CN107942198A - A kind of apparatus and method of the cable local defect assessment based on impedance spectrum analysis - Google Patents

Abstract

The invention relates to the technical field of measurement and analysis of transmission line impedance spectrum, in particular to a device and method for evaluating cable local defects based on impedance spectrum analysis. The device at least includes a signal generation unit, a signal acquisition unit, an analysis control unit and a data storage unit. unit, the signal generation unit is used to transmit an incident signal to one end of the cable under test and a reference signal, the incident signal and the reference signal have the same amplitude and phase; the signal acquisition unit is used for synchronous acquisition The incident signal and the reflection signal, the reflection signal is the signal reflected by the incident signal after passing through the cable under test, the present invention measures and calculates the frequency spectrum of the impedance of the cable under test in the frequency sweep section through the frequency sweep method, and in the frequency domain Transform the spectral function, realize the location of the defect and evaluate its severity in the new domain after the transformation. Compared with the existing cable fault location technology, the present invention has strong noise resistance, wide applicability, and quantitative evaluation .

Description

A Device and Method for Evaluating Cable Local Defects Based on Impedance Spectrum Analysis

technical field

The invention relates to the technical field of measurement and analysis of transmission line impedance spectrum, in particular to a device and method for evaluating local defects of cables through spectrum measurement, frequency domain transformation and analysis.

Background technique

A transmission line is a linear structure that transmits electromagnetic energy, located between a signal source and a load, and a cable is a typical transmission line. Transmission line theory is a relatively mature theory, which is widely used in professional fields such as communication engineering and electronic circuits. The characteristics of a transmission line depend on the difference between its length and the wavelength of the electrical signal it carries. When the length of the transmission line is much smaller than the signal wavelength, the transmission line hardly affects the behavior of the electrical circuit. At this time, viewed from the signal source side, the line impedance Z _In is equal to the impedance of the load; but when the length of the transmission line is greater than the wavelength of the electrical signal, the transmission line will affect the behavior of the electrical circuit. At this time, from the signal source side, except for some special cases, the line impedance and load impedance are different. .

The cable is used as a transmission line, and the voltage vector V and current vector I along the length direction obey the following differential equation:

Where ω is the radial frequency of the signal, R is the conductor resistance, L is the inductance, C is the capacitance, G is the insulation conductivity, and the last four electrical parameters are the values of the cable per unit length. These four electrical parameters can be used to fully characterize the behavior of the cable when passing high frequency signals. In transmission line theory, the behavior of a transmission line is usually described using two functions of a complex variable, the first being the transfer equation:

Usually abbreviated as follows:

γ=α+jβ (0-04)

Among them, the real part α is called the attenuation constant, and the imaginary part β is called the transfer constant. The relationship between β and the phase velocity ν, radial frequency ω and wavelength λ of the signal is as follows:

The second is the characteristic impedance:

Substituting (0-03)～(0-06) into the differential equations (0-01) and (0-02), the impedance expression related to the length direction d from one end of the cable can be obtained as follows:

where Γ _d is the generalized reflection coefficient, and the expansion formula is as follows:

Γ _d ＝ Γ _L e ^-2γd (0-08)

Where Γ _L is the load reflection coefficient, and the expansion formula is as follows:

where Z _L is the impedance of the cable termination load.

From the equations (0-07)~(0-09), it can be seen that when the load matches the characteristic impedance, at any length and any frequency, there are Γ _L ＝Γ _d ＝0, Z _d ＝Z ₀ ＝Z _L . The transmission line impedance can be described by formula (0-07), which is a complex variable, and the curve of amplitude and phase changing with signal frequency is shown in Figure 1.

The existing method based on the transmission line theory tries to locate the local defect of the cable, using a fixed frequency incident signal, and determining the time delay between the incident signal and the reflected signal by measuring the change of the voltage vector V in the equation (0-01) with time. However, under field conditions, signal attenuation in the cable and environmental noise (especially for cables longer than 1000 meters) limit the sensitivity of this method, making it difficult to detect the initial state of local defects in the cable, let alone reflect local defects. development trend.

The evaluation method of cable local defects based on impedance spectrum analysis lacks relevant work in China, and there is still a lack of dedicated research equipment and technology.

Contents of the invention

The invention provides a device for evaluating cable local defects based on impedance spectrum analysis, which adopts a modular structure design and is used to complete the measurement, calculation and analysis of cable impedance spectrum, thereby locating local defects in the tested cable and evaluating their severity .

In order to achieve the above object, the technical solution adopted by the present invention is: a device for evaluating local defects of cables based on impedance spectrum analysis, the device at least includes:

A signal generating unit, which is used to generate a sine wave signal of a specified frequency, one of which is used as an incident signal transmitted to one end of the cable under test, and the other is used as a reference signal, and the incident signal and the reference signal have the same amplitude and phase ;

A signal acquisition unit, which has a first acquisition channel for acquiring the reference signal and a second acquisition channel for synchronously acquiring a reflection signal, wherein the reflection signal is formed by the incident signal passing through the cable under test reflected signal;

An analysis control unit, which is connected to the signal generation unit and the signal acquisition unit respectively, includes a control module that controls the signal frequency and duration of the incident signal and the reference signal, and controls the signal acquisition unit A data analysis module for processing the reflected signal collected by the unit and the reference signal;

An analysis control unit, which is respectively connected to the signal generation unit and the signal acquisition unit, includes a control module that controls the transmission time and signal frequency of the incident signal and the reference signal, and controls the signal acquisition unit A data analysis module for processing the reflected signal collected by the unit and the reference signal;

A data storage unit is connected in communication with the analysis control unit to store the signal data acquired by the analysis control unit and the result data of analysis processing.

Further, the device further includes a communication unit communicating with the data storage unit for sending and receiving data.

Further, the device further includes a human-computer interaction unit that is respectively communicatively connected to the analysis control unit, the data storage unit, and the communication unit, and the human-computer interaction unit has a human-computer interaction interface and a display interface, It is used to receive input information and display the information through the display interface.

Further, the incident signal and the reference signal are swept sine wave signals not higher than 100MHz, and the signal acquisition unit is a dual-channel digital oscilloscope.

The present invention also provides a method for evaluating cable local defects based on impedance spectrum analysis, comprising the following steps:

(1) Using the signal generation unit to generate the incident signal and the reference signal, the incident signal is injected from one end of the cable under test, and the first acquisition channel of the signal acquisition unit is used to collect the incident signal Synchronously, using the second acquisition channel of the signal acquisition unit to acquire the reference signal as the reflected signal after the incident signal passes through the cable under test;

(2) Calculate the impedance of the cable under test in the frequency sweep section according to the reflected signal and the reference signal;

(3) define a new domain transformation, transform the spectral function of the impedance that step (2) calculates into the power spectral function along the length direction of the cable under test;

(4) The point where the power spectrum function is greater than 0dB in the step (3) is regarded as an echo due to the presence of electrical performance discontinuity along the length direction of the tested cable, and the position of the electrical performance discontinuity is is the local defect position of the tested cable.

Further, in the step (2), the impedance calculation method of the tested cable is as follows:

Wherein, _ZDUT is the impedance of the cable under test, Z1 is the impedance of the _first acquisition channel used to collect the reflected signal, and V1 is the voltage vector value measured at the _first acquisition channel place, V ₀ is the voltage vector value measured at the second acquisition channel.

Further, in the step (3), the method for transforming the calculated spectral function into a power spectral function is as follows:

f→t′ (1-02)

where ν _r =ν/ν ₀ ;

f is the frequency of the incident signal, and f is an independent variable;

d is the length of the cable under test;

ν is the phase velocity at which the incident signal propagates in the cable under test;

ν _r is the relative phase velocity of the incident signal in the cable under test;

_ν0 is the speed of light in vacuum.

Further, the location method of defect position in described step (4) is as follows:

Calculate the frequency f' of the function in the domain t',

f' is the fundamental frequency of the phase function in the t' domain, and is also the time when the incident signal propagates from the incident point of the cable under test to the terminal of the cable under test and then returns to the incident point;

When there is a local defect at a distance x from the incident point, an echo is generated there, so that a new frequency component is generated in the power spectrum of the impedance phase, and this frequency is:

By identifying f' and f" in the impedance phase power spectrum in the t' domain, the position of the echo due to the electrical parameter discontinuity is calculated:

The x obtained by the above calculation is the distance between the defect position of the tested cable and the incident point.

Further, when the tested cable has multiple local defects along its length direction, each defect position corresponds to a frequency component in the power spectrum, and the corresponding defect position x _{n is calculated by the frequency component f n :}

The x _n obtained by the above calculation is the distance between the defect corresponding to the frequency component f _n and the incident point.

Further, the evaluation method also includes the following steps: (5) find out the mirror image peak generated by the defect in the power spectrum on the other side of the cable terminal under test, wherein the mirror image peak is located at the same position as the The distance from the terminal of the cable under test is equal to the distance from the defect part to the terminal of the cable under test, and the peak of the defect part is connected with the peak of the mirror image with a straight line as the echo signal along the length direction of the cable under test. There is an intersection point between the trend line and the vertical auxiliary line with the terminal position of the tested cable as the abscissa, and the distance between the intersection point and the peak-to-peak value of the terminal is used to evaluate the The severity of local defects in the cable.

After adopting the above technical scheme, the present invention has the following advantages compared with the prior art:

1. Non-destructive testing. The invention can use an incident signal with a voltage as low as 5V for detection, without damaging the tested cable and terminal load.

2. Strong noise resistance. The impedance calculation method used in the present invention performs filtering through a window function, which can effectively eliminate the influence of field noise on measurement results.

3. Wide range of applicability. The invention is applicable to various types of cables, especially coaxial cables and metal armored single-core power cables. The invention is suitable for cables with a length ranging from 20 meters to several thousand meters, and can provide technical support for electric power enterprises such as power plants and power transmission and distribution.

4. Simple assembly, suitable for harsh environment on site. The present invention adopts a modular structure design, wherein the signal generation unit and the signal acquisition unit can use commercial product modules such as vector network analyzers and digital oscilloscopes, and the system control and data analysis can be realized on a PC through programming, and the device integration degree is high. Can be used in harsh environments.

5. Quantification of evaluation conclusions. The invention can accurately locate the local defect of the cable, and can quantitatively evaluate the severity of the defect at the same time, and has extremely high application value in cable aging management and predictive maintenance activities.

Description of drawings

Accompanying drawing 1 is transmission line (cable) impedance schematic diagram in the technical background of the present invention;

Accompanying drawing 2 is the structural block diagram of detection device among the present invention;

Accompanying drawing 3 is tested cable impedance measurement wiring diagram among the present invention;

Accompanying drawing 4 is the power spectrum figure along the length direction of tested cable;

Figure 5 is a schematic diagram of the calculation method for the severity of local defects of the tested cable.

Among them, 1. Signal generation unit; 2. Signal acquisition unit; 3. Analysis control unit; 4. Data storage unit; 5. Communication unit; 6. Human-computer interaction unit; 7. Tested cable.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

A device for evaluating cable local defects based on impedance spectrum analysis, as shown in Figure 2, at least includes a signal generation unit 1, a signal acquisition unit 2, an analysis control unit 3 and a data storage unit 4.

The signal generation unit 1 is used to generate a sine wave signal of a specified frequency. The sine wave signal is divided into two paths, one of which is used as the incident signal CH0' transmitted to one end of the cable under test, and the other as the reference signal CH0. The incident signal CH0' and The reference signal CH0 has the same amplitude and phase.

The signal acquisition unit 2 has a first acquisition channel for acquiring a reference signal CH0 and a second acquisition channel for synchronously acquiring a reflected signal CH1, the reflected signal CH1 is a signal reflected after the incident signal CH0' passes through the cable 7 under test.

The analysis control unit 3 is connected to the signal generation unit 1 and the signal acquisition unit 2 respectively. The analysis control unit 3 includes a control module that controls the signal frequency and duration of the reference signal CH0 and the incident signal CH0', and controls the signal acquisition unit 2. A data analysis module for processing the collected reference signal CH0 and reflected signal CH1.

The data storage unit 4 is communicatively connected with the analysis control unit 3 for storing data. In this embodiment, the data storage unit 4 uses a reasonable data structure to store the information of the tested cable 7, detected data and analysis results, and has a historical data retrieval function.

In this embodiment, the reference signal CH0 and the incident signal CH0' are sweeping sine wave signals with a frequency not higher than 100 MHz.

In this embodiment, the signal acquisition unit 2 is a dual-channel digital oscilloscope.

In this embodiment, the device also includes a communication unit 5 and a human-computer interaction unit 6 , and the association between each unit module is shown in FIG. 2 .

The communication unit 5 communicates with the data storage unit 4 for sending and receiving data, so that the device of the present application can exchange and share data with other devices.

The human-computer interaction unit 6 is connected to the analysis control unit 3, the data storage unit 4 and the communication unit 5 respectively. The human-computer interaction unit 6 has a human-computer interaction interface and a display interface, and is used to receive the information and information of the tested cable 7 input by the user. Detect instructions, and display the analysis data such as the impedance spectrum diagram of the tested cable 7, the power spectrum diagram of the tested cable 7, and provide the current and historical data comparison results.

In actual operation, the frequency sweep range and number of sweep points of the reference signal CH0 and the incident signal CH0' are specified by the operator. For example, if the frequency sweep range is set to 1-100MHz and the number of sweep points is 1,000,000, then the step of the incident signal CH0' and the reference signal CH0 The input frequency is 1Hz, 100Hz, 200Hz, 300Hz, 400Hz, ... Each frequency lasts at least 10 complete cycles of sine wave. In order to ensure the accuracy of the detection results, the sampling frequency should be greater than 100 times the frequency of the reference signal.

The present invention also provides a method for evaluating local defects of cables using the above-mentioned device, including the following steps:

(1) Use the signal generation unit 1 to transmit the incident signal CH0' and the reference signal CH0, the incident signal CH0' is injected from one end of the cable 7 under test, and the first acquisition channel of the signal acquisition unit 2 is used to collect the incident signal CH0' through the measured The reflected signal CH1 behind the cable 7 is synchronized, and the second acquisition channel of the signal acquisition unit 2 is used to acquire the reference signal CH0;

(2) Calculate the impedance of the cable 7 under test in the frequency sweep section according to the reflected signal CH1 and the reference signal CH0;

(3) define a new domain transform, transform the spectral function of the impedance that step (2) calculates into the power spectral function along the 7 length directions of the cable under test;

(4) The point where the power spectrum function is greater than 0dB in step (3) is regarded as an echo due to the existence of electrical performance discontinuity along the length direction of the tested cable 7, and the position of the electrical performance discontinuity is the measured cable. local defect location.

The reference signal CH0 selects the frequency sweep signal from ω ₁ to ω ₂ , and the determination of ω ₂ depends on the length of the tested cable 7. Generally, the shorter the tested cable 7, the larger the value of ω ₂ ; the reflected signal CH1 is the incident After the signal CH0' passes through the impedance Z _DUT of the cable under test 7, its amplitude and phase have changed; the impedance Z _DUT of the cable under test 7 calculated using the reflected signal CH1 and the reference signal CH0 is the incident signal CH0 'The function of the frequency, the calculation formula is as follows:

Among them, Z _DUT is the impedance of the cable 7 under test, and Z ₁ is the impedance of the first acquisition channel used to collect the reflected signal CH1, which can be selected from 20 ohms or 50 ohms according to the characteristic impedance of the cable 7 under test; V ₁ is the first acquisition The voltage vector value measured at the channel; V ₀ is the voltage vector value measured at the second acquisition channel.

The schematic diagram of the measurement circuit is shown in Fig. 3, wherein Rb is the internal resistance of the signal generating unit 1, and Rg is the parasitic impedance on the circuit. The above formula (1-01) shows that Z _DUT has nothing to do with resistors Rb and Rg, and Z _DUT is only a transfer function from V ₀ to V ₁ .

By selecting an appropriate window function (such as a Hamming window) to eliminate noise, according to formula (1-01), the frequency spectrum of the impedance of the tested cable 7 between ω ₁ and ω ₂ in the frequency sweep can be calculated.

According to the impedance spectrum function of the tested cable 7, the following parameters can be calculated:

(1) Resonant frequency, the frequency value at which the impedance phase angle is zero.

(2) Calculate the impedance Z _DUT of the cable under test 7 , and correspond to the magnitude of the impedance at any local maximum value (or minimum value) of the impedance phase angle.

The present invention also provides a method for locating the local defect position of the tested cable 7 through the impedance Z _DUT of the tested cable 7 .

Equation (0-07) is the expression of the complex variable function in Fig. 1, in which the magnitude and phase of the impedance vary with the frequency of the incident signal CH0'. The pseudo-periodicity of the impedance spectrum function is derived from the periodicity of Γ _d , and the Γ _d equation (0-08) can also be written as:

Γ _d ＝ Γ _L · e ^-2γd ＝ Γ _L · e ^-2αd · e ^-2jβd (0-10)

Due to the attenuation factor α, the magnitude of the impedance drops along the cable length d. If d is taken as an independent variable, the pseudo-period of Γ _d is 1/2β.

Substituting equation (0-05) into equation (0-10) has

where f is the frequency of the incident signal CH0' and ν is the phase velocity of signal propagation in the cable 7 under test. Let f be the independent variable and define a new transformation:

f→t′ (1-02)

Wherein ν _r =ν/ν ₀ , ν _r is the relative phase velocity of the incident signal CH0' in the cable 7 under test, and ν ₀ is the propagation speed of light in vacuum.

Set Γ _L e ^-2αd as A, then the equation (0-11) is transformed by t′ into:

Γ _d ＝A·e ^-jω′t′ (0-12)

Equation (0-12) gives a pseudo-periodic function with radial frequency ω' and amplitude A. In the actual lossy cable 7 under test, the attenuation coefficient α makes the vibration A decrease gradually with t′, thus producing damped oscillation as shown in FIG. 1 . In the t' domain, the frequency f' of this function is:

Where f' is the fundamental frequency of the phase function in the t' domain, and it is also the time for the incident signal CH0' to propagate from the incident point to the terminal d of the cable 7 under test and then return to the incident point, so f' also has a time dimension. As shown in Figure 4, in the t' domain, the terminal d of the horizontal axis of the Fourier transform (power spectrum) of the impedance phase corresponds to the fundamental frequency f' given in equation (1-04).

When there is a local defect (usually manifested as a discontinuity in electrical parameters, such as a small change in the dielectric properties of the insulation) at a distance from the incident point x of the tested cable 7, another echo will be generated there, which will cause the power of the impedance phase A new frequency component is generated in the spectrum. According to equation (1-04), this frequency is:

If the length d of the tested cable 7 is known, by identifying f' and f" in the impedance phase power spectrum in the t' domain, according to the equations (1-04) and (1-05), the electrical discontinuity of the cable can be calculated Sex (i.e. the position at x):

The above method can be used to calculate any position where an echo is generated due to the discontinuity of the electrical parameters, and the frequency components other than the fundamental frequency in the transmission line impedance spectrum function represent an echo generated at a certain position. As shown in FIG. 4 , in the t′ domain, an echo occurs at x on the horizontal axis of the power spectrum of the impedance phase (that is, at a distance from the incident point x) due to the electrical parameter discontinuity of the tested cable 7 .

When performing state monitoring on the cable 7 under test, usually a number of positions where the electrical parameters are discontinuous are calculated, each position corresponds to a frequency component f _n in the power spectrum, and the corresponding defect position x _n is calculated according to the following method:

The x _n obtained by the above calculation is the distance between the defect corresponding to the frequency component f _n and the incident point.

The present invention also provides a method for evaluating the severity of local defects of the tested cable 7 . Any peak higher than 0dB in the power spectrum shown in Fig. 4 can be determined to have a local defect at its location. However, the peak value here cannot be directly used to evaluate the severity of the local defect, because the peak value is also affected by the distance between the defect point and the incident point, the attenuation of the signal in the tested cable 7 and the bandwidth of the sweep signal.

A state indicator DNORM (degree of normalized degradation) is defined for evaluating the severity of the defect of the tested cable 7 . Accompanying drawing 5 has given the definition of DNORM with the power spectrum image: any peak at the local defect will produce a mirror image peak on the other side of the terminal of the cable 7 under test, and this mirror peak is caused by the difference between the defect and the terminal of the cable 7 under test. The distance between the position of the image peak and the terminal of the tested cable 7 is equal to the distance between the local defect and the terminal of the tested cable 7. It is known that the length of the tested cable 7 is d, and the local The position of the defect is x, then the distance between the image peak and the incident point of the tested cable 7 is 2×d-x. The peak top of the peak generated by the defect position is connected with the peak top of the mirror image peak by a straight line, and the slope of the straight line is used to represent the variation trend of the amplitude of the echo peak along the length of the cable. Let TP be the peak value of the terminal of the tested cable 7, and DNORM is defined as the difference between the defect peak along the trend line to the terminal peak and TP. This method can effectively compensate the attenuation of the echo generated at the defect along the length direction of the tested cable 7. The larger the absolute value of DNORM, the stronger the echo generated at the defect and the more serious the defect. Usually, the absolute value of DNORM greater than 10 is used as the criterion for the local defect of the tested cable 7 . For the same cable, the DNORM detected at different times can be used to track the deterioration trend of the defect.

Referring to accompanying drawing 4 and accompanying drawing 5, in the present embodiment, the length of tested cable 7 is d=30.5 meters, and the distance x=22.3 meters of the position of recorded local defect and incident point, then the position of mirror image peak is 2 ×d-x=38.7 meters, the abscissa in the connecting power spectrum is 22.3 meters and the peaks of the two peaks at 38.7 meters form a trend line, DNORM is defined as the abscissa at 30.5 meters, the trend line and the peak value of the cable terminal are in the vertical direction difference in coordinates.

The present invention measures and calculates the frequency spectrum of the measured cable 7 impedance in the sweeping frequency band by means of a frequency sweeping method, and transforms the spectral function in the frequency domain to realize the location of defects in the new domain. The present invention is compatible with the existing cable fault location technology Compared with the invention, it has strong noise resistance, wide applicability, and can evaluate the severity of the defect of the tested cable 7 .

The above-mentioned embodiments are only to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and not to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit of the present invention shall fall within the protection scope of the present invention.

Claims (10)

1. A device for cable local defect assessment based on impedance spectrum analysis, characterized in that said device at least includes: A signal generating unit, which is used to generate a sine wave signal of a specified frequency, one of which is used as an incident signal transmitted to one end of the cable under test, and the other is used as a reference signal, and the incident signal and the reference signal have the same amplitude and phase ; A signal acquisition unit, which has a first acquisition channel for acquiring the reference signal and a second acquisition channel for synchronously acquiring a reflection signal, wherein the reflection signal is formed by the incident signal passing through the cable under test reflected signal; An analysis control unit, which is connected to the signal generation unit and the signal acquisition unit respectively, includes a control module that controls the signal frequency and duration of the incident signal and the reference signal, and controls the signal acquisition unit A data analysis module for processing the reflected signal collected by the unit and the reference signal; A data storage unit is connected in communication with the analysis control unit to store the signal data acquired by the analysis control unit and the result data of analysis processing. 2. A device for evaluating cable local defects based on impedance spectrum analysis according to claim 1, characterized in that said device further comprises a communication unit communicating with said data storage unit for sending and receiving data. 3. A device for evaluating cable local defects based on impedance spectrum analysis according to claim 2, characterized in that: said device also includes a device that communicates with said analysis control unit, said data storage unit and said communication unit respectively A human-computer interaction unit connected by communication, the human-computer interaction unit has a human-computer interaction interface and a display interface, and is used to receive input information and display the information through the display interface. 4. A device for evaluating cable local defects based on impedance spectrum analysis according to claim 1, characterized in that: said incident signal and said reference signal are sweeping sine wave signals not higher than 100MHz, said The signal acquisition unit is a dual-channel digital oscilloscope. 5. A method for evaluating cable local defects using the device according to any one of claims 1 to 4, characterized in that it comprises the steps of: (1) Using the signal generation unit to generate the incident signal and the reference signal, the incident signal is injected from one end of the cable under test, and the first acquisition channel of the signal acquisition unit is used to collect the incident signal Synchronously, using the second acquisition channel of the signal acquisition unit to acquire the reference signal as the reflected signal after the incident signal passes through the cable under test; (2) Calculate the impedance of the cable under test in the frequency sweep section according to the reflected signal and the reference signal; (3) define a new domain transformation, transform the spectral function of the impedance that step (2) calculates into the power spectral function along the length direction of the cable under test; (4) The point where the power spectrum function is greater than 0dB in the step (3) is regarded as an echo due to the presence of electrical performance discontinuity along the length direction of the tested cable, and the position of the electrical performance discontinuity is is the local defect position of the tested cable. 6. The method for cable local defect assessment according to claim 5, characterized in that: in the step (2), the calculation method of the measured cable impedance is as follows: <mrow><msub><mi>Z</mi><mrow><mi>D</mi><mi>U</mi><mi>T</mi></mrow></msub><mo>=</mo><msub><mi>Z</mi><mn>1</mn></msub><mo>&amp;CenterDot;</mo><mrow><mo>(</mo><mfrac><msub><mi>V</mi><mn>0</mn></msub><msub><mi>V</mi><mn>1</mn></msub></mfrac><mo>-</mo><mn>1</mn><mo>)</mo></mrow><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>01</mn><mo>)</mo></mrow></mrow> Wherein, _ZDUT is the impedance of the cable under test, Z1 is the impedance of the _first acquisition channel used to collect the reflected signal, and V1 is the voltage vector value measured at the _first acquisition channel place, V ₀ is the voltage vector value measured at the second acquisition channel. 7. the method for cable local defect assessment according to claim 6, is characterized in that: in described step (3), the method that the spectrum function that calculates is transformed into power spectrum function is as follows: f→t′ (1-02) <mrow><mfrac><mrow><mn>4</mn><mi>&amp;pi;</mi><mi>d</mi></mrow><mrow><msub><mi>v</mi><mi>r</mi></msub><msub><mi>v</mi><mn>0</mn></msub></mrow></mfrac><mo>&amp;RightArrow;</mo><msup><mi>&amp;omega;</mi><mo>&amp;prime;</mo></msup><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>03</mn><mo>)</mo></mrow></mrow> where ν _r =ν/ν ₀ ; f is the frequency of the incident signal, and f is an independent variable; d is the length of the cable under test; ν is the phase velocity at which the incident signal propagates in the cable under test; ν _r is the relative phase velocity of the incident signal in the cable under test; _ν0 is the speed of light in vacuum. 8. The method for cable local defect assessment according to claim 7, characterized in that: the positioning method of the defect position in the step (4) is as follows: Calculate the frequency f' of the function in the domain t', <mrow><msup><mi>f</mi><mo>&amp;prime;</mo></msup><mo>=</mo><mfrac><msup><mi>&amp;omega;</mi><mo>&amp;prime;</mo></msup><mrow><mn>2</mn><mi>&amp;pi;</mi></mrow></mfrac><mo>=</mo><mfrac><mrow><mn>2</mn><mi>d</mi></mrow><mrow><msub><mi>v</mi><mi>r</mi></msub><msub><mi>v</mi><mn>0</mn></msub></mrow></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>04</mn><mo>)</mo></mrow></mrow> f' is the fundamental frequency of the phase function in the t' domain, and is also the time when the incident signal propagates from the incident point of the cable under test to the terminal of the cable under test and then returns to the incident point; When there is a local defect at a distance x from the incident point, an echo is generated there, so that a new frequency component is generated in the power spectrum of the impedance phase, and this frequency is: <mrow><msup><mi>f</mi><mrow><mo>&amp;prime;</mo><mo>&amp;prime;</mo></mrow></msup><mo>=</mo><mfrac><mrow><mn>2</mn><mi>x</mi></mrow><mrow><msub><mi>v</mi><mi>r</mi></msub><msub><mi>v</mi><mn>0</mn></msub></mrow></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>05</mn><mo>)</mo></mrow></mrow> By identifying f' and f" in the impedance phase power spectrum in the t' domain, the position of the echo due to the electrical parameter discontinuity is calculated: <mrow><mi>x</mi><mo>=</mo><mi>d</mi><mo>&amp;CenterDot;</mo><mfrac><msup><mi>f</mi><mrow><mo>&amp;prime;</mo><mo>&amp;prime;</mo></mrow></msup><msup><mi>f</mi><mo>&amp;prime;</mo></msup></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>06</mn><mo>)</mo></mrow></mrow> The x obtained by the above calculation is the distance between the defect position of the tested cable and the incident point. 9. The method for cable local defect assessment according to claim 8, characterized in that: when the cable under test has a plurality of local defects along its length direction, each defect position corresponds to a frequency in the power spectrum Component, calculate the defect position x _{n corresponding to it through the frequency component f n :} <mrow><msub><mi>x</mi><mi>n</mi></msub><mo>=</mo><mi>d</mi><mo>&amp;CenterDot;</mo><mfrac><msub><mi>f</mi><mi>n</mi></msub><msup><mi>f</mi><mo>&amp;prime;</mo></msup></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>-</mo><mn>07</mn><mo>)</mo></mrow></mrow> The x _n obtained by the above calculation is the distance between the defect corresponding to the frequency component f _n and the incident point. 10. The method for cable local defect evaluation according to claim 5, characterized in that, said evaluation method also comprises the steps of: (5) find out the defect position in the other end of said tested cable terminal in the power spectrum. The mirror image peak generated on the side, wherein, the distance from the position of the mirror image peak to the terminal of the tested cable is equal to the distance from the defect site to the terminal of the cable under test, and the peak of the defect site is compared with the peak of the mirror image peak The top is connected with a straight line as the trend line of the attenuation of the echo signal along the length direction of the tested cable, and there is an intersection point between the trend line and the longitudinal auxiliary line with the terminal position of the tested cable as the abscissa, using the The distance between the intersection point and the peak-to-peak value of the terminal is used to evaluate the severity of local defects of the cable.