.

[0052] The transmitted part T of the incident signal S continues to propagate in the line and then encounters a second impedance discontinuity creating a second reflection of the incident signal with a reflection coefficient Γ 2 of opposite sign to the first reflection coefficient Γ 1 . If Γ 1 < 0 then Γ 2 > 0. If Γ 1 > 0 then Γ 2 < 0.

[0053] The reflected signal R is measured by the device M 1 . By observing the reflected signal R, the signature of the non-clear defect DNF is characterized by two successive peaks of inverted signs as shown in figure 3 .

[0054] There figure 3 represents a time domain reflectogram which corresponds either directly to the measurement of the reflected signal R, or to the cross-correlation between the reflected signal R and the signal injected into the cable S.

[0055] In the case where the injected reference signal is a time pulse, which corresponds to the case of a time domain reflectometry method, the reflectogram can correspond directly to the measurement of the reflected signal R. In the case where the injected reference signal is a more complex signal, for example for methods of the MCTDR (Multi Carrier Time Domain Reflectometry) or OMTDR (Orthogonal Multi tone Time Domain Reflectometry) type, then the reflectogram is obtained by inter-correlating the reflected signal R and the injected signal S.

[0056] On the figure 3 , two reflectograms 201,202 are shown corresponding to two different pulse durations for the signal injected into the cable. Curve 201 corresponds to a pulse duration 2.ΔT much greater than the time taken for the signal to cross the non-clear fault DNF. The length of the fault being noted Ld, this duration is equal to Ld / V, with V the speed of propagation of the signal in the cable. Curve 202 corresponds to a pulse duration 2.ΔT much less than the time taken for the signal to cross the non-clear fault DNF.

[0057] In both cases, the signature 203 of the non-clear defect, in the reflectogram, is always composed of the succession of a first peak and a second peak whose signs are reversed.

[0058] The distance between the two peaks represents the length of the non-clear defect and their amplitude represents the severity of the non-clear defect. Indeed, the greater the variation in the characteristic impedance, the greater the amplitude of the non-clear defect signature in the reflectogram.

[0059] As is known in the field of reflectometry diagnostic methods, the position of the non-clear fault on the cable, in other words its distance from the signal injection point P, can be directly obtained from the measurement, on the time domain reflectogram of the figure 3 , of the duration t DNF between the first amplitude peak noted on the reflectogram (at the abscissa 0.5 in the example of the figure 3 ) and the amplitude peak 203 corresponding to the signature of the non-clear defect.

[0060] Different known methods are possible to determine the position of DNF. A first method consists of applying the relationship between distance and time: d DNF = Vt DNF where V is the speed of propagation of the signal in the cable.

[0061] In the case where a signal reflection occurs at the device located at the end of a cable section, another possible method consists of applying a proportionality relationship of the type d DNF / t DNF = I / t 0 where I is the length of the cable section and t 0 is the time, measured on the reflectogram, between the amplitude peak corresponding to the impedance discontinuity at the injection point and the amplitude peak corresponding to the reflection of the signal at the end of the section.

[0062] In the case where the cable section S 1 is healthy, i.e. without fault, the signal R is either reflected on the impedance discontinuity caused by the coupling between the second device M 2 and the cable C, or it is not reflected. These two situations can be identified by analyzing the presence or absence of amplitude peaks in the measured reflectogram as well as the time abscissas of the peaks. For example, if a reflection occurs at a distance greater than the length of a cable section, this means that there is no fault on the analyzed section.

[0063] An advantage provided by the invention is the possibility of providing information on the location of the degradation or fault, located on a particular cable section. The analysis of the reflectogram also makes it possible to locate the fault within the identified cable section.

[0064] There figure 4 represents, in a diagram, an example of an embodiment of a reflectometry device M 1 , M 2 used to monitor the condition of a conductor of a three-phase cable comprising three conductors. The ends of the cable under test are short-circuited CC 1 , CC 2 with a neighboring conductor allowing the current to flow between the two conductors.

[0065] This loop can be made in different ways. A first example of implementation consists of connecting the core of a coaxial cable to its shielding via a resistor at each end of the cable. A second example of implementation consists of connecting two independent conductors via short circuits CC 1 , CC 2 as illustrated in the figure 4 .

[0066] In the example shown in figure 4 , the coupling between each device M 1 , M 2 and the cable is achieved by a contactless induction PLC coupler. Induction couplers are, for example, achieved by ferrite cores T 1 , T 2 , T 3 , T 4 mounted in parallel near a point on the cable. In the example of the figure 4 , the coupler consists of four toroids. Each toroid has several windings of the connection wire that connects it to the device. The number of toroids and the number of windings is a parameter that allows the coupling gain and its frequency constancy to be adapted in order to best control the amplification of the injected or measured signal.

[0067] The example of the figure 4 can be generalized to monitoring all three conductors of the cable by positioning a PLC coupler on each conductor or to a single cable with a single connector.

[0068] Without departing from the scope of the invention, the coupling between the cable and each device may be achieved by other contactless coupling means or by a physical connection to the cable. For example, galvanic coupling may be achieved by stripping the cable to bring it into contact with a metal clamp connected to the device M 1 ,M 2 .

[0069] Each reflectometry device can be implemented by means of an embedded processor. The processor can be a generic processor, a specific processor, an application-specific integrated circuit (also known as an ASIC for "Application-Specific Integrated Circuit") or an in situ programmable gate array (also known as an FPGA for "Field-Programmable Gate Array"). The device according to the invention can use one or more dedicated electronic circuits or a general-purpose circuit. The technique of the invention can be carried out on a reprogrammable computing machine (a processor or a microcontroller for example) executing a program comprising a sequence of instructions, or on a dedicated computing machine (for example a set of logic gates such as an FPGA or an ASIC, or any other hardware module).

[0070] The control organ CTRL and the post-processing organ PTR can be implemented by means of a computer or any other equivalent computing device.

Claims

1. A system for monitoring the state of a cable (C), the system comprising a plurality of reflectometry devices (M1, M2, M3, ... Mn-1, Mn), each capable of injecting a test signal at an injection point of the cable and of measuring a signal back-propagated through the cable at the injection point of the cable, the reflectometry devices being intended to be positioned along the cable at predetermined positions so as to divide the cable into several successive sections (S1, S2 ... Sn-1), wherein each section is defined by a first reflectometry device located at the beginning of the section and a second reflectometry device disposed at the end of the section, the system comprising a control component (CTRL) capable of communicating with the reflectometry devices and configured so as to carry out at least one reflectometry test involving injecting a test signal into the cable by means of a first reflectometry device and measuring the test signal, by means of the first reflectometry device, after it has propagated through the cable and it has reflected on an impedance discontinuity, the system further comprising a post-processing component (PTR) capable of communicating with the reflectometry devices and configured to analyse the measurement of the test signal to detect an amplitude peak corresponding to a fault, with the control component (CTRL) being configured to control, according to a predefined test procedure, each reflectometry device by activating or deactivating the injection of a test signal into the cable by means of said device and by activating or deactivating the acquisition of a measurement of a signal propagating through the cable by means of said device, the test procedure involving successively executing the following commands: - activating the injection of a test signal by a first reflectometry device (M1) located at the beginning of a first section (S1) of the cable; - activating the acquisition of a signal measurement by the first reflectometry device (M1); - deactivating, after a first predetermined monitoring duration, the injection of a test signal by the first device (M1), deactivating the acquisition of a measurement by the first device (M1); - the procedure further comprising reiterating the preceding steps for at least one other section of the cable.

2. The system for monitoring the state of a cable according to claim 1, wherein reiterating the preceding steps is carried out after a second predetermined timing duration.

3. The system for monitoring the state of a cable according to any one of claims 1 or 2, wherein the post-processing component (PTR) is configured to assess, based on the measurement of the test signal, the distance between the impedance discontinuity and the injection point of the signal into the cable and to conclude that a fault is present if the distance is less than the distance between two neighbouring reflectometry devices.

4. The system for monitoring the state of a cable according to any of the preceding claims, wherein the first monitoring duration is predetermined as a function of the propagation speed of the signal through the cable and of the distance between two consecutive reflectometry devices.

5. The system for monitoring the state of a cable according to claim 4 as dependent on claim 2, wherein the second timing duration is predetermined as a function of the propagation speed of the signal through the cable and of a signal attenuation factor.

6. The system for monitoring the state of a cable according to claim 1, wherein the control component (CTRL) is configured to simultaneously activate the reiteration of the preceding steps for several sections of the cable mutually spaced apart by a predetermined distance as a function of the propagation speed of the signal through the cable and of the signal attenuation factor.

7. The system for monitoring the state of a cable according to claim 1, wherein the control component (CTRL) is configured to simultaneously activate the reiteration of the preceding steps for several sections of the cable by means of test signals that are mutually orthogonal.

8. The system for monitoring the state of a cable according to any of the preceding claims, wherein each reflectometry device comprises a coupling means for coupling to the cable for injecting and measuring a signal at a point of the cable.

9. The system for monitoring the state of a cable according to claim 8, wherein the means for coupling is a contactless coupler of the inductive or capacitive type.

10. The system for monitoring the state of a cable according to any of claims 8 or 9, wherein the coupling means (CPL) is implemented by several tori (T1, T2, T3, T4) arranged in parallel in the vicinity of a point of the cable, with each torus being connected to the reflectometry device by a connecting wire forming several windings around the torus.

11. The system for monitoring the state of a cable according to claim 8, wherein the coupling means is implemented by physical contact with the cable.

12. An assembly comprising a cable (C) and a system for monitoring the state of the cable according to any of the preceding claims.