CN114720808B - Nondestructive testing and positioning method for quality of conductor in middle section of cable - Google Patents
Nondestructive testing and positioning method for quality of conductor in middle section of cable Download PDFInfo
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- CN114720808B CN114720808B CN202210643903.6A CN202210643903A CN114720808B CN 114720808 B CN114720808 B CN 114720808B CN 202210643903 A CN202210643903 A CN 202210643903A CN 114720808 B CN114720808 B CN 114720808B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
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- G—PHYSICS
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- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
- G01B7/12—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring diameters
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/20—Metals
- G01N33/202—Constituents thereof
- G01N33/2028—Metallic constituents
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R23/00—Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
- G01R23/16—Spectrum analysis; Fourier analysis
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- G—PHYSICS
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R27/00—Arrangements for measuring resistance, reactance, impedance, or electric characteristics derived therefrom
- G01R27/02—Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant
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Abstract
The invention belongs to the technical field of cable nondestructive testing, and relates to a cable middle section conductor quality nondestructive studying, judging and positioning method, which comprises the steps of measuring an actually measured impedance amplitude spectrum at the head end of a cable, carrying out Hilbert transform on the actually measured impedance amplitude spectrum to obtain an imaginary part of the impedance amplitude spectrum, constructing a complex impedance amplitude spectrum, carrying out fast Fourier transform, and then carrying out normalization processing to obtain the distance between a problem section cable and the head end and the tail end; calculating the degree of conductivity change or radius reduction of the conductor of the cable at the problem section according to the resistance value of the conductor of the cable at the problem section, respectively calculating a theoretical impedance amplitude spectrum under the condition of conductivity change or radius reduction, extracting an impedance module value of a maximum value point as a feature vector, respectively calculating a feature vector deviation mean value of an actually measured impedance amplitude spectrum and the theoretical impedance amplitude spectrum, and judging whether the conductor material problem or the conductor radius reduction problem is solved according to the deviation mean value. The invention solves the problem that the quality problem of the conductor at the middle section of the cable is difficult to judge and position.
Description
Technical Field
The invention relates to a method for nondestructively studying, judging and positioning the quality of a conductor at the middle section of a cable, belonging to the technical field of nondestructive testing of cables.
Background
With the increase of the cabling rate of the power distribution network year by year, the cable consumption is larger and larger, the quality of the cable produced by a cable manufacturer is uneven, and the cable conductor is used as a core component of the cable, so that the cost-occupation ratio is high, and the quality problem is outstanding. At present, whether the conductor material is qualified or not is generally judged by cutting an end cable and measuring the direct current resistance of the conductor, but the sampling inspection generally can only reflect the quality condition of the detected sample. At present, in order to avoid quality detection, copper conductors with qualified quality and standard cross sections are used at the end parts of some manufacturers, aluminum conductors (namely 'aluminum replaces copper') or single wires are reduced in number (namely 'thick at two ends and thin at the middle') in the middle section of a cable, so that the cost is reduced, and the conventional detection cannot find the problems.
In recent two years, the existing method only can cut open the cable and then intercept a sample for inspection and test, and the existing technology can not diagnose the quality problem and the severity of the conductor in the middle section of the cable in a nondestructive mode. If the position of the dissecting cable is inaccurate, trouble is brought to problem finding, subsequent use of the cable is inevitably influenced, and serious waste is caused.
CN104133155B discloses a cable local defect diagnosis method, which comprises (1) extracting inherent propagation coefficient of a cable to be tested under intact condition; (2) measuring an input impedance amplitude or a phase frequency spectrum of the head end of the cable to be measured; (3) constructing a line characteristic diagnosis function reflecting the change condition of the transmission coefficient of the cable to be tested along the line; (4) and searching a peak value point of the characteristic diagnostic function along the line, wherein the position of the peak value point is the position of the defect, and the defect is more serious when the peak value is higher. The method needs cables with the same length and the same model for comparison, needs a large amount of basic data which are difficult to obtain, and interference peaks exist in the obtained cable state diagnosis curve, so that great errors exist in defect positioning. Meanwhile, the positioning method is to directly perform integral transformation on data obtained by measurement to obtain a positioning curve, the application object of the method is a real impedance amplitude spectrum, but positive frequency and negative frequency parts can occur when the real number is directly applied to Fourier transformation, so that interference of negative frequency can be caused, judgment errors can be caused, and particularly under the condition that the cable length is not clear. In addition, the severity of the defects is only qualitative, and the method cannot be generally applied to various defects and cannot judge the properties of the defects. Finally, the method needs to measure the impedance of the same type of fault-free cable when the tail end is open or short-circuited, and the operation is troublesome.
Therefore, the prior art is difficult to effectively diagnose, locate and judge the cause of the quality problem of the conductor in the middle section of the cable, and can not effectively guide the development of the detection work.
Disclosure of Invention
The invention aims to solve the problem that the quality problems of replacing copper by aluminum, reducing the cross section and the like of a conductor at the middle section of a cable are difficult to judge and accurately position, and provides a method for nondestructively judging and positioning the quality of the conductor at the middle section of the cable.
Aiming at the actual problem that the quality problem of the conductor in the middle section of the cable is difficult to judge and position, the invention adopts the technical scheme that: a method for nondestructively studying, judging and positioning the conductor quality of a cable middle section comprises the steps of measuring an actually measured impedance amplitude spectrum of a cable head end on the basis of an impedance amplitude spectrum when the quality of a conductor in the cable middle section is analyzed, carrying out Hilbert transform on the actually measured impedance amplitude spectrum to obtain an imaginary part of the impedance amplitude spectrum, combining the imaginary part with the actually measured impedance amplitude spectrum to obtain a complex impedance amplitude spectrum, carrying out fast Fourier transform on the complex impedance amplitude spectrum, normalizing to obtain a normalized positioning amplitude curve, and respectively obtaining the distance from a problem section cable to the cable head end and the distance from the problem section cable tail end to the cable tail end;
obtaining the resistance value of the conductor of the cable at the section with the problem according to the resistance measurement values of the whole cable and the cable at the unit length of the end part; calculating the degree of conductivity change or radius reduction of the cable conductor at the problem section according to the resistance value of the cable conductor at the problem section, and respectively calculating a theoretical impedance amplitude spectrum under the condition of conductivity change or radius reduction; respectively extracting impedance mode values of maximum points of the actually measured impedance amplitude spectrum and the theoretical impedance amplitude spectrum under the condition of conductivity change or radius reduction as characteristic vectors, respectively calculating deviation mean values of the characteristic vectors of the actually measured impedance amplitude spectrum and the characteristic vectors of the theoretical impedance amplitude spectrum, and judging whether the conductor material problem or the conductor radius reduction problem is solved according to the deviation mean values.
The invention relates to a cable middle section conductor quality nondestructive studying, judging and positioning method, which comprises the following steps:
step S1, obtaining the resistance standard requirement of the cable with unit length according to the cable model specificationR 0 Detecting the resistance of the entire cableRAnd testing the resistance of the end cableR ’ ；
Step S2, testing the actually measured impedance amplitude spectrum of the cable at the head end of the cable; when the cable has middle section quality problem, the distance from the cable with the problem section to the head end of the cable is assumed to bel 1 The distance between the end of the cable of the problem section and the end of the cable isl 2 ；
Step S3, verifyThe impedance magnitude spectrum is measured and Hilbert transform is carried out to construct the imaginary part H [ alpha ], [ beta ] of the impedance magnitude spectrum isZ(f)]：
WhereinZ(f) In order to measure the impedance magnitude spectrum,fin order to be the frequency of the radio,τfor the variable to be used for the integration,Z(τ) To be composed ofτImpedance magnitude as an independent variable; combining the imaginary part of the impedance amplitude spectrum with the measured impedance amplitude spectrum to construct a complex impedance amplitude spectrum for fast Fourier transformZ R (f)：
Where j is the unit of the imaginary number.
Step S4, the complex impedance amplitude spectrum of the structure
Fast Fourier transform is carried out to obtain a normalized positioning amplitude curve FFTZ R (f))；
Step S5, under the condition that the full length of the cable is known, FFT (according to the normalized positioning amplitude curve)Z R (f) Finding an obvious peak value, namely an impedance discontinuous point, in the range from 0 to the full length of the cable to obtain the distance between the cable at the problem section and the head end of the cablel 1 The distance between the end of the cable of the problem section and the end of the cable isl 2 ；
Step S6, obtaining the resistance of the whole cable disc according to the detectionRCalculating the resistance R of the cable conductor at the section of the problem m Calculating the problem of section reduction or aluminum-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the problem section cable conductor when the conductor is copperrAnd when the cross section of the cable conductor is standard round, the conductivity of the cable conductor at the section of the problemσ；
In step S7, countHead end theoretical impedance amplitude spectrum when radius of problem section cable conductor is calculated to be rZ r Extracting ofZ r Impedance maximum value sequence of
，
Respectively 1 st, 2 nd and 2 nd when the radius of the conductor of the cable in the problematic section is r,
m impedance magnitude spectral maxima;
in step S8, the conductivity of the cable conductor in the problematic section is calculated as
Head end theoretical impedance magnitude spectra of timeZ σ Extracting ofZ σ Impedance maximum value sequence of
，
The electrical conductivity of the cable conductor is respectively the problem section
1, 2, respectively,
m impedance magnitude spectral maxima;
step S9, extracting the actually measured impedance amplitude spectrumZ(f) Impedance maximum value sequence of
，
Respectively the measured 1 st, 2 nd,
m impedance magnitude spectral maxima;
in step S10, the closeness degree of the impedance amplitude spectrum is evaluated, and the deviation mean value of the impedance amplitude spectrum when the radius of the conductor of the problem section is reduced is respectively calculated
And the deviation mean value of the impedance amplitude spectrum when the conductivity of the cable conductor at the problem section is reduced
：
Z ri For the radius of the cable conductor of the problem section of riThe maximum of the amplitude spectrum of the impedance,Z i obtained for measurement ofiThe amplitude of each of the impedances is a maximum of the spectrum, Z σi the electrical conductivity of the cable conductor for the problematic section is
First of timeiM is the total number of the impedance amplitude spectrum maximum values;
when in use
Is less than
When the cross section of the cable is smaller than a set value epsilon, the cross section of the cable in the problem section is judged to be reduced to the radius r, and the cross section of the middle section is reduced according to the number of single wires at the end part of the cable and the cross section of the middle sectionCalculating the number of single wires of the conductor at the middle section; when the temperature is higher than the set temperature
Is less than
And when the conductivity is less than the set value epsilon, judging that the problem section cable has the material problem, and calculating the increase value of the conductivity.
More preferably, in step S6, the resistance of the entire cable is detectedRCalculating the resistance R of the cable conductor at the section of the problem m ：
Calculating the problem of section reduction or aluminum-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the problem section cable conductor when the conductor is copperr，
Wherein the content of the first and second substances,σ copper (Cu) Which is the electrical conductivity of the copper,lthe actual length of the cable;
assuming that the cross section of the cable conductor is in a standard circle shape, calculating the conductivity of the cable conductor at the section of the problemσ：
Wherein r is 0 Is the standard radius of the cable conductor.
Compared with the prior art, the invention has the beneficial effects that: the invention provides an impedance amplitude spectrum transformation method based on Hilbert transformation aiming at the problem that the quality problem of a conductor in the middle section of a cable is difficult to study and position, constructs a complex impedance amplitude spectrum, eliminates the influence of negative frequency in Fourier transformation, is convenient for determining the length of the cable, has more obvious impedance peak value, and is more suitable for being applied under the difficult test condition that a middle conductor is poor. Meanwhile, the problem type and the severity are quantitatively researched and judged according to characteristic vectors such as impedance maximum points of an impedance amplitude spectrum, rather than qualitative description in the prior art, the method has the advantages of rapidness, accuracy, no damage and the like, is a powerful cable quality detection means, and can effectively guide quality verification work.
Drawings
Fig. 1 is a schematic diagram of a problem section of a cable.
FIG. 2 is a measured impedance magnitude spectrum of an embodiment of the invention.
Fig. 3 is a direct fourier transform of a measured impedance magnitude spectrum of an embodiment of the present invention.
Fig. 4 shows the result of fourier transform of the complex impedance magnitude spectrum obtained after transformation according to the embodiment of the present invention.
FIG. 5 is a locally amplified complex impedance magnitude spectrum location curve of an embodiment of the present invention.
The specific implementation mode is as follows:
the invention is explained in more detail below with reference to the figures and examples.
A cable middle section conductor quality nondestructive studying and judging and positioning method comprises the following steps:
step S1, obtaining the resistance standard requirement of the cable with unit length (1 meter) according to the cable model specificationR 0 Detecting the resistance of the entire cableRAnd testing the resistance of the end cableR ’ ；
Step S2, referring to figure 1,xthe location of =0 is the cable head end,x=lthe position is the tail end of the cable, and the actually measured impedance amplitude spectrum of the cable is tested at the head end of the cable; when the cable is present in the middle sectionl 3 When the quality is in question, the distance between the cable section of the question and the head end of the cable is assumed to bel 1 The distance between the tail end of the cable at the problem section and the tail end of the cable isl 2 ；
Step S3, performing Hilbert transform on the measured impedance amplitude spectrum to construct the imaginary part H [ phi ] of the impedance amplitude spectrumZ(f)]：
WhereinZ(f) In order to measure the impedance magnitude spectrum,fis a function of the frequency of the received signal,τfor the variable to be used for the integration,Z(τ) To be composed ofτIs the impedance magnitude of the independent variable. The imaginary part of the impedance amplitude spectrum is constructed by the formula, and the imaginary part is combined with the actually measured impedance amplitude spectrum to construct a complex impedance amplitude spectrum for fast Fourier transformZ R (f)：
Where j is the unit of the imaginary number.
Step S4, the constructed complex impedance amplitude spectrum
Fast Fourier transform is carried out to obtain a normalized positioning amplitude curve FFTZ R (f)). Complex impedance amplitude spectrum obtained based on Hilbert transformZ R (f) Is resolvable, i.e. can be converted into a natural indexeIn Euler form ofe jωt ，ωIn order to be the angular frequency of the frequency,tfor time, such complex impedance magnitude spectraZ R (f) Its frequency can be extracted directly, while if only the real part, the imaginary part needs to be appended to constitute the euler form. The imaginary part of the impedance amplitude spectrum counteracts the negative frequency part in Fourier transform, which is helpful to find the actual end of the cable and obtain the actual length of the cablel。
Step S5, under the condition that the full length of the cable is known, FFT (according to the normalized positioning amplitude curve)Z R (f) Finding an obvious peak value, namely an impedance discontinuous point, between the distance 0 and the full length range of the cable to obtain the distance between the cable of the problem section and the head end of the cablel 1 The distance between the tail end of the cable at the problem section and the tail end of the cable isl 2 。
Step S6, obtaining the resistance of the whole cable disc according to the detectionRCalculating the resistance R of the cable conductor at the section of the problem m Calculating the problem of section reduction or aluminum-to-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the cable conductor in the problem section when the cable conductor is copperrAnd when the cable section is round, the conductivity of the cable conductor at the section of the problemσ。
According to the resistance of the whole cable obtained by detectionRCalculating the resistance R of the cable conductor at the problematic section m ：
Calculating the problem of section reduction or aluminum-to-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the cable conductor in the problem section when the cable conductor is copperr，
Wherein the content of the first and second substances,σ copper (Cu) Is the electrical conductivity of the copper, and,lthe actual length of the cable;
assuming that the cross section of the cable conductor is in a standard circle shape, calculating the conductivity of the cable conductor at the section of the problemσ：
Wherein r is 0 Is the standard radius of the cable conductor.
In step S7, a head end theoretical impedance amplitude spectrum when the radius of the cable conductor of the problem section is r is calculatedZ r Extracting ofZ r Impedance maximum value sequence of
Respectively 1 st, 2 nd and 2 nd when the radius of the conductor of the cable in the problematic section is r,
m impedance magnitude spectral maxima.
In step S8, the conductivity of the cable conductor in the problematic section is calculated as
Head end theoretical impedance magnitude spectra of timeZ σ Extracting ofZ σ Impedance maximum value sequence of
The electrical conductivity of the cable conductor is respectively the problem section
1, 2, respectively,
m impedance magnitude spectral maxima.
Step S9, extracting the actually measured impedance amplitude spectrumZ(f) Impedance maximum value sequence of
Respectively the measured 1 st, 2 nd,
m impedance magnitude spectral maxima.
In step S10, the closeness degree of the impedance amplitude spectrum is evaluated, and the deviation mean value of the impedance amplitude spectrum when the radius of the conductor of the problem section is reduced is respectively calculated
And the deviation mean value of the impedance amplitude spectrum when the conductivity of the cable conductor at the problem section is reduced
：
Z ri For the radius of the cable conductor of the problem section of riThe maximum of the amplitude spectrum of the impedance,Z i obtained for measurement ofiThe maximum of the amplitude spectrum of the impedance, Z σi the electrical conductivity of the cable conductor for the problem section is
First of timeiM is the total number of the impedance amplitude spectrum maximum values;
when in use
Is less than
When the cross section of the cable is smaller than a set value epsilon, the cross section of the cable at the problem section is judged to be reduced to the radius r, and the number of single wires of the conductor at the middle section is calculated according to the number of the single wires at the end part of the cable and the reduction degree of the cross section of the middle section; when in use
Is less than
And when the conductivity is less than the set value epsilon, judging that the problem section cable has the material problem, and calculating the increase value of the conductivity.
In order to make the content of the invention more intuitive, 2 pieces of specification with the lengths of 33m and 23m are respectively 35mm 2 The copper core 10kV crosslinked polyethylene cable and a section of 32m crosslinked polyethylene cable with the specification of 35mm 2 The aluminum core 10kV crosslinked polyethylene cables are spliced together, a cable with the total length of 88m is simulated, the end conductor quality is qualified, and aluminum replaces copper in the middle section.
Firstly, measuring the direct current resistance of the whole cableAnd 0.0565 omega. The impedance amplitude spectrum of the cable was then tested at the head end of the cable, with a frequency range of 0.3MHz-10MHz, with the test results shown in fig. 2. For measured impedance amplitude spectrumZ(f) Performing Hilbert transform to construct a complex impedance magnitude spectrumZ R (f). Are respectively paired withZ(f) AndZ R (f) The fourier transform is performed to obtain the normalized positioning amplitude curves, which are respectively shown in fig. 3 and fig. 4, and it can be seen that directly performing the fourier transform on the test impedance amplitude spectrum will introduce negative frequency influence, which will bring about great confusion to the case that the cable length is unknown.
According to the normalized positioning amplitude curve shown in fig. 4, local amplification is carried out to obtain the normalized positioning amplitude curve shown in fig. 5, and the distance between the cable at the problem section and the head end is obtainedl 1 =32.9m, distance of problem section cable from cable endl 2 =23.3m 。 Calculating the resistance value =0.0271 Ω of the middle section cable, calculating the theoretical radius r =2.54mm and the theoretical conductivity σ =30095642S/m of the conductor of the problem section cable, calculating the theoretical impedance amplitude spectrum of the cable by using the theoretical radius and the theoretical conductivity, respectively, extracting data with the frequency of 4MHz or more, and obtaining the impedance maximum value sequence F = [ 68.6061200674.1632177379.0125159881.0755213187.4976904792.4920053595.94592047101.076469399.97583944103.368497108.8575013112.2641716119.8954681128.3324326137.4857156145.9053446158.8384604170.9976325182.8615617199.5807315218.3685723238.7499919 ] of the theoretical impedance amplitude spectrum at the head end when the radius of the conductor of the problem section cable is r (theoretical radius)]；
An impedance maximum value sequence H = [ 63.9386021169.1821060973.5000148775.4893122181.544911986.199445889.4183788194.2877512493.2610442596.42583677101.5461765104.7240407111.8427874119.7130901128.2516004136.2328148148.3085532159.661655170.7390866186.3498894203.8922244222.9224948 ] of a head-end theoretical impedance amplitude spectrum when the conductivity of the cable conductor at the problem section is (theoretical conductivity);
extracting impedance maximum value sequence Z = [ 64.2599016269.5297548773.869 ] of measured impedance amplitude spectrum36167 75.86865549 81.95468533 86.63260885 89.8677174 94.66641691 93.6355866 96.81308912 101.9539925 105.1446191 112.2919552 120.1938656 128.6375129 136.642743 148.7548176 160.1420812 171.2528451 186.9106213 204.5057416 223.5932746]Calculating the mean deviation of the two cases
= -6.78% and
=0.39%。
much less than
And if the conductivity is less than 0.5 percent of the set value, the conductivity of the conductor in the middle section of the cable is judged to be not satisfactory, the conductivity is 32369268S/m, aluminum is suspected to replace copper, and the problem section is 32.9m away from the head end and 23.3m away from the tail end.
Claims (2)
1. A cable middle section conductor quality nondestructive study and determination and positioning method is characterized in that on the basis of an impedance amplitude spectrum when the quality problem of a cable middle section conductor is analyzed, an actually measured impedance amplitude spectrum of a cable head end is measured, Hilbert transform is carried out on the actually measured impedance amplitude spectrum to obtain an imaginary part of the impedance amplitude spectrum, the imaginary part and the actually measured impedance amplitude spectrum are combined to obtain a complex impedance amplitude spectrum, fast Fourier transform is carried out on the complex impedance amplitude spectrum, normalization is carried out to obtain a normalized positioning amplitude curve, and the distance from a problem section cable to the cable head end and the distance from the problem section cable tail end to the cable tail end are respectively obtained;
obtaining the resistance value of the conductor of the cable at the section with the problem according to the resistance measured values of the whole cable and the cable at the end part; calculating the degree of conductivity change or radius reduction of the cable conductor at the problem section according to the resistance value of the cable conductor at the problem section, and respectively calculating a theoretical impedance amplitude spectrum under the condition of conductivity change or radius reduction; respectively extracting impedance module values of maximum points of the actually measured impedance amplitude spectrum and the theoretical impedance amplitude spectrum under the condition of conductivity change or radius reduction as characteristic vectors, respectively calculating deviation mean values of the characteristic vectors of the actually measured impedance amplitude spectrum and the characteristic vectors of the theoretical impedance amplitude spectrum, and judging whether the conductor material problem or the conductor radius reduction problem is solved according to the deviation mean values;
the method comprises the following specific steps:
step S1, obtaining the resistance standard requirement of the cable with unit length according to the cable model specificationR 0 Detecting the resistance of the entire cableRAnd testing the resistance of the end cableR ’ ；
Step S2, testing the actually measured impedance amplitude spectrum of the cable at the head end of the cable; when the cable has middle section quality problem, the distance from the cable with the problem section to the head end of the cable is assumed to bel 1 The distance between the tail end of the cable at the problem section and the tail end of the cable isl 2 ；
Step S3, performing Hilbert transform on the measured impedance amplitude spectrum to construct the imaginary part H [ phi ] of the impedance amplitude spectrumZ(f)]：
WhereinZ(f) In order to measure the impedance magnitude spectrum,fin order to be the frequency of the radio,τas a variable to be used for the integration,Z(τ) To be composed ofτImpedance magnitude as an independent variable; combining the imaginary part of the impedance amplitude spectrum with the measured impedance amplitude spectrum to construct a complex impedance amplitude spectrum for fast Fourier transformZ R (f)：
Wherein j is the unit of an imaginary number;
step S4, the complex impedance amplitude spectrum of the structure
Fast Fourier transform is carried out to obtain a normalized positioning amplitude curve FFTZ R (f))；
Step S5, under the condition that the full length of the cable is known, FFT (according to the normalized positioning amplitude curve)Z R (f) Finding an obvious peak value, namely an impedance discontinuous point, in the range from 0 to the full length of the cable to obtain the distance between the cable at the problem section and the head end of the cablel 1 The distance between the end of the cable of the problem section and the end of the cable isl 2 ；
Step S6, obtaining the resistance of the whole cable disc according to the detectionRCalculating the resistance R of the cable conductor at the section of the problem m Calculating the problem of section reduction or aluminum-to-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the cable conductor in the problem section when the cable conductor is copperrAnd when the cross section of the cable conductor is standard round, the conductivity of the cable conductor at the section of the problemσ；
In step S7, a head end theoretical impedance amplitude spectrum when the radius of the cable conductor of the problem section is r is calculatedZ r ExtractingZ r Impedance maximum value sequence of
，
Respectively 1 st, 2 nd and 2 nd when the radius of the conductor of the cable in the problematic section is r,
m impedance magnitude spectral maxima;
in step S8, the conductivity of the cable conductor in the problematic section is calculated as
Head end theoretical impedance magnitude spectrum of timeZ σ ExtractingZ σ Impedance maximum value sequence of
，
The electrical conductivity of the cable conductor is respectively the problem section
1, 2, respectively,
m impedance magnitude spectral maxima;
step S9, extracting the actually measured impedance amplitude spectrumZ(f) Impedance maximum value sequence of
，
Respectively the measured 1 st, 2 nd,
m impedance magnitude spectral maxima;
in step S10, the closeness degree of the impedance amplitude spectrum is evaluated, and the deviation mean value of the impedance amplitude spectrum when the radius of the conductor of the problem section is reduced is respectively calculated
And the deviation mean value of the impedance amplitude spectrum when the conductivity of the cable conductor at the problem section is reduced
：
Z ri For the radius r of the cable conductor of the problem sectioniThe maximum of the amplitude spectrum of the impedance,Z i obtained for measurement ofiThe maximum of the amplitude spectrum of the impedance, Z σi the electrical conductivity of the cable conductor for the problematic section is
First of timeiM is the total number of the impedance amplitude spectrum maximum values;
when the temperature is higher than the set temperature
Is less than
When the cross section of the cable is smaller than a set value epsilon, the cross section of the cable at the problem section is judged to be reduced to the radius r, and the number of the single wires of the conductor at the middle section is calculated according to the number of the single wires at the end part of the cable and the reduction degree of the cross section at the middle section; when in use
Is less than
And when the conductivity is less than the set value epsilon, judging that the problem section cable has the material problem, and calculating the increase value of the conductivity.
2. The method as claimed in claim 1, wherein in step S6, the resistance of the whole cable is measured according to the test resultRCalculating the resistance R of the cable conductor at the section of the problem m ：
Calculating the problem of section reduction or aluminum-to-copper substitution according to the curve of the impedance amplitude spectrum, and calculating the radius of the cable conductor in the problem section when the cable conductor is copperr，
Wherein the content of the first and second substances,σ copper (Cu) Which is the electrical conductivity of the copper,lthe actual length of the cable;
assuming that the cross section of the cable conductor is in a standard circle shape, calculating the conductivity of the cable conductor at the problem sectionσ：
Wherein r is 0 Is the standard radius of the cable conductor.
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