Low-voltage cable fault diagnosis positioning method and device based on stepping frequency wave reflection

Abstract

The invention relates to a low-voltage cable fault diagnosis positioning method and device based on stepping frequency wave reflection. The low-voltage cable fault diagnosis and positioning method based on stepping frequency wave reflection comprises the following steps: inputting a stepping frequency signal to a first end of a cable, and receiving a reflection signal of the stepping frequency signal; establishing an equivalent parameter model of the to-be-detected cable; calculating a reflection coefficient L of the first end according to the equivalent parameter model and the reflection signal; judging whether the cable has a fault according to the L; if gamma is equal to 0, the cable is healthy, and if gamma is not equal to 0, the cable has a fault; and determining the fault distance according to the time difference between the step frequency signal and the reflection signal. By injecting the detection signal into the to-be-detected cable and extracting the reflected wave, local insulation damage of the low-voltage cable can be effectively detected, potential fault points can be found in time, accidental power failure and accidents caused by insulation damage can be prevented and reduced, and the safety of a power grid is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a cable, in particular to a low-voltage cable fault diagnosis positioning method based on stepped frequency wave reflection and a low-voltage cable fault diagnosis positioning device based on stepped frequency wave reflection. BACKGROUND

[0002] With the development of cable defect detection technology and the improvement of the national safety requirements for power transmission and distribution network, the problems such as many intermediate branches, significant stress damage, frequent tree barriers and mice, high probability of electric leakage, and high risk of electricity stealing in low-voltage distribution cables need to be solved. The existing overhead cables in the transformer area are affected by the weather, and faults such as insulation damage, loose joints, short circuits, and open circuits occur. In addition, some low-voltage cables are buried in the ground, and it is difficult to detect and locate the faults after the faults occur. In the traditional detection method, the cable line needs to be powered off for detection, and the detection is carried out by a large-scale excavation after power-off fuzzy detection, which has low accuracy, high difficulty in excavation, time-consuming, labor-intensive, and high service pressure. SUMMARY

[0003] Therefore, it is necessary to provide a low-voltage cable fault diagnosis positioning method and device based on stepped frequency wave reflection to solve the problems of low accuracy, time-consuming, and labor-intensive of the existing low-voltage cable fault diagnosis positioning method based on stepped frequency wave reflection.

[0004] In a first aspect, the present application provides a low-voltage cable fault diagnosis positioning method based on stepped frequency wave reflection, which comprises:

[0005] Input a stepped frequency signal to a first end of a cable, and receive a reflected signal of the stepped frequency signal.

[0006] Establish an equivalent parameter model of the cable to be detected; calculate the reflection coefficient G of the first end according to the equivalent parameter model and the reflected signal. The calculation method of G comprises:

[0007] Calculate the propagation coefficient γ0 of a normal cable according to the parameter difference between the reflected signal and the stepped frequency signal; calculate the characteristic impedance Z0 according to the equivalent parameter model; regard the cable to be detected as a first normal cable, a defect cable, and a second normal cable in cascade; first calculate the reflection coefficient G0 of the second normal cable according to Z0 and the load Z L of the second normal cable, and then calculate the input impedance Z(l3) at the junction of the second normal cable and the defect cable according to G0, Z0, and γ0; calculate the reflection coefficient G d of the defect cable according to Z(l3) and the characteristic impedance Z l2 of the defect cable; calculate the input impedance Z(l) of the first normal cable far away from the defect cable according to G l2 , Z0, and γ0; and calculate G according to Z(l) and Z0.

[0008] According to the step frequency signal and the time difference of the reflection signal, the fault distance is determined.

[0009] In a second aspect, the application further provides a low-voltage cable fault diagnosis and positioning device based on step frequency wave reflection, which uses the low-voltage cable fault diagnosis and positioning method based on step frequency wave reflection in the first aspect, and comprises a transmitting unit, a sampling unit and a data processing unit.

[0010] The transmitting unit is configured to transmit the step frequency signal. The sampling unit is configured to receive the reflection signal. The data processing unit is configured to calculate the reflection coefficient G of the first end according to the equivalent parameter model and the reflection signal; determine whether the cable has a fault according to the G; if the G is 0, it indicates that the cable is healthy, and if the G is not 0, it indicates that the cable has a fault; and determine the fault distance according to the time difference of the step frequency signal and the reflection signal.

[0011] The application has the following beneficial effects:

[0012] The application can effectively detect the local insulation damage of the low-voltage cable by injecting a detection signal into the cable to be tested and extracting the reflected wave, so as to timely find the potential fault point, prevent and reduce the unexpected power outage and accidents caused by insulation damage, and improve the safety of the power grid. Moreover, the detection of the low-voltage cable buried in the ground does not need to stop power supply and dig, and the detection of the insulation damage of the cable can be completed in a short time, thereby improving the work efficiency, reducing the workload of the maintenance personnel, producing significant economic benefits, helping to reduce the long-term maintenance cost, and avoiding more serious faults caused by untimely maintenance. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute some embodiments of the application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0014] Figure 1 A schematic diagram of the low-voltage cable defect positioning principle;

[0015] Figure 2 An equivalent parameter model of the low-voltage cable in the embodiment;

[0016] Figure 3 A schematic diagram of the low-voltage cable containing defects in the embodiment;

[0017] Figure 4This is a schematic diagram of the low-voltage cable fault diagnosis and location device in the embodiment. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "or / and" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] This invention introduces a stepped-frequency signal into the detection of local defects in low-voltage cables. It uses a low-voltage cable fault diagnosis and location device to inject a detection signal into the cable under test and extract the reflected wave. The reflected wave is then processed and measured to achieve online detection of local cable defects, thereby diagnosing and locating local insulation faults in low-voltage cables. Based on the above technical concept, this embodiment provides a low-voltage cable fault diagnosis and location method based on stepped-frequency wave reflection. The principle of this method is as follows: when the cable experiences insulation damage or has an intermediate joint, the characteristic impedance at that location changes. A variable-frequency sinusoidal signal is injected into the cable's beginning. When the sinusoidal signal reaches the impedance mismatch point, reflection occurs. Figure 1 As shown, an incident signal is injected at the beginning of the cable, and a reflected signal is collected. By processing the incident and reflected signals, the location of the cable defect is obtained.

[0021] Specifically, the focus of this invention is on how to process the received reflected signal to obtain the required data. The fault diagnosis logic will be described in detail below.

[0022] First, establish an equivalent parameter model for a normal cable, such as... Figure 2 As shown, the equivalent parameter model of a unit length cable includes: resistance R, inductance L, and capacitance C connected in series with the power source, and conductance G connected in parallel with C. Taking the power source location as the starting point, the voltage phasor at a distance x from the power source on the cable is... Current phasor The following transmission equations are satisfied:

[0023]

[0024] In the formula, wL represents inductive reactance, wC represents capacitive reactance, and j represents the imaginary unit. Solving the above differential equation, we get:

[0025]

[0026] In the formula, K1 and K2 represent coefficients.

[0027] Furthermore, based on the equivalent parameter model and the reflected signal, the propagation coefficient γ0 and characteristic impedance Z0 of the normal cable are calculated using the following formulas:

[0028] γ0=α+jβ.

[0029]

[0030] In the formula, α represents the attenuation constant, indicating the amplitude attenuation characteristics of the voltage and current waves. β represents the phase constant, indicating the phase change characteristics of the voltage and current waves. α and β are obtained by analyzing and comparing the amplitude and phase changes of the waveforms between the step-frequency signal and the reflected signal. wL represents the inductive reactance, wC represents the capacitive reactance, and j represents the imaginary unit.

[0031] After analyzing the parametric model of a normal cable, such as Figure 3 As shown, the cable under test can be considered as the first normal cable or the defective cable. Figure 3 The black segment in the middle), and the second normal cable cascaded together. l1, l2, and l3 represent the lengths of the three cable segments, respectively. Figure 3 Second normal cable ( Figure 3 Taking the right end of the normal cable (located at the right end) as the starting point, the formula for calculating the reflection coefficient Г0 at x=0 is:

[0032]

[0033] In the formula, Z L This represents the load on the cable under test. At this point, the input impedance Z(l3) at the cable end, viewed from x = l3 (the boundary between the second normal cable and the defective cable), is:

[0034]

[0035] If Z(l3) at x = l3 is equivalent to a load, then the reflection coefficient Γ at x = l3 is... l2 for:

[0036]

[0037] The input impedance Z(l2) from x = l2 (the boundary between the first normal cable and the defective cable) to x = l3 is:

[0038]

[0039] In the formula, Z d This represents the characteristic impedance of the defective cable. γ d This represents the propagation coefficient of the defective cable. Both values ​​are fixed and can be found in relevant technical manuals.

[0040] Similarly, following the above logic, we can deduce that from the cable's starting end, x = l ( Figure 3 The input impedance Z(l) at the left end of the first normal cable (located on the left) looking towards the defect location x = l2 is:

[0041]

[0042] Therefore, the reflection coefficient Γ at the cable head end can be calculated as follows:

[0043]

[0044] After calculating the value of Γ, it can be determined whether there is a fault in the cable. When the cable is operating healthily and the load is matched, Γ = 0. When there is a local defect in the cable, Γ will change accordingly.

[0045] After determining whether the cable under test is faulty based on Γ, it is also necessary to measure the distance to the fault location. This embodiment determines the fault distance using the time difference between the step frequency signal and the reflected signal. Specifically, the propagation speed v of the step frequency signal is first calculated based on the relative permittivity of the cable, using the following formula:

[0046]

[0047] In the formula, c represents the speed of light in a vacuum. ε r This represents the relative permittivity of the cable insulation material. It is worth noting that ε... r The inherent parameters such as R, L, C, and G mentioned above can be obtained by consulting the cable's technical specifications or manufacturer's instructions.

[0048] Then, by multiplying v by the time difference Δt between the recorded step frequency signal and the reflected signal, the distance between the fault location and the injected step frequency signal location can be obtained.

[0049] In another embodiment, a low-voltage cable fault diagnosis and location device based on step frequency wave reflection is also proposed, which uses the low-voltage cable fault diagnosis and location method based on step frequency wave reflection as described in the above embodiment to diagnose faults in low-voltage cables. Figure 4 As shown, the low-voltage cable fault diagnosis and location device includes: a transmitting unit, a sampling unit, a data processing unit, a power supply unit, and a display unit.

[0050] The system comprises the following components: a transmitting unit for transmitting stepped-frequency signals, which can employ a high-speed DAC (digital-to-analog converter) module; a sampling unit for receiving transmitted signals, which can employ a high-speed ADC (analog-to-digital converter) module; a data processing unit for calculating the reflection coefficient Γ at the first end based on the equivalent parameter model and the reflected signal, and for determining whether a cable fault exists based on Γ; if Γ = 0, the cable is healthy; if Γ is not equal to 0, the cable is faulty. The data processing unit also determines the fault distance based on the time difference between the stepped-frequency signal and the reflected signal; its main control core can be a Xilinx FPGA, model XC7A35T-2FGG484. A power supply unit can be a switching power supply to provide power at various levels for the entire low-voltage cable fault diagnosis and location device. A display unit displays the detection results of cable defects, the location of the defects, and information wave velocity information; its user interface (UI) can be implemented using C language.

[0051] In another embodiment, a live-line detector for local defects in low-voltage cables based on step-frequency wave reflection is proposed, comprising a housing, a processing chip, and an inductive coupler. The inductive coupler includes a ferrite toroidal core, a copper core winding, a metal shielding shell, a coaxial connector, and an insulating outer shell. The outer conductor of the coaxial connector connects to the metal shielding shell, and the inner conductor connects to the copper core winding, forming an electrical circuit. The inductive coupler can be fixed inside the housing using a snap-fit ​​design, used to inject a detection signal into the cable under test and extract the reflected wave. The processing chip is also installed inside the housing, used to calculate the reflection coefficient Γ at the first end based on an equivalent parameter model and the reflected wave signal, and to determine whether a fault exists in the cable based on Γ. If Γ = 0, the cable is healthy; if Γ is not equal to 0, the cable is faulty. The processing chip is also used to determine the fault distance based on the time difference between the step-frequency signal and the reflected wave signal.

[0052] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0053] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for diagnosing and locating faults in low-voltage cables based on the reflection of stepped-frequency waves, characterized in that, It includes: Input a step frequency signal to the first end of the cable and receive the reflected signal of the step frequency signal; Establish an equivalent parameter model for the cable to be tested; calculate the reflection coefficient Γ at the first end based on the equivalent parameter model and the reflected signal; wherein, the calculation method for Γ includes: The propagation coefficient γ0 of the normal cable is calculated based on the parameter difference between the reflected signal and the step frequency signal; the characteristic impedance Z0 is calculated based on the equivalent parameter model; the cable under test is considered as a cascaded structure of a first normal cable, a defective cable, and a second normal cable; first, based on Z0 and the load Z of the second normal cable... L Calculate the reflection coefficient Γ0 of the second normal cable, and then calculate the input impedance Z(l3) at the junction of the second normal cable and the defective cable based on Γ0, Z0, and γ0; based on Z(l3) and the characteristic impedance Z of the defective cable... d Calculate the reflection coefficient Γ of the defective cable l2 According to Г l2 Calculate the input impedance Z(l) of the end of the first normal cable furthest from the defective cable using Z(l) and Z0; calculate Γ based on Z(l) and Z0. The presence of a cable fault is determined by the value of Γ. If Γ = 0, the cable is healthy; if Γ is not equal to 0, the cable is faulty. The fault distance is determined based on the time difference between the step frequency signal and the reflected signal.

2. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 1, characterized in that, Cable faults are diagnosed using step frequency signals of at least two different frequencies.

3. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 1, characterized in that, The equivalent parameter model of a cable per unit length includes: resistance R, inductance L, and capacitance C connected in series with the power source, and conductance G connected in parallel with C; Voltage phasor at a distance x from the power source on the cable Current phasor satisfy: In the formula, K1 and K2 represent coefficients.

4. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 3, characterized in that, The formula for calculating γ0 is: γ0=α+jβ; In the formula, α represents the attenuation constant and β represents the phase constant; α and β are obtained based on the parameter differences between the reflected signal and the step frequency signal.

5. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 4, characterized in that, The formula for calculating Z0 is: In the formula, wL represents inductive reactance, wC represents capacitive reactance, and j represents the imaginary unit.

6. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 1, characterized in that, The formula for calculating Г0 is: Г l2 The calculation formula is: The formula for calculating Г is:

7. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 1, characterized in that, The formula for calculating Z(l3) is: The formula for calculating Z(l) is:

8. The low-voltage cable fault diagnosis and location method based on step frequency wave reflection according to claim 1, characterized in that, The method for calculating the fault distance includes: First, calculate the propagation speed of the step frequency signal based on the relative permittivity of the cable; then multiply the propagation speed by the time difference to obtain the fault distance.

9. A low-voltage cable fault diagnosis and location device based on step frequency wave reflection, characterized in that, It uses the low-voltage cable fault diagnosis and location method based on step frequency wave reflection as described in any one of claims 1 to 8; It includes: The transmitting unit is used to transmit step frequency signals; A sampling unit, used to receive reflected signals; The data processing unit is used to calculate the reflection coefficient Γ at the first end based on the equivalent parameter model and the reflected signal; to determine whether there is a fault in the cable based on Γ; if Γ = 0, it indicates that the cable is healthy, and if Γ is not equal to 0, it indicates that there is a fault in the cable; it is also used to determine the fault distance based on the time difference between the step frequency signal and the reflected signal.

10. The low-voltage cable fault diagnosis and location device based on step frequency wave reflection according to claim 9, characterized in that, The low-voltage cable fault diagnosis and location device also includes: The power supply unit is used to supply power to the transmitting unit, sampling unit, and data processing unit. And / or, it also includes a display unit for displaying the detection results of whether the cable is defective and the location of the defect in the cable.

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