Abstract:
A method and apparatus ( 12-20, 32 ) for automatically locating faults in a network of signal carriers (cables) utilises an interrogation waveform which is pulse-like in form and receives composite reflected signals from the carriers which are a time distributed collection of individual reflections. These are sampled and compared using the function of an adaptive filter ( 52 ) to minimise the difference signal. The difference signals are analysed to determine significant points of departure which identify the fault location which is then represented as a distance value.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for locating faults in a network of signal carriers such as electrical or optical cabling systems. In particular, it relates to networks which contain multiple joints, such as low voltage electrical signal or power distributions systems. 
     BACKGROUND OF THE INVENTION 
     Locating faults in underground cabling systems is currently achieved using a pulse echo technique. In the pulse echo technique, an interrogation pulse is transmitted along the system from a test site, this pulse is reflected by any fault in the cable system, and the elapsed time between the initial pulse and the reflected pulse being received at the test site is used to calculate the distance of the fault from the test site. 
     This technique works well in systems which do not have nodes (such as joints) which generate reflections of the pulse even though they are not faulty. However, when this technique is used in systems with multiple joints (such as low voltage electrical distribution systems or telecommunications systems) then each joint may introduce a reflection of the pulse in addition to the reflection generated by the fault or disruption. 
     Thus, data produced by the pulse echo technique performed on a system with multiple joints is difficult to interpret. Skilled interpretation of this data is required to distinguish between reflections from joints and reflections from faults. 
     Even with skilled interpretation of the data from a multijoined system where there may be more than one possible location of the fault, it is difficult to locate the fault to an accuracy of less than several meters for each of these possible locations. 
     Using standard echo pulse techniques for tracing faults in multijoined systems (particularly where the cable is buried underground or is otherwise inaccessible) is very expensive because of the uncertainty in distinguishing between reflections due to joints and reflections due to faults, and because of the inaccuracy in determining the precise location of the faults. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve fault-finding in cabling systems. It is another object to distinguish between faultless nodes (which produce reflections) and faults. It is a further object to reduce the amount of skill needed to interpret the results of fault location apparatus. It is a further object to automate the fault locating process. 
     The general solution to the above problem is to compare pairs of signals from different signal carriers of the network under test and adaptively process the signals to determine any key deviations. These deviations may indicate the locations of the faults. 
     According to a first aspect of the present invention there is provided a method for locating faults in a network of signal carriers comprising establishing a test site where signal carriers of the network can be accessed: delivering an interrogation pulse-like waveform along a first set of signal carriers; receiving a first composite reflected signal from the first set of signal carriers and which is a time-distributed collection of individual reflections; sampling the first composite reflected signal at time intervals and storing the samples; delivering the same interrogation pulse-like waveform along a second set of signal carriers; receiving a second composite reflected signal from the second set of signal carriers and which is a time-distributed collection of individual reflections; sampling the second composite reflected signal at time intervals and storing the samples; consecutively comparing in pairs, samples of the first composite reflected signal with those of the second composite reflected signal using the function of an adaptive filter and comparator arranged consecutively to adapt the samples of the first composite reflected signal to minimise the different signal, between the output of the adaptive filter and the second composite reflected signal, at the output of the comparator; identifying the time interval at which the difference signal exceeds a threshold value; and scaling the time domain position of the identified time interval to represent the distance to a fault in the signal carriers from the test site. 
     According to a second aspect of the present invention there is provided a fault locator for carrying out the method of the first aspect comprising an adaptive filter and a comparator. 
     By virtue of the present invention the location of the fault position is automatically obtained from the filtered error signal due to the adaptive processing effectively learning about the similarities between cable characteristics for all locations prior to the fault and utilising this knowledge to determine where significant departure from the norm exists. 
     It will be understood that the pulse-like waveform is a finite-duration waveform and for example may be a simple pulse of square, rectangular or triangular shape or it may be of damped sinusoid form for example having only two sinusoidal periods of which the amplitude of the second is less than that of the first. 
     The method and apparatus may be operated and controlled remotely from the test site and may be operated in a trigger mode where the trigger signal is provided automatically resulting from an event of interest such as a voltage or current surge. 
     Furthermore the network of signal carriers may be arranged to carry electrical signals and/or power in which case the interrogation waveform is itself an electrical signal or the network may be arranged to carry optical signals in which case the interrogation waveform is itself an optical signal for example a transformed electrical signal, and of course the composite reflected signals are transformed to electrical signals for processing. 
     The network may have any number of signal carriers per cable, for example, ten or more as is conventionally used in telecommunications, or it may have only three carriers per cable as is conventionally used in electrical power distribution networks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and to show how the same may be carried into effect, reference will be made to the accompanying diagrams, in which: 
     FIG. 1 is a block diagram of a known print-cut TDR (Time Domain Reflectometer); 
     FIG. 2 a  is a block diagram of a TDR according to the present invention; 
     FIG. 2 b  illustrates part of FIG. 2 a  in greater detail; 
     FIG. 3 a  illustrates a basic adaptive processing element used in FIG. 2 b;    
     FIG. 3 b  illustrates part of FIG. 2 b  in greater detail and utilising the elements of FIG. 3 a  for a  3  cable system; 
     FIG. 4 shows two composite reflected signals from a cable system with multiple joints; 
     FIG. 5 a  shows the unsmoothed signal resulting from an adaptive processed comparison of the FIG. 4 signals; and 
     FIG. 5 b  shows the method signal resulting from an adaptive processed comparison of the FIG. 4 signals. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a conventional TDR  10  for connection to a pair of signal carriers or cables to be tested. A typical network containing a fault has many cables, therefore, pairs of cables are usually tested in sequence until all of the cables have been tested. A pulse transceiver  12  is used to interrogate (by launching an electrical pulse at an established test site where the signal carrier or cables of the network can be accessed) all possible pairs of cables (one pair at a time) via test leads  14 . The resulting composite reflected analogue signals are converted by an A/D converter  16  and stored in the TDR storage area  18 . The stored data is displayed on the TDR display  20  as pairs of signals. 
     Conventional TDRs are used in the well-established compare and contrast scheme of fault finding. In the compare and contrast scheme the signals from pairs of phases of a 3-phase electrical distribution network are combined (added or subtracted) to form composite signals which are compared to each other by a skilled person. The skilled person endeavours to isolate any key deviations (deviation points) between the two composite signals which may indicate the fault location. The skilled person locates the deviation points manually using a cursor control provided by the TDR. Once the deviation points have been located manually, the actual distances to the faults are displayed by the TDR using a standard formula. 
     A TDR  30  in accordance with the present invention, shown in FIG. 2 a  is used to automate and improve the existing compare and contrast method. FIG. 2 a  shows the modified TDR  30  which is similar to the standard TDR  10  having components  12 ,  14 ,  16 ,  18  and  20  but with an additional component  32  for signal processing and automatic fault distance computation and which operates by processing individual stored samples held in the storage area  18  before issuing the result to display  20 . 
     The additional component for signal processing, smoothing and fault distance computation (the processing component  32 ) is shown in more detail in FIG. 2 b  and comprises an adaptive processor  40  receiving signals of leads  33  from the transceiver and A/D convertor  12 ,  16 . The processor  40  generates difference signals which are delivered to a store/comparator and distance computation unit  42  which is provided with a reference input  44 . The output from unit  42  is delivered to the display  20 . 
     The reference input  44  represents a threshold level and a velocity factor and can be altered, although it is envisaged that most users will not need to alter at least the threshold level, therefore in some embodiments of the present invention the means to alter the threshold may not be apparent to the user. The threshold level may need to be altered to optimise performance as will be explained. For example, if use of the TDR is being transferred from one cable type to another or from one geographical location to another. Altering the threshold level may offset changes introduced by various factors, including use in a different environment, differing cable characteristics, differing numbers of nodes in each cable system, and differing numbers of users of each node. The velocity factor which forms part of the reference input  44  is determined by cable type and is a measure of how fast the pulse travels along the cable. 
     The adaptive processor  40  is composed of a sequence of individual adaptive processing elements  50  one of which is shown in FIG. 3 a.  Adaptive processing is a known technique which used a conventional filter  52  whose characteristics can be altered adaptively. That is, there is a mechanism which enables the filter to be adapted in use. A first signal  54  is input to the adaptive filter  52 , a second signal  56  is used as a reference signal. The adaptive filter output  58  is compared with the reference signal  56  to produce a difference signal  60  which is used to alter the characteristics of the adaptive filter  52  in such a way as to minimise the difference signal  60 . The adaptive processing elements  50  may be linear or non-linear. The inputs to the adaptive filter  52  are both sampled versions of the signals received from the pairs of cables under test. 
     FIG. 3 b  shows the structure of an adaptive processor  40  for testing a three cable system, the individual phases of which are denoted R, Y, and B. The processor  40  has three adaptive processing elements  50   a,    50   b,    50   c,  one for each combination of pairs of the three cables. The difference signals  60   a,    60   b,    60   c  from each adaptive processing element is input to the processing component  42  which stores, compares, and smoothes the difference signals  60   a,    60   b,    60   c  and then calculates the distance to the fault or faults automatically. 
     FIG. 4 shows an example of the signals received from two phases under test. The x axis of FIG. 4 represents distance, the y axis represents the amplitude of the signal. One of the signals (labelled a) is used as a reference signal  56  for the adaptive filter  52 , the other signal (labelled b) is the input  54  to the adaptive filter. 
     A each adaptive processing element  50  receives sampled input signals it operates on corresponding pairs of sampled data. That is, the adaptive processing element  50  receives a sampled data point from each of the two sampled input signals; it consecutively operates on pairs of sampled data points; it automatically filters one sampled signal (signal b) on an individual sample basis, and compares each filtered sample point with the corresponding sample point from the second (reference) sampled signal (signal a); and it automatically alters the characteristics of the adaptive filter  52  to minimise the difference between the inputs. Standard techniques exist for performing this function. The result of minimising the difference signal  60  using adaptive filtering is to provide a more accurate difference signal  60  which is a clean measure of the key differences between the two signals a and b, for example, as shown in FIG. 5 a.    
     The difference signals  60  (for the three cable system) input to the processing component  42  are smoothed by the processing component  42  to produce smoothed difference signals  62 , for example, as shown in FIG. 5 b.  Three smoothed difference signals result from the difference signals  60   a,    60   b,  and  60   c  of FIG. 3 b.  The predetermined threshold level from the reference input  44  is then applied to the smoothed difference signals  62 , as indicated in FIG. 5 b.  In FIG. 5 b  the predetermined threshold is shown by y axis lines at plus and minus two units. 
     In FIGS. 5 a  and  5   b  the x axis represents distance and the y axis represents amplitude. 
     The processing component  42  compares the smoothed difference signals  62   a,    62   b,    62   c  from each adaptive processing element  50   a,    50   b,    50   c  with the threshold or reference level input  44 , and determines whether the particular smoothed difference signal  62   a,    62   b,    62   c  is outside, i.e., above the predetermined threshold or not. 
     If the particular smoothed difference signal  62  is outside the predetermined threshold (plus and minus 2 units in this case), then there are no faults in the cables being tested by the corresponding adaptive processing element  50 . 
     If a particular smoothed difference output  62  is outside the predetermined threshold then the processing component  42  identifies the point in which the smoothed difference output  62  exceeds the threshold as a possible fault location. The component  42  then calculates the distance to the possible fault location from the test site and displays the result in the display  20 . The calculation of the distance to the possible fault location is performed using standard techniques including use of a velocity factor as previously described. By comparing the possible fault location results of the various smoothed difference signals  62 , the processing component  42  determines the most probable fault location in the cable. 
     By way of illustration and referring to FIG. 4, the TDR of FIG. 2 a  identifies the most probable fault location as that denoted ‘A’ whereas there are actually three possible fault locations each marked with an asterisk which would be considered by the human observer using the prior art TDR of FIG.  7 . However, due to the use of the adaptive filtering mechanism in the TDR of FIG. 2 a,  the system learns the similarities in the cable characteristics and identifies that the departure at A is the only significant departure between the waveforms a and b and, therefore, identifies A as the fault location. 
     Due to the fact that multiple joints exist there will inevitably be the existence of ambiguity of the fault location as determined by the processing component  42 . This ambiguity is addressed by considering the secondary or higher order reflected signal components of the composite reflected signals in relation to the result from the primary composite reflected signals. For example, consider a cable section comprising a single 10 meters branch at 12 meters along the 30 meter length. Assuming that a fault occurs at the branch end, then analysing the composite reflected signals will detect a fault at 22 meters. This is the primary difference signal. The ambiguity to be resolved is whether this fault occurs at 22 meters on the main cable or at the branch end. This is achieved by analysing the secondary or higher order difference signals. If we determine through the instrument that there exists a significant difference signal at 30 meters then this will have resulted from a reflection at the end of the main cable and thus the fault is along the main cable at 22 meters. If we determine through the instrument that there exists a significant difference signal at 32 meters then this will have resulted from a reflection at the end of the branch cable and thus the fault is at the end of the branch cable. 
     There is a certain learning period associated with the adaptive processing element. During the learning period no faults will be detected. The effect of this is that there is a certain distance from the test site (the location at which the cables are pulsed) along the cables under test over which faults cannot be located. 
     There is a trade-off in determining the learning period. The shorter the learning period the greater the variations in the difference signal  60  before the adaptive filter  52  has optimised the matching process (minimised the difference signal  60 ), but the shorter the learning distance. The learning distance is the distance from the test site over which faults cannot be detected. Use of a slower learning period produces smaller variations in the difference signal  60 , but increases the learning distance. 
     If no fault is detected in a known faulty line then the fault may have occurred in the learning distance. To determine if the fault did occur in the learning distance the test site is moved to another position, for example at the other end of the cables. The location of the fault should then be identified using the procedure described above. 
     The present invention may also be incorporated into existing equipment, for example the transceiver function may be performed by a conventional time domain reflectometer. 
     It will be appreciated that the adaptive filters  52  could be fixed after a certain period of time, and pulse compression may be used to facilitate long range, high resolution detection of faults.