A non-contact cable fault location method

Abstract

A non-contact cable fault location method, which solves the problem of complex existing cable fault detection and belongs to the technical field of cable fault detection. The present invention includes: S1. Excitation signal #imgabs0##imgabs1# S2. Inject the excitation signal into the cable to be measured while measuring the reflection signal; S3. The reflection signal is y i (t), construct an estimation function y i (t) with the same frequency and the same duration as the excitation signal x iE (t), calculate the Euclidean space distance between y i (t) and y iE (t). The amplitude and phase in y iE (t) corresponding to the minimum distance are equal to the amplitude and phase of the reflection signal. Obtain the return loss parameter S of the cable to be measured according to the amplitude and phase of the reflection signal; S4. Perform fault location according to the return loss parameter S of the cable to be measured. It is applicable to various cables and reduces the implementation difficulty.

Description

Technical Field

[0001] The present invention relates to a non-contact cable fault location method, belonging to the technical field of cable fault detection. Background Art

[0002] As an important carrier for power supply and signal transmission, cables are widely used in fields such as electricity, transportation, aerospace, and national defense. Since cables are mainly responsible for transmitting electrical energy and signals in the system, the normal operation of the cables will seriously affect the working state of the system. To improve the reliability and safety of system operation, it is necessary to detect and eliminate cable faults at the early stage before they develop into short circuits or open circuits.

[0003] Most of the current cable fault detection technologies are contact-based, which require using specific cable connectors to connect the measuring instrument to the cable under test. Moreover, different cables may require different connectors. For cables with different characteristic impedances, in order to ensure the measurement accuracy, a complex impedance transformation circuit also needs to be set up. The above operations not only increase the complexity of the operation but also increase the cost.

[0004] On the other hand, the existing contact-based fault detection technologies usually require expensive equipment such as impedance analyzers, vector network analyzers, and high-precision sensors, which further increases the implementation cost and limits the development and application of this technology. Summary of the Invention

[0005] Aiming at the problem of complex cable fault detection in the prior art, the present invention provides a non-contact cable fault location method.

[0006] A non-contact cable fault location method of the present invention includes:

[0007] S1. Determine the excitation signal x i (t);

[0008]

[0009] where A is the signal amplitude, f i is the frequency of the i-th sine signal, θ i is the initial phase of the i-th sine signal, TD i is the waveform duration of the i-th sine signal, L is the starting time difference between the i-th and (i + 1)-th sine waveforms, t is time, l min 、l max are respectively the lengths of the first impedance discontinuity point causing reflection and the cable end, v op represents the transmission speed of electromagnetic waves in the cable, t stepis the time-step resolution, and NP represents the number of consecutive periods of each sinusoidal signal; T i is the period of the sine wave;

[0010] S2. Inject the excitation signal into the cable under test and measure the reflected signal simultaneously;

[0011] S3. Extract the amplitude and phase of the reflected signal to obtain the return loss parameter S of the cable under test 11 ;

[0012] The reflected signal y i (t) is:

[0013]

[0014] where B i is the amplitude of the reflected signal, and the time delay between the reflected signal and the excitation signal is τ;

[0015] Construct an estimation function y i (t) with the same frequency and duration as the excitation signal x iE (t):

[0016]

[0017] In the formula, B Ei is the estimated value of the amplitude of the reflected signal, with a value range of [0, A], and the time constant τ i has a value range of [0, L - TD i ]; Then, calculate the Euclidean space distance d i between y iE (t) and y iE (t):

[0018] d iE = ||y i (t) - y iE (t)||

[0019] d iE The minimum value of corresponds to the amplitude B iE in the estimation function y Ei (t) and the values of the time constant τ i are equal to the amplitude B i of the reflected signal and the time delay τ, respectively;

[0020] The return loss parameter S of the cable under test:

[0021]

[0022] S4. Perform fault location based on the return loss parameter S of the cable under test.

[0023] Preferably, in S2, an excitation signal is injected into the cable under test using an arbitrary function generator, and at the same time, a digital oscilloscope is used to measure the reflected signal; wherein, the arbitrary function generator injects the excitation signal into the cable under test through an inductive coupler, and the reflected signal is input into the digital oscilloscope through the inductive coupler.

[0024] Advantages of the present invention: The present invention will propose a non-contact cable fault location method based on an inductive coupler, which can realize the injection and measurement of detection signals without using specific connectors, and is applicable to various cables, reducing the implementation difficulty. At the same time, the present invention only needs to use an arbitrary function generator and an oscilloscope to realize the acquisition of measurement signals. Compared with expensive impedance analyzers and vector network analyzers, this method greatly reduces the hardware cost. In addition, compared with other non-contact fault detection methods, the present invention can achieve a fault location accuracy at the millimeter (mm) level. In summary, the present invention has strong engineering application capabilities and high engineering application value. Description of the Drawings

[0025] Figure 1 Schematic diagram of the excitation signal;

[0026] Figure 2 Structure of the signal injection and measurement system;

[0027] Figure 3 Schematic diagram of the signal injection and measurement method;

[0028] Figure 4 The excitation signal and the reflected signal measured by the coupler, (a) is measured at port 1 of the cable under test, and (b) is measured at port 2 of the cable under test;

[0029] Figure 5 Estimation results of the reflected signal, (a) is measured at port 1 of the cable under test, and (b) is measured at port 2 of the cable under test;

[0030] Figure 6 Spatial spectrum distribution;

[0031] Figure 7 Kurtosis distribution map;

[0032] Figure 8 Impedance distribution along the cable. Detailed Implementation Manner

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0034] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0035] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but it is not a limitation of the present invention.

[0036] The basic principle of this embodiment: Regarding the cable as a uniform transmission line, when a cable fails, the characteristic impedance at the fault point will change, resulting in impedance mismatch, which will cause part of the transmitted signal to be reflected. By measuring and analyzing the reflected signal, the detection and location of the fault can be achieved. The reflected signal can be measured and processed in the time domain and the frequency domain. Compared with time-domain reflection, the measurement performance of frequency-domain reflection is better, and it can better reflect the subtle changes in the characteristic impedance. This embodiment is also based on this principle, and this embodiment will operate on the reflected signal in the frequency domain to achieve the detection of the fault.

[0037] A non-contact cable fault location method in this embodiment includes:

[0038] Step 1: Determine the excitation signal according to the cable to be measured;

[0039] This embodiment will use a series of sine signals with the same frequency interval as the excitation signal, and its expression is as follows:

[0040]

[0041] where A is the signal amplitude, f i is the frequency of the i-th sine signal, θ i is the initial phase of the i-th sine signal, TD i is the waveform duration of the i-th sine signal, L is the start time difference between the i-th and (i + 1)-th sine waveforms, and t is time;

[0042] The schematic diagram of the excitation signal is as Figure 1 shown. It can be seen from Figure 1 that if L is too small or TD i is too large, it will cause the reflected signal of the previous excitation to overlap with the next excitation, which is not conducive to subsequent data processing. Therefore, it is necessary to perform a constrained design on L and TD i according to the length of the cable:

[0043]

[0044]

[0045] l min 、l max are respectively the length of the first impedance discontinuity point (i.e., the fault point) causing reflection and the end of the cable, and v op represents the propagation speed of electromagnetic waves in the cable;

[0046] TD defined by the above (2) i Although signal overlap is avoided, the cut-off position of the sine wave is uncertain, resulting in increased difficulty in subsequent processing. Therefore, it is necessary to process it so that the waveform cuts off at the zero crossing, that is, the number of continuous sine signal periods NP is always an integer multiple of 1 or 1.5. Therefore, there is:

[0047]

[0048]

[0049] t step is the time step resolution, and NP represents the number of continuous sine signal periods; T i is the period of the sine waveform. After determining the number of periods NP of the first sine waveform, the number of periods of the remaining waveforms can be calculated according to equations (4) and (5).

[0050] Step 2: Inject the excitation signal into the cable under test and measure the reflected signal simultaneously;

[0051] Step 3: Extract the amplitude and phase of the reflected signal to obtain the return loss parameter S 11 ;

[0052] When the injected excitation signal encounters an impedance discontinuity point (fault point), part of the signal is reflected, and the reflected signal y i (t) The mathematical expression is as follows:

[0053]

[0054] where B i is the amplitude of the reflected signal, and the time delay between the reflected signal and the excitation signal is τ;

[0055] can be expressed as:

[0056]

[0057] where l o is the distance between the signal reflection point and the injection point, that is, the fault distance.

[0058] The above is the theoretical expression of the reflection signal. However, the digital oscilloscope measures a series of discrete points and cannot directly obtain the amplitude B of the reflection signal i and the time delay τ between the reflection signal and the excitation signal. Therefore, it is necessary to estimate and process the reflection to obtain the amplitude B of the reflection signal i and the time delay τ between the reflection signal and the excitation signal

[0059] As can be seen from Equations (1) and (7), the excitation signal and its corresponding reflection signal have the same frequency and waveform duration. The difference between the two lies in the amplitude and the starting time point of the waveform. Therefore, the frequency and the number of continuous cycles of the reflection signal can be directly obtained from the excitation signal, and an estimation function with the same frequency and the same duration as the excitation function is constructed

[0060]

[0061] where, is the estimated value of the reflection amplitude B Ei with a value range of [0, A], and the time constant τ i has a value range of [0, L - TD i . Then, the Euclidean space distance d i between y iE (t) and y iE is calculated for each of the sine waveforms used

[0062] d iE = ||y i (t) - y iE (t)|| (9)

[0063] Then, the estimation function y iE (t) corresponding to the minimum value of d iE is the best estimation result of the reflection signal. From the amplitude B Ei and the time constant τ i of the function, the amplitude B of the reflection signal can be obtained i and the time delay τ between the reflection signal and the excitation signal

[0064] According to the above estimation results and combined with the engineering definition of the S-parameters, the return loss parameter of the cable under test can be obtained

[0065]

[0066] S4. Further processing of the parameters shown in Equation (10) can obtain the location of the cable fault point and the impedance information of the fault point

[0067] In this embodiment, in step 2, an arbitrary function generator is used to inject an excitation signal into the cable under test, and at the same time, a digital oscilloscope is used to measure the reflected signal. Among them, the arbitrary function generator injects the excitation signal into the cable under test through a coupler, and the reflected signal is input into the digital oscilloscope through the coupler.

[0068] As for non-contact signal injection and measurement, an inductive coupler is used. The coupler and the cable under test form a transformer, which can realize the injection of the excitation signal and the measurement of the reflected signal.

[0069] Time step resolution t step Is the reciprocal of the sampling rate of the function generator. According to the actual situation of the cable, the sampling rate of the arbitrary function generator, and the selected signal frequency range, the time function data points of the excitation signal can be automatically generated by programming according to formulas (1)-(5). Then, writing this data file into the arbitrary function generator can obtain the programmed excitation signal.

[0070] After using the arbitrary function generator to elaborate the edited signal, the signal is coupled to the cable under test through an inductive coupler, and another inductive coupler connected to the digital oscilloscope is used to collect the signal on the cable at the signal coupling end. The arbitrary function generator and the digital oscilloscope are controlled by a processor through the GPIB bus. The processor is responsible for sending the programmed arbitrary function to the arbitrary function generator to generate the excitation signal, and at the same time controlling the digital oscilloscope to collect the signal and transmit it to the processor for subsequent processing. The system configuration is as Figure 2 shown.

[0071] The installation methods of the coupler are divided into Figure 3 the 3 types shown. (a) Couples the signal to the core wire of the cable under test through the coupler. The shielding layer of the cable under test is grounded, and this ground is the same as the ground of the arbitrary function generator and the oscilloscope. (b) The core wire and the shielding layer of the cable under test form a loop, and the coupler couples the signal to this loop. (c) Couples the signal to the shielding layer of the cable under test through the coupler. The core wire of the cable under test is grounded, and this ground is the same as the ground of the function generator and the oscilloscope. The signal waveforms coupled to the cable by the above 3 coupling methods are the same, and the difference lies in the amplitude of the signal. The amplitude of (b) is the highest, and (a) and (c) are the same. At the same time, in order to avoid strong reflections caused by the open circuit at the end of the cable, a matching resistor can be connected to the end of the cable.

[0072] After completing the estimation of the amplitude and phase (time difference from the excitation signal) of the reflected signal and synthesizing the return loss, fault detection and location can be carried out. The specific fault location implementation methods include 2 types: inverse fast Fourier transform (IFFT) and time reversal multi-signal classification method (TR-MUSIC).

[0073] When using the inverse fast Fourier transform, step 4 includes:

[0074] The TDR time domain reflection method is used to process the return loss parameter S to obtain Z(t):

[0075] TDR Z(t) = Z C ×[(1 + IFFT(A × S)) / (1 - IFFT(A × S))] (11)

[0076] Wherein, Z(t) represents the distribution of the impedance along the cable to be measured over time, and Z C is the characteristic impedance of the cable to be measured, and IFFT(·) represents the inverse fast Fourier transform. From equation (11), the distribution curve of the impedance of the cable over time can be obtained. Combining with the wave velocity v op the impedance distribution Z(x) along the cable to be measured is obtained:

[0077] Z(x) = v op ×TDR Z(t) (12)

[0078] Based on the impedance distribution Z(x), not only the location of the cable fault can be obtained, but also the characteristic impedance of the cable fault point can be obtained, which is used to evaluate the degree of its fault.

[0079] Using IFFT, fault detection and location can be achieved only by using single - end measurement. However, to achieve high - precision fault location and fault degree evaluation, a high bandwidth is required.

[0080] When using the time reversal multi - signal classification method, step 4 includes:

[0081] The time reversal multi - signal classification (TR - MUSIC) can also be used to process the data in equation (10) for fault location. However, this requires measuring once at each end of the cable respectively, and then synthesizing the return losses of the two ports.

[0082] Measure once at each end of the cable to be measured using steps 2 and 3. For the reflected signal measured at port 1, the return loss parameter s 11 is obtained, and for the reflected signal measured at port 2, the return loss parameter s 22 is obtained, and a vector u i that only retains the phase information is obtained:

[0083]

[0084] Wherein, the subscript i = 1, 2, …, N, and N represents the sampling length of the discrete signal;

[0085] Then, construct the Green's function g i (r), and calculate the spatial spectrum distribution Φ(x):

[0086]

[0087] Obtain the spatial kurtosis distribution K of the spatial spectrum at all frequencies:

[0088]

[0089] where r i is the discrete signal value, is the signal mean, and σ t is the standard deviation of the sampling signal;

[0090] Multiply the kurtosis distributions of each frequency component to obtain the final kurtosis:

[0091]

[0092] At this time, the position where the peak of the kurtosis K out is located is the fault position.

[0093] This time reversal multi-signal classification method can easily achieve super-resolution and reach fault location at the mm level. However, this time reversal multi-signal classification method cannot estimate the fault degree. It can be combined with the above IFFT method to achieve high-precision fault location and evaluate the severity of the fault.

[0094] Experiment:

[0095] Taking an RG-58 cable with a length of 51 m as an example, it is preset that the fault is about 20.5 m away from Port 1. Use the method proposed in the present invention to construct an excitation signal within 10 - 30 MHz with a frequency step of 0.1 MHz, and then use an inductive coupler to inject the excitation signal generated by the function generator into the cable in the manner of Figure 3 (b), and the digital oscilloscope measures the excitation signal and the reflected signal as shown in Figure 4 .

[0096] Then use the reflected signal estimation method in Step 3 to estimate it, and the estimation effect is as shown in Figure 5 . The solid line is the measured data, and the dotted line is the result estimated by the method proposed in the present invention. It can be seen that the estimation result is very good.

[0097] And synthesize s 11 and s 22 according to the estimation result according to Equation (10), and use TR-MUSIC to calculate the spatial spectrum as shown in Figure 6 .

[0098] Furthermore, perform kurtosis analysis on Figure 6 to obtain the final kurtosis as shown in Figure 7 . It can be seen that the deviation between the kurtosis peak and the fault position is only 0.17 m, achieving fault location.

[0099] The result obtained by using IFFT is as follows Figure 8 As shown. It can be seen that due to the relatively narrow bandwidth, the resolution of its fault location is very low. However, it can roughly estimate the impedance information of the fault point. Combining it with TR-MUSIC can achieve high-precision fault location and impedance estimation.

[0100] Although the present invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed, as long as they do not deviate from the spirit and scope of the present invention as defined by the appended claims. It should be understood that different dependent claims and the features described herein can be combined in a manner different from that described in the original claims. It should also be understood that the features described in connection with a single embodiment can be used in other described embodiments.

Claims

1. A non-contact cable fault location method, characterized in that, The method includes: S1. Determine the excitation signal x i (t) according to the cable to be measured; where A is the signal amplitude, f i is the frequency of the i-th sine signal, θ i is the initial phase of the i-th sine signal, TD i is the waveform duration of the i-th sine signal, L is the starting time difference between the i-th and the (i + 1)-th sine waveforms, t is time, l min 、l max are the lengths of the first impedance discontinuity point causing reflection and the cable end respectively, v op represents the transmission speed of electromagnetic waves in the cable, t step is the time step resolution, NP represents the number of continuous periods of each sine signal; T i is the period of the sine waveform; S2. Inject an excitation signal into the cable under test and measure the reflected signal at the same time; S3. Extract the amplitude and phase of the reflected signal to obtain the return loss parameter S of the cable under test 11 ; Reflected signal y i (t) is as follows: Among them, B i is the amplitude of the reflected signal, and the time delay between the reflected signal and the excitation signal is τ; Structure and excitation signal x i (t) Estimation function y iE (t) that is of the same frequency and duration as: where B Ei is the estimated value of the reflected signal amplitude, with a value range of [0, A], and the time constant τ i has a value range of [0, L - TD i ; Then, calculate the Euclidean space distance d i between y iE (t) and y iE : d iE = ||y i (t) - y iE (t)|| d iE The estimated function y corresponding to the minimum value iE (t) has an amplitude B Ei and a time constant τ i whose values are respectively equal to the amplitude B i of the reflected signal and the time delay τ; The return loss parameter S of the cable under test: S4. Perform fault location according to the return loss parameter S of the cable under test; In S2, an arbitrary function generator is used to inject the excitation signal into the cable under test, and a digital oscilloscope is used to measure the reflected signal at the same time. Among them, the arbitrary function generator injects the excitation signal into the cable under test through an inductive coupler, and the reflected signal is input to the digital oscilloscope through the inductive coupler.

2. The non-contact cable fault location method according to claim 1, wherein Couple a signal to the core wire of the cable under test through an inductive coupler. The shield layer of the cable under test is grounded, and this ground is the same as the grounds of the arbitrary function generator and the oscilloscope.

3. The non-contact cable fault location method according to claim 1, characterized in that The core wire and the shield layer of the cable under test form a loop, and the inductive coupler couples a signal to this loop.

4. The non-contact cable fault location method according to claim 1, wherein Couple a signal to the shield layer of the cable under test through an inductive coupler. The core wire of the cable under test is grounded, and this ground is the same as the grounds of the function generator and the oscilloscope.

5. The non-contact cable fault location method according to claim 1, characterized in that, An arbitrary function generator and a digital oscilloscope are controlled by a processor via a GPIB bus. The processor sends an arbitrary function to the arbitrary function generator to generate an excitation signal x i (t), and the reflected signal measured by the digital oscilloscope is sent to the processor.

6. The non-contact cable fault location method according to claim 1, wherein The said S4 includes: Adopt the TDR time domain reflection method to process the return loss parameter S to obtain Z(t): TDR Z(t) = Z C ×[(1 + IFFT(A × S)) / (1 - IFFT(A × S))] where \(Z(t)\) represents the distribution of the impedance along the cable under test over time, and \(Z\) C is the characteristic impedance of the cable under test, and IFFT(·) represents the inverse fast Fourier transform; Obtain the impedance distribution Z(x) along the cable under test: Z(x) = v op × TDR Z(t) Obtain the fault location of the cable under test and the characteristic impedance of the fault point of the cable under test according to the impedance distribution Z(x).

7. The non-contact cable fault location method according to claim 1, characterized in that The said S4 includes: Measure once at each end of the cable under test using S2 and S3. For the reflected signal measured at port 1, obtain the return loss parameter s 11 , and for the reflected signal measured at port 2, obtain the return loss parameter s 22 , and obtain the vector u that only retains the phase information i : In the formula, the subscript i = 1, 2, L, N, and N represents the sampling length of the discrete signal; Then, construct the Green's function g i (r), and calculate the spatial spectral distribution Φ(x): Obtain the kurtosis distribution K of space for the spatial spectra at all frequencies: where r i is the discrete signal value, is the signal mean, and σ t is the standard deviation of the sampled signal; the kurtosis distributions of each frequency component are multiplied to obtain the final kurtosis: At this time, the peak position of the kurtosis K out is the fault position.

8. According to the non-contact cable fault location method described in claim 1, characterized in that, where l o is the distance between the signal reflection point and the injection point, i.e., the fault distance.

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