GB2601058A - A method and system for assessing the integrity of overhead power line conductors - Google Patents

Abstract

The integrity or condition of aluminium-based or aluminium alloy-based overhead power line conductors is assessed using a detector head to induce eddy currents in the conductor. A detector head 30 comprises a chamber 28 for receiving the power line conductor and a primary coil 40 surrounding the chamber. Applying an alternating current to the primary coil generates an alternating magnetic field within the chamber and induces eddy currents in the cable under examination. A secondary coil 42 detects a magnetic field response generated by eddy currents. A measure of the cross sectional area of the conductor is obtained which can indicate e.g. pits or cracks. The frequency of the AC may be varied to examine eddy currents at different depths. The detector head may attached to the overhead line and moved along the line using a wheel. The conductor may be a stranded aluminium cable.

Description

A METHOD AND SYSTEM FOR ASSESSING THE INTEGRITY OF OVERHEAD POWER LINE CONDUCTORS

TECHNICAL FIELD

The present disclosure relates to a method and system for assessing the integrity of overhead powerline conductors and in particular, but not exclusively, to a method and system for assessing the integrity of aluminium or aluminium-alloy overhead powerline conductors. Aspects of the invention relate to a method of assessing the integrity of overhead powerline conductors, to a detector head for assessing the integrity of overhead powerline conductors and to a detector system for assessing the integrity of overhead powerline conductors

BACKGROUND

Overhead electrical powerline conductors are used to transmit and distribute electricity across an electrical network or grid. Historically, overhead powerlines have been made from strands of aluminium and steel which are twisted together to form the conductor known as Aluminium Conductor Steel Reinforced (ACSR) conductors. These conductors worked well, however, over time these conductors are being replaced progressively by more modern aluminium-based equivalents. The replacement of overhead powerlines with aluminium-based equivalents has been gradually taking place over the last 30-40 years and it is currently estimated that more than 15% of the UK network is comprised of aluminium-based conductors.

Aluminium-based conductors include, but are not limited to, All Aluminium Alloy Conductor (AAAC), All Aluminium Conductor (AAC) and Aluminium Conductor Alloy reinforced (AGAR). These cables are comprised of aluminium or aluminium alloy wires, which are configured in concentric layers twisted in alternating helical-lay patterns.

These stranded conductors are available in different sizes, depending upon the amount of power that needs to be transported along the conductor. At the smallest, there may be one central wire with a single layer of six wires around the outside. The larger types have up to six layers, or more, of wires.

The cables are installed between support structures and suspended in the air at a safe height to allow energisation without risk to the infrastructure or population below. Furthermore, the conductors are often not insulated meaning the aluminium-based conductors are exposed to the external environment. Sustained exposure to the environment over time causes the wires of the cable to degrade as the aluminium alloy gradually reacts with the air and airborne pollutants to form aluminium-based compounds. Conversion of the base metal to corrosion products over time reduces the thickness of the metal within the wires which in turn reduces the mechanical performance of the wires and overall cable performance.

With sufficient loss of the overall bulk cross section, or the formation and advancement of cracks or pits, the cable can eventually break from a variety of mechanical and chemical mechanisms. In addition, such degradation can reduce the efficiency of the cable in terms of the electrical conductivity, which in turn can cause excessive heating and can accelerate the degradation process further. As the older of the aluminium-based conductors are reaching approximately 40 years of service a condition assessment is necessary to determine the expected future life in order to prevent unexpected failure of the cables.

Currently, the most common method for assessing the condition of aluminium-based conductors is to remove a piece of the conductor from the network and perform a range of destructive tests. This can be done either by removing and replacing a piece of the existing main conductor or by removing a small piece of connecting conductor that can be removed and replaced more easily. However, both approaches are undesirable as their destructive nature can bring significant disruption to the network and potentially to the power supply of customers. Furthermore, removing a small piece of connecting conductor can provide poor results due to the uncertainty that the connecting "jumper" is not representative of the actual load bearing conductor, so is not a fair test of condition.

The sample size that can be assessed using the aforementioned destructive tests is only a small fraction of the whole line, so the use of this information to support condition decisions involves considerable assumption that the entire line is equally degraded. As such, there is a risk that a portion of the line that has not been tested is significantly more pitted and degraded than the portion tested in the destructive test which can lead to inaccurate predictions in relation to the remaining operational life of the conductor.

There is therefore a need to develop a technique for accurately assessing the remaining operational life of aluminium based conductors without having to remove sections of the conductor for tests.

Historically, techniques and devices for determining steel cross-section loss or the loss of zinc protective coating on aluminium conductor steel reinforced (ACSR) conductors fall broadly into two categories, namely: permanent magnet techniques and alternating magnetic field techniques.

Permanent magnetic techniques There are many devices available which use strong permanent magnets to create a magnetic field within a steel conductor or cable. These originate from the detection of condition within wire ropes such as those used in mining hoist ropes. Such devices usually use a strong, stationary permanent magnet to create a magnetic field within the cable and then detect the changes in magnetic flux to distinguish local pitting or defects and loss of metallic area (cross-section) via a sensor or array of sensors.

Although cross sectional area detection is possible using permanent magnets, it is notably only possible on materials which have high magnetic permeability. For ACSR conductors, the steel strands have sufficient magnetic permeability and are the fundamental load bearing elements within the conductor. For aluminium-based conductors, there are no known techniques developed using permanent magnets, likely because of the low magnetic permeability of aluminium and aluminium alloys.

Alternating magnetic field techniques

By exciting a coil of conductive wire with an alternating electrical current, a similarly alternating magnetic field is produced. If a conductive material test piece is placed near to or within the coil, eddy currents are generated within the test material. These are opposing in nature, in accordance with Lenz's Law and are dependent upon the conductivity and magnetic permeability of the test material, therefore producing a detectable change in the coil's impedance.

This technique has been developed and used for the detection of steel corrosion in ACSR conductors since the 1980's. In this instance, the alternating magnetic field is usually supplied through a "primary" coil and the effects from the test piece detected on a "secondary" or "detection" coil. The original version of this technique was developed by the Central Electricity Generating Board (Now National Grid) in the UK.

US 4652823 describes a device designed as a tug, which pulls the system along the conductor, a passive unit, which houses the processing and communication electronics and a sensing head. The sensing head is a hollow cylinder, split into hinged halves.

When closed around a conductor, multiple coils of copper wire within the head form the two separate primary and secondary windings.

The purpose of the device of US 4652823 is to measure the remaining zinc galvanising which is routinely coated onto the steel wires in ACSR conductors. The measurement of this zinc thickness can be taken as an indicator of overall conductor condition, based on the principles of galvanic corrosion and protection. According to this, when the three metals which comprise ACSR conductors, aluminium, zinc and steel (which is predominantly iron-based) are in contact with each other in the presence of a suitable electrolyte, the corrosion of the zinc will take place preferentially, followed by the corrosion of the aluminium and then the steel. In practice, the contact between the three metals can vary due to the complex geometry of the twisted wires, alternating in directions with each layer, and the presence of an electrolyte will naturally vary in the open environment. But the principle that the zinc protects the steel has been proven through many years of observation, test and measurement to be true and to preserve the underlying steel strength.

To detect the zinc thickness, a high frequency alternating current provided to the primary coil creates an axial magnetic field which induces an electromagnetic response on the materials within the conductor. As a result, the secondary coil detects an impedance, which is interpreted as a voltage which is comprised of an "in-phase" and "quadrature" voltage. The in-phase voltage (P) voltage is a measure of the induced magnetic field resulting from the resistance of the test materials. The quadrature (Q) voltage is a measure of the inductive reactance of the steel due to its high magnetic permeability. As zinc has a negative magnetic permeability, the galvanising layer on the steel wires produces a "negative" Q voltage, which has the effect of reducing the magnetic flux detected in the secondary coil from the steel. The greater the thickness of zinc on the steel wires, the greater the "countering" effect detected. The measurement of the reactance from both the steel and the zinc enables the thickness of the galvanising layer can be determined.

US 20170010240 describes a device which uses a similar alternating magnetic field technique to evaluate the thickness of remaining zinc layer. The main differentiation from US 4652823 is the introduction of a 'Hybrid circuit' which is used to increase the sensitivity of the measurement.

All of the devices or techniques outlined above which use alternating magnetic field induction principles to assess the condition of overhead line conductors, are only for use on ACSR conductors and would not work on aluminium or aluminium-alloy conductor types. They are also all based on the principle of measuring an electromagnetic response from the steel and from the zinc and balancing the two responses to determine the thickness of the zinc layer as an indicator of conductor corrosion.

There are no devices, techniques or principles yet available for the measurement of aluminium and aluminium alloy loss of cross section or surface flaws in aluminium-based 20 conductors.

It is an aim of the present invention to address one or more of the disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a method of testing or determining the integrity of an aluminium-based or aluminium alloy-based overhead power line conductor, the method comprising: producing an alternating magnetic field; passing the alternating magnetic field through a section of the overhead power line so as to produce eddy currents within the section of the overhead power line; measuring a magnetic field created by the resulting eddy currents; and determining the integrity of the section of the overhead power line using the measured magnetic field created by the resulting eddy currents wherein determining the integrity of the section comprises determining an integrity parameter of the section of the overhead power line conductor indicative of a cross-sectional area of the section of the overhead power line conductor.

The method may be used with aluminium-based or aluminium alloy-based stranded conductors. Aluminium based or aluminium alloy stranded conductors comprise a plurality of aluminium strands or wires in a bundle or helically wound to form the conductor.

The method beneficially allows the integrity of a section of conductor to be assessed quickly without the requirement to remove a section of the conductor. As such, the method provides a non-destructive method of assessing the integrity of an aluminium or aluminium alloy conductor thereby minimising the disruption to the conductor, and therefore electrical network, when assessing the integrity of a conductor. Furthermore, the method advantageously allows degradation caused by weathering and fatigue to be detected within the conductor such that the remaining service life of a conductor may be accurately determined.

The method also beneficially allows areas of the conductor that are cracked, pitted or have experienced a loss of overall cross-sectional area to be detected. Thereby allowing small sections of the conductor to be identified as requiring repairing or replacing without the need to replace an entire length of conductor.

Examples of aluminium or aluminium alloy-based overhead powerline conductors that the aforementioned method may be used to assess include, but is not limited to, All Aluminium Alloy Conductors (AAAC), All Aluminium Conductors (AAC) and Aluminium Conductor Alloy reinforced (ACAR). The skilled reader will appreciate that other forms of aluminium or aluminium-alloy conductors may be assessed using the aforementioned method.

In an embodiment the step of determining the integrity may comprise determining the bulk integrity of the section of the overhead power line. The bulk integrity may be a cross-sectional area of the conductor. Determining the bulk integrity of the conductor may comprise checking for the presence of a loss of bulk thickness of the section of the conductor. This is beneficial as the cross-sectional area of the conductor may decrease over time due to weathering and fatigue thus it is desirable to monitor fluctuations in the cross-sectional area of the conductor to determine a remaining service life of the conductor.

In one embodiment the step of determining the integrity may comprise determining the surface integrity of the section of the overhead power line. Furthermore, determining the surface integrity may comprise checking for the presence of pits and/or cracks in the surface of the section of the overhead power line in dependence on the measured magnetic field. This is beneficial as defects in the surface of the conductor are at the furthest point from the longitudinal neutral axis of the conductor and thus bending stresses on the conductor are at a maximum at the surface of the conductor. As such, identifying stress concentrations such as cracks or pits in the surface of the conductor is beneficial such that the conductor may be repaired or replaced safely.

Determining the integrity may comprise calculating an integrity parameter which is proportional to the volume or cross-sectional area of the material present in the section of the overhead power line. The integrity parameter may be a value indicative of the geometrical properties of the conductor. For example, the integrity parameter may be indicative of one or more of: a reduction in the cross-sectional area of the conductor, a surface crack in the conductor and a pit in the conductor's surface. The integrity parameter may be compared with a reference or threshold parameter to determine the level of degradation of the structure of the conductor.

In one embodiment the method may comprise advancing the alternating magnetic field along a length of conductor. For example, the method may comprise: passing the alternating magnetic field through a second section of the overhead power line so as to produce second eddy currents within the second section of the overhead power line; measuring the magnetic field created by the resulting second eddy currents; and determining the integrity of the second section of the overhead power line using the measured magnetic field created by the resulting second eddy currents. Determining the integrity of the second section of the overhead power line comprises determining an integrity parameter of the second section of the overhead power line conductor, the integrity parameter of the second section of the overhead power line conductor being indicative of a cross-sectional area of the second section of the overhead power line conductor The method may further comprise comparing the magnetic field created by the resulting eddy currents with the magnetic field created by the resulting second eddy currents so as to determine the relative integrity of the section of the overhead power line and the second section of the overhead power line; wherein the relative integrity is indicative of the relative cross-sectional area of the section of the overhead power line and the second section of the overhead power line. This is beneficial as a length or span of a conductor may be assessed to determine variations in the integrity of the conductor along its length.

In one embodiment the method may comprise passing the alternating magnetic field through a section of a calibration piece of overhead power line conductor so as to produce calibration eddy currents within the calibration piece of overhead power line conductor; measuring the magnetic field created by the resulting calibration eddy currents; and comparing the magnetic field created by the resulting eddy currents with the magnetic field created by the resulting calibration eddy currents so as to determine the relative integrity of the section of the overhead power line and the calibration piece of overhead power line. The calibration piece may be a section of overhead powerline conductor known to have 100% integrity. Determining the relative integrity of the section of the overhead power line and the calibration piece of the overhead power line may comprise determining the relative cross-sectional area of the section of the overhead power line and the calibration piece of the overhead power line. Determining the relative cross-sectional area between the calibration piece and the section of the overhead conductor may be used to determine a level of degradation in the overhead power line conductor.

In an embodiment the method may comprise: producing the alternating magnetic field using a primary coil; and measuring the magnetic field created by the resulting eddy currents using a secondary coil. This is beneficial as an alternating current may be applied to the primary coil to generate a uniform alternating magnetic field which can be passed through the conductor. The secondary coil may then be used to detect the magnetic field generated by the eddy currents. The parameters and characteristics of the magnetic field generated by the eddy currents is indicative of the resistivity of the conductor which is in turn indicative of the geometrical shape of the conductor. Thus, the integrity of the conductor may be determined from the magnetic field response from the eddy currents.

The method may further comprise advancing the primary coil and the secondary coil along the overhead power line so as to determine the integrity of a length of the overhead power line conductor. The method may further comprise retracting the primary coil and secondary coil back along the length of conductor.

In an embodiment the method may comprise reversing or generating an alternating magnetic field having a reverse direction of the alternating field at opposing end regions of the primary coil. This is beneficial as the reversed alternating magnetic field at the opposing end regions of the primary coil acts to restrict the alternating magnetic field to a central portion of the primary coil. As such, the method may comprise generating a substantially uniform magnetic field between the opposing end regions of the primary coil. This is beneficial as the substantially uniform alternating magnetic field invokes a consistent or predictable eddy current response in the section of conductor thereby improving the accuracy of the variations detected in the magnetic field of the eddy currents.

The method may comprise measuring the magnetic field created by the resulting eddy currents at a central region of the primary coil. The method may comprise measuring the magnetic field of the eddy currents at a midpoint between the opposing ends of the primary coil. This is beneficial as the midpoint between the opposing ends of the primary coil may be considered to be the point having the most highly uniform magnetic field generated by the primary coil.

In an embodiment the alternating magnetic field generated by the primary coil may comprise a frequency and the method may comprise selecting the frequency in dependence on a target depth of penetration of the eddy current. The target depth of penetration is a depth of penetration measured radially from the surface of the conductor towards the central longitudinal axis of the conductor. Varying the frequency of the alternating magnetic field may vary the volume of material within the conductor excited by the alternating magnetic field. The frequency may be selected in dependence on the region of the conductor that is to be assessed. For example, a relatively high frequency may be used to excite a surface region of the conductor such that the surface integrity may be determined or a relatively low frequency may be used to increase the depth of penetration to assess the cross-sectional area of the conductor.

The method may comprise determining the integrity of the conductor at a first region using a first frequency and subsequently determining the integrity of the conductor at a second region using a second frequency wherein the first region and second region are at different depths within the conductor. For example, the first region may be a surface region and the first frequency may be a relatively high frequency such that the surface integrity may be assessed. The second region may be a core region such that substantially the entire cross-sectional area of the conductor is excited by the alternating magnetic field using the relatively low frequency such that the overall cross-sectional area of the conductor may be assessed. The determined integrity of the section of power line at the first region may be indicative of a surface crack and/or pit and the determined integrity of the power line at the second region may be indicative of the cross-sectional are of the section of the power line.

In one embodiment the frequency may be between approximately 0.5 MHz to approximately 1.5 MHz. This frequency range may be considered to be the relatively high frequency used to excite the surface region of the section of conductor. Optionally the frequency may be, approximately 0.8 MHz to approximately 1.2 MHz.

In another embodiment the frequency may be between approximately 10 kHz to approximately 200 kHz. This frequency range may be considered to be the relatively low frequency range used to excite substantially entire cross-sectional area of the section of conductor. The frequency may be approximately 50 kHz to approximately 100 kHz, and further optionally, approximately 60 kHz to approximately 80 kHz.

In one embodiment the primary coil may have a length of approximately 50 mm to approximately 300 mm between the opposing ends of the primary coil. Optionally, the length may be approximately 100 mm to approximately 250 mm, and further optionally, approximately 150 mm to approximately 200 mm. The primary coil may have an inner diameter of approximately 20 mm to approximately 60 mm, optionally: approximately 20 mm to 30 mm, preferably approximately 24 mm; or approximately 30 mm to 40 mm, preferably approximately 35 mm; or approximately 45 mm to 55 mm, preferably approximately 50 mm.

In an embodiment the secondary coil may be a pick-up wire having about five turns or fewer. For example, the pick-up wire may comprise a single turn. In another embodiment the secondary coil may have an inner diameter of approximately 20 mm to approximately 60 mm, optionally: approximately 20 mm to 30 mm, preferably approximately 24 mm; or approximately 30 mm to 40 mm, preferably approximately 35 mm; or approximately 45 mm to 55 mm, preferably approximately 50 mm.

According to a further aspect of the present invention there is provided a detector head for determining the integrity of an aluminium-based or aluminium alloy-based overhead power line conductor, the detector head comprises: a chamber for receiving the power line conductor; a primary coil surrounding at least a portion of the chamber such that applying an alternating current to the primary coil generates an alternating magnetic field within the chamber; and a secondary coil surrounding at least a portion of the chamber; wherein the secondary coil is configured to detect a magnetic field generated by eddy currents created within the conductor when the conductor is positioned within the chamber by the alternating magnetic field such that the integrity of the conductor may be determined in dependence on the detected magnetic field response; wherein the integrity is indicative of a cross-sectional area of the conductor.

The chamber may be a longitudinally extending chamber or aperture for receiving the conductor. The conductor may extend through the chamber such that the conductor protrudes from opposing sides of the chamber or aperture. An air gap may be provided between the surface of the conductor and the inner walls of the chamber to allow the chamber to be moved longitudinally relative to the conductor such that a length of conductor may be assessed by the conductor head.

In an embodiment the secondary coil may be positioned between opposing end regions of the primary coil. The primary coil may comprise a plurality of turns arranged perpendicularly to a longitudinal axis of the conductor and/or chamber. The secondary coil may comprise one or more turns arranged substantially parallel to the plurality of turns of the primary coil In an embodiment one or more turns of the primary coil located at the opposing end regions of the primary coil may be reversed such that the alternating magnetic field generated by the primary coil is reversed at the opposing end regions. The number of reversed turns of the primary coil may be equal to half the total turns in the primary coil such that the number of reversed turns is equal to the number of forward turns. This is beneficial as the alternating magnetic field generated by the reversed turns may be equal and opposite to the alternating magnetic field generated by the forward turns.

In one embodiment the secondary coil may be positioned substantially at a mid-point between the opposing end regions of the primary coil. This is beneficial as the mid-point has the most highly uniform alternating magnetic field.

In another embodiment the secondary coil may be a pickup wire surrounding at least a portion of the chamber. The pickup wire may be configured to detect an impedance indicative of the magnetic field generated by the eddy currents. The impedance may be interpreted as a voltage and the integrity of the conductor may be determined from the detected impedance. The pickup wire may comprise, for example, five turns or fewer. In an embodiment the pickup wire may comprise a single turn or be a single piece of wire extending around at least a substantial section of the chamber.

According to a yet further aspect of the present invention there is provided a detector system for determining the integrity of an aluminium-based or aluminium alloy-based overhead power line conductor, the detector system comprising: a detector head according to any one of the aforementioned aspects or embodiments; and a processor configured to receive data indicative of the detected magnetic field and to determine, in dependence on the received data, an integrity parameter of the conductor wherein the integrity parameter is indicative of the cross-sectional area of the conductor. The data may be a signal indicative of the magnetic field created by the eddy currents and detected by the secondary coil.

The detector system may further comprise an alternating current source connected to the primary coil. The processor may be configured to vary the frequency of the alternating current source. The processor may be configured to control the frequency of the alternating current source such that a depth of penetration of the alternating magnetic field within the conductor may be varied in dependence on the frequency of the alternating current source.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a detector system according to an embodiment of the invention being used to assess the integrity of a conductor suspended between two adjacent towers; Figure 2 is a perspective view of the detector system of Figure 1 suitable for use with an embodiment of the invention; Figure 3 is a block diagram of the detector system of Figure 1; Figure 4 is a schematic cross-sectional plan view of the conductor located within a primary coil of the detector head of the detector system of Figure 1; Figure 5 is a perspective schematic view of the primary coil of Figure 4; Figure 6 is a plan schematic view of the primary coil of Figure 4 and the alternating magnetic field generated by the primary coil; Figure 7 is a diagram showing example fluctuations in the integrity of a conductor along the span of the conductor; and Figure 8 is a flow chart outlining a method of assessing the integrity of a conductor according to an embodiment of the invention.

DETAILED DESCRIPTION

In general terms embodiments of the invention relate to a method and system for assessing the integrity of overhead electrical power line conductors of the type which are predominantly aluminium or aluminium alloy based stranded conductors, installed on electricity distribution and transmission networks. The method comprises generating an alternating magnetic field by passing an alternating current through a primary coil or winding. The alternating magnetic field is passed through a section of the overhead power line that is to be assessed. Passing the alternating magnetic field through the power line produces eddy currents within the section of the overhead power line. The eddy currents create a magnetic field response which may be measured to determine geometrical parameters of the cable indicative of the integrity of the section of the overhead conductor.

This method of assessing aluminium or aluminium-alloy based stranded overhead power line conductors advantageously allows parameters, such as the integrity, level of degradation or resistance of the lines to be determined without the requirement to remove a section of the conductor. As such, this method may be considered to be a non-destructive method for determining parameters of the conductor without the requirement of removing a portion of the conductor for testing. This beneficially minimises the disruption to the electrical network and allows the integrity of the conductors to be assessed quickly.

To place embodiments of the invention in a suitable context reference will firstly be made to Figure 1 which shows two electricity towers 12 with three overhead power line conductors 10 suspended between them. The conductors 10 are aluminium or aluminium-alloy based conductors 10 comprising aluminium or aluminium alloy wires or strands that form the conductor 10. A detector system 14 is mounted on one of the conductors 10 to assess the integrity of that conductor 10 without the need to remove a section of the conductor 10 for analysis.

Turning now to Figure 2 there is shown a schematic perspective view of a detector system 14 suitable for use with embodiments of the invention. The detector system 14 comprises a housing 20 which may be mounted on a conductor 10 to assess the integrity of a section of conductor 10. The housing 20 comprises a drive wheel 21 for moving or advancing the detector housing 20 along the conductor 10. The detector housing 20 comprises a pair of guides 24 which may engage the conductor 10 and guide the detector housing 20 along the conductor 10.

The detector housing 20 may be hinged or formed from two parts (not shown) to facilitate attachment of the housing 20 to the conductor 10. A pair of clips 23 secure the housing 20 together such that the conductor 10 is received within the central aperture 28 or chamber of the housing 20. The housing 20 may then be advanced, in either a forward or rearward direction, along the conductor 10 such that the entire length of the conductor 10 between the towers 12 may be assessed. When the housing 20 reaches a tower 12 it may be removed from the conductor 10 and reattached on the opposing side of the tower 12. In this manner the skilled reader will appreciate that the entire length of a conductor 10 may be assessed.

Figure 3 shows a block diagram of the detector system 14. As shown in Figure 3 the detector system 14 comprises a detector head 30 coupled to a control module 32. The detector head 30 comprises a primary coil (not shown in Figure 3) for generating an alternating magnetic field and a secondary coil or pick-up wire (not shown in Figure 3) for detecting the magnetic field response generated by eddy currents within the conductor 10 caused by the alternating magnetic field from the primary coil. The detector head 30 may further comprise an alternating current (AC) source for powering the primary coil. The control module 32 is configured to receive data from the detector head 30 and to determine, in dependence on the received data, the integrity of the conductor 10.

The control module 32 is further coupled to a communication module 34 and a motor unit 36. The motor unit 36 may comprise the drive wheel 21 to control the position of the detector housing 20 on the conductor 10. The control module 32 may determine the position of the detector housing 20 on the conductor 10 such that the data generated by the detector head 30 may be plotted against a position on the conductor 10. The communication module 34 may comprise a transmitter and receiver configured to transmit data wirelessly to a user of the detector system 14. This is beneficial as it may allow the user to control the position of the detector system 14 on the conductor 14 and also may allow the user to view the integrity of the conductor 10 in real-time as the conductor 10 is assessed.

Turning now to Figure 4, the detector head 30 is shown schematically in further detail. As mentioned above, the detector head 30 comprises a primary coil 40 and a secondary coil 42. The conductor 10 may be received within the aperture 28 such that the primary coil 40 surrounds and extends longitudinally along a section of conductor 10. The primary coil 10 is coupled to an alternating current power source 44 to provide an alternating current to the primary coil 10. The primary coil 40 creates an alternating magnetic field within chamber or aperture 28 which passes through the conductor 10 when the conductor 10 is received within the aperture 28. The magnetic field generated by the primary coil 24 is a substantially uniform magnetic field that extends longitudinally along the length of the aperture 28 and thus section of conductor 10.

When the conductor 10 is positioned within the aperture 28 of the detector head 30 the alternating magnetic field produced by the primary coil 24 passes through the conductor 10. Passing the alternating magnetic field through the conductor 10 induces eddy currents within the conductor 10. Each eddy current created within the conductor 10 has its own magnetic field response. The characteristics of the magnetic field produced by the eddy currents is dependent on the resistivity of the conductor 10 at that given point. Furthermore, the resistivity of the conductor 10 at a given point along the conductor 10 is dependent on the geometry of the conductor 10. As such, variations in the geometry of the conductor 10, and thus variations in the magnetic field produced by the eddy currents, are indicative of the integrity of the conductor 10 at a given point. For example, variations in the magnetic field produced by the eddy currents may be used to determine a cross-sectional area loss, surface cracks, pits or general fatigue of the conductor 10.

As shown in Figure 4, the detector head 30 comprises a secondary coil 42 positioned in a central region of the primary coil 40. The secondary coil 42 is configured to detect the secondary magnetic field generated by the eddy currents within the conductor 10. The secondary coil 42 detects the impedance of the secondary magnetic field as a voltage within the secondary coil 42. The waveform of the voltage detected by the secondary coil 42 may be input to the control module 32 and processed to determine integrity of the conductor 10 by determining the presence of, for example, a localised loss of section, such as a pit or crack in the surface of the conductor 10, or a larger loss of cross-section within the conductor 10.

Positioning the secondary coil 42 in a central region of the primary coil 40 is beneficial as it is where the magnetic field produced by the primary coil 40 is considered to be the most highly uniform. This in turn is the point in which the magnetic field generated by eddy currents within the conductor 10 is most predictably generated. Thus, fluctuations in the magnetic field response generated by the eddy currents may be used to accurately infer geometrical fluctuations in the conductor 10.

The detector system 14 is configured to detect both fluctuations in the cross-sectional area of the conductor 10 and the presence of surface pits or cracks. The magnetic field generated by the eddy currents in the conductor 10 causes an impedance response in the secondary coil 42. The impedance may be interpreted or measured as a voltage response which may be input to the control module 32 to determine the integrity of the conductor 10 at a given point. The voltage response is converted into a section thickness or pit/crack identification based on a predetermined calibration curve stored within the control module 32. For example, the control module 32 may compare the received voltage response with a calibration curve to determine the nature of the geometrical fluctuation in the conductor 10 such that the control module 32 may detect a surface crack in the conductor 10 or an overall loss in cross-sectional area.

The calibration curve stored within the control module 32 may be determined through experimentation for a given conductor type. For example, the control module 32 may comprise a series of calibration curves associated with the type of conductor 10 to be assessed. The calibration range may be determined through experimentation and sets the maximum and zero levels for the measurable volume for the conductor 10 under test.

For example, the calibration curve may include an expected magnetic response from the eddy currents induced in the conductor 10 by the primary coil 40. The calibration curve may be used to determine the integrity or degradation level of the conductor 10 by comparing the measured magnetic response generated by the eddy currents with the calibration curve. The control module 32 may comprise multiple calibration curves indicative of the type of conductor 10 and also the depth of penetration of the alternating magnetic field. For example, the control module 32 may comprise a calibration curve representative of a low depth of penetration and also a curve representative of a high depth of penetration. The resistance of an aluminium conductor 10 varies substantially linearly with respect to cross-sectional area.

The alternating current source 44 may be connected to the control module 32 such that the frequency of the current supplied to the primary coil 40 may be varied by the control module 32. This in turn varies the frequency of the alternating magnetic field generated by the primary coil 40. The frequency of the alternating current source 44 is selected by considering the desired or target depth of penetration of the eddy current within the conductor 10. Thus, the depth of penetration of the magnetic field within the conductor 10 may be varied by varying the frequency of the alternating current source. The depth of penetration of the eddy current is governed by the following equation: 1.

VVhere:d = standard depth of penetration (mm) f= test frequency (Hz) m = magnetic permeability (H/mm) s = electrical conductivity (c/cilACS) When assessing the integrity of a conductor 10 it is often desirable to assess the flaws nearest the surface of the conductor 10 to determine if there are any pits or cracks in the surface of the conductor 10. This is because surface flaws are at the furthest point from the neutral longitudinal axis 44 of the conductor 10 and thus oscillations and movements in the conductor 10 over its lifetime results in the maximum bending stresses on the conductor 10 being close to the surface of the conductor 10.

To assess a surface region of the conductor 10 a relatively high current frequency of between about 0.8MHz and 1.2MHz is used to charge the primary coil 40 which in turn generates an alternating magnetic field with the same frequency. This frequency range achieves a relatively low depth of penetration in most aluminium or aluminium alloy conductor types such that eddy currents are produced in the surface region of the conductor 10. Furthermore, this frequency range provides a high resolution of geometrical variations of the surface of the conductor 10 such that critical size cracks of between about 0.1mm and 1 mm and other surface flaws may be detected. This frequency range excites an external torus of the conductor such that the magnetic field induced by the eddy currents relates to the geometrical fluctuations of the outer torus of the conductor 10. Furthermore, this frequency range provides generally sufficient accuracy for assessing the overall cross-sectional area of the conductor 10.

The overall cross-sectional area of the conductor 10 may be assessed with greater precision by reducing the frequency of the alternating current source 44 thereby increasing the penetration of the alternating magnetic field. Increasing the penetration of the alternating magnetic field causes eddy currents to form deeper in the conductor 10 such that the secondary magnetic field produced by the eddy currents, and detected by the secondary coil 42, is deeper within the conductor 10. A relatively low frequency of between about 10kHz and 20kHz may be used to determine the condition of the conductor 10 at a greater through thickness depth. This beneficially allows the overall cross-sectional area of the conductor 10 to be accurately assessed. Generation of the eddy currents at a greater depth results in the excitation of a greater material volume in terms of generating eddy currents. As such, the magnetic response induced by the eddy currents is related to the full thickness of the conductor material rather than the external torus.

The curvature of the conductor 10 under typical stringing applications may vary depending on the tower type and size of conductor 10. Furthermore, the resistivity of each conductor type may vary which will affect the penetration depth. The skilled reader will understand that the frequency of the alternating current source may be adjusted to achieve a target penetration depth depending on the characteristics of a given conductor type.

As shown in Figure 4, the conductor 10 is positioned within the aperture 28 or chamber of the detector head 30 such that the primary coil 40 and secondary coil 42 surround at least a major portion of the aperture 28 and thus conductor 10. The aperture 28 is dimensioned such that the conductor 10 may be accommodated within the aperture 28 of the detector head 30. The ratio of the cross-sectional area of the aperture 28 and the conductor 10 is known as the fill factor. It is desirable to have a fill factor close to unity as a higher fill factor provides a greater response or sensitivity to variations in the detected magnetic field response generated by the eddy currents within the conductor 10.

However, in practice a fill-factor of unity is unachievable as the conductor 10 is a catenary or curved along its span between adjacent towers 12 and if the diameter of the conductor 10 equalled the diameter of the aperture 28 the detector head 30 would not be capable of being advanced along the conductor 10. Furthermore, an aged conductor may have defects such as bulges which cause variations in the diameter of the conductor 10. As such, in practice the fill factor may be about 0.3 or above. The fill factor may be selected in dependence on a target signal:noise ratio and also the length of the primary coil 40. Longer primary coils 40 improve the signal:noise ratio and as such allow the fill factor to be reduced. This is beneficial as the reduced fill factor, for example around 0.3, ensures that the catenary of relatively slack conductors 10 can pass through the aperture 28 of the detector head 30.

The detector system 14 may be advanced along a conductor 10 with a relatively high field frequency in the range of 0.8MHz and 1.2MHz to firstly assess the integrity of the surface of the conductor 10. If an operator wishes to subsequently accurately assess the overall cross-sectional area of the conductor 10 the field frequency may be reduced to a relatively low frequency of between about 50kHz and 100kHz and the detector system 14 may be retracted back along the length of conductor 10. Alternatively, the detector system 14 may be advanced along the conductor 10 at a relatively low frequency to firstly assess the overall cross-sectional area of the conductor 10 before subsequently assessing areas of the conductor 10 determined to have a reduced cross-sectional area using the high frequency to check for surface cracks or pits.

In a further embodiment, the detector system 14 may comprise two detector heads 30 each having primary coils 40 and secondary coils 42 such that the surface integrity and cross-sectional area of the conductor 10 can be accurately assessed by charging the primary coils 40 in each detector head 30 with a different field frequency.

In an embodiment the detector system 14 may comprise a single detector head 30 and the field frequency of the alternating magnetic field generated by the primary coil 40 may be switched between the high frequency range and the low frequency range as the detector head 30 is advanced along the conductor 10. As such, the surface of the conductor 10 may be assessed for cracks or pits and the overall cross-sectional area of the conductor 10 may be assessed as the detector head 30 is advanced along the length of the conductor 10. For example, the field frequency may be switched between the high frequency and the low frequency every 0.5 seconds such that the surface of the conductor 10 and the cross-sectional area of the conductor 10 may be assessed in a single pass of the detector head 10.

Furthermore, the frequency of the alternating magnetic field may be predominantly in the high frequency range such that the surface of the conductor 10 may be assessed in a relatively high resolution to detect cracks in the surface of the conductor 10. The frequency may be switched to the low frequency, for example, between about every 1cm to 20cm along the length of the conductor 10 such that the variation of the cross-sectional area of the conductor 10 may be estimated whilst the surface integrity of the conductor 10 may be assessed along substantially the entire length of the conductor 10.

The primary coil 40 and secondary coil 42 are designed in dependence on the resistance of the conductor 10 that is to be assessed by the detector system 14 in dependence on the following equation: VVhere R = Resistance of the conductor 10 (Q) r = resistivity of the conductor 10 (Om) I = length of the conductor 10 between opposing ends of the secondary coil 42 measured along the longitudinal axis 44 (m) a = cross-sectional area of the conductor 10 (m2) The resistivity of the conductor 10 and length of the conductor 10 between opposing ends of the secondary coil 42 are constants. As such, the measured resistance or variations in the measured resistance, indicative of the measured magnetic field response of the generated eddy currents, may be used to determine the cross-sectional area or variations in the cross-sectional area of the conductor 10. The length of the primary coil 40 along the longitudinal axis 44is typically about 50 -300mm in length, depending on the type of conductor 10 to be assessed.

Furthermore, the longitudinal length of the secondary coil 42 may be between about 1mm and 200mm, depending on the number of turns in the secondary coil 42 and diameter of the wire in the secondary coil 42. Conductors 10 are typically up to approximately 40mm in diameter and the inner diameter of the aperture 28 is typically about 50mm or less depending on the type of conductor to be assessed.

Turning now to Figure 5 a perspective view of the primary coil 40 is shown schematically.

The secondary coil 42 has been removed from Figure 5 for clarity. Figure 5 shows the relative direction of the AC current through the primary coil 40. As shown in Figure 5 the current is reversed at the opposing end regions 50 of the primary coil. Reversing the current within the primary coil 40 at the end regions 50 of the primary coil 40 also reverses the direction of the magnetic field at the end regions 50. Reversing the direction of the magnetic field at the end regions 50 of the primary coil 40 has the effect of restricting the magnetic field to the central portion 52 of the primary coil 40 thereby promoting the uniformity of the magnetic field generated by the primary coil 40 in the central portion 52. This is beneficial as the resultant uniform magnetic field generated by the primary coil 40 is uniform thereby increasing the accuracy of the secondary

magnetic field generated within the conductor 10.

The total number of turns carrying the reverse current in the end portions 50 is equal to the number of turns carrying the forward current in the central portion 52. For illustrative purposes, Figure 5 illustrates four turns carrying a reverse current in each end region 50 and eight turns carrying the forward current in the central portion 52. As such, the magnetic field generated by the reverse turns is substantially equal and opposite to the magnetic field generated by the forward turns thereby cancelling the magnetic field out in the end regions 50 of the primary coil 40. This has the effect of restricting the magnetic field generated by the primary coil 40 by the forward turns to the central region 52 of the primary coil 40 such that the resultant magnetic field is highly uniform in the central region 52.

Figure 6 is a schematic plan view of the primary coil 40 and the alternating magnetic field generated by the primary coil 40. As shown in Figure 6, the central portion 52 of the primary coil 40 has a substantially uniform magnetic field 60 that extends along the central longitudinal axis 44 of the aperture 28. The end region 50 of the primary coil 40 comprises a reverse current which generates an alternating magnetic field that opposes the magnetic field in the central portion 52 of the primary coil 40 thereby promoting uniformity of the magnetic field 60 generated within the central portion 52. This is beneficial as the secondary coil 42 (not shown in Figure 6) may detect fluctuations in the uniform magnetic field 60 caused by eddy currents generated within the conductor 10.

Figure 7 shows an example diagram outlining the variation in cross-sectional area of a conductor 10 tested by the detector system 14 along a span of conductor 10 between two towers 12. Line 60 shows where the cross-sectional area of the conductor 10 has degraded to below 100% of its theoretical value. Line 62 shows an example threshold value of 95% of the theoretical cross-sectional area of the conductor 10 and Line 64 shows an example threshold value of 80% of the theoretical cross-sectional area of the conductor 10. The threshold values may be set by a user of the detector system 14 and set accordingly for the type of conductor or the area the conductor 10 is in, for example, is the conductor 10 close to critical infrastructure such as a major road or railway crossing, and used to determine when a conductor 10 requires repair or replacing.

Figure 8 shows a flow chart outlining a method of testing an aluminium-based or aluminium alloy-based overhead power line conductor 10. In Step 101 an alternating magnetic field is produced. The alternating magnetic field may be produced by applying an alternating current to a primary coil 40 via an alternating current source 44. The primary coil 40 may be housed within a detector head 30.

In Step 102 the alternating magnetic field is passed through the section of the overhead powerline conductor 10 that is to be tested. Passing the alternating magnetic field through the section of the overhead powerline conductor 10 creates eddy currents in the section of conductor 10 that is to be assessed. The eddy currents generated by the alternating magnetic field vary in dependence on the resistance of the conductor 10 which is in turn indicative of the geometrical shape of the conductor 10 at a given location. Thus surface cracks, surface pits, reduction in cross-sectional area and general degradation or fatigue of the conductor 10 may be inferred from variations in the secondary magnetic field generated by the eddy currents resulting from an increase in the resistance of the conductor 10.

In Step 103 secondary magnetic field generated by the eddy currents is measured by a secondary coil. The secondary coil measures the secondary magnetic field as an impedance which is interpreted as a voltage. The waveform of the voltage detected by the secondary coil 42 may be input to a control module 32. Finally, in Step 104 the integrity of the section of the overhead powerline conductor 10 is determined in dependence on the measured magnetic field. The characteristics of the measured secondary magnetic field are processed by the control module 32 into results indicating the presence of either a localised loss of section, such as a pit or crack and/or a larger loss of cross-section. The control module 32 may determine the integrity of the conductor by comparing parameters of the detected secondary magnetic field with reference or calibration parameters. For example, the control module 32 may compare the received data with a calibration curve, a look-up table or the like.

The method may be preceded by calibrating the detector head 30. Calibrating the detector head 30 may comprise passing the alternating current through free space, when the conductor 10 is not present in the aperture 28 and measuring the response with the second coil 42. Furthermore, a test section of conductor 10 with a known cross-sectional area may be positioned within the aperture 28 such that the magnetic response from eddy currents generated within the calibration section of conductor 10 may be measured with the secondary coil 42. The calibration section may, for example, be a new section of conductor 10 having a no or minimal cross-sectional area losses or surface defects.

The control module 32 may compare the known test or calibration cases with known points on the calibration curve such that the control module 32 may calibrate the calibration curve prior to assessing the integrity of the conductor 10.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims (5)

1. CLAIMS1. A method of determining the integrity of a stranded aluminium-based or aluminium alloy-based overhead power line conductor, the method comprising:producing an alternating magnetic field;passing the alternating magnetic field through a section of the overhead power line so as to produce eddy currents within the section of the overhead power line; measuring a magnetic field created by the resulting eddy currents; and determining the integrity of the section of the overhead power line using the measured magnetic field created by the resulting eddy currents; wherein determining the integrity of the section comprises determining an integrity parameter of the section of the overhead power line conductor indicative of a cross-sectional area of the section of the overhead power line conductor.
2. The method of Claim 1, wherein the step of determining the integrity comprises determining the bulk integrity of the section of the overhead power line.
3. 3. The method of Claim 1 or 2, wherein the step of determining the integrity further comprises determining the surface integrity of the section of the overhead power line.
4. 4. The method of Claim 3, wherein determining the surface integrity comprises checking for the presence of pits and/or cracks in the surface of the section of the overhead power line in dependence on the measured magnetic field.
5. 5. The method of any preceding claim, wherein the integrity parameter is proportional to the cross-sectional area of the section of the overhead power line. 35 6. The method of any preceding claim, further comprising: advancing the alternating magnetic field along the overhead power line; passing the alternating magnetic field through a second section of the overhead power line so as to produce second eddy currents within the second section of the overhead power line; measuring the magnetic field created by the resulting second eddy currents; and determining the integrity of the second section of the overhead power line using the measured magnetic field created by the resulting second eddy currents; wherein determining the integrity of the second section of the overhead power line comprises determining an integrity parameter of the second section of the overhead power line conductor, the integrity parameter of the second section of the overhead power line conductor being indicacitve of a cross-sectional area of the second section of the overhead power line conductor.7. The method of Claim 6, further comprising comparing the magnetic field created by the resulting eddy currents with the magnetic field created by the resulting second eddy currents so as to determine the relative integrity of the section of the overhead power line and the second section of the overhead power line; wherein the relative integrity is indicative of the relative cross-sectional area of the section of the overhead power line and the second section of the overhead power line.8. The method of any preceding claim, further comprising: passing the alternating magnetic field through a section of a calibration piece of overhead power line so as to produce calibration eddy currents within the calibration piece of overhead power line; 9. 10. 11. 12. 13. 14.measuring the magnetic field created by the resulting calibration eddy currents; and comparing the magnetic field created by the resulting eddy currents with the magnetic field created by the resulting calibration eddy currents so as to determine the relative integrity of the section of the overhead power line and the calibration piece of overhead power line.The method of Claim 8, wherein determining the relative integrity of the section of the overhead power line and the calibration piece of the overhead power line comprises determining the relative cross-sectional area of the section of the overhead power line and the calibration piece of the overhead power line.The method of any preceding claim, comprising: producing the alternating magnetic field using a primary coil; and measuring the magnetic field created by the resulting eddy currents using a secondary coil.The method as claimed in Claim 10, comprising advancing the primary coil and the secondary coil along the section of the overhead power line so as to determine the integrity along a length of the section of the overhead power line.The method as claimed in Claim 10 or 11, comprising reversing the direction of the alternating field at opposing end regions of the primary coil.The method as claimed in Claim 12, comprising generating a substantially uniform magnetic field between the opposing end regions of the primary coil.The method as claimed in Claim 12 or 13, comprising measuring the magnetic field created by the eddy currents in the section of power line at a central region of the primary coil located between the opposing end regions of the primary coil. 15. 16. 17. 18. 19. 20. 21.The method as claimed in any one of Claims 10 to 14, wherein the alternating magnetic field generated by the primary coil comprises a frequency and wherein the method comprises selecting the frequency in dependence on a target depth of penetration of the eddy current.The method as claimed in Claim 15, wherein the method comprises determining the integrity of the power line at a first region using a first frequency and subsequently determining the integrity of the power line at a second region using a second frequency wherein the first region and second region are at different depths within the conductor.The method as claimed in Claim 16, wherein the determined integrity of the section of power line at the first region is indicative of a surface crack and/or pit and wherein the determined integrity of the power line at the second region is indicative of the cross-sectional area of the section of the power line.The method in any one of Claims 15 to 17, wherein the frequency is between approximately 0.5 MHz to approximately 1.5 MHz, optionally, approximately 0.8 MHz to approximately 1.2 MHz, and further optionally, approximately 0.9 MHz to approximately 1.1 MHz.The method in any one of Claims 15 to 17, wherein the frequency is between approximately 50 kHz to approximately 100 kHz, optionally, approximately 60 kHz to approximately 90 kHz, and further optionally, approximately 70 kHz to approximately 80 kHz.The method of any one of Claims 10 to 19, wherein the primary coil has a length of approximately 50 mm to approximately 300 mm, optionally, approximately 100 mm to approximately 250 mm, and further optionally, approximately 150 mm to approximately 200 mm.The method of any one of Claims 10 to 20, wherein the primary coil has an inner diameter of approximately 20 mm to approximately 60 mm, optionally: approximately 20 mm to 30 mm, preferably approximately 24 mm; or approximately 30 mm to 40 mm, preferably approximately 35 mm; or approximately 45 mm to 55 mm, preferably approximately 50 mm.22. The method of any one of Claims 10 to 21, wherein the secondary coil is a pick-up wire comprising five turns or fewer.23. The method of any one of Claims 10 to 22, wherein the secondary coil has an inner diameter of approximately 20 mm to approximately 60 mm, optionally: approximately 20 mm to 30 mm, preferably approximately 24 mm; or approximately 30 mm to 40 mm, preferably approximately 35 mm; Or approximately 45 mm to 55 mm, preferably approximately 50 mm.24. A detector head for determining the integrity of a stranded aluminium-based or aluminium alloy-based overhead power line conductor, the detector head comprising: a chamber for receiving the power line conductor; a primary coil surrounding at least a portion of the chamber such that applying an alternating current to the primary coil generates analternating magnetic field within the chamber; anda secondary coil surrounding at least a portion of the chamber; wherein the secondary coil is configured to detect a magnetic field response generated by eddy currents created within the power line by the alternating magnetic field when the conductor is positioned within the chamber such that the integrity of the conductor may be determined in dependence on the detected magnetic field response; wherein the integrity is indicative of a cross-sectional area of the conductor.25. A detector head as claimed in Claim 24, wherein the secondary coil is positioned between opposing end regions of the primary coil. 26. 27. 28. 29. 30. 31. 32.A detector head as claimed in Claim 24 or Claim 25, wherein the primary coil comprises a plurality of turns and wherein one or more turns at the opposing end regions of the primary coil are reversed such that the alternating magnetic field generated by the primary coil is reversed at the opposing end regions.A detector head as claimed in Claim 26, wherein the number of reversed turns is equal to half the total number of turns in the primary coil.A detector head as claimed in any one of Claims 24 to 27, wherein the secondary coil is positioned substantially at a mid-point between the opposing end regions of the primary coil.A detector head as claimed in any one of Claims 26 to 28, wherein the secondary coil is a pickup wire surrounding at least a portion of the chamber and wherein the pickup wire comprises five turns or fewer.A detector head as claimed in Claim 29, wherein the pickup wire comprises a single turn.A detector system for determining the integrity of an aluminium-based or aluminium alloy-based overhead power line conductor, the detector system comprising: a detector head according to any one of Claims 24 to 30-and a processor configured to receive data indicative of the detected magnetic field and to determine, in dependence on the received data, an integrity parameter of the power line; wherein the integrity parameter is indicative of the cross-sectional area of the conductor.A detector system as claimed in Claim 31, the detector system comprising: an alternating current source connected to the primary coil and being configured to provide an alternating current to the primary coil to generate the alternating magnetic field; the processor being configured to control a frequency of the alternating current source such that a depth of penetration of the alternating magnetic field within the conductor can be varied in dependence on the frequency of the alternating current source.