CN110763931A - Method and system for testing high-frequency transmission characteristics of coaxial cable - Google Patents

Method and system for testing high-frequency transmission characteristics of coaxial cable Download PDF

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CN110763931A
CN110763931A CN201910957983.0A CN201910957983A CN110763931A CN 110763931 A CN110763931 A CN 110763931A CN 201910957983 A CN201910957983 A CN 201910957983A CN 110763931 A CN110763931 A CN 110763931A
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coaxial cable
frequency
test
input impedance
tail end
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张丹丹
黎立
莫石
王贤妮
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Huazhong University of Science and Technology
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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
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • 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
    • 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
    • G01R31/12—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing
    • G01R31/1227—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing of components, parts or materials
    • G01R31/1263—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing of components, parts or materials of solid or fluid materials, e.g. insulation films, bulk material; of semiconductors or LV electronic components or parts; of cable, line or wire insulation
    • G01R31/1272—Testing dielectric strength or breakdown voltage ; Testing or monitoring effectiveness or level of insulation, e.g. of a cable or of an apparatus, for example using partial discharge measurements; Electrostatic testing of components, parts or materials of solid or fluid materials, e.g. insulation films, bulk material; of semiconductors or LV electronic components or parts; of cable, line or wire insulation of cable, line or wire insulation, e.g. using partial discharge measurements

Abstract

The invention discloses a method and a system for testing the high-frequency transmission characteristic of a coaxial cable, wherein the method comprises the steps of forming a test loop by a conductor core wire and an outer conductor shielding layer of the coaxial cable, and respectively measuring the input impedance of the coaxial cable under any frequency when the tail end is open-circuited and the input impedance of the coaxial cable under any frequency when the tail end is short-circuited according to a frequency sweeping method; and calculating the high-frequency transmission characteristics of the coaxial cable according to the measurement result, namely the change characteristics of the propagation constant, the attenuation constant, the phase shift coefficient, the phase speed and the characteristic impedance along with the frequency at high frequency. The system comprises a coaxial cable with any length and a test module, wherein the test module forms a test loop by using a conductor core wire and an outer conductor shielding layer of the coaxial cable, and measures the input impedance of the coaxial cable at an open circuit at the tail end and the input impedance of the coaxial cable at a short circuit at the tail end under any frequency to obtain the high-frequency transmission characteristic of the coaxial cable. The test method provided by the invention can solve the problems of complex coaxial cable test system, time-consuming test, serious high-frequency noise interference, low precision and the like.

Description

Method and system for testing high-frequency transmission characteristics of coaxial cable
Technical Field
The invention belongs to the field of power cable high-frequency signal transmission, and particularly relates to a method and a system for testing high-frequency transmission characteristics of a coaxial cable.
Background
Coaxial cables are widely used in power systems, but failure of the cables often occurs due to external force damage, environmental changes, and the like. The high-frequency transmission characteristic of the cable is the basis for researching the fault location of the cable, and the existing popular cable fault location methods, such as a traveling wave method, a partial discharge technology and an impedance spectrum technology, all relate to the propagation of high-frequency signals with different frequencies in the cable. Under high frequency, due to the existence of line resistance and conductance loss, attenuation exists when signals are transmitted along a cable, the frequency-varying characteristics of line capacitance and inductance parameters cause the signals to be subjected to dispersion, and the frequency-varying effect of phase velocity directly influences the fault positioning precision, so that the high-frequency characteristics of the cable are accurately obtained, and the method has important significance for improving the precision of cable fault diagnosis.
The transmission characteristics of the high-frequency signal in the cable depend on the characteristic impedance Z of the cablecPropagation constant γ and phase velocity v, which are parameters that vary with frequency. The existing cable high-frequency transmission characteristic obtaining methods, such as a time domain reflection method, a scattering parameter method and an insulating material dielectric spectrum measuring method, have the defects of complex testing system, difficult frequency sweeping, time consumption, serious high-frequency noise interference, low precision and the like. In the chinese patent application, CN201710318944.7 (application number: 2017.05.08), signals at two ends of a cable are tested and fourier transformed, and then a phase difference between the signals at the two ends is obtained, so that a phase velocity of the signal in the cable can be obtained.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a method and a system for testing the high-frequency transmission characteristic of a coaxial cable, and aims to solve the problems of complex test system and low test precision.
To achieve the above object, according to an aspect of the present invention, there is provided a method for testing high frequency transmission characteristics of a coaxial cable, including the steps of:
(1) forming a test loop by using a conductor core wire and an outer conductor shielding layer of the coaxial cable, and respectively measuring the input impedance of the coaxial cable under any frequency when the tail end is open-circuited and the input impedance of the coaxial cable under any frequency when the tail end is short-circuited according to a frequency sweeping method;
(2) and (4) calculating the high-frequency transmission characteristic of the coaxial cable according to the measurement result of the step (1).
Preferably, the high frequency transmission characteristics of the coaxial cable include a propagation constant, an attenuation constant, a phase shift coefficient, a phase velocity, and a characteristic impedance.
Preferably, the propagation constant γ of the coaxial cable is given by the formula:
Figure BDA0002228001280000021
wherein Z isshFor input impedance of coaxial cable in the event of end short-circuit, ZopThe input impedance of the coaxial cable at the open end is l is the length of the coaxial cable.
Preferably, the attenuation constant α of the coaxial cable is given by the formula:
α＝Re[γ]
the formula for phase shift factor β is:
β＝Im[γ]
wherein, gamma is the propagation constant of the coaxial cable, Re represents a complex real part, and Im represents a complex imaginary part.
Preferably, the formula of the phase velocity v of the coaxial cable is: v 2 pi f/Im [ gamma ]
Wherein, gamma is the propagation constant of the coaxial cable, Im represents the imaginary part of the complex number, and f is the test frequency.
Preferably, the formula of the series impedance z per unit length of the coaxial cable is:
Figure BDA0002228001280000022
wherein r iscIs the radius of the conductor core of the coaxial cable, rsInner radius of the outer conductor, mu0For vacuum permeability, ω is the angular frequency at the test frequency, ρc、ρsThe resistivity of the core conductor and the outer conductor shield, respectively.
Preferably, the characteristic impedance z of the coaxial cablecThe formula of (1) is:
Zc＝z/γ
where z is the series impedance per unit length of the coaxial cable and γ represents the propagation constant.
Preferably, the input impedance of the coaxial cable when the tail end is open-circuited and the input impedance of the coaxial cable when the tail end is short-circuited are measured for at least 3 times according to a plurality of groups of discrete data of the change of the test frequency. When the radius of the core wire of the coaxial cable conductor and the inner radius of the outer conductor are obtained, the average value of the measurement results is also obtained by measuring at least 3 times.
The evaluation method can accurately acquire the propagation constant, the attenuation constant, the phase shift coefficient, the phase velocity and the characteristic impedance of the coaxial cable under high frequency by measuring the discrete data of the single-ended input impedance of the coaxial cable changing along with the test frequency, measuring the radius of the conductor core wire of the cable and the inner radius of the shielding layer of the outer conductor and combining a related theoretical calculation formula, accurately acquire the high-frequency transmission characteristic of the cable, and provide supporting parameter data for the methods for evaluating faults, local defects, intermediate structures and insulation states of the cable.
According to another aspect of the present invention, a system for testing a high-frequency transmission characteristic of a coaxial cable is provided, which includes a coaxial cable of any length and a testing module, the coaxial cable includes a conductor core and an outer conductor, the testing module forms a testing loop with the conductor core and the outer conductor shielding layer of the coaxial cable, and measures an input impedance of the coaxial cable at an open end and an input impedance of the coaxial cable at a short end at any frequency according to a frequency sweep method, so as to obtain the high-frequency transmission characteristic of the coaxial cable.
Preferably, the test cable is selected from any known length of healthy cable, no fault or intermediate joint, and the type and service life of the cable are not limited.
Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:
1. the coaxial cable high-frequency transmission characteristic testing system provided by the invention has the advantages of simple wiring, short measuring time, accurate measurement of parameters along with the change of frequency and high measuring precision;
2. the method for testing the high-frequency transmission characteristic of the coaxial cable is less influenced by high-frequency noise, namely, the method has strong anti-noise interference capability and strong reliability;
3. the method for testing the high-frequency transmission characteristic of the coaxial cable has the advantages that the voltage applied to the cable is low, and the cable is not damaged;
4. the coaxial cable high-frequency transmission characteristic testing method provided by the invention has universality, is suitable for coaxial cables of any type and any voltage level, and can be used for testing and obtaining the signal transmission characteristics of the coaxial cable under any frequency.
Drawings
FIG. 1 is a schematic block diagram of a process of testing high-frequency transmission characteristics of a coaxial cable according to the present invention;
FIG. 2(a) is a graph of input impedance as a function of frequency for a 10kV coaxial cable tested with the end open;
FIG. 2(b) is a graph of input impedance as a function of frequency for a 10kV coaxial cable tested with a short at the end;
FIG. 3 is a graphical representation of the attenuation constant test results for a 10kV coaxial cable of the present invention;
FIG. 4 is a graphical representation of the phase shift coefficient test results for a 10kV coaxial cable of the present invention;
FIG. 5 is a graphical representation of phase velocity test results for a 10kV coaxial cable of the present invention;
FIG. 6(a) is a graph showing the characteristic impedance amplitude test results for a 10kV coaxial cable of the present invention;
fig. 6(b) is a graph showing the characteristic impedance phase test results of the 10kV coaxial cable of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
As shown in fig. 1, the present invention provides a method for testing high-frequency transmission characteristics of a coaxial cable, which specifically includes the following steps:
step 1, selecting a coaxial cable with any known length, wherein the length is marked as l. The conductor core wire and the outer conductor shielding layer of the test coaxial cable form a test loop, and the sweep frequency measurement method is utilized to measure the input impedance Z of the known length coaxial cable under any higher frequency f when the tail end is openopAnd measuring the input impedance Z of the cable in the case of short circuit at the end by the same methodshThe changes of the two with frequency are shown in fig. 2(a) and fig. 2(b), respectively.
Specifically, the characteristic parameters of the coaxial cable are changed along with the frequency, and a suitable frequency band can be selected according to the requirement. When the cable is short and the frequency of the transmission signal is low, the length of the cable is less than the wavelength of the signal, the transmission signal cannot complete a whole period of oscillation on the conductor, and the cable has little influence on the whole circuit response and the input impedance, and the whole loop impedance is equal to the load impedance. If the conductor is long enough or the signal frequency is high, the cable impedance will have a significant weight in the overall loop impedance, and the cable input impedance will be primarily related to the characteristics of the cable itself.
Step 2, according to the tested coaxial input impedance parameter obtained by measurement in step 1, the propagation constant gamma of the coaxial cable with the length of l is obtained through the following formula:
Figure BDA0002228001280000051
and 3, calculating the obtained parameters according to the step 2, wherein the attenuation constant α of the coaxial cable is obtained by a formula α ═ Re [ gamma ], the phase shift coefficient β of the coaxial cable is obtained by a formula β ═ Im [ gamma ], and the phase velocity v of the coaxial cable is obtained by a formula v ═ 2 pi f/Im [ gamma ], wherein Re represents a complex real part, Im represents a complex imaginary part, f represents a test frequency, and the test results of the three are shown in FIGS. 3 to 5.
Step 4, measuring and obtaining the radius r of the conductor core wire of the coaxial cable by utilizing a micrometer caliper or a calipercInner radius r of outer conductorsThe average of at least 3 measurements. The series impedance z per unit length of the coaxial cable is obtained by the following formula:
Figure BDA0002228001280000052
wherein, mu0For vacuum permeability, ω is the angular frequency at the test frequency, ρc、ρsElectrical resistivity of the core conductor and the outer conductor shielding layer, respectively;
and 5, calculating the obtained parameters according to the step 2 and the step 4, wherein the characteristic impedance of the coaxial cable is obtained through the following formula: zcWhere z represents the series impedance per unit length of the axial cable and γ represents the propagation constant. Fig. 6(a) and 6(b) are the amplitude and phase test results, respectively, of the characteristic impedance of the 10kV coaxial cable.
The input impedance of the coaxial cable when the tail end is open-circuited and the input impedance of the coaxial cable when the tail end is short-circuited are measured for at least 3 times to obtain the average value of the measurement results. When the radius of the core wire of the coaxial cable conductor and the inner radius of the outer conductor are obtained, the average value of the measurement results is also obtained by measuring at least 3 times.
The invention also provides a test system of the high-frequency transmission characteristic of the coaxial cable, which comprises the coaxial cable with any length and a test module, wherein the coaxial cable comprises a conductor core wire and an outer conductor, the test module forms a test loop by using the conductor core wire and the outer conductor shielding layer of the coaxial cable, and the input impedance of the coaxial cable under any frequency when the tail end is open-circuited and the input impedance of the coaxial cable under any frequency when the tail end is short-circuited are respectively measured according to a frequency sweeping method to obtain the high-frequency transmission characteristic of the coaxial cable.
Specifically, the test cable needs to be selected from any known length of healthy cable, no fault or intermediate joint, and the type and service life of the cable are not limited.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (10)

1. A method for testing the high-frequency transmission characteristic of a coaxial cable is characterized by comprising the following steps:
(1) forming a test loop by using a conductor core wire and an outer conductor shielding layer of the coaxial cable, and respectively measuring the input impedance of the coaxial cable under any frequency when the tail end is open-circuited and the input impedance of the coaxial cable under any frequency when the tail end is short-circuited according to a frequency sweeping method;
(2) and (4) calculating the high-frequency transmission characteristic of the coaxial cable according to the measurement result of the step (1).
2. The test method of claim 1, wherein the high frequency transmission characteristics of the coaxial cable include propagation constants, attenuation constants, phase shift coefficients, phase velocities, and characteristic impedance versus frequency characteristics at high frequencies.
3. The test method according to claim 2, wherein the propagation constant γ of the coaxial cable is expressed by the formula:
Figure FDA0002228001270000011
wherein Z isshFor input impedance of coaxial cable in the event of end short-circuit, ZopThe input impedance of the coaxial cable at the open end is l is the length of the coaxial cable.
4. The test method of claim 3, wherein the attenuation constant α of the coaxial cable is expressed by the formula:
α＝Re[γ]
the formula for phase shift factor β is:
β＝Im[γ]
wherein, gamma is the propagation constant of the coaxial cable, Re represents a complex real part, and Im represents a complex imaginary part.
5. A test method according to claim 3, characterized in that the formula of the phase velocity v of the coaxial cable is: v 2 pi f/Im [ gamma ]
Wherein, gamma is the propagation constant of the coaxial cable, Im represents the imaginary part of the complex number, and f is the test frequency.
6. The test method of claim 2, wherein the formula of the series impedance per unit length z of the coaxial cable is:
Figure FDA0002228001270000021
wherein r iscIs the radius of the conductor core of the coaxial cable, rsInner radius of the outer conductor, mu0For vacuum permeability, ω is the angular frequency at the test frequency, ρc、ρsThe resistivity of the core conductor and the outer conductor shield, respectively.
7. The test method of claim 6, wherein the characteristic impedance z of the coaxial cablecThe formula of (1) is:
Zc＝z/γ
where z is the series impedance per unit length of the coaxial cable and γ represents the propagation constant.
8. The method of claim 1, wherein the input impedance of the coaxial cable at the open end and the input impedance at the short end vary with the test frequency for at least 3 measurements, and averaging the measurements.
9. The system for testing the high-frequency transmission characteristic of the coaxial cable is characterized by comprising the coaxial cable with any length and a testing module, wherein the coaxial cable comprises a conductor core wire and an outer conductor, the testing module forms a testing loop by using the conductor core wire and an outer conductor shielding layer of the coaxial cable, and the input impedance of the coaxial cable under any frequency when the tail end is open-circuited and the input impedance of the coaxial cable under any frequency when the tail end is short-circuited are respectively measured according to a frequency sweeping method to obtain the high-frequency transmission characteristic of the coaxial cable.
10. The test system of claim 9, wherein the coaxial cable is free of faults or intermediate joints.
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CN111929553A (en) * 2020-10-19 2020-11-13 四川大学 Partial discharge positioning method based on phase velocity frequency-varying characteristics
CN113075501A (en) * 2021-03-26 2021-07-06 华中科技大学 Cable fault positioning method and system based on impedance spectrum periodic characteristics
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CN111398722A (en) * 2020-04-14 2020-07-10 西安交通大学 Equipment for on-site measurement of transmission characteristics of power cable
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CN116011266B (en) * 2023-03-28 2024-05-17 西安热工研究院有限公司 Method for inverting electric parameters of long cable by using scattering parameters
CN116643132A (en) * 2023-07-26 2023-08-25 四川省机场集团有限公司成都天府国际机场分公司 Cable insulation on-line monitoring method and device based on high-frequency signals
CN116643132B (en) * 2023-07-26 2023-10-13 四川省机场集团有限公司成都天府国际机场分公司 Cable insulation on-line monitoring method and device based on high-frequency signals

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