WO2011015820A1

WO2011015820A1 - Current detector

Info

Publication number: WO2011015820A1
Application number: PCT/GB2010/001476
Authority: WO
Inventors: Graham Earp
Original assignee: Ea Technology Limited
Priority date: 2009-08-04
Filing date: 2010-08-04
Publication date: 2011-02-10
Also published as: GB0913571D0

Abstract

A current detector (10) comprises a loop of Rogowski coil (11) provided within a suitable protective cover and connected to an operator handset (100) incorporating a filter unit (110) and an indicator unit (120). In use, the loop of Rogowski coil (11) is fitted around a support structure (1) of an overhead electricity distribution network. Any leakage current flowing in the support (1) generates a corresponding signal in the coil (11), the coil signal being of the same frequency as the leakage current. The filter unit (110) is connected to the coil (11) and is operable to filter the coil signals such that only signals in a narrow band around a preset centre frequency are passed. Since leakage current is generally of a substantially equal frequency to the line frequency, if the preset frequency of the filter unit (110) is substantially equal to the line frequency, the filter unit (110) should only pass signals indicative of the existence of a leakage current derived from the line in the support (1).

Description

Current Detector

The present invention relates to a current detector and in particular to a non- contact current detector operable to detect weak alternating or time-varying currents, or the weak alternating component of direct currents. More particularly, the present invention relates to such a detector suitable for measuring earth currents flowing in supports (or earth conductor of supports) used on overhead electricity transmission and distribution lines.

HV insulation faults on supporting structures (including wood, concrete and composite poles) on the high voltage overhead electricity distribution network may under certain circumstances lead to the passage of a small leakage current to earth through the support. This might occur, for instance, if one or more of the insulators on the top of the support was cracked so as to permit the passage of an undesired leakage current from the live conductor to earth through the support. Normally, any such current is minor and will not cause the protection equipment fitted to the power line to trip out. Consequently the fault will go undetected. This situation is highly undesirable for a number of reasons. There is a danger that persons or animals that come into contact with the support may receive an electric shock. Under wet conditions (rain) it is likely the magnitude of the leakage current will increase and in some circumstances the electric shock could be severe enough to cause harm. In limited circumstances such as where there are stay wires or nearby trees, this might lead to some arcing or flashover between the support and the stay wires/trees, again with the risk that a potentially high voltage associated with the leakage current could manifest itself near the ground level where it could represent a danger to persons or animals. If left undetected it is likely that the faulty insulation on the support will continue to deteriorate further until it fails catastrophically, resulting in the failure of the power line and disruption to the electricity supply.

In alternative situations, such as where a wooden support is drying unevenly, an uneven voltage and leakage current distribution on the surface of the support can result. Where the current is more concentrated, more electrical power will be dissipated leading to localised heating and the formation of 'hot spots'. Under certain conditions (e.g. when fanned by a light wind) this can cause the timber of a wooden support to ignite giving rise to what is known as a pole top fire which will destroy the support.

In addition to these potential dangers posed by leakage currents, it is highly desirable to know if there is a pole top fault before deciding to commence any live-line work (for example working wearing insulated gloves from a bucket truck). For safety reasons it is not permissible to perform live-line work on a section of line where there is any chance that the line itself may fail whilst the work is underway as this could be potentially dangerous for the linesmen involved.

Typically such leakage currents are of the order of milliamps. As such, these currents are too small to trip conventional protection equipment or fault detectors fitted to the line. Presently, there are no practical methods that can be used in the field to reliably detect the presence of such small currents within a support. Whilst it is possible to perform some voltage measurements at the top of the support using voltmeters mounted on long insulating rods called hot sticks, the results are usually inconclusive, the approach means resorting to specialised live line working methods to ensure safety, and the hot sticks and equipment are not easy to carry far (which is frequently a requirement in rural areas).

In other circumstances, it is desirable to detect a larger current (up to say a few amps) flowing in a conductor. In one such example this may be an earth conductor, securely fixed to the surface of a supporting structure for the overhead network. It may be desirable to measure the current flowing in the earth conductor in order to monitor the condition of the overhead line and support generally or to establish whether it is safe to carry out live line work. It may also be useful to measure earth conductor currents to enable a fault to be localised and repaired before a protection system trip (and so disruption to customers) occurs.

Often, the earth conductor is enclosed in a protective plastic or wooden sheath fixed to the structure. These protective features make it difficult to measure the current flowing in the earth conductor.

It is therefore an object of the present invention to provide a current detector that at least partially overcomes or alleviates the above problems.

According to a first aspect of the preset invention there is provided a current detector, suitable for detecting the presence of an alternating current or other type of time-varying current in a structure, the detector comprising: a Rogowski coil adapted to be positioned around the structure such that currents flowing in the structure generate a corresponding signal in the Rogowski coil; filter means connected to the Rogowski coil and operable to filter the signals from the Rogowski coil so as to pass only a preset band of frequencies related to an expected current frequency; and indicator means operable to indicate at least the presence or absence of passed signals in the preset band.

In this manner, if the passed frequency band is centred on the transmission frequency of an overhead electricity distribution network, the current detector of the present invention may be utilised to detect the presence of a leakage current in a support. In particular, the detector of the present invention may detect a small current, say of the order of 0.1 milliamps.

In particular, the invention provides a method of detecting the presence of leakage current in the support, whilst the conductors are still live, from the safety of the ground, without need for live line tools or procedures, without needing to approach the live conductors, using a sensor that can quickly and simply be clipped round the base of the support, and using a measuring device that is lightweight and easily portable for a person on foot.

Furthermore, the present invention, by measuring the total current in the structure and conductors passing within the coil, removes the need to gain access to an earth conductor to measure current flowing in an earth conductor attached to an overhead line support.

In some embodiments, the filter means is operable to filter the signals from the Rogowski coil so as to pass only preset frequency bands related to a set of expected current frequencies. In such an embodiment, it may be possible to further analyse the nature of any leakage current and use this information to diagnose the nature of the failure or the condition of the equipment (such as insulators, surge arresters, isolators and transformers) mounted on the structure. A differential amplifier may be connected between the Rogowski coil and the filter means. The differential amplifier may have a fixed gain or an adjustable gain. The differential amplifier may thus boost the signal output by the Rogowski coil. The differential action of the amplifier enables it to reject external interfering signals picked up from the environment, as these are coupled identically into the two terminals of the coil. In this manner, a detector of the present invention may more readily and reliably detect the presence of currents of the order of 0.1 milliamps.

Conventionally, a Rogowski coil is only used to measure large magnitude currents, typically of the order of tens to thousands of Amps. Furthermore, in conventional practice, the output of a Rogowski coil is connected directly to an integrator, since the output is the differential of the detected current. Since the present invention is concerned with the detection of the presence or absence of small currents rather than the accurate recreation of the current signal, an integrator is unnecessary and can be replaced with a differential amplifier which rejects interference and can have a higher sensitivity. Hence in this invention an unorthodox approach has been employed that sacrifices accurate measurement of the current waveform information in order to maximise the sensitivity of the coil for detecting the presence of currents at the pre-selected frequency or frequencies. The filter means may comprise one or more digital or analogue components. In the event that the filter means is digital it may comprise a digital signal processing unit. In one digital implementation of the invention, the detected current signal may be sampled at four times the desired current measurement frequency. Vector addition of the samples may then be used to obtain the component of the signal at the desired current measurement frequency.

In the event that the filter means is analogue, it may comprise a band-pass filter. The band-pass filter may be a Butterworth filter. In particular the Butterworth filter may be an 8^th order Butterworth filter. The Butterworth filter may comprise a switched capacitor integrated circuit. Preferably, the band-pass filter is adapted to pass a band centred on the transmission frequency of an electricity distribution line supported by the structure. As an example, if the detector is to be used on an overhead line of the UK overhead electricity distribution network, the band-pass filter may be centred on 50Hz and have a bandwidth of, say, 2Hz. Alternatively the filter means may comprise a phase-sensitive detector or a lock-in amplifier. Such components may be provided with a local reference oscillator running at the desired measurement frequency. Preferably, the local reference oscillator is a high-accuracy oscillator. In this manner, these components may be operable to detect only the part of the signals from the Rogowski coil which is at the same frequency as the reference signal.

The centre frequency and bandwidth of the filter means may be fixed. In some embodiments, the centre frequency and/or the bandwidth of the filter means may be adjustable. For adjustable centre frequency filter means, the centre frequency may be continuously adjustable or may be adjusted between one or more preset alternative centre frequencies. In an alternative embodiment, one or more filter means having alternative centre frequencies may be provided in parallel and switching means may be provided to switch between them. In such embodiments each filter means preferably has a fixed centre frequency and bandwidth. The switching means can thus be used to select a filter means having a desired centre frequency and bandwidth. In either embodiment the alternative centre frequencies may comprise one or more harmonics of a base centre frequency. In one preferred embodiment, the lowest selectable centre frequency corresponds to a distribution line transmission frequency and the other selectable centre frequencies correspond to harmonics of the line transmission frequency.

The centre frequency of the filter means or the filter means with the desired centre frequency may be selected using a suitable user actuable interface. Additionally or alternatively, the centre frequency of the filter means or the filter means with the desired centre frequency may be selected automatically by a suitable control unit. The control unit may comprise a suitable microprocessor.

In some embodiments, the control unit may be operable to sweep the centre frequency across a desired range of frequencies. Such operation can generate a current frequency spectrum. In such embodiments, the control unit may be operable to process the current frequency spectrum to provide an indication as to the how the detected current has been generated. This may enable a type of fault at, on or near the structure to be diagnosed from analysis of the frequency spectrum of the generated leakage current. Alternatively, it may allow the condition of equipment mounted at, on or near the structure to be assessed.

The filter means may further comprise one or more anti-aliasing filters. The arrangement of the anti-aliasing filters may be determined by the performance of the filter means and/or the differential amplifier. In one embodiment, the or each antialiasing filter may comprise a 1^st order RC filter.

The indicator means may take any suitable form. In some embodiments, this may be one or more indicator lights indicative of the present or absence of a detected current or of a detected current above a predetermined threshold. In other embodiments, the indicator means may comprise a display means operable to display one or more symbols indicative of the magnitude of the detected current. The display may be a numerical display, and may comprise a liquid crystal (LCD), LED or other suitable form of display. The indicator means may additionally or alternatively be operable to provide information relating to the centre frequency and/or bandwidth of the filter means. In embodiments incorporating a control unit, the display may be operable to display information related to the mode of operation of the control unit and/or the results of any processing carried out by the control unit. Additionally or alternatively, and especially in digital implementations, the RMS, rolling average or other statistics of the readings may be computed and displayed to the user. A rectifier may be provided to rectify the output of the filter means before it is passed to the indicator means. A low-pass filter may be provided to convert the rectified output to a steady DC signal. The voltage of the DC signal may then be measured to provide an indication of the magnitude of the detected current. The voltage may be measured by a voltmeter.

The low-pass filter may be a 1^st order RC filter. The low-pass filter may have a break frequency of say 2Hz or lower to provide a consistent reading from a fluctuating input current.

The structure may be an insulating support, or a partially insulating support, or a conductive support of an overhead electricity distribution network. Typically, the structure is an unearthed support such as a wooden, concrete or composite pole. Alternatively, the support may be an earthed support. If earthed, the support may be a metal, wooden, concrete or composite pole. In the present context and in practice, an insulating support allows at least some current flow, for instance, wood and concrete poles are partially conducting in practice although they would normally be described as insulating.

The Rogowski coil may comprise a flexible loop. The loop may be provided with a protective cover. Typically, this may be a flexible plastic protective cover.

The Rogowski coil may be adapted to be fitted around the structure. In the event that the structure is an unearthed support of an overhead electricity distribution line, the coil may be used to detect leakage current in the support. In the event that the support is an earthed support, the Rogowski coil may be used to measure earth current in the earth conductor.

The Rogowski coil may be screened. This can help to shield the coil from detecting other sources of current or electric field in the vicinity. This may include current carried or any electric field generated by overhead electricity distribution lines. The screening may be achieved by use of semiconducting tape.

The ends of the Rogowski coil may be provided with suitable connectors adapted to allow the coil to be connected to a dedicated filter unit incorporating the filter means. The filter unit may incorporate the indicator means. Alternatively, the indicator means may be provided in a separate indicator unit connected to the filter unit. The filter unit or the separate indicator unit may be provided with a communicator unit. The communicator unit may be operable to communicate data indicative of a detected current to remote equipment. This can be achieved by means of a suitable wired or wireless link. In a preferred embodiment, the filter unit and the indicator unit (and if appropriate the communicator unit) are all mounted within a protective housing to provide an operator handset. The operator handset may further incorporate a suitable power source.

The filter unit or the separate indicator unit may be provided with a suitable user interface. The user interface may enable the centre frequency and/or bandwidth of the filter means to be adjusted. In embodiments incorporating a control unit, the user interface may be operable to control the operation of the control unit and/or the information displayed by the indicator means.

The current detector may incorporate a calibration unit. The calibration unit enables the checking of the correct operation and calibration of the detector. The calibration unit may comprise a reference current source operable to output to a test probe an alternating current or a time varying current signal of known magnitude and frequency. The calibration unit may provide a fixed reference signal or may provide a reference signal of adjustable magnitude and/or frequency. The test probe may comprise a length of wire passed through the Rogowski coil. The operator can then check that the current detector is measuring the correct signal level by comparing the measured value with the known reference current produced by the reference source. The reference current may be output to the test probe via a suitable connection arrangement which may be a switch and/or a suitable electrical plug and socket arrangement.

According to a second aspect of the present invention there is provided a method of detecting the presence of an alternating current or other type of time varying current in a structure, the method comprising the steps of: positioning a

Rogowski coil around the structure; and filtering the output of the Rogowski coil to pass only a preset range of frequencies related to an expected current frequency.

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

The passed frequency range may be centred on the transmission frequency of an electricity distribution line supported by the structure. There may be more than one preset passed frequency range, each preset passed frequency range being related to a set of expected current frequencies. The centre frequency and/or the bandwidth of the or each passed frequency range may be adjustable. Additional or alternative centre frequencies may comprise one or more harmonics of a base centre frequency. A passed frequency range centre frequency may be swept across a desired range of frequencies. The filtered output at different frequencies or different frequency ranges may processed to provide an indication as to the how the detected current has been generated. This may allow a fault at, on or near the structure to be diagnosed and/or may allow the condition of equipment mounted at, on or near the structure to be assessed.

According to a third aspect of the present invention there is provided a method of monitoring an overhead electricity distribution network comprising the steps of: using a current detector according to the first aspect of the present invention to detect leakage current in one or more selected support structures of the network. According to a fourth aspect of the present invention there is provided a method of monitoring an overhead electricity distribution network by applying the method of the second aspect of the present invention to detect leakage current in one or more selected support structures of the network.

The methods of the third and fourth aspects of the present invention may incorporate any or all features of the first or second aspects of the present invention as are desired or as are appropriate.

The methods may be carried out by use of dedicated coils and filter units permanently fixed to one or more selected support structures of the network.

Additionally or alternatively the methods may be carried out by one or more portable dedicated filter units transported to one or more coils permanently fixed to one or more selected support structures of the network. As a further possibility the methods may be carried out by transporting a coil and a dedicated filter unit to one or more selected support structures of the network.

In order that the invention may be more clearly understood, one embodiment is described in more detail below, by way of example only, and with reference to the accompanying drawings:

Figure I shows a current detector according to the present invention fitted to a support of an overhead electricity distribution network for detection of a leakage current; and

Figure 2 is a schematic block diagram of a current detector according to the present invention.

Referring now to figure 1, a current detector 10 comprises a loop of Rogowski coil 11 provided within a suitable protective cover and connected to an operator handset 100 incorporating a filter unit 110 and an indicator unit 120. In use, the loop of Rogowski coil 11 is fitted around an unearthed support structure 1 of an overhead electricity distribution network. The unearthed support structure 1 may typically comprise a wooden, concrete or composite pole. The coil 11 may equally be deployed in the same way around a structure that is earthed or has an earth conductor attached to it lengthways such that the earth conductor passes within the coil 1 1 in the same way as the structure. Any leakage current flowing in the support 1 generates a corresponding signal in the coil 11 , the coil signal being of the same frequency as the leakage current. Typically, the coil 11 is screened, perhaps by use of semiconducting tape, to reduce the influence of unwanted signals outside the coil 11. The filter unit 110 is connected to the coil 11 and is operable to filter the coil signals such that only signals in a narrow band around a preset centre frequency are passed. Since leakage current is generally of a substantially equal frequency to the line frequency, if the preset frequency of the filter unit 1 10 is substantially equal to the line frequency, the filter unit 110 should only pass signals indicative of the existence of a leakage current derived from the line in the support 1.

The indicator unit 120 is connected to the filter unit 110. In the event that a signal is passed by the filter unit 110, the indicator unit 120 is operable to output a visual indication thereof. In this manner, the detector 10 may detect the presence of a leakage current in the support 1.

Turning now to figure 2, a schematic block diagram of the detector 10 is shown. The coil 11 is connected to a differential amplifier 101, which is in turn connected to the filter unit 110. In the arrangement shown, the filter unit comprises an anti-aliasing unit 115, a band-pass filter 114 and an oscillator 116. The anti- aliasing unit 115 shown comprises filters 111, 112, 113 and amplifiers 119, 118. The particular properties of the filters 111, 112, 113 and amplifiers 119, 118 are selected in view of the particular properties of the band-pass filter 114, Typically, the filters 111, 112, 113 are 1^st order RC filters.

The band-pass filter 114 shown is an 8^th order Butterworth filter. Such a filter may incorporate either precision resistors and capacitors or more preferably a switched capacitor integrated circuit. The centre frequency of the band-pass filter 114 is tuned with reference to the oscillator 116. In the case of a transmission line operating at 50Hz, the bandwidth passed by the filter 1 14 might be say 2Hz.

The indicator unit 120 incorporates a rectifier 121 which rectifies the signal passed by band-pass filter 114. A low-pass filter 122 is provided to convert the rectified signal to a DC signal. In the example shown, the filter 122 may have a break frequency of say 2Hz.

In the example shown, the output of filter 122 is input to a digital display voltmeter unit 123. This therefore displays a numerical value indicative of the magnitude of the signal detected in the coil and passed by band-pass filter 114. In alternative embodiments different forms of display may be provided. In a simpler display an indicator lamp may be lit only when the output of filter 122 exceeds a predetermined threshold. In a more complex display, an indication of the output of filter 122 may be provided numerically or graphically alongside additional information as to the configuration and/or operation of the detector 10. In the example shown, the operator handset 100 also incorporates a reference current source 124 operable to output a reference alternating/time varying current of known frequency and amplitude. To check the accuracy and correct operation of the current detector, the reference current is output to a test probe 125 comprising a length of conducting wire passed through the Rogowski coil 11. The test probe 125 is connected to the reference current source 124 by means of a suitable connection arrangement (not shown) such as a switch and/or electrical plug and socket connections on the operator handset 100. As an alternative to the detector 10 shown in figure 2, the filter 114 may be replaced with a tuneable band-pass filter. The tuneable filter can then be tuned to one or more different frequencies to thereby detect leakage currents of different frequencies. As a further possibility, the tuneable filter could be swept across a range of frequencies to provide a frequency spectrum of any detected leakage current. A variation on such an embodiment would incorporate a plurality of differently tuned band-pass filters in parallel, which may be switched into or out of use by a suitable switching unit. In each case, the rectifier 121 and turnover filter 122 may be suitably adapted (or additional such means may be provided) to deal with a signal at a different frequency.

In such embodiments, particular attention may be paid to harmonics of the line frequency. By measuring the relative magnitudes of leakage current at the line frequency and one or more harmonics, it is possible to determine the nature of a fault that has generated the leakage current. In such embodiments, a processing unit may be incorporated into the handset 100 along with a suitable user interface and a display providing information as to the configuration and/or operation of the detector 10.

In a further embodiment, any or all of the previously described signal processing features of the invention could be implemented by digitising the signal produced by coil 11 and performing some or all of the functions depicted in the block diagram of Figure 2 in software running on a digital signal processor (DSP microprocessor). As a further alternative, in place of the band pass filter 114 other equivalent filter means may be used such as a phase-sensitive detector or a lock-in amplifier. As a further possibility, the detector 10 may be provided with a communication means (not shown) mounted within the handset 100. The communication means can enable the output of the indicator means to be communicated via a wired or wireless communication link to external devices. The skilled man will of course appreciate that whilst the present invention has been primarily described in terms of detecting currents flowing in unearthed support structures of an overhead electricity distribution network, it may also be applied to detecting currents flowing in earth conductors of earthed support structures of such a network. The current detector 10 of the present invention is of particular use where the earth conductor is enclosed in a protective plastic or wooden sheath fixed to the structure as these protective features make it difficult to measure the current flowing in the earth conductor.

Detecting and measuring earth currents in earthed support structures can be useful where live line work is to be carried out. In some circumstances, it may be necessary to disconnect the earth conductor from an earthed supporting structure before carrying out live-line work. This is safe to do provided that the current in the earth conductor is small. If a substantial current is flowing then a dangerous voltage may occur when the conductor is cut and there is also a danger of electrical arcing occurring whilst attempting to break the electrical circuit. The current detector 10 of the present invention can be readily used to determine whether the current flowing in the earth conductor allows for safe disconnection. Detecting and measuring earth currents in earthed support structures can be useful in order to locate faults in the network, for example, in some networks power may be supplied to a given point via more than one route to provide redundancy. If a partial fault (e.g. the failure of one of the three power conductors) occurs on one of these supplies then an imbalance currents will flow between the earth connections (commonly fitted to supporting structures at the points where supplies are taken) on the two sides of the fault. This current depends on the loading of the supplies and is often less than the protection equipment trip current at times of low load. The current detector 10 of the present invention can be readily used to detect such currents and enable the fault to be localised and repaired before a protection system trip (and so disruption to customers) occurs.

Similarly, use of the current detector 10 of the present invention is not restricted to supports used on an electricity distribution network. It may used on any type of support carrying live conductors where leakage current through the support itself is undesirable and may need to be detected. Another example would be the overhead electrification systems used on railway lines.

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

Claims

1. A current detector, suitable for detecting the presence of an alternating current or other type of time-varying current in a structure, the detector comprising: a Rogowski coil adapted to be positioned around the structure such that currents flowing in the structure generate a corresponding signal in the Rogowski coil; filter means connected to the Rogowski coil and operable to filter the signals from the Rogowski coil so as to pass only a preset band of frequencies related to an expected current frequency; and indicator means operable to indicate at least the presence or absence of passed signals in the preset band.

2. A current detector as claimed in claim 1 wherein a differential amplifier is connected between the Rogowski coil and the filter means.

3. A current detector as claimed in claim 1 or claim 2 wherein the filter means comprises a digital signal processing unit. .

4. A current detector as claimed in claim 1 or claim 2 wherein the filter means comprises a band-pass filter.

5. A current detector as claimed in claim 1 or claim 2 wherein the filter means comprises a phase-sensitive detector or a lock-in amplifier.

6. A current detector as claimed in claim 5 wherein the phase-sensitive detector or the lock-in amplifier are provided with a local reference oscillator.

7. A current detector as claimed in any one of claims 3 to 6 wherein the filter means is adapted to pass a band centred on the transmission frequency of an electricity distribution line supported by the structure.

8. A current detector as claimed in any one of claims 3 to 7 wherein the centre frequency and bandwidth of the filter means are fixed.

9. A current detector as claimed in any one of claims 3 to 7 wherein the centre frequency and/or the bandwidth of the filter means are adjustable.

10. A current detector as claimed in claim 9 wherein the centre frequency is continuously adjustable or is adjusted between one or more preset alternative centre frequencies.

11. A current detector as claimed in claim 8 wherein one or more filter means are provided in parallel and switching means are provided to switch between them.

12. A current detector as claimed in claim 11 wherein each filter means has a fixed centre frequency and bandwidth.

13. A current detector as claimed in claim 10, claim 11 or claim 12 wherein the alternative centre frequencies comprise one or more harmonics of a base centre frequency.

14. A current detector as claimed in claim 13 wherein the centre frequency of the filter means or the filter means with the desired centre frequency is selected using a user actuable interface.

15. A current detector as claimed in claim 13 wherein the centre frequency of the filter means or the filter means with the desired centre frequency is selected automatically by a suitable control unit.

16. A current detector as claimed in claim 15 wherein the control unit is operable to sweep the centre frequency of the band pass filter across a desired range of frequencies.

17. A current detector as claimed in claim 15 or claim 16 wherein the control unit is operable to process the detected current at different frequencies or different frequency ranges to provide an indication as to the how the detected current has been generated.

18. A current detector as claimed in any preceding claim wherein the filter means further comprises one or more anti-aliasing filters.

19. A current detector as claimed in any preceding claim wherein the indicator means comprises one or more indicator lights indicative of the present or absence of a detected current or of a detected current above a predetermined threshold.

20. A current detector as claimed in any preceding claim wherein the indicator means comprises a display means operable to display one or more symbols indicative of the magnitude of the detected current.

21. A current detector as claimed in any preceding claim wherein the indicator means is operable to provide information relating to the centre frequency and/or bandwidth of the filter means.

22. A current detector as claimed in any preceding claim wherein the display is operable to display information related to the mode of operation of the control unit and/or the results of any processing carried out by the control unit.

23. A current detector as claimed in any preceding claim wherein a rectifier is provided to rectify the output of the filter means before it is passed to the indicator means.

24. A current detector as claimed in claim 23 wherein a low-pass filter is provided to convert the rectified output to a steady DC signal.

25. A current detector as claimed in claim 24 wherein the voltage of the DC signal is measured by a voltmeter to provide an indication of the magnitude of the detected current.

26. A current detector as claimed in any preceding claim wherein the structure is an insulating support, or a partially insulating support, or a conductive support of an overhead electricity distribution network.

27. A current detector as claimed in any preceding claim wherein the Rogowski coil comprises a flexible loop provided with a protective cover.

28. A current detector as claimed in any preceding claim wherein the Rogowski coil is screened.

29. A current detector as claimed in any preceding claim wherein the ends of the coil are provided with suitable connectors adapted to allow the coil to be connected to a dedicated filter unit incorporating the filter means.

30. A current detector as claimed in claim 29 wherein the filter unit incorporates the indicator means or the indicator means is provided in a separate indicator unit connected to the filter unit.

31. A current detector as claimed in claim 30 wherein the filter unit or the separate indicator unit are provided with a communicator unit operable to communicate data indicative of a detected current to remote equipment.

32. A current detector as claimed in claim 31 wherein the filter unit, indicator unit and the communicator unit are mounted within a protective housing to provide an operator handset.

33. A current detector as claimed in any preceding claim wherein the current detector incorporates a calibration unit comprising a reference current source operable to output to a test probe an alternating current or a time varying current signal of known magnitude and frequency.

34. A current detector as claimed in claim 33 wherein the test probe comprises a length of wire passed through the Rogowski coil.

35. A method of detecting the presence of an alternating current or other type of time varying current in a structure, the method comprising the steps of: positioning a Rogowski coil around the structure; and filtering the output of the Rogowski coil to pass only a preset range of frequencies related to an expected current frequency.

36. A method as claimed in claim 35 wherein the passed frequency range is centred on the transmission frequency of an electricity distribution line supported by the structure.

37. A method as claimed in claim 35 or claim 36 wherein there is more than one preset passed frequency range, each preset passed frequency range related to a set of expected current frequencies.

38. A method as claimed in claim 37 wherein the centre frequency and/or the bandwidth of the or each passed frequency range is adjustable.

39. A method as claimed in claim 37 or claim 38 wherein alternative centre frequencies comprise one or more harmonics of a base centre frequency.

40. A method as claimed in claim 38 wherein a passed frequency range centre frequency is swept across a desired range of frequencies.

41. A method as claimed in claim 39 or claim 40 wherein the filtered output at different frequencies or different frequency ranges is processed to provide an indication as to the how the detected current has been generated.

42. A method as claimed in claim 41 wherein a fault at, on or near the structure is diagnosed from the outcome of the processing.

43. A method as claimed in claim 41 wherein the condition of equipment mounted at, on or near the structure is assessed from the outcome of the processing.

44. A method of monitoring an overhead electricity distribution network comprising the steps of: using a current detector according to any one of claims 1 to 34 to detect leakage current in one or more selected support structures of the network.

45. A method of monitoring an overhead electricity distribution network by applying the method of any one of claims 35 to 43 to detect leakage current in one or more selected support structures of the network.

46. A method as claimed in claim 45 wherein the method is carried out by use of dedicated coils and filter units permanently fixed to one or more selected support structures of the network.

47. A method as claimed in claim 45 wherein the method is carried out by one or more portable dedicated filter units transported to one or more coils permanently fixed to one or more selected support structures of the network.

48. A method as claimed in claim 45 wherein the method is carried out by transporting a coil and a dedicated filter unit to one or more selected support structures of the network.