Cable local insulation defect live detection method based on capacitive coupling

Abstract

The invention discloses a cable local insulation defect live detection method based on capacitive coupling, and the method comprises the steps: firstly generating an initial signal, dividing the initial signal into an incident signal and a reference signal, transmitting the incident signal to a to-be-detected cable through an annular coupling capacitor, and receiving a reflected signal reflected by the to-be-detected cable through the annular coupling capacitor; performing frequency mixing on the reflection signal and the reference signal to obtain a frequency mixing signal, and obtaining the distance between the annular coupling capacitor and the cable insulation defect point according to the frequency mixing signal; the method can detect and position the defects of the to-be-detected cable, effectively avoids sudden power failure accidents, can detect and position the local insulation defects of the cable in a live operation state, does not need power failure, does not affect the power supply reliability, can carry out detection at any time, and is high in detection efficiency and high in reliability. The problem of low power failure cable detection efficiency in the prior art is effectively improved.

Description

Technical Field

[0001] The present application belongs to the field of power equipment detection, and specifically relates to a method for detecting local insulation defects of cables based on capacitive coupling. Background Art

[0002] The acceleration of urbanization has made the urban distribution network larger and larger. As a carrier of power transmission, distribution cables will endanger the reliable operation of the distribution network once a fault occurs. Rapidly locating and identifying faults is the key to ensuring the safe and reliable operation of cables. At present, the detection and positioning of cable insulation defects are mainly carried out by power outages. In actual projects, power outages must be specially arranged for cable insulation fault detection, which takes a long time to detect, has a short detection distance, and has low detection efficiency. In the existing online monitoring of cable insulation faults, wireless transmission technology with magnetic and electric fields as transmission media is the most widely used, but high-frequency magnetic fields will be shielded by conductors with low resistivity and cause eddy current losses. Therefore, it will be greatly restricted when used in metal environments, resulting in low accuracy in detecting local insulation defects of cables. Summary of the invention

[0003] In order to solve the problems of low power outage detection efficiency of cable local insulation defects and low detection accuracy of cable local insulation defects caused by high-frequency magnetic field in the prior art, a live detection method for cable local insulation defects based on capacitive coupling was proposed.

[0004] A method for detecting local insulation defects of cables under voltage based on capacitive coupling, comprising: an annular coupling capacitor and a cable to be tested, wherein the annular coupling capacitor is sleeved outside the cable to be tested;

[0005] Step 1: Generate an initial signal and divide the initial signal into an incident signal and a reference signal;

[0006] Step 2: Input the incident signal to the annular coupling capacitor, the annular coupling capacitor transmits the input incident signal to the cable to be tested, the impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflected signal, and transmits the reflected signal to the annular coupling capacitor;

[0007] Step 3: Collect the reference signal and the reflected signal received by the ring coupling capacitor, mix the reference signal and the reflected signal to obtain a mixed signal, and calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point based on the mixed signal.

[0008] A device for detecting local insulation defects of cables under voltage based on capacitive coupling, comprising a signal generator, a power divider, a circulator, a ring coupling capacitor, a mixer, a cable to be tested, an oscilloscope and a host computer; the ring coupling capacitor is sleeved on the outside of the cable to be tested;

[0009] The signal generator is used to generate an initial signal and send the initial signal to the power divider; the power divider is used to divide the received initial signal into an incident signal and a reference signal, and input the incident signal into a ring coupling capacitor through a circulator, and input the reference signal into a mixer; the ring coupling capacitor transmits the input incident signal to the cable to be tested, and the impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflected signal, and transmits the reflected signal to the ring coupling capacitor; the ring coupling capacitor inputs the received reflected signal into the mixer through the circulator;

[0010] The mixer is used to mix the received reference signal and the reflected signal to obtain a mixed signal, and input the mixed signal to the oscilloscope; the oscilloscope is used to display the waveform of the received mixed signal and send the mixed signal to the host computer;

[0011] The host computer is used to calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point according to the mixing signal; the host computer is also used to control the signal generator to generate the initial signal.

[0012] Beneficial Effects

[0013] The present invention discloses a method for detecting local insulation defects of cables under power on the basis of capacitive coupling. First, an initial signal is generated, and the initial signal is divided into an incident signal and a reference signal. The incident signal is transmitted to the cable to be tested through a ring-shaped coupling capacitor. The impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflection signal, and the ring-shaped coupling capacitor receives the reflection signal. The reflection signal is mixed with the reference signal to obtain a mixed signal, and the distance between the ring-shaped coupling capacitor and the impedance discontinuity point of the cable is obtained according to the mixed signal. The method can detect and locate the defects of the cable to be tested, effectively avoiding sudden power outage accidents, and the method can be carried out under power on operation. The detection and positioning of local insulation defects in cables does not require power outages and does not affect power supply reliability. Detection can be carried out at any time with high detection efficiency, which effectively improves the problem of low detection efficiency of power outage cables in the existing technology. The initial signal is a frequency modulated continuous wave, and the capacitive coupling technology reduces the energy loss of the frequency modulated continuous (FMCW) signal during the transmission process. The capacitive coupling method directly attaches the metal electrode to the outer surface of the sheath of the cable accessories without destroying the inherent structure of the cable accessories, so it will not affect the insulation performance of the cable. The method can be widely used in on-site and online detection of distribution cables, improving the reliability of the power grid and having significant social and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a flow chart of a method for detecting local insulation defects of cables based on capacitive coupling according to a specific implementation of the present application;

[0015] Figure 2 This is a structural diagram of a live cable local insulation defect detection device based on capacitive coupling according to a specific implementation of the present application;

[0016] Figure 3 This is an equivalent circuit diagram of a capacitive coupling-based cable local insulation defect live detection device for verifying frequency modulated continuous wave input according to a specific implementation of the present application;

[0017] Figure 4 This is a diagram showing the verification result of the frequency modulated continuous wave input of the live detection device for local insulation defects of cables based on capacitive coupling according to the specific implementation mode of the present application;

[0018] Figure 5 This is a diagram of the signal reception verification results of the live cable local insulation defect detection device based on capacitive coupling according to the specific implementation method of the present application. DETAILED DESCRIPTION

[0019] Specific implementation method 1: The following will be combined with the attached embodiment of the present invention Figure 1 To Attachment Figure 5 , illustrate this implementation mode, and clearly and completely describe the technical solutions in the embodiments of the present invention:

[0020] A method for detecting local insulation defects of cables under voltage based on capacitive coupling, comprising: an annular coupling capacitor and a cable to be tested, wherein the annular coupling capacitor is sleeved outside the cable to be tested;

[0021] Step 1: Generate an initial signal and divide the initial signal into an incident signal and a reference signal;

[0022] Step 2: Input the incident signal to the annular coupling capacitor, the annular coupling capacitor transmits the input incident signal to the cable to be tested, the impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflected signal, and transmits the reflected signal to the annular coupling capacitor;

[0023] Step 3: Collect the reference signal and the reflected signal received by the ring coupling capacitor, mix the reference signal and the reflected signal to obtain a mixed signal, and calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point based on the mixed signal.

[0024] Furthermore, the initial signal is a frequency modulated continuous wave. Specifically, a method for detecting local insulation defects of cables based on capacitive coupling realizes live detection by installing a ring-shaped coupling capacitor on a high-voltage cable and inputting a frequency modulated continuous wave (FMCW) to the ring-shaped coupling capacitor.

[0025] Specifically, the core issue of measurement under power is to solve the injection and reception of FMCW frequency modulated continuous waves; signal injection and measurement require covering the commonly used frequency band of frequency domain reflection (about 1 to 50 MHz), and isolating the 50 Hz high-voltage signal in the cable to prevent high voltage from causing personal injury accidents or destroying equipment.

[0026] Capacitance is a measure of the ability of a capacitor to store charge. It represents the total amount of charge stored per unit voltage, represented by the symbol C, and the unit is F. The definition of capacitance is:

[0027] Among them, Q is the charge carried by the capacitor, and U is the potential difference between the two plates; in the circuit, the capacitive reactance of the capacitor is the impedance of the capacitor to the alternating current of the circuit, and its calculation formula is:

[0028] Among them, X c It represents the capacitive reactance of the capacitor, f represents the frequency of the circuit, and the unit of capacitive reactance is ohm.

[0029] The electric field coupling type mostly uses metal plates for power transmission. A natural dielectric isolator can be formed between the metal plates, and an isolation layer for contactless signal transmission can be formed between the two plates. The size of the metal plates and the distance between the two plates affect the signal transmission performance of the electric field.

[0030] Reasonably design the loop coupling capacitor parameters according to the capacitance of the cable to be tested to minimize the coupling loss of the FMCW sweep signal and achieve efficient injection and measurement;

[0031] The capacitance calculation formula of the ring coupling capacitor is: Where S is the capacitance of the parallel plate capacitor and the area directly opposite to the plate; ε r is the dielectric constant, d is the distance between the two plates, and k is the correction factor.

[0032] Furthermore, the frequency domain waveform expression of the incident signal is: Where s1(t) is the incident signal, A is the incident signal amplitude, and ω0(t) is the angular frequency of the incident cosine signal; is the initial phase of the incident signal, t is the duration of the incident signal;

[0033] The frequency of the incident signal is calculated as:

[0034] Specifically, the incident signal is a linear frequency modulation signal; the incident signal has the following expression in the time domain:

[0035]

[0036] Among them, f rect (t / τ) is a rectangular pulse with a width of τ, f0 is the center frequency, μ is the bandwidth of the incident signal, and j is a complex number.

[0037] Furthermore, the waveform expression of the reflected signal is: Among them, s r(t) is the reflected signal, α is the signal transmission attenuation factor, ω0(tt d ) is the angular frequency of the cosine signal of the reflected signal;

[0038] The frequency of the reflected signal is calculated as: Among them, f r (t) is the reflected signal frequency, f0 is the center frequency, and μ is the incident signal bandwidth.

[0039] Furthermore, the mixing signal waveform expression is: Among them, s out (t) is the mixing signal, f b (t) is the frequency difference between the reference signal and the reflected signal, is the phase of the mixing signal;

[0040] t w is the time difference between the reference signal and the reflected signal.

[0041] Further, the distance between the annular coupling capacitor and the cable insulation defect point is calculated according to the mixed signal, including:

[0042] Where D is the distance between the annular coupling capacitor and the cable impedance discontinuity point, T s is the frequency modulation period of the mixing signal, and Δf is half the bandwidth of the frequency modulation of the mixing signal. Specific implementation method 2:

[0044] A device for detecting local insulation defects of cables under voltage based on capacitive coupling, comprising a signal generator, a power divider, a circulator, a ring coupling capacitor, a mixer, an oscilloscope and a host computer; the ring coupling capacitor is sleeved on the outside of the cable to be tested;

[0045] The signal generator is used to generate an initial signal and send the initial signal to the power divider; the power divider is used to divide the received initial signal into an incident signal and a reference signal, and input the incident signal to the annular coupling capacitor through a circulator, and input the reference signal to the mixer; the annular coupling capacitor transmits the input incident signal to the cable to be tested, the incident signal is transmitted to the cable insulation defect point along the cable extension direction, and is reflected by the cable insulation defect point to generate a reflection signal, and the reflection signal is transmitted to the annular coupling capacitor in a direction opposite to the cable extension direction; the annular coupling capacitor inputs the received reflection signal to the mixer through the circulator;

[0046] The mixer is used to mix the received reference signal and the reflected signal to obtain a mixed signal, and input the mixed signal to the oscilloscope; the oscilloscope is used to display the waveform of the received mixed signal and send the mixed signal to the host computer;

[0047] The host computer is used to calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point according to the mixing signal; the host computer is also used to control the signal generator to generate the initial signal.

[0048] Specifically, the signal generator generates a continuous frequency modulation signal, and the signal is divided into a reference signal and an incident signal through a power divider. The incident signal is transmitted to the cable impedance mismatch point through a circulator through a coupling capacitor. The cable impedance mismatch point reflects the incident signal to generate a reflected signal, and the reflected signal is transmitted to the circulator to the mixer through a coupling capacitor again. The reference signal and the reflected signal are mixed by the mixer to obtain a mixed signal, and the cable impedance mismatch point is located through the mixed signal.

[0049] The frequency modulated continuous wave input of the cable local insulation defect live detection device based on capacitive coupling was verified. The verification results are as follows: Figure 4 As shown; the signal generator acts as the power supply side, one end of the signal generator is connected to the coupling capacitor, and the other end of the signal generator is connected to the copper shielding layer of the test cable. The frequency modulated continuous wave is injected at the head end of the test cable through capacitor coupling. The signal is collected through an oscilloscope at the end of the test cable. One end of the oscilloscope is connected to the core wire of the test cable, and the other end of the oscilloscope is connected to the copper shielding layer of the test cable. The equivalent circuit diagram is shown as follows Figure 3 As shown, the validity of the FMCW input is verified in this way, and the verification results are as follows Figure 4 As shown;

[0050] Verify the reception of reflected signals of the live detection device for local insulation defects of cables based on capacitive coupling. Input frequency modulated continuous wave at the head end of the test cable, connect the cable core wire at one end of the power supply side, and connect the cable copper shielding layer at the other end of the power supply side. Receive at the end of the test cable by capacitive coupling. Connect the coupling capacitor at one end of the oscilloscope and the cable copper shielding layer at the other end of the oscilloscope. In this way, verify the success of signal reception and the verification result. Figure 5 shown.

[0051] Furthermore, the initial signal is a frequency modulated continuous wave. Specifically, a method for detecting local insulation defects of cables based on capacitive coupling realizes live detection by installing a ring-shaped coupling capacitor on a high-voltage cable and inputting a frequency modulated continuous wave (FMCW) to the ring-shaped coupling capacitor.

[0052] Furthermore, the signal generator is a RIGOLDG1062 signal generator; and the oscilloscope is a PicoScope2000 Series oscilloscope.

[0053] Further, the distance between the annular coupling capacitor and the cable insulation defect point is calculated according to the mixed signal, including:

[0054] Where D is the distance between the annular coupling capacitor and the cable impedance discontinuity point, T s is the frequency modulation period of the mixing signal, Δf is the half bandwidth of the frequency modulation of the mixing signal, and f b (t) is the frequency difference between the reference signal and the reflected signal.

[0055] Although the present invention is 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. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A method for detecting local insulation defects of cables based on capacitive coupling, characterized in that: include: Ring coupling capacitor and the cable to be tested, the ring coupling capacitor is set outside the cable to be tested; Step 1: Generate an initial signal and divide the initial signal into an incident signal and a reference signal; Step 2: Input the incident signal to the annular coupling capacitor, the annular coupling capacitor transmits the input incident signal to the cable to be tested, the impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflected signal, and transmits the reflected signal to the annular coupling capacitor; Step 3: Collect the reference signal and the reflected signal received by the ring coupling capacitor, mix the reference signal and the reflected signal to obtain a mixed signal, and calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point based on the mixed signal.

2. The method for detecting local insulation defects of cables based on capacitive coupling according to claim 1 is characterized in that: The initial signal is a frequency modulated continuous wave.

3. The method for detecting local insulation defects of cables based on capacitive coupling according to claim 1 is characterized in that: The frequency domain waveform expression of the incident signal is: Where s1(t) is the incident signal, A is the incident signal amplitude, and ω0(t) is the angular frequency of the incident cosine signal; is the initial phase of the incident signal, t is the duration of the incident signal; The frequency of the incident signal is calculated as:

4. The method for detecting local insulation defects of cables based on capacitive coupling according to claim 3 is characterized in that: The waveform expression of the reflected signal is: Among them, s r (t) is the reflected signal, α is the signal transmission attenuation factor, ω0(tt d ) is the angular frequency of the cosine signal of the reflected signal, t d is the two-way delay between the reflected signal and the incident signal, D is the distance between the annular coupling capacitor and the cable impedance discontinuity point, and v is the average propagation speed of the signal in the cable; The frequency of the reflected signal is calculated as: Among them, f r (t) is the reflected signal frequency, f0 is the center frequency, and μ is the incident signal bandwidth.

5. The method for detecting local insulation defects of cables based on capacitive coupling according to claim 3 is characterized in that: The waveform expression of the mixing signal is: Among them, s out (t) is the mixing signal, f b (t) is the frequency difference between the reference signal and the reflected signal, is the phase of the mixing signal; t w is the time difference between the reference signal and the reflected signal.

6. The method for detecting local insulation defects of cables based on capacitive coupling according to claim 4 is characterized in that: Calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point based on the mixed signal, including: Where D is the distance between the annular coupling capacitor and the cable impedance discontinuity point, T s is the frequency modulation period of the mixing signal, and Δf is half the bandwidth of the frequency modulation of the mixing signal.

7. A device for detecting local insulation defects of cables based on capacitive coupling, characterized in that: It includes a signal generator, a power divider, a circulator, a ring coupling capacitor, a mixer, a cable to be tested, an oscilloscope and a host computer; the ring coupling capacitor is set outside the cable to be tested; The signal generator is used for generating an initial signal and sending the initial signal to the power divider; The power divider is used to divide the received initial signal into an incident signal and a reference signal, and input the incident signal into the annular coupling capacitor through the circulator, and input the reference signal into the mixer; the annular coupling capacitor transmits the input incident signal to the cable to be tested, and the impedance discontinuity point of the cable to be tested reflects the incident signal to generate a reflected signal, and transmits the reflected signal to the annular coupling capacitor; The ring coupling capacitor inputs the received reflected signal to the mixer via the circulator; The mixer is used to mix the received reference signal and the reflected signal to obtain a mixed signal, and input the mixed signal to the oscilloscope; The oscilloscope is used to display the received mixed signal waveform and send the mixed signal to the host computer; The host computer is used to calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point according to the mixing signal; the host computer is also used to control the signal generator to generate the initial signal.

8. The device for detecting local insulation defects of cables based on capacitive coupling according to claim 7 is characterized in that: The initial signal is a frequency modulated continuous wave.

9. The device for detecting local insulation defects of cables based on capacitive coupling according to claim 7, characterized in that: The signal generator is RIGOL DG1062 signal generator; the oscilloscope is PicoScope 2000 Series oscilloscope.

10. The device for detecting local insulation defects of cables based on capacitive coupling according to claim 7, characterized in that: Calculate the distance between the ring coupling capacitor and the cable impedance discontinuity point based on the mixed signal, including: Where D is the distance between the annular coupling capacitor and the cable impedance discontinuity point, T s is the frequency modulation period of the mixing signal, Δf is the half bandwidth of the frequency modulation of the mixing signal, and f b (t) is the frequency difference between the reference signal and the reflected signal.

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