WO2014009726A1

WO2014009726A1 - Detection of tampering with or theft of an electrical conductor

Info

Publication number: WO2014009726A1
Application number: PCT/GB2013/051829
Authority: WO
Inventors: Joe AINSWORTH; Graham Earp
Original assignee: Ea Technology Limited
Priority date: 2012-07-11
Filing date: 2013-07-10
Publication date: 2014-01-16
Also published as: GB201212341D0

Abstract

A system (100) for detecting tampering with or theft of an earth conductor (10) for an electricity substation comprises: an integrity testing unit (110), a processing unit (120), and a communication unit (130). The testing unit (110) comprises an acoustic signal sensor (111) and a signal conditioning unit (112) and is operable to conduct integrity tests of the conductor (10) and output a succession of outcome values indicative thereof. The processing unit (120) comprises a decision unit (121), a data storage means (122), and an alarm output unit (123). The decision unit (121) is operable to compare outcome values against one or more reference values stored in the data storage means (122). The reference value may be a predetermined characteristic value or a prior outcome value. If the change in value is greater than a predetermined threshold, the decision unit (121) activates the alarm output unit (123) to control local external alarms and the communications unit (130) to communicate the alarm signal via any one or more suitable wired (e.g. Ethernet 131) or wireless communication (e.g. GSM 132) links to one or more additional systems or devices (141-142).

Description

Detection of Tampering with or Theft of an Electrical Conductor

Technical Field of the Invention

The present invention relates to a method and system for detecting tampering with or theft of an electrical conductor, such as an earth conductor, from an electricity substation or electricity distribution network. In particular, the system and method is adapted to be relatively low cost and thus suitable for use on smaller substations.

Background to the Invention

Metal theft is becoming an increasing problem in view of the present high prices of scrap metal. One common form of metal theft is the theft of copper earth conductors from electricity substations. As a preventative measure, many substations now are provided with security systems including such features as movement sensors, lights, CCTV (closed circuit television), loudspeakers and the like. Whilst such sensors can detect attempted thefts, they may also result in false alarms, for instance, due to passing animals such as cats or foxes. Providing sensors able to distinguish such events may reduce the number of false alarms but adds considerable expense. Similarly providing additional human oversight or building more secure fences or wall around such substations may also reduce the number of false alarms but will add considerable expense. Since this is the case and since there is increased cost and time in responding to false alarms at smaller and typically more remote substations, significant alarms are only installed on a selection of larger more important substations. Smaller substations must therefore either operate without significant security or must potentially allow a large number of false alarms.

It is an object of the present invention to provide a method and/or system that at least partially overcomes or alleviates the above problem. Summary of the Invention

According to a first aspect of the present invention there is provided a sensing system suitable for detecting tampering with or theft of an electrical conductor of an electricity substation or electricity distribution network, the system comprising: an integrity testing unit, said integrity testing unit being operable to conduct a succession of integrity tests on the conductor and output a value indicative of the instantaneous integrity in response to each test; a processing unit operable to: receive and store said outcome values; compare the outcome values of one or more selected tests to a reference value; and output an alarm signal if the difference between the outcome value and the reference value is greater than a preset threshold.

The present system thus allows for very frequent integrity tests to be conducted. Even if such tests are relatively low accuracy, this can enable an attempt to tamper with or steal an electrical conductor to be rapidly detected, without significant danger of generating false alarms due to passing wildlife.

The system may be particularly adapted to monitoring earth conductors of electricity substations or electricity distribution networks but may additionally be adapted to monitor other types of conductor in such locations or otherwise. Whilst it is known to conduct integrity testing on earth conductors this is conventionally implemented on an infrequent basis using high accuracy equipment for the purpose of assessing the overall condition of an earth conductor in situ. In particular this testing is directed to assessing the condition of any underground or unseen portions of the earth conductor and to ensure that it remains capable of adequately performing its intended role in the event of a fault condition arising on the network. The integrity testing unit may take any suitable form and use any suitable technology. Suitable technologies include but are not limited to: acoustic; electrical; or magnetic.

Suitable forms of acoustic integrity testing unit include but are not limited to: passive acoustic units and active acoustic units. Preferably, the acoustic signals have an ultrasonic frequency.

The integrity testing unit may comprise one or more acoustic signal sensors mechanically attached to the conductor. The one or more acoustic signal sensors may be operable so as to detect acoustic signals that are imparted into the conductor. Such a unit may be described as a passive acoustic unit. Actions associated with attempts to tamper with or steal an earth conductor (such as but not limited to sawing, cutting, grinding, hammering or prising the conductor) are capable of inducing acoustic signals into the conductor that are detectable by such a system. By suitable processing, it is possible to recognise and discriminate such signals from background acoustic signals present in the conductor under normal conditions.

The integrity testing unit may comprise one or more signal injection units operable to inject an acoustic signal onto the earth conductor and one or more acoustic signal sensors operable to detect acoustic signals on the conductor. The or each signal injection unit and the or each acoustic signal sensor may both be mechanically attached to the conductor. Such a unit may be described as an active acoustic unit.

In a preferred embodiment, the signal injection unit and the acoustic signal sensor may be provided in a single acoustic transducer. Alternatively, the signal injection unit and the acoustic signal sensor may be provided at substantially the same point on the conductor. In such units, the reflection of the injected signal from discontinuities in the conductor may be monitored, preferably by using time domain reflectometry (TDR) techniques or other techniques such as Frequency Domain Reflectometry, Spatial Domain Reflectometry. In TDR, the time difference between signal injection and detection of reflected signals may be used to generate a characteristic spatial map of discontinuities in the earth. In such units, the characteristic map may be used as a reference value for comparing with detected reflections.

In alternative embodiments, the signal injection unit may be provided at a first point on the conductor and the acoustic signal sensor may be provided at a second point on the conductor. In such units, the direct transmission of the injected signal may be monitored.

In the acoustic integrity testing units, the or each acoustic signal sensor may be provided with a signal conditioning unit operable to condition the detected acoustic signal. The conditioning may involve amplification, filtering or other processing to extract desired features within the signal. In one embodiment, detection of interference to the conductor may be achieved by detecting changes in the acoustic resonance characteristics of the network being monitored as a result of changes to its mechanical configuration resulting from said interference.

Mechanical connection of the acoustic signal injection units, acoustic signal sensor and/or acoustic transducers may be achieved by means of, but not limited to: a clamping arrangement; adhesive; and/or a mounting stud. Preferably, an electrically nonconductive material is provided between the acoustic signal injection units, acoustic signal sensor and/or acoustic transducers and the conductor. This is advantageous for a number of reasons including: preventing undesirable electrical noise signals naturally present on the conductor from directly coupling to the sensor/transducer; and to provide electrical safety isolation from high transient voltages that may occur momentarily on the conductor during fault conditions. The nonconductive material may comprise a thin layer of substantially similar extent to the acoustic signal injection units, acoustic signal sensor and/or acoustic transducers. Alternatively, the nonconductive material may comprise or be mounted on an angled block in contact with the conductor. This can allow acoustic signals to be injected into the conductor at a desired angle. In the case wherein the nonconductive material is mounted on the angled block, the angled block may be conductive. The nonconductive material may be adapted or selected to match the acoustic properties of the conductor and/or the sensor. Similarly, the angled block may be adapted to match the acoustic properties of the nonconductive material and/or the conductor and/or the sensor

The acoustic signal injection units, acoustic signal sensor and/or acoustic transducers preferably comprise piezoelectric devices. In particular, a suitable piezoelectric device may comprise a piece of piezoelectric material sandwiched between two conductive plates. Electrical connections can then be made to the conductive plates to harness the acoustic signal in the form of an electrical waveform when acting as a sensor, and to stimulate the piezoelectric material using an applied electrical signal when acting as an emitter.

The conductive plates may be attached to the piezoelectric material using an electrically conductive adhesive. In particular said electrically conductive adhesive may be an epoxy resin laden with silver. In one embodiment the conductive plates may take the form of copper foil. The whole assembly may be sandwiched between two electrically insulating plates. Preferably, the electrically insulating plates are made from a material that readily propagates acoustic signals.

The acoustic integrity testing unit may be provided within an isolating housing. This can help to isolate the acoustic integrity testing unit from external acoustic noise. The housing may be a conductive housing. This can help to isolate the acoustic integrity testing unit from external electrical noise. The housing may be directly attached to the conductor by any suitable means.

Suitable forms of magnetic integrity testing unit include but are not limited to: magnetic flux leakage units; eddy current testing units; and magnetic signal injection units.

The testing unit may be operable to test integrity substantially continuously or at regular intervals, as is appropriate. For instance, a passive unit may be operable substantially continuously whilst an active unit may be operable at regular intervals. The testing interval may be selected to be any suitable time interval. Preferably, the testing interval is significantly shorter than the estimated time taken by a thief to remove an earth conductor. For example, the testing interval may be in the range of around 1 second to around 2 minutes. In particular, the testing interval may be of the order of 30 seconds.

The processing unit may comprise a decision unit operable to undertake the comparison of outcome values with a reference value. The comparison step may operate on the basis of threshold comparison or on the basis of pattern recognition. The comparison step may be based on signal amplitude, signal frequency or on a combination of both. In some embodiments, the reference value may be selected from one or more stored reference values. The preset threshold may be defined as an absolute difference between outcome value and reference value. Alternatively, the preset threshold may be defined as a relative difference between outcome value and reference value. The selection of either alternative and/or the selection of the magnitude of the threshold in either alternative may be determined by considering the qualities of the particular integrity testing unit and/or the expected difference between outcome value and reference value during removal of the earth conductor.

The difference between outcome value and reference value may be measured against the threshold after each integrity test. Alternatively, the difference between outcome value and reference value may be measured against the threshold periodically. This period may be determined by a preselected time interval or by a preselected number of tests. As a further alternative, the change in outcome values may be measured against the threshold at random intervals.

The reference value may be an earlier outcome value. The reference value may be the immediately preceding outcome value or another selected outcome value. For example this may be the outcome value of the nth preceding test.

Alternatively, the reference value may be an average of selected preceding outcome values. The average of preceding outcome values may be an overall average, an average over a present time interval or an average over a present number of tests. In a further alternative, it is possible for the processing unit to be operable to compare an average of the most recent outcome value and one or more immediately preceding values with an average of a larger number of preceding values.

Additionally or alternatively, the reference value may be determined by a learning algorithm over a limited time after installation. In another implementation, the reference value may be a characteristic value of the conductor being monitored. The characteristic value can be established by prior testing of the conductor or may be established by an algorithm based on previous outcome values.

The system may be provided with a communication unit connected to the processing unit. The communication unit may be operable to pass the alarm signal to one or more additional devices or systems. This can allow the system to be used to trigger the operation or activation of such additional devices or systems.

The additional devices or systems may include but are not limited to: an external monitoring system; cameras (still or video; visible or infrared); illumination means or other visible alarm output means; loudspeakers or other audible alarm output means; microphones or the like.

The communication unit may be operable to communicate the alarm signal via any suitable communication link. The link may be wired or wireless. If the link is wired it may comprise any suitable electrical or optical cable, including, in particular power line carrier (PLC) communications. If the link is wireless it may comprise any suitable frequency band or protocol including but not limited to: Bluetooth™, ZigBee™, WiFi, GSM, GPRS, 3G or the like. The communicated alarm signal may take any suitable format including but not limited to an SMS or other alphanumeric text based format.

In some embodiments, triggering of additional devices or systems may be delayed or staggered. This can allow, for instance, cameras to be activated in advance of visible or audible alarm output means thereby obtaining images of a suspected thief without alerting the thief. The system may be provided with a data storage means. This can provide a library of outcome values which may be used for audit or monitoring purposes as desired or required. Alternatively or additionally the system may be provided with the means to transmit data (such as images or video for example) to a remote location via the communication unit. This provides an additional precaution against a thief removing any potential evidence captured by the system that might be useful to any subsequent incrimination proceedings.

In some embodiments, the communication means may be operable to transmit successive outcome values to an external monitoring system. This can allow remote monitoring of the earth conductor for tampering and/or theft.

In some embodiments, the communication means may be operable for the system to receive commands from an external controller. This can allow remote adjustment, including fine tuning, of the detection trigger thresholds for the alarm settings.

The system may be powered by any suitable power source. In some embodiments, this may be a mains supply. In other embodiments an internal battery may be provided either in addition to or in place of a mains supply. Additionally or alternatively, the system may derive some or all of its power by power scavenging. Power scavenging may be provided by an inductive or capacitive coupler to a power frequency circuit or from low-level currents flowing within the conductor itself that is being monitored.

According to a second aspect of the present invention there is provided a method for detecting tampering with or theft of an electrical conductor of an electricity substation or electricity distribution network, the method comprising: conducting a succession of integrity tests and outputting a value indicative of the instantaneous integrity in response to each test; storing said outcome values; comparing the outcome values of one or more selected tests to a reference value; and outputting an alarm signal if the difference between the outcome value and the reference value is greater than a preset threshold.

The method of the second aspect of the present invention may incorporate any or all features of the first aspect of the present invention as are desired or as are appropriate.

Detailed Description of the Invention

In order that the invention is more clearly understood, selected embodiments are described in more detail below, by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a system according to the first aspect of the present invention using passive acoustic testing means;

Figure 2 is a schematic view of a system according to the first aspect of the present invention using active acoustic testing means;

Figure 3 is a schematic illustration of an arrangement for mechanically connecting an acoustic testing unit to a conductor for implementation of the present invention;

Figure 4 is a schematic illustration of another arrangement for mechanically connecting an acoustic testing unit to a conductor for implementation of the present invention; Figure 5 is a schematic illustration of a further arrangement for mechanically connecting an acoustic testing unit to a conductor for implementation of the present invention;

Figure 6 is a schematic illustration of a still further arrangement for mechanically connecting an acoustic testing unit to a conductor for implementation of the present invention; and

Figure 7 is a schematic illustration of an arrangement including an isolating housing for mechanically connecting an acoustic testing unit to a conductor for implementation of the present invention.;

Turning now to figure 1, a system 100 for detecting tampering with or theft of an earth conductor 10 for an electricity substation comprises: an integrity testing unit 110, a processing unit 120, and a communication unit 130.

The testing unit 110 is operable to conduct integrity tests of the conductor 10 and output a succession of outcome values indicative thereof. In the present example, the testing unit 110 comprises an acoustic signal sensor 111 and a signal conditioning unit 112. The acoustic signal sensor 111 is mechanically connected to the conductor 10 and is operable to detect acoustic signals, typically ultrasonic. The signal conditioning unit 112 is operable to condition the detected acoustic signal. The conditioning may involve amplification, filtering or other processing to extract desired features within the detected signal. The skilled man will appreciate that whilst, for clarity, a single acoustic sensor 111 is shown in figure 1, a network of such sensors 111 (and a corresponding network of conditioning units 112, if required or desired) may alternatively be provided. The processing unit 120 comprises a decision unit 121, a data storage means 122, and an alarm output unit 123. The decision unit 121 is operable to compare outcome values against one or more reference values stored in the data storage means 122. Actions associated with attempts to tamper with or steal an earth conductor (such as but not limited to sawing, cutting, grinding, hammering or prising the conductor) are capable of inducing acoustic signals into the conductor that are detectable by such a system. By suitable processing, it is possible to recognise and discriminate such signals from background acoustic signals present in the conductor under normal conditions.

The reference value may be a predetermined characteristic value or a prior outcome value and determine whether the change in the compared values is greater than a predetermined threshold. In the event that the change in values is greater than a predetermined threshold, the decision unit 121 activates the alarm output unit 123. When activated, the alarm output unit 123 controls local external alarms via connection to a SCAD A network 143 or direct connection to audio/visual alarm means 144.

In addition to local alarms, the alarm output unit 123 can activate the communications unit 130. The communication means 130 is operable to communicate the alarm signal via any one or more suitable wired (e.g. Ethernet 131) or wireless communication (e.g. GSM 132) links to one or more additional systems or devices 141-142. These additional systems or devices may be local or may be remote as required. In the present example, the additional systems include a remote monitoring system 141, which may be at a control centre for the electricity network; and a back up data store 142 which may be local or at the said control centre. In a typical implementation, attempted theft of the earth conductor 10 would result in a change in the outcome values output by testing unit 110. Consequently, processing unit 120 will output an alarm signal, which is in turn communicated by communication unit 130. The alarm signal can result in the issuance of an alert on the remote monitoring system 141. This can enable control centre personnel or the system 141 to despatch personnel to investigate and/or monitor related systems to determine whether this is a false alarm.

The alarm signal can also activate the local security system 143. Typically, this may cause the initial activation of security cameras to capture images of the earth conductor 10 and/or the surrounding area. These images can be stored for later use in evidence and/or transmitted to the control centre.

At some point after the cameras have been activated, the illumination means and/or the audible alarms can also be activated. The delay in activation may allow the cameras to capture images of the thief before they are aware that their presence is detected. The activation of these additional devices may be after a preset interval or may be in response to a further signal from the control centre. In this way, the control centre may be able to view the images, determine that there has been a false alarm and reset the systems.

Turning now to figure 2, a system 200 for detecting tampering with or theft of an earth conductor 10 for an electricity substation comprises: an integrity testing unit 210, a processing unit 220, and a communication unit 230.

The system 200 differs from the system 100 primarily in the integrity testing unit 210. As before, the testing unit 210 is operable to conduct integrity tests of the conductor 10 and output a succession of outcome values indicative thereof. In the example of figure 2, the testing unit 210 comprises an acoustic signal sensor 211 and a signal conditioning unit 212 but additionally comprises an acoustic signal injecting unit 213 connected to a signal generator 214. Both the acoustic signal sensor 211 and the signal injection unit 213 are mechanically connected to the conductor 10. As in the previous embodiment, the acoustic signal sensor 211 is operable to detect acoustic signals, typically ultrasonic. The signal conditioning unit 212 is operable to condition the detected acoustic signal. The conditioning may involve amplification, filtering or other processing to extract desired features within the detected signal. The signal injecting unit 213 is operable to inject acoustic signals, typically ultrasonic, on to the conductor 10 under the control of the signal generator 214.

The skilled man will appreciate that whilst, for clarity, a single acoustic sensor 211 and a single acoustic signal injection unit 213 are shown in figure 2, a network of such sensors 211 and injection units 213 (and a corresponding network of conditioning units 212, if required or desired) may alternatively be provided. The skilled man will further appreciate that the sensor 211 and injection unit 213 may be comprised of a single transducer unit, if required or desired.

In embodiments where the signal injection unit 213 and the acoustic signal sensor 211 are provided in a single acoustic transducer or at substantially the same point on the conductor, the reflection of the injected signal from discontinuities in the conductor may be monitored, preferably by using time domain reflectometry (TDR) techniques. In TDR, the time difference between signal injection and detection of reflected signals may be used to generate a characteristic spatial map of discontinuities in the earth. In such units, the characteristic map may be used as a reference value for comparing with detected reflections. In alternative embodiments, the signal injection unit 213 is provided at a first point on the conductor 10 and the acoustic signal sensor 211 is provided at a second point on the conductor 10 and the direct transmission of the injected signal may be monitored.

The processing unit 220 comprises a decision unit 221, a data storage means

222, an alarm output unit 223, and a timing unit 224. The timing unit 224 is operable to control the injection of signals. As before, the decision unit 221 is operable to compare outcome values against one or more reference values stored in the data storage means 222. Actions associated with attempts to tamper with or steal an earth conductor (such as but not limited to sawing, cutting, grinding, hammering or prising the conductor) are capable of inducing acoustic signals into the conductor that are detectable by such a system. Additionally, such actions may alter the reflection of or transmission of the injected signals. By suitable processing, it is possible to recognise and discriminate such signals from background acoustic signals present in the conductor under normal conditions.

The reference value may be a predetermined characteristic value or a prior outcome value and determine whether the change in the compared values is greater than a predetermined threshold. In the event that the change in values is greater than a predetermined threshold, the decision unit 221 activates the alarm output unit 223. When activated, the alarm output unit 223 controls local external alarms via connection to a SCADA network 243 or direct connection to audio/visual alarm means 244.

In addition to local alarms, the alarm output unit 223 can activate the communications unit 230. The communication means 230 is operable to communicate the alarm signal via any one or more suitable wired (e.g. Ethernet 231) or wireless communication (e.g. GSM 232) links to one or more additional systems or devices 241-242. These additional systems or devices may be local or may be remote as required. In the present example, the additional systems include a remote monitoring system 241, which may be at a control centre for the electricity network; and a back up data store 242 which may be local or at the said control centre. The output of an alarm can be dealt with in a similar manner to that described for the embodiment of figure 1.

Turning now to figure 3, a first embodiment of an arrangement 50 for mechanically connecting a piezoelectric device 20 (suitable for acting as an acoustic signal sensor, injection unit or transducer) to a conductor 10 is shown. In the present example the conductor 10 is a copper earth conductor tape 10 of an electricity substation, the conductor provided adjacent to a wall 11 of the substation.

The arrangement 50 comprises a clamp 53 provided with a bolt 54. The clamp 53 fits around the conductor 10 and piezoelectric device 20. The bolt 54 can be tightened so as to hold the piezoelectric device 20 in place on the conductor 10. This attachment arrangement 50 has the advantage that it can readily be applied and removed when required. The piezoelectric device 20 is provided with a layer of nonconductive material 51 between the piezoelectric device 20 and the conductor 10. The nonconductive layer 51 is formed from a material chosen to act as an electrical insulator but also to readily allow the propagation of acoustic signals. This provides galvanic isolation to the piezoelectric device 20 both as a safety measure in the event of an earth fault and to reduce electrical interference present in the substation coupling on to the acoustic measurements. In one preferred embodiment the nonconductive layer 51 might be a ceramic material such as an alumina substrate or the like.

A second nonconductive layer 52 may also be provided. The second layer 52 is formed from a material chosen to act as an electrical insulator but also compliant so as to prevent the clamping arrangement overly restricting the movement of the piezoelectric device 20. In one preferred embodiment the second layer 52 might be a rubber washer.

The thickness of the layers 51, 52 can be chosen so as to give the required electrical insulation for safety purposed to protect against a rise in touch potential of the conductor 10 under high fault current conditions. Also shown in figure 3 are electrical connections 59 to the piezoelectric device 20.

Turing now to figure 4, another arrangement 60 for attachment of a piezoelectric device 20 to a conductor 10 is shown. In this arrangement 60, a stud 63 comprising a flat flange 64 and threaded shaft 65 is attached to the conductor 10 by use of a suitable adhesive. The adhesive is selected to have good adhesion properties and also to readily conduct acoustic signals. In one preferred embodiment the adhesive may be a cyanoacrylate which has the advantage of rapid deployment.

The piezoelectric device 20 is provided with a layer of nonconductive material 61 between the piezoelectric device 20 and the conductor 10. The nonconductive layer 61 is formed from a material chosen to act as an electrical insulator but also to readily allow the propagation of acoustic signals. This provides galvanic isolation to the piezoelectric device 20 both as a safety measure in the event of an earth fault and to reduce electrical interference present in the substation coupling on to the acoustic measurements. In one preferred embodiment the nonconductive layer 61 might be a ceramic material such as an alumina substrate or the like.

A mounting block 66 may be provided to enable the piezoelectric device 20 to be attached and removed from the stud 63. The material for the mounting block 66 is chosen to be a good conductor of acoustic signals. In one embodiment the mounting block 66 may comprise a short length of metal rod, for instance brass, copper or steel.

Also shown in figure 4 are electrical connections 69 to the piezoelectric device 20. In a further embodiment (not shown) the mounting block 66 may be adapted such that it surrounds the whole of the remainder of the piezoelectric device 20 in order to provide mechanical protection and also to provide strain relief to the electrical connections 69. Additionally, such an arrangement may provide a measure of electrical screening for the connections 69.

Turning now to figure 5, a further arrangement 70 for attachment of a piezoelectric device 20 to a conductor 10 is shown. The arrangement 70 is adapted to vary the angle of incidence 78 of the acoustic signal on to the conductor 10. This is advantageous as it allows the excitation and reception of different acoustic modes within the conductor 10.

In arrangement 70, a nonconductive layer 71 is provided between the piezoelectric device 20 and the conductor 10. The nonconductive layer 71 is formed from a material chosen to act as an electrical insulator but also to readily allow the propagation of acoustic signals. This provides galvanic isolation to the piezoelectric device 20 both as a safety measure in the event of an earth fault and to reduce electrical interference present in the substation coupling on to the acoustic measurements. In one preferred embodiment the nonconductive layer 71 might be a ceramic material such as an alumina substrate or the like.

The piezo electric device 20 is attached to mounting block 72 by use of a suitable adhesive. The adhesive is selected to have good adhesion properties and also to readily conduct acoustic signals. In one preferred embodiment the adhesive may be a cyanoacrylate which has the advantage of rapid deployment. Also shown in figure 5 are electrical connections 79 to the piezoelectric device 20.

Mounting block 72 is formed from a material that may be chosen to match the acoustic properties of the conductor 10, or alternatively may be chosen to have significantly different properties so that the direction of travel of acoustic waves crossing the boundary between the block 72 and conductor 10 is altered (i.e. the angle of incidence 78). This method can be advantageous in coupling certain types of acoustic signal between the conductor 10 and the piezoelectric device 20.

In one embodiment , the mounting block 72 may be held in place against the copper conductor 10 using a clamp arrangement 73 and provided with tightening bolt 74 similar in design to that described for figure 3 above. In an alternative embodiment (not shown) the mounting block 72 may be attached to the conductor 10 using a adhesive similar to the approach described in relation to figure 4 above.

Figure 6 shows another example of an arrangement 80 for attaching a piezoelectric device 20 to a conductor 10. In this figure, the piezoelectric device 20 is a piezoelectric transducer operable to act as both an acoustic sensor and or emitter and features of the piezoelectric device 20 are shown in greater detail. In this example the device 20 can be attached directly to the conductor 10 on the substation wall 11 using suitable adhesive. Typically, this might be cyanoacrylate adhesive resulting in a simple and quick means of deployment. The device 20 comprises piezoelectric material 21 sandwiched between two conducting plates or discs 87 held in place with an electrically conducting adhesive. In a preferred embodiment the plates/discs may be fabricated from brass, copper or steel. Electrical connections 89 are made to each plate/disc 87. In this example each plate/disc 87 includes a threaded hole to allow an electrical connection by way of a soldered lug held in place by a screw 88. The assembly includes an insulating plate/disc 81 to provide electrical isolation between the device 20 and the conductor 10. In a preferred embodiment the insulating plate/disc 81 is fabricated from ceramic and is held in place using cyanoacrylate adhesive. It will be obvious to one skilled in the art that the particular form of device 20 shown in figure 6 may be used as the piezoelectric device 20 in the arrangements 50, 60, 70 shown in figures 3 - 5. In each case, it is also possible for the particular arrangement of the piezoelectric material, the electrical connections and the insulating layer to be adapted to conform with the required or desired attachment arrangement 50, 60, 70, 80 as necessary.

Turning now to figure 7, a further alternative arrangement 90 for attaching a piezoelectric device 20 to a conductor 10 is shown. The arrangement 90 provides improved immunity to electrical noise pick up and acoustic noise pick up. As in figure 6, the device 20 comprises piezoelectric material 21 sandwiched between two conducting plates or discs 87 held in place with an electrically conducting adhesive. Electrical connections 89 are made to each plate/disc 87 through a threaded hole by way of a soldered lug held in place by a screw 88. An insulating plate/disc 81 provides electrical isolation and reduces electrical interference present in the substation coupling on to the acoustic measurements. The device 20 is provided within a conductive housing 99. In the example shown, the housing 99 comprises a cylindrical tube 91 provided with end caps 92, 93. The tube 91 and end caps 92, 93 can be made from any suitable materials. In a preferred embodiment the tube 91 and end caps 92, 93 may be made from copper as this is a good electrical conductor and so provides good electrical screening of the sensor. Cooper additionally has matched ultrasonic propagation speed to the typically copper earth conductor 10. This improves acoustic coupling. The housing 99 isolates the device 20 from background acoustic noise sources thereby drastically reducing the susceptibility of the device to interference from external noise sources. The housing 99 does not have to be tubular in design and in other embodiments could be different shapes. The cavity within the housing assembly may be left empty or it could be filled with some acoustic absorbing material that could further improve the rejection of unwanted external noise. The housing 99 may further be shaped so as to reflect or otherwise inhibit the transmission of external acoustic noise to the piezoelectric device 20. The housing 99 may be clad or coated (internally and/or externally) with an anechoic material or other means for dampening or impeding sound transmission so as to further acoustically isolate the piezoelectric device 20.

The connecting cable 95 (incorporating connecting wires 89) may enter the housing 99 through a gland 94 in one end cap 92. This arrangement offers mechanical strain relief. The screen 96 of the connecting cable 95 may be electrically connected to the housing to ensure electrical screening of the piezoelectric device 20.

The housing 99 can be held in place on the conductor 10 by any suitable means. In the example shown, straps 97 are adapted to loop around bracket 98 and conductor 10. As such, the straps 97 urge the housing 99 and conductor 10 into contact. The straps 97 may have some form of linkage or joining mechanism such as hooks (not shown) to allow them to be passed behind the conductor 10. The straps 97 are typically formed from a stretchable material.

It should of course be understood that the invention is not to be restricted to the details of the above embodiments which are described by way of example only.

Claims

1. A sensing system suitable for detecting tampering with or theft of an electrical conductor of an electricity substation or electricity distribution network, the system comprising: an integrity testing unit, said integrity testing unit being operable to conduct a succession of an integrity tests and output a value indicative of the instantaneous integrity in response to each test; a processing unit operable to: receive and store said outcome values; compare the outcome values of one or more selected tests to a reference value; and output an alarm signal if the difference between the outcome value and the reference value is greater than a preset threshold.

2. A sensing system as claimed in claim 1 wherein the integrity testing unit comprises one or more acoustic signal sensors mechanically attached to the conductor and operable so as to detect acoustic signals that are imparted into the conductor.

3. A sensing system as claimed in claim 1 wherein the integrity testing unit comprises one or more signal injection units operable to inject an acoustic signal onto the earth conductor and one or more acoustic signal sensors operable to detect acoustic signals on the conductor.

4. A sensing system as claimed in claim 3 wherein the or each signal injection unit and the or each acoustic signal sensor are mechanically attached to the conductor

5. A sensing system as claimed in claim 3 or claim 4 wherein the signal injection unit and the acoustic signal sensor are provided in a single acoustic transducer.

6. A sensing system as claimed in claim 3 or claim 4 wherein the signal injection unit and the acoustic signal sensor are provided at substantially the same point on the conductor.

7. A sensing system as claimed in claim 5 or claim 6 wherein the reflection of the injected signal from discontinuities in the conductor is monitored using time domain reflectometry (TDR) techniques.

8. A sensing system as claimed in claim 3 wherein the signal injection unit is provided at a first point on the conductor and the acoustic signal sensor is provided at a second point on the conductor.

9. A sensing system as claimed in claim 8 wherein the direct transmission of the injected signal is monitored.

10. A sensing system as claimed in any one of claims 2 to 9 wherein the or each acoustic signal sensor is provided with a signal conditioning unit operable to condition the detected acoustic signal.

11. A sensing system as claimed in any one of claims 2, or 4 to 10 wherein mechanical connection to the conductor is achieved by means of a clamping arrangement; adhesive; and/or a mounting stud.

12. A sensing system as claimed in any one of claims 2 to 11 wherein a nonconductive material is provided between the acoustic unit and the conductor.

13. A sensing system as claimed in claim 12 wherein the nonconductive material comprises a thin layer of substantially similar extent to the acoustic unit.

14. A sensing system as claimed in claim 12 or claim 13 wherein the nonconductive material comprises or is mounted on an angled block.

15. A sensing system as claimed in any one of claims 12 to 14 wherein the nonconductive material is adapted or selected to match the acoustic properties of the conductor and/or the sensor.

16. A sensing system as claimed in any one of claims 2 to 15 wherein the acoustic units comprise piezoelectric devices.

17. A sensing system as claimed in any one of claims 2 to 16 wherein the acoustic integrity testing unit is provided within an isolating housing.

18. A sensing system as claimed in claim 17 wherein the isolating housing is a conductive housing.

19. A sensing system as claimed in claim 1 wherein integrity testing unit comprises: a magnetic flux leakage unit; an eddy current testing unit; or a magnetic signal injection type unit.

20. A sensing system as claimed in any preceding claim wherein the testing interval is significantly shorter than the estimated time taken by a thief to remove an earth conductor.

21. A sensing system as claimed in any preceding claim wherein the preset threshold is defined as an absolute difference between outcome value and reference value.

22. A sensing system as claimed in any preceding claim wherein the preset threshold is defined as a relative difference between outcome value and reference value.

23. A sensing system as claimed in any preceding claim wherein the reference value is an earlier outcome value.

24. A sensing system as claimed in any one of claims 1 to 22 wherein the reference value is an average of selected preceding outcome values.

25. A sensing system as claimed in claim 24 wherein the average of preceding outcome values is: an overall average, an average over a present time interval or an average over a present number of tests.

26. A sensing system as claimed in any preceding claim wherein the reference value is a characteristic value of the conductor being monitored.

27. A sensing system as claimed in claim 26 wherein the characteristic value is established by prior testing of the earth or is established by an algorithm based on previous outcome values.

28. A sensing system as claimed in any preceding claim wherein the system is provided with a communication unit connected to the processing unit, the communication unit operable to communicate the alarm signal to one or more additional devices or systems.

29. A sensing system as claimed in claim 28 wherein the additional devices or systems include any one or more of: an external monitoring system; cameras (still or video; visible or infrared); illumination means or other visible alarm output means; loudspeakers or other audible alarm output means; microphones or the like.

30. A sensing system as claimed in claim 28 or claim 29 wherein the communication unit may be operable to communicate the alarm signal via a wired or wireless communication link.

31. A sensing system as claimed in any one of claims 28 to 30 wherein the triggering of additional devices or systems in response to the alarm signal is delayed or staggered.

32. A sensing system as claimed in any preceding claim wherein the system is provided with a data storage means.

33. A sensing system as claimed in any preceding claim wherein the communication means is operable to transmit successive outcome values and or data (for example but not limited to images or video) to an external monitoring system.

34. A sensing system as claimed in any preceding claim wherein the communication means is operable for the system to receive commands from an external controller.

35. A sensing system as claimed in any preceding claim wherein the system is powered by a mains supply.

36. A sensing system as claimed in any preceding claim wherein the system is powered by an internal battery.

37. A sensing system as claimed in any preceding claim wherein the system is powered fully or partially by scavenging energy from the conductor being monitored or nearby power conductors.

38. A method for detecting tampering with or theft of an electrical conductor of an electricity substation or electricity distribution network, the method comprising: conducting a succession of an integrity tests at regular intervals and outputting a value indicative of the instantaneous integrity in response to each test; storing said outcome values; comparing the outcome values of one or more selected tests to a reference value; and outputting an alarm signal if the difference between the outcome value and the reference value is greater than a preset threshold.

39. A method as claimed in claim 38 wherein the integrity tests are carried out by a passive acoustic integrity testing unit operable to detect acoustic signals that are imparted into the conductor.

40. A method as claimed in claim 38 wherein the integrity tests are carried out by an active acoustic integrity testing unit operable to inject an acoustic signal onto the earth conductor and to detect acoustic signals on the conductor.

41. A method as claimed in claim 40 wherein the reflection of the injected signal from discontinuities in the conductor is monitored using time domain reflectometry

(TDR) techniques.

42. A method as claimed in claim 40 wherein the direct transmission of the injected signal is monitored.

43. A method as claimed in claim 38 wherein the integrity tests are carried out by a magnetic integrity testing unit.

44. A method as claimed in any one of claims 38 to 43 wherein the testing interval is significantly shorter than the estimated time taken by a thief to remove an earth conductor.

45. A method as claimed in any one of claims 38 to 44 wherein the preset threshold is defined as an absolute difference between outcome value and reference value.

46. A method as claimed in any one of claims 38 to 45 wherein the preset threshold is defined as a relative difference between outcome value and reference value.

47. A method as claimed in any one of claims 38 to 46 wherein the reference value is an earlier outcome value.

48. A method as claimed in any one of claims 38 to 46 wherein the reference value is an average of selected preceding outcome values.

49. A method as claimed in any one of claims 38 to 48 wherein one or more additional devices or systems are triggered by the alarm signal.

50. A method as claimed in any one of claims 38 to 49 wherein the reference value is a characteristic value of the earth being monitored.

51. A method as claimed in claim 50 wherein the characteristic value is established by prior testing of the earth or is established by an algorithm based on previous outcome values.