METHOD FOR ELECTRICAL MEASUREMENT AND USE OF A MEASURING TECHNOLOGY FOR DETERMINING THE WEAR STATE OF ELECTRICAL CONDITIONS, AS WELL AS A CABLE WEAR STATE MEASURING DEVICE

Description

[0001] The invention relates to a method for electrically measuring the wear condition of electrical cables, in particular the mechanical wear condition of insulated electrical conductors, as specified in the preamble of claim 1. Furthermore, the invention relates to a device according to claim 7 and a use of a measuring device according to claim 10.

[0002] Electrical cables are used in a wide variety of stationary and mobile installations, such as machines, systems, buildings, and devices. In many installation situations, electrical cables are subjected to considerable dynamic loads, which cause mechanical wear and ultimately lead to failure.

[0003] But not only mechanical wear is problematic, but also environmental influences, fatigue of insulation due to aging or wear due to regular or special stress as a power and / or signal cable.

[0004] A cable failure usually results in a malfunction of the affected installation. Malfunctions often cause considerable costs due to necessary repairs and, in some cases, downtime. If safety-critical installations are affected, malfunctions can potentially pose a threat to life and limb.

[0005] There is therefore considerable interest in checking the condition of cables in-situ and, based on the wear progression thus determined, being able to determine the expected remaining service life and, if necessary, a replacement time for the cable in question.

[0006] Common testing procedures for electrical cables are usually divided into cable testing and cable diagnostics.

[0007] Cable testing is used to determine the functionality of the electrical cable under test. For example, the non-destructive VLF (Very Low Frequency) testing technique, as taught in DE 44 37 355 C2 and DE 44 13 585 C2, can be used for cable testing.

[0008] Conventional cable diagnostics determine the current cable condition, particularly its aging state. Common non-destructive diagnostic methods include the OWTS method mentioned in DE 10 2013 008 968 A1 and the tan-delta method explained in DE 10 2015 202 597 B4. OWTS technology (OWTS = Oscillating Wave Test System) and the tan-delta method enable easy detection and localization of both aging-related weak points and damage to the electrical cable, sometimes exclusively for medium-voltage cables.

[0009] A common feature of known testing and diagnostic procedures is that they only determine the condition of the insulation(s) of the test object. The degree of wear of the current-carrying part of an electrical cable is only secondarily determined based on the condition of the insulation. A prognosis for the further service life of the electrical cable is based on the determined condition of the insulation(s).

[0010] If the electrical conductor inside an electrical cable wears out faster than the surrounding insulation, for example due to dynamic mechanical loads, the point of failure or the remaining service life cannot be reliably determined using the testing and diagnostic procedures described.

[0011] From US Pat. No. 6,242,924 B1, it is known that voids in electrical conductors can be electronically detected to automate quality assurance in the manufacture of computer chips. Layered conductors on the chips are only permitted for further manufacturing once sufficient electrical continuity has been verified. A skin effect is used to detect manufacturing defects that occur only once during the life cycle of the tested conductor during its manufacturing process. Changes over time are not addressed, nor is an electronic assessment of cables.

[0012] Based on the testing and diagnostic methods for cables shown, the object of the present invention is to propose a device and a method that enables an economically sensible and reliable determination of the wear condition of cables.

[0013] This object is achieved according to the invention by a cable wear condition measuring method according to claim 1, comprising its characterizing features. Using a skin effect, the surface roughness of the electrical lines or the electrical conductor of an electrical cable is measured within the scope of the invention. Based on the determined roughness values, a degree of wear of the tested conductor material is determined in this preferred embodiment. This object is also achieved by a cable wear condition measuring device according to claim 7 or by an inventive use of a measuring device according to claim 10. Advantageous embodiments are the subject of the respective subclaims.

[0014] Despite protection by insulation and despite soft materials, the studies on which the invention is based have shown that the surface roughness of metallic cables changes considerably under dynamic mechanical stress. The invention uses this to determine wear. The electrical conductors of electrical cables sometimes exhibited this effect clearly and, in a verifiable manner, increased over a period of mechanical stress. The invention identifies average surface roughness in the range of up to 2 µm as particularly relevant for assessing the degree of wear and for predicting a remaining service life. If electrical conductors of stressed cables had average surface roughness of 2 µm, a remaining service life of 10% to 20% could be classified as ready for replacement according to current findings. Engineers refer to a cable that is so stressed that it has become a risk as ready for discard.

[0015] The well-known skin effect causes currents to concentrate on the surfaces of electrical conductors. It occurs in electrical conductors through which high-frequency alternating currents flow. As the frequency increases, the current density of a conductor decreases in the interior of the conductor compared to the peripheral region. The skin effect is caused by eddy currents that limit the penetration depth of alternating magnetic fields. The resistance values ​​of conductors are therefore frequency-dependent and depend, among other things, on the amplitude of the magnetic field and the material properties of the conductor.

[0016] According to the invention, to measure wear, regardless of whether mechanical stress or usage stress has occurred without external influences, a signal in the form of a high-frequency alternating current is preferably introduced into the electrical conductor(s) of an electrical cable under test. Due to the skin effect, the current flow is concentrated on the surfaces of the electrical conductor under test. The penetration depth of the alternating magnetic fields is referred to as the skin depth and depends on the frequency of the introduced signal.

[0017] In a preferred embodiment, an impedance measuring device and an electrical measurement signal with a frequency between 800 and 1000 MHz are used. In this frequency range, for example, the electrical currents flow along the surface of the electrical conductor with a skin depth of approximately 2 µm, thus covering a relevant range for the inventive detection of the surface properties, in particular the surface roughness, which has been particularly evaluated by the invention to date. The description of the measurement signal, in particular the mention of the frequency range of 800 to 1000 MHz and the skin depth of 2 µm, do not represent a limitation of the invention, but merely describe a preferred embodiment.

[0018] According to the investigations underlying the invention, as the surface roughness of the conductors increases during the cable's service life, the resistance value of the area through which the current flows increases; the measured wear value changes accordingly.

[0019] In the monitoring of standard cables enabled by the invention, i.e., electrical cables that are not equipped with special additional test conductors, the measured values ​​determined using the method according to the invention are preferably recorded in a largely permanent monitoring system and compared with reference measured values. The reference measured values ​​represent a signal curve over the entire service life of an electrical cable. A comparison of the signal curve of the monitored electrical cable with a reference signal curve provides the desired conclusions regarding the degree of wear, the rate of wear, and the expected remaining service life.

[0020] In a further preferred embodiment of the method according to the invention, particularly applicable to electrical cables with integrated test conductors, instead of permanent monitoring, a cable diagnosis is carried out at freely definable intervals, for example monthly. The test specimen is subjected to the diagnostic signal according to the invention for a few seconds and simultaneously the resistance value or attenuation of the electrical cable is measured. In this method, the measurement result is compared with a reference value such as would occur when a brand-new, undamaged electrical cable of the same design and length is subjected to the signal. The reference value is determined either by a measurement immediately after the installation of a new electrical cable or by a calculation if the starting value can only be determined by simulation, for example because no stored sample or data sheet is available.

[0021] Both continuous monitoring and spot checks at regular intervals can be performed using the inventive methods during ongoing operation. Shutting down the line being tested is not necessary.

[0022] The method according to the invention is non-destructive, and the current intensity of the test or diagnostic signal is in the milliampere range.

[0023] The method according to the invention can be applied to all common electrical cables with metallic conductors. This includes, in particular, power and data cables, both with rigid metallic conductors and with metallic stranded conductors.

[0024] In a preferred embodiment, a device for performing the cable testing or cable diagnostics according to the invention consists of a signal generator, a crossover filter for introducing the test or diagnostic signal, connecting terminals or connectors for contacting the test object, a data logger for recording relevant data, in particular the RF resistance, at least one measured value display device, preferably optically or acoustically operating, and an interface for connecting the device to a network, a backup computer, or the like. Further preferred embodiments of the device can be equipped with fewer or more functions, depending on the desired scope of testing or diagnostics.

[0025] For flexible in-situ diagnostics or tests, the device components can be provided in a portable design with their own power supply, for example, via batteries. For parallel implementation of a measurement method according to the invention on multiple electrical conductors, the device must be equipped with a corresponding number of input and output interfaces for the test or diagnostic signals.

[0026] The invention is explained in detail below with reference to the drawings.

[0027] It shows Figure 1 - a perspective view of electrical cable sections in alphabetical order with a schematic representation of the increasing surface roughness of the electrical cable from 1A to 1C, for which the Figure 1D a realistic image for comparison with the state in 1A and the Figure 1Efor comparison with 1C, each with a section entered, which section in the Figures 1F (from 1D) and 1G (from 1E) are shown enlarged, Figure 2 - a schematic detailed view of a known structural change on the surface of an electrical conductor due to dynamic mechanical loading, Figure 3 - a plan view in axial direction of a conductor cross-section with schematically drawn penetration depth δ (skin depth) of a higher frequency electric current, for better recognition without the usual section hatching, Figure 4 - a schematic view of a production plant with a symbolic representation of the cable routing and a cable monitoring device integrated into the plant, Figure 5 - a diagram for visualising a signal curve of an exemplary cable monitoring according to the method according to the invention with a jump in the attenuation specified in dBW over the number of load cycles, and Figure 6 - a table to illustrate a relationship between the signal frequency and the skin depth = penetration depth when applying the method according to the invention.

[0028] Figure 1shows, in partial figures 1A to 1C, each in a perspective schematic view of the seemingly identical section of an electrical cable 1, wherein the mechanical load on the electrical cable 1 increases alphabetically from 1A to 1C. The electrical cable 1, which is subject to wear due to mechanical stress here, is shown according to a simple embodiment as consisting solely of an electrical conductor 2 with conventional, usually axially aligned, material- and / or manufacturing-related surface features 14, and an insulation 3 radially enclosing the conductor 2 on the outside. Possible bending directions 5 of the electrical cable 1 due to dynamic mechanical loads are indicated by arrows. Dynamic mechanical loads on metallic electrical conductors 2 lead, beginning at the surface of the electrical conductor 2, to structural changes in the metal structure of the electrical conductor 2.Such structural changes are reflected in an increasing surface roughness 4 of the electrical conductor 2.

[0029] Realistic images of such conductors 2 are shown in the further sub-figures 1D to 1G in pairwise comparison for comparison with the schematizations according to the Figures 1A us 1C. The realistic Figure 1D Compared to the schematically depicted state in Figure 1A, it also shows rather axially aligned depressions. Here, a certain degree of roughness is present, rather locally, due to material or manufacturing. In the macroscopic image ( Figure 1D ) Production-related drawing marks can be seen in the axial direction of a brand-new copper conductor. Figure 1F shows an enlarged (microscopic) section of the brand-new conductor and the grooves present therein. However, the dimensions are not representative. Figure 1EHowever, in comparison with Figure 1C, the accumulation of such surface changes indicated in Figures 1A to 1C is put into perspective. Rather than originating from local production-related phenomena, but possibly even independent of them, there is now no discernible pattern when looking at the Figure 1E with its enlarged section in Figure 1G a possibly another type of roughness is recognizable, and possibly even beyond that even in the non-visible radial outer area near the conductor surface 15, which in Figure 3 as a skin depth range is indicated.

[0030] In Figure 1 The increasing surface roughness 4 of an electrical conductor is symbolically represented using the schematic views A - C of sections of an electrical cable 1.

[0031] Figure 2shows, in a schematic detailed view as a greatly enlarged fragment, conceivable changes in a metal structure due to dynamic mechanical stress. The metal structure, consisting of grains 8, forms, upon dynamic mechanical stress, partial structural changes in the form of sliding zones 9, beginning at a surface 7 of the electrical cable 1. The material of the sliding zones 9 shifts relative to each other due to the dynamic mechanical stress, forming depressions referred to as intrusions 6 and elevations referred to as extrusions 10 relative to the surrounding material of the electrical conductor 2.

[0032] Relevant for a wear diagnosis according to a particular method according to the invention is the average distance 11 between the highest points of the extrusions 10 and the lowest points of the intrusions 6 at various locations over a certain length of an electrical conductor to be evaluated. If this average distance is 2 µm for a metallic electrical conductor 2, the electrical conductor 2 in question has completed approximately 80% of its service life and is therefore ready for discard. Therefore, significant wear has already been diagnosed, and a conclusion derived from the method according to the invention has been made.

[0033] But even in the case of extrusions or intrusions that are not as serious on the surface as would be expected from observation Figure 1G corresponds, a method according to the invention for assessing the surface or the peripheral area close to the surface allows a conclusion to be drawn about the degree of cable wear as a result of the invention.

[0034] Figure 3shows a schematic cross-sectional view of the electrical cable 1. The electrical conductor 2 and the insulation 3 surrounding it are clearly shown radially on the inside. A circumferential dashed line in the radially outer region of the circumference of the electrical conductor indicates a skin depth 15. Alternating currents are known to concentrate, depending on their frequency, in a certain area along the surface of the electrical conductor 2. The value for the thickness or depth of the area flowing through, measured from the surface of the electrical conductor 2 radially towards the center, is referred to as the skin depth 15. The method according to the invention particularly preferably uses alternating currents with a frequency of 800 to 1000 MHz, which reach a skin depth of approximately 2 µm and thus, according to a large number of tests on which the invention is based, cover a surface area of ​​an electrical conductor 2 that is relevant for a wear diagnosis according to the invention.

[0035] Figure 4 shows, in a schematic view, an exemplary production plant and, integrated therein, a first particularly preferred embodiment of a device according to the invention for measuring the mechanical wear condition of electrical cables 1. Symbolically represented are plant components 20 with, for example, a control cabinet 30 and electrical cables 1 connecting them.

[0036] In the Figure 4 In the embodiment shown, the device according to the invention consists of a measuring device 35 with an interface 40 for connecting an electronic system to a plurality of electrical cables 1 to be monitored.

[0037] The electrical cables 1 are shown only schematically to illustrate the functional principle and do not represent a limitation of the applicability of the method or device according to the invention.

[0038] The measuring device 35 includes as electronics in the embodiment shown a power supply 36, a signal generator 37, a signal evaluation 38 and a data logger 39. Not visible in Figure 4 shown, but nevertheless part of the preferred measuring device are acoustic or optical display components such as a screen and at least one connection option with external devices such as a network, a machine control system, a backup computer.

[0039] The measuring device 35 is in the Figure 4In the illustrated embodiment, the measuring device 35 is integrated as a stationary component into a control cabinet 30. In a second preferred embodiment (not shown), the measuring device 35 is designed as a mobile unit without its own control cabinet. The power supply of such a mobile measuring device can be either wired, via batteries, rechargeable batteries, or other suitable methods such as induction.

[0040] Not shown are further preferred embodiments of a measuring device according to the invention, which, depending on the required range of functions, can have fewer components or functions. For example, in the case of regular monitoring of an electrical cable 1, a data logger could be dispensed with, since the measured value acquisition can be done manually in this case. Likewise conceivable are further embodiments of a measuring device 35 according to the invention with additional functions. For connecting the measuring device 35 to the electrical cables 1, likewise signal lines 31 or power lines 32, which are subjected to wear monitoring according to the invention, Figure 4the interface 40 is shown. In a particularly preferred embodiment, the interface 40 is formed by crossover networks for introducing the test or diagnostic signals into the electrical cables 1, 31, 32 and connecting terminals or plug connections for the preferably separable physical connection of the electrical cables 1, 31, 32 to the measuring device 35. The device according to the invention can thus also be retrofitted to systems already in operation. Several methods according to the invention can often be applied in-situ. The electrical cables 1, 31, 32 do not need to be disconnected from the system or de-energized in order to connect a measuring device 35. The test or diagnostic signals to be introduced for monitoring or diagnostic purposes do not disrupt the current flows of power lines 32 or data lines 31. Monitoring and diagnostics can be carried out during ongoing operation of the respective electrical cables 1, 31, 32.

[0041] In the first particularly preferred embodiment shown according to Figure 4 An electrical cable 1, 31, 32 to be monitored is connected to the measuring device 35 via the interface 40. In the third particularly preferred embodiment, not shown here, an electrical cable 1 to be monitored is equipped with an interface at the beginning of the cable for feeding in the test or diagnostic signal from a signal generator of a measuring device and at the end of the cable with a further interface for feeding an output signal into a signal evaluation unit of a measuring device.

[0042] Figure 5shows, in diagram form, the signal curve of a test signal according to the invention in an exemplary wear monitoring of an electrical cable 1. The abscissa 50 of the diagram represents the movement cycles of a dynamic mechanical load on the monitored electrical cable 1. The ordinate shows the measured values ​​in dBW resulting from the evaluation of the test signal. The test signal is evaluated by comparing the input signal fed into the electrical cable 1 and the output signal emerging from the electrical cable 1. The reduction in the signal level of the output signal compared to the input signal is referred to as power level attenuation or power attenuation and is specified in decibels (dB). It is described as the ratio of a first power P1 (input signal) to a second power P2 (output signal) in logarithm to base 10. Thus, power attenuation is a relative and dimensionless quantity.

[0043] As can be seen from the measured value curve 52, there is a range of increasing damage 55, i.e. wear of the monitored electrical cable 1, running from approximately 24,000 to approximately 48,000 movement cycles.

[0044] From approximately 48,000 movement cycles, the measured value curve 52 signals the discard readiness 56 of the wear-monitored electrical cable 1 with further increasing negative measured values. At 57,600 movement cycles, the measured value curve 52 indicates the break 57 of the electrical cable 1 through a drastic change (break) in the measured values.

[0045] Figure 6shows, in tabular form, the change in penetration depth / skin depth of higher-frequency alternating currents as a function of the frequency of the alternating current. It can be seen that frequencies between 500 MHz and 1 GHz result in penetration depths of slightly more than 2 µm, thus completely detecting the approximately 2 µm surface layers of the electrical conductors of the electrical cables 1 to be monitored, which are relevant for wear monitoring according to the invention.

[0046] Possible application locations for the measuring devices according to the invention can be selected at various voltage levels of the general electrical power grid, with the generators and consumers often connected to them. Application is possible at any supply level. Furthermore, the system is used in all generator and consumer networks. Depending on the application, various coupling mechanisms for the measurement signal can be implemented. A direct, galvanic connection and / or capacitive and inductive devices are being developed for the coupling of measurement signals according to the invention.

[0047] In the European interconnected grid of the high and extra-high voltage level or in a metropolitan network infrastructure, the wear condition measuring device according to the invention is used at strategically important nodes of the grid to monitor the lines. List of reference symbols

[0048] 1Electrical cable 2Electrical conductor 3Insulation 4Surface roughness 5Bending direction 6Intrusion 7Surface 8Grain 9Slip zone 10Extrusion 11Mean distance 14Material- and / or manufacturing-related surface feature 15Skin depth 20System component 30Control cabinet 31Signal line 32Power line 35Measuring device 36Power supply 37Signal generator 38Signal evaluation 39Data logger 40Interface 50Abscissa 51Ordinate 52Measured value curve 55Area of ​​increasing damage 56Area of ​​discard 57Cable break A1 enlarged section A2 enlarged section

Claims

1. Cable state of wear measuring method for determining cable wear, characterized in that, in order to determine the wear of a cable (1), at least one metallic conductor (2) of the cable, which is not necessarily insulated, is measured in the case of section-by-section stripping or without stripping in a cable section affected by the measurement or at a cable measuring point with respect to at least one of its surface properties or with respect to properties of radially external conductor properties.

2. Cable state of wear measuring method according to Claim 1, characterized in that the change in the surface roughness of at least one portion of the at least one electrical conductor is recorded over time.

3. Cable state of wear measuring method according to Claim 1 or 2, characterized in that an alternating current is used as a measuring signal, the frequency of which is determined such that, using the skin effect, the measuring signal flows exclusively or at least predominantly through the surface areas of the electrical conductor to be tested that are affected by wear, and that an indication of the degree of wear of the electrical conductor is made on the basis of the evaluation of the impedance values.

4. Cable state of wear measuring method according to Claim 3, characterized in that the skin depth in the case of a copper conductor is about 2 µm and the measuring signal has a frequency of about 800 to 1000 MHz.

5. Cable state of wear measuring method according to any one of the preceding claims, characterized in that a resistance value of a cable portion through which electrical measuring current flows is measured.

6. Cable state of wear measuring method according to any one of the preceding claims, characterized in that the current intensity of the test signal is in the milliampere range.

7. Cable state of wear measuring device for carrying out a method according to any one of Claims 1 to 6.

8. Cable state of wear measuring device according to Claim 7, characterized in that a signal generator (37) and an impedance measuring unit are electrically connected to the electrical conductor to be assessed with respect to the surface roughness.

9. Cable state of wear measuring device according to Claim 7 or 8, characterized by its own energy supply unit (36) ,which allows it to be used as a transportable measuring device.

10. Use of a measuring unit (35) according to any one of Claims 7-9, which examines the surface or a radially outer material region near the surface of at least one conductor running in a cable in order to determine a state of wear of the cable.