Abstract:
A system and method for fault detection including a transmitter adapted to transmit a composite signal on a buried cable so that fault leakage of the composite signal returns to the transmitter through the ground surrounding the buried cable. The composite signal is composed of a low frequency first component that alternates at a first frequency and that defines a phase, and a low frequency second component that alternates at a second frequency higher than the first frequency and that defines a duty cycle having a predetermined relationship with the phase of the first component. A receiver receives the composite signal from the probe and filters the first component, filters the second component, defines an alternating reference signal having a phase based on the duty cycle of the filtered second component, and compares the phase of the reference signal with the phase of the filtered first component.

Description:
BACKGROUND OF THE INVENTION 
     Underground cables, such as power, telephone and cable TV lines (CATV), are in wide use and typically include one or more wires encased in a metallic sheath covered by insulating material. Although the lines are manufactured to endure the harshness of underground burial, they do not always escape physical damage that may allow moisture intrusion through the insulation cover, thereby causing a short to ground. 
     A fault detection system described in U.S. Pat. No. 3,299,351 to Williams applies to an underground cable, a low frequency AC test signal and a reference signal at one-half the frequency of the test signal. Differential probes detect the signals when they leak to ground from a cable fault, and circuitry attached to the probes doubles the reference signal&#39;s frequency. Because the phase of the leaked test signal, but not the phase of the doubled reference signal, changes depending on the relative position of the probes and the fault, a comparison of the reference signal phase to the phase of the test signal indicates the direction to the fault. 
     A fault detection circuit disclosed in U.S. Pat. No. 3,991,363 to Lathrop also relies on a phase comparison between test and reference signals applied to an underground cable. The test signal is carried by a high frequency RF carrier. The high frequency reference signal radiates from the earth and is detected by above ground antennas. Differential probes detect the lower frequency test signal when it leaks to ground from a cable fault. A phase reversal takes place between these two signals at the fault. Thus, comparison of the signal phases indicates the fault&#39;s direction with respect to the receiver. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses disadvantages of prior art constructions and methods. Accordingly, it is an object of the present invention to provide an improved fault detection system. 
     This and other objects are achieved by a system and method that applies a composite signal to a cable. The composite signal is comprised of a low frequency first component that alternates at a first frequency and that defines a phase, and a low frequency second component that alternates at a second frequency higher than the first frequency and that defines a duty cycle having a predetermined relationship with the phase of the first component. The composite signal returns to a transmitter through a conducting medium from a fault in the cable. A probe, when inserted into the conducting medium proximate the cable, receives the composite signal leaked from the fault. Receiver circuitry that receives the composite signal from the probe filters the first component, filters the second component, defines an alternating reference signal having a phase based on the duty cycle of the filtered second component, and compares the phase of the reference signal with the phase of the filtered first component. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended drawings, in which: 
         FIG. 1  is a schematic illustration of a fault detection system according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of a signal generator for use in the system shown in  FIG. 1 ; 
         FIG. 3  is a block diagram of a receiver for use in the system shown in  FIG. 1 ; 
         FIG. 4  is a graphical illustration of waveforms at various points within the fault detection system shown in  FIG. 1 ; 
         FIG. 5A  is an electrical schematic illustration of a band pass filter for use in the receiver shown in  FIG. 3 ; 
         FIG. 5B  is a response graph of the band pass filter shown in  FIG. 5A ; 
         FIG. 6A  is an electrical schematic illustration of a band pass filter for use in the receiver shown in  FIG. 3 ; and 
         FIG. 6B  is a response graph of the band pass filter shown in FIG.  6 A. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the present invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     The present system both detects faults and locates underground cables. Cable location is ancillary to the present invention. For purposes of clarity, therefore, the system is described below only as it relates to fault detection. 
     Referring to  FIG. 1 , a fault detection system  5  is used to detect a fault  11  along an underground cable  10 . System  5  includes a transmitter  12  and a receiver  13 . Transmitter  12  connects to cable  10  and a conducting medium, such as ground  15 , by probes  14  and  16 , respectively. An A-frame detector  30  connects receiver  13  to ground  15  so that a ground path is formed between the fault and the A-frame. A-frame detector  30  has two ground probes  32  and  34  that are placed into ground  15 . 
     As should be well understood, electric current generally leaks to ground from an underground cable, even when a fault is not present, due to capacitive coupling between the conductor and ground. Capacitive coupling is an indirect transfer of energy from the cable to ground by means of the mutual capacitance between the cable and ground. Capacitive coupling favors the transfer of higher frequency components of a signal and tends to attenuate the signal as it travels along the cable. That is, the level of capacitive coupling increases with signal frequency so that higher frequency signals attenuate at a faster rate, while lower frequency signals tend to travel a longer distance along the cable. Cable faults, however, can cause a much more direct current path to ground. A “fault,” as used herein, refers to any unintended perturbation in the path of a conductor from which current flowing in the conductor leaks into the conducting medium and does not include capacitive coupling. 
     The embodiment of the fault detection system described herein applies an electrical signal onto an underground cable so that a detectable phase change occurs in the voltage gradient of the leaked signal from one side of the fault to the other. In general, the ability to detect a fault from a signal leaked to ground from the fault depends on the signal&#39;s frequency. Leaks caused by capacitive coupling for lower frequency signals tend to be lower in magnitude than leaks caused by faults, whereas at higher frequencies, a leak created by a fault may be indistinguishable from the general capacitive leak. Thus, at lower frequencies, the effect of the fault leak can be distinguished from the effect of the capacitive leak. For higher frequency signals, however, the fault may be said to be “masked” by the capacitively coupled signal. 
     Accordingly, reference herein to a “low” or “high” frequency signal reflects the different effect a fault may have upon the signal. A fault causes a detectable leaked signal of a “low” frequency signal, whereas a fault leak is masked in a “high” frequency signal. As will be understood by those skilled in this art, the boundary between such low and high frequency signals will depend on the equipment used to measure the signals and on the conditions that determine, on one hand, the severity of a fault leak and, on the other hand, the normal capacitive coupling between the cable and ground. Thus, the boundary may depend upon the fault&#39;s conditions, upon the cable&#39;s construction (including insulation), and upon soil conditions (e.g. moisture content, soil resistively, soil stratification, chemical content and temperature). It is believed that signals up to approximately 45 kHz may be considered low frequency signals for most fault, cable and soil conditions, and the presently described embodiment utilizes signals within a range of 10 Hz to 45 kHz. 
     More specifically, the presently described embodiment applies to an underground cable an electrical signal that is a composite of two low frequency signals of 512 Hz and 9.5 kHz, respectively. At these frequencies, it has been found that capacitively coupled ground currents are detectable over the first one-fifth to one-third of the cable&#39;s length. Such currents are relatively weak, however, and therefore do not interfere with fault detection. 
     Returning to  FIG. 1 , cable  10  comprises a conductor surrounded by insulation material. Cable  10  may be, for example, a coaxial cable, a single insulated wire, or a group of insulated wires encased in a metallic sheath covered by an insulating cover. Above-ground transmitter  12  applies a signal to cable  10  through probe  14 . Probe  14  may consist of a clamp, a clip, or any other means of applying the signal to cable  10 . The attachment of probe  14  to cable  10  may take place before the cable enters the ground or through an excavated hole in the ground so that probe  14  may be attached to cable  10  below ground level. Probe  16  is placed in ground  15  to provide a return path for signals passed to ground through cable fault  11 . Probe  16  may be a ground stake or any suitable metal object set into ground  15 , such as a stop sign post. Probe  16  is preferably placed away from the cable being tested and away from other utility lines. 
     Referring also to  FIG. 2 , transmitter  12  applies a composite signal  29  to the conductor in cable  10 . The composite signal is composed of a 512 Hz square wave test signal and a 9.5 kHz±512 Hz reference signal generated by the transmitter as described below in greater detail. Although 512 Hz and 9.5 kHz are used in this example, it should be understood that other low frequencies could also be used. 
     Still referring to  FIG. 2 , an oscillator  18  outputs a 9.5 kHz sinusoidal signal through a resister  20 . A square wave generator  22  outputs the 512 Hz test signal through a resister  24 . When the 512 Hz square wave test signal is high, the 9.5 kHz sinusoidal signal combines with the square wave, and the result is passed to a 9.5 kHz low pass filter  26 . When the 512 Hz test signal is low, however, a diode  28  shorts the 9.5 kHz signal, thereby passing a low signal to amplifier  26 . The result is composite signal  29  consisting of the 9.5 kHz±512 Hz reference signal and the 512 Hz test signal. That is, composite signal  29  includes a 512 Hz test signal and a 9.5 kHz signal that turns on and off at a 50% duty cycle at a rate of 512 Hz. “Duty cycle,” as used herein, refers to the relationship between the time a signal is transmitted to the time it is not transmitted. Thus, for example, the 9.5 kHz signal is transmitted at a 50% duty cycle since it is on or off half the time. In the described embodiment, the transition between active and inactive portions of the duty cycle occur at the transition between the high and low portions of the test signal, and the time in which the 9.5 kHz signal is active corresponds to the high portions of the test signal. It should be understood, however, that other relationships between the reference signal&#39;s duty cycle and the test signal may be used. 
     Fault  11  occurs at some point along cable  10  and provides a path to ground  15 . Because both components of signal  29  are low frequency signals, fault  11  is the primary leakage path for the entire composite signal  29 . Accordingly, when probes  32  and  34  of A-frame detector  30  are disposed in the ground proximate the cable, the A-frame detector receives a voltage gradient corresponding to composite signal  29  as it returns to the transmitter&#39;s ground probe  16  from fault  11 . To obtain a stable measurement at receiver  13 , receiver  13  should sense a return signal of at least one milliamp. The magnitude of the resistance between fault  11  and ground  15  is generally greater than 50 kOhms. Therefore, composite signal  29  is stepped up to about 130 volts using a power amplifier  25 , which has a maximum output of about three watts, and a 44:1 transformer circuit  27 . 
     The A-frame detector receives a signal that is either in phase or 180 degrees out of phase with transmitted composite signal  29 . The change in phase occurs at the fault, when the voltage gradient across probes  32  and  34  changes polarity. That is, when probes  32  and  34  are between transmitter  12  and fault  11 , probe  34  is at a higher voltage potential than probe  32 , and the detected voltage gradient across the probes is positive. If the probes are moved beyond fault  11 , the detected voltage gradient becomes negative since probe  32  is now at a higher voltage potential than probe  34 . Thus, the polarity of the signal picked up by probes  32  and  34  changes at the point of the fault, and the received signal&#39;s phase changes by 180 degrees. 
     Accordingly, and referring also to  FIG. 4 , A-frame  30  outputs a modified square wave  36 , corresponding to composite signal  29  (FIG.  2 ), when the A-frame is in front of the cable fault. Beyond the fault, the A-frame outputs a modified square wave  38  that is 180 degrees out of phase with respect to signal  36 . Referring to  FIGS. 3 and 4 , the output signal  36  or  38  is input to receiver circuitry having first and second frequency channels. The first frequency channel includes a 512 Hz band pass filter (BPF)  40  that passes a 512 Hz square wave test signal  42  or  44 , depending on whether the A-frame outputs signal  36  or  38 , to a gain control circuit  46 . A second 512 Hz band pass filter  62  removes noise from the first channel signal, converts square wave test signal  42  or  44  to a sinusoidal test signal  64  or  66 , respectively, and synchronizes test signal  64  or  66  to the output signal of a band pass filter  70  in the second frequency channel. BPF  70  is similar to BPF  62  and is discussed in more detail below. A schematic illustration of BPF&#39;s  62  and  70  and a response graph are provided at  FIGS. 5A and 5B . A schematic illustration of BPF  40  and a response graph are provided at  FIGS. 6A and 6B . 
     The second frequency channel includes a 9.5 kHz band pass filter  48  that outputs base reference signal  50  or  52  to a gain control  54 . Signal  50  or  52  is the 9.5 signal having a 512 Hz 50% duty cycle. A first stage  56  of a wave detector circuit half-wave-rectifies reference signal  50  or  52  and applies the rectified signal to a second stage integrator  58  that uses the rectified signal to generate a modified square wave reference signal  60  that is always in phase with composite signal  29 . That is, reference signal  60  is not affected by the position of probes  32  and  34  with respect to fault  11 . It is always in phase with received test signal  36  and always 180 degrees out of phase with received test signal  38 . 
     Reference signal  60  passes through 512 Hz BPF  70 , which removes extraneous noise, increases the signal-to-noise ratio, and produces an 512 Hz sinusoidal reference signal  72 . BPF  70 , which has the same frequency characteristics as BPF  62 , further synchronizes the phase of reference signal  72  with that of test signal  64  or  66  output from BPF  62 . 
     The output of BPF  70  is split and sent to a half-wave rectifier circuit  84  and a square wave forming circuit  74 . Half-wave rectifier circuit  84  outputs to an analog-to-digital (A/D) converter  86  that outputs to CPU  78  a digital signal that is representative of the amplitude of reference signal  72  and is used to adjust gain control circuits  46  and  54 . That is, the gain applied to the output of BPFs  40  and  48  by gain control circuits  46  and  54  increases or decreases depending on the output amplitude of reference signal  72  so that the receiver&#39;s signal levels remain within an operative range. 
     Wave forming circuit  74  uses sine wave reference signal  72  to produce a square wave reference signal  76  that controls a switching multiplexer circuit  68 . The frequency of reference signal  76  is equal to that of test signals  64  and  66  and is respectively in phase and 180 degrees out of phase with those signals. When reference signal  76  is high, switch multiplexer circuit  68  closes, allowing a portion of test signal  64  or  66  to pass from BPF  62  through to an integrator  69 . If, however, signal  76  is low, switch multiplexer  68  is open, and no portion of test signal  64  or  66  passes to integrator  69 . 
     If A-frame ground probes  32  and  34  are in front of cable fault  11  (FIG.  1 ), BPF  62  outputs test signal  64 . Because reference signal  76  is in phase with test signal  64 , switch multiplexer circuit  68  closes only during the positive amplitude portions of test signal  64 . Thus, a signal  87  having only a positive amplitude is sent to an A/D converter  80 . If, however, the A-frame ground probes are beyond the fault, BPF  62  outputs test signal  66 . Since reference signal  76  is 180 degrees out of phase with test signal  66 , switch multiplexer circuit  68  closes only during the negative amplitude portions of test signal  66 . Thus, a signal  88  having only a negative amplitude is sent to AID converter  80 . 
     A/D converter  80  outputs a digital signal corresponding to signal  87  or  88  to CPU  78 . Thus, the signal received by the CPU reflects whether the A-frame probes are in front of or beyond the cable fault, and the CPU drives a display  82  to so notify the operator. Display  82  may notify the user by lights that indicate a positive phase, a negative phase, and a null phase. Display  82  may also contain a numerical readout that shows the magnitude of the phase: negative, positive, or zero. Display  82  may use other suitable techniques to inform the user of the probe&#39;s location relative to fault  11 . 
     It should be understood that modifications and variations of the present invention may be practiced by those of ordinary skill in the art without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims, and that the aspects of varying embodiments may be interchanged in whole or in part.