CN115639500A - Cable detection system and identification method based on variable-frequency pulse frequency modulation excitation - Google Patents
Cable detection system and identification method based on variable-frequency pulse frequency modulation excitation Download PDFInfo
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Abstract
The invention belongs to the field of cable defect and fault detection, and discloses a cable detection system based on variable frequency pulse frequency modulation excitationRCThe device comprises a waveform conditioning unit, a high-frequency IV conversion unit, an MCU signal processing main control unit and an upper computer terminal output unit. Meanwhile, an identification method based on the system is disclosed, and the method comprises the steps of firstly obtaining pseudo-trapezoidal variable frequency pulse excitation and head end response current waveform data; and then obtaining the complete head end input impedance spectrum of the tested cable by time-frequency domain conversion and impedance spectrum digital reconstruction technology(ii) a And finally, calculating and analyzing the actually measured head end input impedance spectrum and the healthy cable reference impedance spectrum by using a multi-parameter fusion diagnosis model, and carrying out accurate mode identification on the cable defects and faults. By adopting the cable detection system and the identification method based on variable-frequency pulse frequency modulation excitation, the cable defects and faults can be accurately detected and identified.
Description
Technical Field
The invention relates to the technical field of cable defect and fault detection, in particular to a cable detection system and an identification method based on variable frequency pulse frequency modulation excitation.
Background
With the rapid development of urban modernization in China, the ratio of power cable transmission is rapidly increased, and the power cable is developed towards high voltage, large capacity and long distance. The service cable is affected by factors such as heating caused by nuclear radiation and overload, moisture permeation in the environment, self insulation oxidation and the like, the insulation damage and aging problems of insulation defects are easily caused, and further permanent faults are caused to further cause electric power accidents. Typical types of cable faults are short circuit faults, open circuit faults, high resistance faults and low resistance faults. In addition, compared with the system fault of the cable line, the insulation defects of the cable body, such as moisture, thermal aging, water tree/electrical tree aging and the like, are not ignored.
A Partial Discharge (PD) method for detecting the insulation of a common cable is mainly used for identifying defects (such as impurities and tool marks) of cable accessories and the like, and is difficult to detect insulation reduction defects such as aging, moisture, copper shielding layer defect and corrosion of a cable body. In addition, when a partial discharge pulse propagates in a cable, high-frequency attenuation characteristics exist, and the detection sensitivity of the partial discharge pulse is also extremely easily influenced by a complex electromagnetic environment on site.
The cable positioning technique using the reflection method can be classified into a Time Domain Reflection (TDR) method and a Frequency Domain Reflection (FDR) method. The TDR method is difficult to identify a local weak defect of a cable body because the injected pulse contains a small amount of high-frequency components and is seriously affected by dispersion. The FDR method is only used for analyzing in a frequency domain, reflected signals are easily interfered by noise and then distorted, and the FDR method can only be used for fault location of short-distance cable lines and cannot identify defects and fault modes. Therefore, it is very important for detecting defects and faults of long-distance cable lines and identifying patterns.
The current defect and fault detection of long-distance cable lines still has the following problems:
(1) In a wide frequency range, the gain bandwidth product of the traditional frequency spectrum testing instrument is limited, the amplitude of the excitation voltage is too low, the signal attenuation is serious, and the anti-electromagnetic interference level needs to be further improved.
(2) The limited output capability of the existing instrument causes serious distortion of the impedance spectrum of a long-distance cable line test.
Therefore, a new detection system and a defect and fault pattern recognition technology are urgently needed to realize the detection and pattern recognition method of the defects and faults in the long-distance cable lines.
Disclosure of Invention
The present invention is directed to solving the above-mentioned problems of the prior art.
In order to achieve the aim, the invention provides a cable detection system based on variable frequency pulse frequency modulation excitation, which comprises a direct current-alternating current variable frequency pulse modulation unit and a composite typeRCThe device comprises a waveform conditioning unit, a high-frequency IV conversion unit, an MCU signal processing main control unit and an upper computer terminal output unit;
the output end of the DC-AC variable frequency pulse modulation unit and the composite typeRCThe input ends of the waveform conditioning units are connected, and the composite typeRCThe output end of the waveform conditioning unit is connected with a head end metal wire core of a tested cable, a head end outer copper shielding layer of the tested cable is connected with an input end of the high-frequency IV conversion unit, an output end of the high-frequency IV conversion unit is connected with an input end of the MCU signal processing main control unit, and the upper computer terminal output unit is connected with the composite typeRCAnd the waveform conditioning units are connected with the MCU signal processing main control unit.
Preferably, the direct current-alternating current variable frequency pulse modulation unit adopts an MOSFET as an inverter bridge arm and outputs variable frequency pulse square waves in a wide frequency band of 100kHz-10 MHz.
Preferably, the composite typeRCThe waveform conditioning unit passes through the fixed resistorRValue and switched capacitanceCAdjusting the charge-discharge oscillation time to 3τ-4τIn the method, the frequency conversion pulse square wave is conditioned into frequency conversion pseudo trapezoidal wave excitation.
Preferably, the high-frequency IV conversion unit is configured to collect a high-frequency insulating micro-current flowing through the copper shielding layer on the cable head end, and convert the high-frequency insulating micro-current into a response output voltage without phase shift.
Preferably, the MCU signal processing main control unit synchronously acquires discrete data of frequency conversion pseudo-trapezoidal wave excitation and response output voltage in a frequency range of 100kHz-10MHz in real time, and the MCU signal processing main control unit obtains a complete frequency range impedance spectrum in the frequency range of 100kHz-90MHz through time-frequency domain conversion and impedance spectrum digital reconstruction.
An identification method based on the cable detection system comprises the following specific steps:
step S1: acquiring cable test data in a time domain, wherein the cable test data comprises frequency conversion pseudo-trapezoidal wave excitation and response output voltage signals in the time domain of the head end of a tested cable;
step S2: input impedance spectrum Z for determining complete frequency band of tested cable d (f) Processing cable test data through time-frequency domain conversion and impedance spectrum digital reconstruction, performing discrete FFT (fast Fourier transform), frequency band continuation and polar coordinate system mapping recombination, and then reversely mapping to a two-dimensional coordinate system again to obtain an input impedance spectrum Z in a complete frequency band of 100kHz-90MHz d (f)；
S3, establishing a cable defect and fault diagnosis judgment model;
the decision formula is as follows:
D(f)= Z d (f) - Z h (f)
wherein Z is d (f) Is the input impedance spectrum, Z, of the cable under test h (f) Is the input impedance spectrum of the intact cable;
if D (f) is constantly 0 in the complete frequency band, the diagnosis and identification of defects and faults of the tested cable are not needed, otherwise, the step S4 is carried out;
and step S4: identifying cable defects and fault types;
the cable defects comprise C + defects and C-defects, the C + defects comprise heat aging, water tree branches and electrical tree aging, and the C-defects comprise copper shielding breakage;
the fault types comprise high-resistance faults, low-resistance faults, open-circuit faults and short-circuit faults;
establishing a multi-dimensional characteristic parameter fusion mode identification criterion comprising an impedance spectrum amplitude, an impedance spectrum resonance period, an impedance spectrum initial phase and a resonance peak number transformation ratio:
wherein the content of the first and second substances,k N the ratio of the number of resonance peaks of the impedance spectrum of the fault cable to the number of resonance peaks of the impedance spectrum of the intact cable in the complete frequency band is obtained;k f the degree of change of the impedance spectrum amplitude of the first resonance peak;k t to the extent that the resonance period of the impedance spectrum varies,k ψ is the change of the initial phase of the impedance spectrum;
when the defect is a C + defect, the defect is formed,k N <1， k ψ = -90 °, impedance spectrum shifts to the left;
when the defect is C-, the defect is,k N <1， k ψ = -90 °, impedance spectrum shifts to the right;
when the short-circuit is in fault,k N >1，k f >1，k t >1 andk ψ =+90°；
when the low-resistance is in a fault state, k N >1， k f <1，k t >1 andk ψ =0°~+90°；
when the high-resistance fault occurs, the fault can be detected,k N <1,k f <1，k t =1 andk ψ =-90°~0°；
in the event of an open circuit fault,k N >1，k f >1，k t >1 andk ψ =-90°。
preferably, in step S2,
the voltage amplitude is taken as a polar coordinate axis, the sweep frequency interval is taken as a stepping angle,
firstly, reconstructing a frequency band interval, wherein the expression is as follows:
in the formula (I), the compound is shown in the specification,f min in order to sweep the starting frequency of the fundamental wave,f max the fundamental frequency sweep termination frequency.r i Is the first in polar coordinatesiThe radius of the sub-harmonic wave,θ[i,i+1]for the reconstructed second in polar coordinatesiSubharmonic waveA sector arc of (a);
secondly, further digitally reconstructing the impedance spectrum in the complete frequency band for the obtained required frequency band interval, wherein the expression is as follows:
in the formula, deltafFor fundamental frequency sweep step frequency, discrete points in the digitally reconstructed impedance spectrumnThe expression is as follows:
obtaining an impedance spectrum Z within a complete frequency band d (f) And comprises an amplitude impedance spectrum | Zd (f) | and a phase spectrum Pd (f), wherein f is the frequency of the frequency sweep signal.
Preferably, in step S4,
impedance spectrum Z based on digital reconstruction d (f) And the input impedance spectrum Z of the intact cable h (f) Extractingk f 、k t 、k ψ Andk N and combining the characteristic parameters in a multi-dimensional mode, and constructing a multi-dimensional characteristic diagnosis chart.
Preferably, perfect cable input impedance spectrumZ h (f) The following three acquisition modes are provided:
the first method comprises the following steps: testing before the new cable is put into operation to obtain the input impedance spectrum of the cable under the intact stateZ h (f)；
And the second method comprises the following steps: by testing cables of the same type, the input impedance spectrum of the cable under a complete state is obtainedZ h (f)；
And the third is that: calculating the unit length of these cables with reference to the structural dimensions of the cables or to specifications provided by the manufacturerR 0 、L 0 、C 0 、G 0 And further according to the characteristic impedanceZ 0h And propagation coefficientγ h Obtaining the input impedance spectrum of the cable under the intact stateZ h (f)。
Calculating the parameters of the intact cable transmission line model comprises the following steps: resistance of cable per unit lengthR 0 InductorL 0 Capacitor and method for manufacturing the sameC 0 And electrical conductanceG 0 Can be approximately equal to
Characteristic impedance of intact cableZ 0h And propagation constantγ h Comprises the following steps:
coefficient of reflection at end of cableГ L Comprises the following steps:
the cable signal injection end is taken as the origin of coordinates, and the length under no-load condition is taken asLAt any position on the cablexThe input impedance at (a) is:
therefore, the cable detection system and the identification method based on the variable frequency pulse frequency modulation excitation have the following beneficial effects:
(1) Compared with the existing low-voltage analog signal broadband impedance spectrum test system, the invention provides a new hardware topological structure for generating a pseudo-trapezoidal pulse frequency modulation frequency-sweep excitation signal. The output voltage amplitude of the high-voltage power supply can reach 1200V within the frequency range of 100kHz-10 MHz. The problem that the output voltage in a high-frequency interval is too low and the signal attenuation is severe when a long-distance cable is tested is effectively solved.
(2) The invention provides a method for analyzing a broadband impedance spectrum by replacing the traditional IFFT method, which utilizes an impedance spectrum digital reconstruction technology to expand the high-frequency range of impedance spectrum measurement and extends the original test data of 100kHz-10MHz to a complete non-attenuation impedance spectrum of 100kHz-90 MHz.
(3) Based on the digitally reconstructed impedance spectrum (amplitude impedance spectrum | Zd (f) | and phase spectrum Pd (f)) and the intact cable input impedance spectrumZ h (f) To extract outk f 、k t 、k ψ 、k N And combining the characteristic parameters in a multidimensional way, constructing a multidimensional characteristic diagnosis chart, and realizing accurate mode identification of cable defects and faults.
(4) The invention greatly improves the excitation test voltage on the basis of a non-destructive test mode, has stronger anti-interference performance and extremely high sensitivity, is not limited by the length and the voltage grade of the tested cable, has small equipment volume, and is suitable for testing in a complex environment on site.
The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.
Drawings
FIG. 1 is a block diagram of a cable detection system based on variable frequency pulse frequency modulation excitation according to the present invention;
FIG. 2 is a flow chart of an identification method of the present invention;
FIG. 3 is a schematic diagram of a digital reconstruction method;
fig. 4 is a diagram showing type determination of cable insulation defects and faults.
Detailed Description
Examples
In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings or orientations or positional relationships conventionally put in use of products of the present invention, and are only for convenience of description and simplification of description, but do not indicate or imply that the devices or elements referred to must have specific orientations, be constructed in specific orientations, and be operated, and thus, should not be construed as limiting the present invention.
In the description of the present invention, it should also be noted that, unless otherwise explicitly specified or limited, the terms "disposed," "mounted," and "connected" are to be construed broadly, e.g., as meaning fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
In this example, the detection targets are 200 meters long YJLV8.7/15kV XLPE power cables (phase a: C + defect (thermal aging), phase B: C-defect (copper shield damage), phase C: high resistance, low resistance, open circuit, short circuit fault) containing different defects and faults.
A cable detection system based on variable frequency pulse frequency modulation excitation comprises a direct current-alternating current variable frequency pulse modulation unit and a composite typeRCThe device comprises a waveform conditioning unit, a high-frequency IV conversion unit, an MCU signal processing main control unit and an upper computer terminal output unit.
The output end of the DC-AC variable frequency pulse modulation unit and the composite typeRCThe input ends of the waveform conditioning units are connected, and the composite typeRCThe output end of the waveform conditioning unit is connected with the metal wire core at the head end of the tested cable, the copper shielding layer outside the head end of the tested cable is connected with the input end of the high-frequency IV conversion unit, and the output end of the high-frequency IV conversion unit is connected with the MCU signal processing unitThe input end of the main control unit is connected, the output unit of the upper computer terminal is connected with the composite typeRCAnd the waveform conditioning units are connected with the MCU signal processing main control unit.
The direct current-alternating current variable frequency pulse modulation unit adopts the MOSFET as an inverter bridge arm and outputs variable frequency pulse square waves in a wide frequency band of 100kHz-10MHz, thereby ensuring that the output amplitude and the capacity of the system are not attenuated along with the increase of frequency. The present embodiment selects the dc side voltage amplitude as 500 v.
Composite typeRCThe waveform conditioning unit passes through the fixed resistorRValue and switched capacitanceCAdjusting the charge-discharge oscillation time to 3τ-4τIn the method, the oscillation peak of the SiC MOSFET is effectively absorbed, and the frequency conversion pulse square wave is conditioned into frequency conversion pseudo trapezoidal wave excitation. Effectively prevent overshoot voltage to cause the damage to the cable body.
The high-frequency IV conversion unit is used for collecting high-frequency insulating micro-current flowing through the copper shielding layer outside the head end of the cable and converting the high-frequency insulating micro-current into response output voltage without phase shift. And accurate sampling data is provided for a subsequent impedance spectrum digital reconstruction technology.
The MCU signal processing main control unit synchronously collects discrete data of variable-frequency pseudo-trapezoidal wave excitation and response output voltage in a frequency range of 100kHz-10MHz in real time, obtains a complete frequency range impedance spectrum in the frequency range of 100kHz-90MHz through time-frequency domain conversion and impedance spectrum digital reconstruction, and transmits the impedance spectrum to an upper computer terminal through an upper computer input unit to carry out defect and fault identification.
An identification method based on the cable detection system comprises the following specific steps:
step S1: acquiring cable test data under a time domain, wherein the cable test data comprises frequency conversion pseudo-trapezoidal wave excitation and response output voltage signals under the time domain of the head end of the tested cable, injecting pseudo-trapezoidal wave pulse frequency modulation signals (100 kHz-10 MHz) into a YJLV8.7/15kV XLPE cable, and acquiring the frequency conversion pseudo-trapezoidal wave excitation and response output voltage signals under the time domain of the head end of the tested cable.
Step S2: input impedance spectrum Z for determining complete frequency band of tested cable d (f)，And processing the cable test data through time-frequency domain conversion and impedance spectrum digital reconstruction, and representing the digital reconstruction method of the impedance spectrum by a polar coordinate graph. A schematic diagram of the digital reconstruction method is shown in fig. 3. After discrete FFT conversion, frequency band continuation and polar coordinate system mapping recombination processing, the input impedance spectrum Z in the complete frequency band of 100kHz to 90MHz is obtained by reversely mapping the input impedance spectrum Z to a two-dimensional coordinate system again d (f)。
The voltage amplitude is taken as a polar coordinate axis, the sweep frequency interval is taken as a stepping angle,
firstly, reconstructing a frequency band interval, wherein the expression is as follows:
in the formula (I), the compound is shown in the specification,f min in order to sweep the starting frequency of the fundamental wave,f max the fundamental frequency sweep termination frequency.r i Is the first in polar coordinatesiThe radius of the sub-harmonic wave,θ[i,i+1]is the reconstructed second in polar coordinatesiThe segmental arc of the subharmonic.
Secondly, further digitally reconstructing the impedance spectrum in the complete frequency band for the obtained required frequency band interval, wherein the expression is as follows:
in the formula, deltafFor fundamental frequency sweep step frequency, discrete points in the digitally reconstructed impedance spectrumnThe expression is as follows:
from the analysis, when the cable is subjected to frequency sweep test by frequency conversion pseudo-trapezoidal wave excitation based on the impedance spectrum digital reconstruction method, the complete frequency band impedance spectrum can be obtained only by utilizing the frequency sweep information of the fundamental wave.
In addition, as the number of harmonics increases, the response amplitude decreases. In thatExcitation voltageU dc Under the condition of =1200V, the nine harmonic amplitude with the highest frequency can still reach more than 120V. The defects that the amplitude of the excitation voltage of the existing impedance analyzer is too low and the output capacity is insufficient are overcome, and meanwhile, the output signal-to-noise ratio is improved. The impedance spectrum information after digital reconstruction in the polar coordinate system is reversely mapped to the two-dimensional coordinate system again so as to obtain the input impedance spectrum Z in the complete frequency band of 100kHz to 90MHz d (f) (ii) a Includes amplitude impedance spectrum-Z d (f) I, phase spectrumP d (f) WhereinfIs the frequency of the swept frequency signal.
S3, establishing a cable defect and fault diagnosis judgment model;
the decision formula is as follows:
D(f)= Z d (f) - Z h (f)
wherein Z is d (f) Is the input impedance spectrum, Z, of the cable under test h (f) Is the input impedance spectrum of the intact cable.
Intact cable input impedance spectroscopyZ h (f) The following three acquisition modes are provided:
the first method comprises the following steps: testing before the new cable is put into operation to obtain the input impedance spectrum of the cable under the intact stateZ h (f)；
And the second method comprises the following steps: by testing cables of the same type, the input impedance spectrum of the cable under a complete state is obtainedZ h (f)；
And the third is that: calculating the unit length of these cables with reference to the structural dimensions of the cables or to specifications provided by the manufacturerR 0 、L 0 、C 0 、G 0 And further according to the characteristic impedanceZ 0h And propagation coefficientγ h Obtaining the input impedance spectrum of the cable under the intact stateZ h (f)。
If D (f) is constantly 0 in the complete frequency band, the diagnosis and identification of defects and faults of the tested cable are not needed, otherwise, the step S4 is carried out.
In the embodiment, a simulation calculation method is selected to obtain a cable input impedance spectrum of A, B, C three-phase cable in a perfect stateZ h (f) Electric powerResistance of cable unit length cableR 0 InductorL 0 Capacitor and method for manufacturing the sameC 0 Electrical conductanceG 0 Can be approximately equal to
Characteristic impedance of intact cableZ 0h And propagation constantγ h Comprises the following steps:
coefficient of reflection at end of cableГ L Comprises the following steps:
the cable signal injection end is taken as the origin of coordinates, and the length under no-load condition is taken asLAt any position on the cablexThe input impedance at (a) is:
for the embodiment, the calculated D of the a-phase cable A (x) D of B-phase cable B (x) D of C-phase cable C (x) The average is not 0, so the A, B, C three-phase cable needs to be defected and the defect is causedPattern recognition of the barrier type.
And step S4: and identifying the cable defects and fault types.
The cable defects comprise C + defects and C-defects, the C + defects comprise thermal aging, water tree branches and electrical tree aging, and the C-defects comprise copper shielding breakage;
the fault types comprise high-resistance faults, low-resistance faults, open-circuit faults and short-circuit faults;
establishing a multi-dimensional characteristic parameter fusion mode identification criterion comprising an impedance spectrum amplitude, an impedance spectrum resonance period, an impedance spectrum initial phase and a resonance peak number transformation ratio:
wherein the content of the first and second substances,k N the ratio of the number of resonance peaks of the impedance spectrum of the fault cable to the number of resonance peaks of the impedance spectrum of the intact cable in the complete frequency band is obtained;k f the degree of change of the impedance spectrum amplitude of the first resonance peak;k t to the extent that the resonance period of the impedance spectrum varies,k ψ is the change of the initial phase of the impedance spectrum;
when the defect is a C + defect, the defect is formed,k N <1， k ψ = -90 °, impedance spectrum shifts to the left;
when the defect is C-, the defect is,k N <1， k ψ = -90 °, impedance spectrum shifts to the right;
when the short-circuit is in fault,k N >1，k f >1，k t >1 andk ψ =+90°；
when the low-resistance is in a fault state, k N >1， k f <1，k t >1 andk ψ =0°-+90°；
when the high-resistance fault occurs, the fault can be detected,k N <1,k f <1，k t =1 andk ψ =-90°-0°；
in the event of an open circuit fault,k N >1，k f >1，k t >1 andk ψ =-90°。
impedance spectrum Z based on digital reconstruction d (f) And the input impedance spectrum Z of the intact cable h (f) Extractingk f 、k t 、k ψ Andk N and combining the characteristic parameters in a multi-dimensional mode, and constructing a multi-dimensional characteristic diagnosis chart.
A. 5363 comprehensive determination table of defect and pattern recognition of B, C three-phase cable is shown in table 1,
TABLE 1 impedance spectrum variation law for different fault types
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the invention without departing from the spirit and scope of the invention.
Claims (9)
1. A cable detection system based on frequency conversion pulse frequency modulation excitation is characterized in that: comprises a DC-AC variable frequency pulse modulation unit and a composite typeRCThe device comprises a waveform conditioning unit, a high-frequency IV conversion unit, an MCU signal processing main control unit and an upper computer terminal output unit;
the output end of the DC-AC variable frequency pulse modulation unit and the composite typeRCThe input ends of the waveform conditioning units are connected, and the composite typeRCThe output end of the waveform conditioning unit is connected with a head end metal wire core of a tested cable, a head end outer copper shielding layer of the tested cable is connected with an input end of the high-frequency IV conversion unit, an output end of the high-frequency IV conversion unit is connected with an input end of the MCU signal processing main control unit, and the upper computer terminal output unit is connected with the composite typeRCAnd the waveform conditioning units are connected with the MCU signal processing main control unit.
2. The system according to claim 1, wherein the cable detection system based on the variable frequency pulse frequency modulation excitation comprises: the direct current-alternating current variable frequency pulse modulation unit adopts MOSFET as an inversion bridge arm and outputs variable frequency pulse square waves in a wide frequency band of 100kHz-10 MHz.
3. The system according to claim 2, wherein the cable detection system based on the variable frequency pulse frequency modulation excitation comprises: the composite typeRCThe waveform conditioning unit passes through the fixed resistorRValue and switched capacitanceCAdjusting the charge-discharge oscillation time to 3τ-4τIn the method, the frequency conversion pulse square wave is conditioned into frequency conversion pseudo trapezoidal wave excitation.
4. The system according to claim 3, wherein the cable detection system based on the variable frequency pulse frequency modulation excitation comprises: the high-frequency IV conversion unit is used for collecting high-frequency insulating micro-current flowing through the copper shielding layer outside the head end of the cable and converting the high-frequency insulating micro-current into response output voltage without phase shift.
5. The system according to claim 4, wherein the system comprises: the MCU signal processing main control unit synchronously acquires discrete data of frequency conversion pseudo-trapezoidal wave excitation and response output voltage in a frequency range of 100kHz-10MHz in real time, and obtains a complete frequency range impedance spectrum in the frequency range of 100kHz-90MHz through time-frequency domain conversion and impedance spectrum digital reconstruction.
6. A method for identifying a cable detection system based on variable frequency pulse frequency modulation excitation according to claim 5, which comprises the following steps:
step S1: acquiring cable test data in a time domain, wherein the cable test data comprises frequency conversion pseudo-trapezoidal wave excitation and response output voltage signals in the time domain of the head end of a tested cable;
step S2: input impedance spectrum Z for determining complete frequency band of tested cable d (f) The cable test data is processed through time-frequency domain conversion and impedance spectrum digital reconstruction,after discrete FFT conversion, frequency band continuation and polar coordinate system mapping recombination processing, the input impedance spectrum Z in a complete frequency band of 100kHz-90MHz is obtained by reversely mapping to a two-dimensional coordinate system again d (f)；
S3, establishing a cable defect and fault diagnosis judgment model;
the decision formula is as follows:
D(f)= Z d (f) - Z h (f)
wherein Z is d (f) Is the input impedance spectrum, Z, of the cable under test h (f) Is the input impedance spectrum of the intact cable;
if D (f) is constantly 0 in the complete frequency band, the diagnosis and identification of defects and faults of the tested cable are not needed, otherwise, the step S4 is carried out;
and step S4: identifying cable defects and fault types;
the cable defects comprise C + defects and C-defects, the C + defects comprise thermal aging, water tree branches and electrical tree aging, and the C-defects comprise copper shielding breakage;
the fault types comprise high-resistance faults, low-resistance faults, open-circuit faults and short-circuit faults;
establishing a multi-dimensional characteristic parameter fusion mode identification criterion comprising an impedance spectrum amplitude, an impedance spectrum resonance period, an impedance spectrum initial phase and a resonance peak number transformation ratio:
wherein the content of the first and second substances,k N the ratio of the number of resonance peaks of the impedance spectrum of the fault cable to the number of resonance peaks of the impedance spectrum of the intact cable in the complete frequency band is obtained;k f the degree of change of the impedance spectrum amplitude of the first resonance peak;k t to the extent that the resonance period of the impedance spectrum varies,k ψ is the change of the initial phase of the impedance spectrum;
when the defect is a C + defect, the defect is formed,k N <1， k ψ = -90 °, impedance spectrum shifts to the left;
when the defect is C-, the defect is,k N <1， k ψ = -90 °, impedance spectrum shifts to the right;
when the short-circuit is in fault,k N >1，k f >1，k t >1 andk ψ =+90°；
when the low-resistance is in a fault state, k N >1， k f <1，k t >1 andk ψ =0°~+90°；
when the high-resistance fault occurs, the fault can be detected,k N <1,k f <1，k t =1 andk ψ =-90°~0°；
in the event of an open circuit fault,k N >1，k f >1，k t >1 andk ψ =-90°。
7. the identification method according to claim 6, characterized in that: in the step S2, the process is carried out,
the voltage amplitude is taken as a polar coordinate axis, the sweep frequency interval is taken as a stepping angle,
firstly, reconstructing a frequency band interval, wherein the expression is as follows:
in the formula (I), the compound is shown in the specification,f min in order to sweep the starting frequency of the fundamental wave,f max in order to obtain the fundamental frequency sweep termination frequency,r i is the first in polar coordinatesiThe radius of the sub-harmonic wave,θ[i,i+1]for the reconstructed second in polar coordinatesiA sector arc of subharmonics;
secondly, further digitally reconstructing an impedance spectrum in the complete frequency band for the obtained required frequency band interval, wherein the expression is as follows:
in the formula, deltafFor fundamental frequency sweep step frequency, discrete points in the digitally reconstructed impedance spectrumnThe expression is as follows:
obtaining an impedance spectrum Z within the complete frequency band d (f) And comprises an amplitude impedance spectrum | Zd (f) | and a phase spectrum Pd (f), wherein f is the frequency of the frequency sweep signal.
8. The identification method according to claim 7, characterized in that: in the step S4, the process is repeated,
impedance spectrum Z based on digital reconstruction d (f) And the input impedance spectrum Z of the intact cable h (f) Extractingk f 、k t 、k ψ Andk N and combining the characteristic parameters in a multi-dimensional mode, and constructing a multi-dimensional characteristic diagnosis chart.
9. The identification method according to claim 8, characterized in that:
intact cable input impedance spectroscopyZ h (f) The following three acquisition modes are provided:
the first method comprises the following steps: testing before the new cable is put into operation to obtain the input impedance spectrum of the cable under the intact stateZ h (f)；
And the second method comprises the following steps: by testing cables of the same type, the input impedance spectrum of the cable under a complete state is obtainedZ h (f)；
And the third is that: calculating the resistance of the cables per unit length by referring to the structural size of the cables or the specification of parameters provided by manufacturersR 0 InductorL 0 Capacitor and method for manufacturing the sameC 0 Electrical conductanceG 0 And further according to the characteristic impedanceZ 0h And propagation coefficientγ h Obtaining the input impedance spectrum of the cable under the intact stateZ h (f)。
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