New method for measuring resistance of high voltage cable sheath circuit based on injection of hetero-frequency signal coupling

Abstract

This invention discloses a novel method for measuring the resistance of high-voltage cable sheath circuits based on heterogeneous frequency signal coupling injection. The method includes five steps: Step 1, optimization and determination of the heterogeneous operating frequency; Step 2, heterogeneous frequency excitation signal coupling injection; Step 3, response current acquisition and phase-locked loop extraction; Step 4, lumped parameter equivalent circuit modeling and circuit voltage equation establishment; and Step 5, multi-point injection matrix decoupling and impedance parameter solution. This invention injects a specific frequency excitation signal, distinct from the power frequency, into the tested energized sheath circuit through non-invasive electromagnetic coupling. Combined with a matrix decoupling algorithm for cross-connection systems, the actual resistance value of each segment of the sheath circuit is accurately calculated from the measurement data. This invention achieves uninterrupted power supply detection, possesses strong anti-power frequency interference capabilities, and can quantitatively detect resistance changes at the milliohm level. It can effectively detect defects in their early stages, such as slight bolt loosening and early oxidation of contact surfaces, achieving true early warning and defect location.

Description

Technical Field

[0001] This invention relates to the field of power system testing technology, and specifically to a novel method for measuring the circuit resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection. Background Technology

[0002] High-voltage power cables are the core of urban power transmission networks. To suppress induced voltage, their metal sheaths commonly employ cross-interconnected grounding. During long-term operation, grounding loop connection points are prone to early defects such as poor contact due to corrosion and vibration, manifesting as an increase in milliohm-level resistance. However, existing detection technologies have significant bottlenecks: while the power outage detection method is accurate, it requires the line to be de-energized, failing to meet the requirements of condition-based maintenance; the sheath circulating current live detection method has low sensitivity, and early small resistance changes causing circulating current fluctuations are easily masked by normal factors such as load fluctuations; infrared thermometry has a lag effect, as the initial heat generation of defects is extremely small, failing to generate a recognizable temperature rise, resulting in "late" diagnosis. Furthermore, the fundamental technical obstacle to directly measuring milliohm-level resistance under energized conditions lies in the presence of power frequency induced voltages of tens of volts and power frequency circulating currents of tens of amperes in the sheath loop, creating an extremely strong electromagnetic interference background that drowns out any weak measurement signals, making effective measurement impossible. Summary of the Invention

[0003] To address this issue, the present invention provides a novel method for measuring the resistance of high-voltage cable sheath circuits based on heterogeneous frequency signal coupling injection. This method solves the problem in the prior art where the sheath circuit contains power frequency induced voltages of up to tens of volts and power frequency circulating currents of tens of amperes, creating an extremely strong electromagnetic interference background that drowns out any weak measurement signals, making effective measurement impossible.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] A novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection includes the following steps:

[0006] Step 1: Collect the structural parameters and soil environment parameters of the high-voltage cable under test, establish an electromagnetic wave propagation model of the sheath-ground loop, calculate the attenuation coefficient and wavelength of electromagnetic waves at different frequencies, and select the upper limit of the frequency of the injected signal based on the effectiveness of the lumped parameter model and the signal-to-noise ratio requirements, and determine a working frequency that is different from the power frequency and its integer multiples of harmonics.

[0007] Step 2: Using electromagnetic coupling, without interrupting power to the cable line, sequentially inject sinusoidal excitation voltage signals of the operating frequency selected in Step 1 into the three-phase sheath grounding circuit of the high-voltage cable under test.

[0008] Step 3: Synchronously acquire the response current signal generated in the three-phase sheath grounding circuit each time an excitation signal is injected, and use lock-in amplification technology to extract the amplitude and phase information of each response current from the strong power frequency background noise;

[0009] Step 4: Equivalent the three-phase sheath cross-interconnection grounding system to a linear time-invariant lumped parameter circuit network containing self-impedance and mutual impedance. For each injection, establish a loop voltage equation based on Kirchhoff's voltage law, which includes self-impedance parameters, mutual impedance parameters, injection excitation voltage phasor and response current phasor.

[0010] Step 5: Perform at least three independent injections and measurements on different phase circuits to obtain a set of independent circuit voltage equations. Combine these equations to form a system of equations and solve for the real and imaginary parts of the self-impedance and the interphase mutual impedance of each phase sheath circuit using matrix solving methods.

[0011] Step 6: Compare the real parts of the self-impedance of the three-phase sheathing circuit obtained by the solution with each other and compare them with their historical values ​​or preset thresholds. When the real part of the self-impedance of a certain phase is significantly greater than that of other phases or exceeds the preset threshold, it is determined that there is a poor contact defect in the sheathing grounding circuit of that phase.

[0012] Preferably, the method for selecting the upper limit of the injected signal frequency in step one is as follows: calculate the wavelength λ of the electromagnetic wave in the sheath-ground loop, ensuring that it is greater than the length L of the maximum cross-interconnection segment being measured, satisfying the applicable condition λ≥L for the lumped parameter model, while ensuring that the injected frequency avoids integer harmonics of the power frequency, and selecting a frequency range that makes the ground loop impedance calculation model valid. The ground loop impedance calculation formula is:

[0013]

[0014] Where K0 is the modified zeroth-order Bessel function; Let be the propagation constant of electromagnetic waves in soil; d represents the soil conductivity; d represents the burial depth of the cable. ω is the radius of the cable sheath; ω is the angular frequency. It is the permeability in vacuum; j is the imaginary unit.

[0015] Preferably, in step four, the induced electromotive force generated in the three-phase sheath circuit by the signal coupled into the circuit is calculated in the equivalent lumped parameter circuit network of the cross-interconnect system.

[0016] Preferably, the matrix solution method in step five includes: performing an independent coupling injection and full-phase response measurement on the sheathing circuits of phase A, phase B, and phase C in sequence, obtaining multiple circuit voltage equations, which constitute a matrix equation set. This equation set uses self-impedance and mutual impedance as unknowns, and the injected electromotive force vector and response current vector as known coefficients. The numerical solution of the impedance parameters of each circuit is obtained by solving this equation set.

[0017] Preferably, the phase-locked amplification technique in step three uses a reference signal with the same frequency as the injected signal to perform phase-sensitive detection on the acquired response current, and extracts the weak amplitude and phase information of the different frequency response current from the 50Hz power frequency and its harmonic interference background, which are several orders of magnitude higher than the amplitude of the different frequency response signal.

[0018] Preferably, the defect diagnosis criterion in step six is ​​as follows: the real part of the self-impedance of each phase sheath circuit measured in real time is compared with the average value of the real part of the self-impedance of the other two phases in the same group. When the difference of a certain phase exceeds a preset threshold, it is determined that the phase has a poor contact defect; or the real-time resistance value of the circuit under test is compared with the historical resistance value of the previous detection cycle. When the resistance increment exceeds a preset threshold, it is determined that a defect has occurred or worsened.

[0019] Preferably, the operating frequency range determined in step one is 100Hz to 1kHz, and this frequency does not coincide with any of the 50Hz power frequency and its 2nd to 20th integer multiples of harmonics.

[0020] Preferably, the electromagnetic coupling method in step two adopts an open-close magnetic core coupling clamp. This coupling clamp serves as both a signal injection and isolation device. One side of its winding is connected to a signal generator to inject a different frequency voltage, and the other side forms a single-turn coupling circuit with the grounding wire of the sheath of the cable under test.

[0021] Preferably, the matrix solution method in step five employs a least squares estimation algorithm, which uses redundant measurement data to solve the overdetermined system of equations, thereby reducing the impact of random noise and measurement errors on the impedance parameter calculation results.

[0022] Preferably, the injected excitation signal is a signal sequence whose frequency components are different from the power frequency and its harmonics. The signal sequence is one of a sine wave, a square wave, a swept frequency signal, or a pseudo-random binary sequence. When a square wave or a pseudo-random binary sequence is used as the excitation signal, the response signal extraction method in step three adopts the cross power spectrum analysis method or the time-domain synchronous averaging method.

[0023] This invention has the following advantages: It injects a specific frequency excitation signal, distinct from the power frequency, into the tested live sheath circuit via non-invasive electromagnetic coupling. Combined with a matrix decoupling algorithm for the cross-connection system, it accurately calculates the actual resistance value of each section of the sheath circuit from the measurement data. This invention achieves uninterrupted power supply detection, possesses strong resistance to power frequency interference, can quantify resistance changes at the milliohm level, and can effectively detect nascent defects such as slight bolt loosening and early oxidation of contact surfaces, achieving true early warning and defect location. Attached Figure Description

[0024] To more intuitively illustrate the prior art and this application, exemplary drawings are provided below. It should be understood that the specific shapes and structures shown in the drawings should not generally be regarded as limiting conditions for implementing this application; for example, based on the technical concept disclosed in this application and the exemplary drawings, those skilled in the art are able to easily make conventional adjustments or further optimizations to the addition / reduction / classification, specific shapes, positional relationships, connection methods, size ratios, etc. of certain units (components).

[0025] Figure 1 A flowchart illustrating a novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection, provided in this application embodiment;

[0026] Figure 2 The graph shows the variation of the attenuation coefficient of electromagnetic signals of different frequencies propagating between the sheath and the ground loop in the new method for measuring the resistance of high-voltage cable sheath loop based on heterogeneous frequency signal coupling injection provided in the embodiments of this application.

[0027] Figure 3 A schematic diagram of the signal coupling injection cable sheath cross-interconnection grounding circuit for a new method of measuring the resistance of high-voltage cable sheath based on heterogeneous frequency signal coupling injection provided in the embodiments of this application;

[0028] Figure 4 Equivalent circuit diagram of lumped parameters of cable sheath cross-interconnection grounding loop for a new method of measuring high-voltage cable sheath loop resistance based on heterogeneous frequency signal coupling injection provided in the embodiments of this application;

[0029] Figure 5 The flowchart illustrates the principle of selecting the coupling injection signal parameters and solving the loop impedance of the cable sheath grounding loop in the new method for measuring the resistance of high-voltage cable sheath loop based on heterogeneous frequency signal coupling injection provided in this application embodiment. Detailed Implementation

[0030] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. It should be understood that these embodiments are merely for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Technical engineers in the field can make some non-essential improvements and adjustments to the present invention based on the above-described content. 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.

[0031] Please see Figures 1-5 A novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection includes the following steps:

[0032] Step 1: Collect the structural parameters and soil environment parameters of the high-voltage cable under test, establish an electromagnetic wave propagation model of the sheath-ground loop, calculate the attenuation coefficient and wavelength of electromagnetic waves at different frequencies, and select the upper limit of the frequency of the injected signal based on the effectiveness of the lumped parameter model and the signal-to-noise ratio requirements, and determine a working frequency that is different from the power frequency and its integer multiples of harmonics.

[0033] Step 2: Using electromagnetic coupling, without interrupting power to the cable line, sequentially inject sinusoidal excitation voltage signals of the operating frequency selected in Step 1 into the three-phase sheath grounding circuit of the high-voltage cable under test.

[0034] Step 3: Synchronously acquire the response current signal generated in the three-phase sheath grounding circuit each time an excitation signal is injected, and use lock-in amplification technology to extract the amplitude and phase information of each response current from the strong power frequency background noise;

[0035] Step 4: Equivalent the three-phase sheath cross-interconnection grounding system to a linear time-invariant lumped parameter circuit network containing self-impedance and mutual impedance. For each injection, establish a loop voltage equation based on Kirchhoff's voltage law, which includes self-impedance parameters, mutual impedance parameters, injection excitation voltage phasor and response current phasor.

[0036] Step 5: Perform at least three independent injections and measurements on different phase circuits to obtain a set of independent circuit voltage equations. Combine these equations to form a system of equations and solve for the real and imaginary parts of the self-impedance and the interphase mutual impedance of each phase sheath circuit using matrix solving methods.

[0037] Step 6: Compare the real parts of the self-impedance of the three-phase sheathing circuit obtained by the solution with each other and compare them with their historical values ​​or preset thresholds. When the real part of the self-impedance of a certain phase is significantly greater than that of other phases or exceeds the preset threshold, it is determined that there is a poor contact defect in the sheathing grounding circuit of that phase.

[0038] The method for selecting the upper limit of the injected signal frequency in step one is as follows: Calculate the wavelength λ of the electromagnetic wave in the sheath-ground loop, ensuring that it is greater than the length L of the maximum cross-interconnection segment being measured, satisfying the applicable condition λ≥L for the lumped parameter model, while ensuring that the injected frequency avoids integer harmonics of the power frequency, and select a frequency range that makes the ground loop impedance calculation model valid. The ground loop impedance calculation formula is:

[0039]

[0040] Where K0 is the modified zeroth-order Bessel function; Let be the propagation constant of electromagnetic waves in soil; d represents the soil conductivity; d represents the burial depth of the cable. ω is the radius of the cable sheath; ω is the angular frequency. It is the permeability in vacuum; j is the imaginary unit.

[0041] Based on the above formula, a sheath ground loop model was established to study the propagation speed and attenuation of electromagnetic waves at different frequencies. The relationship between the electromagnetic wave wavelength and the length of the cable crossover section at different frequencies was obtained, thereby determining the upper frequency limit of the signal injected into the cable grounding loop. The sheath radius was set at 0.5085m, the burial depth at 1m, and the soil resistivity at 100Ω. Substituting these parameters into the above formula, the ground loop impedance per unit length can be calculated. Combined with the ground admittance parameter, the attenuation coefficient and phase constant can be calculated, and thus the electromagnetic wave wavelength can be determined.

[0042] As shown in the figure, the attenuation coefficient per unit length increases rapidly in the high-frequency band as the frequency changes from 10Hz to 1MHz. At 1MHz, the signal amplitude attenuation per km is as high as -237.1dB, indicating that the signal frequency band should not be too high. Here, 17kHz signal is selected as the upper limit of the injection frequency. At 17kHz, the signal amplitude attenuation per km is -3.0508dB.

[0043] The following three basic technical principles are used to determine an optimal operating frequency range.

[0044] Criterion 1: Ensure the validity of the lumped parameter model (determine the upper limit of frequency).

[0045] This model is based on representing the cable sheath loop as an equivalent lumped-parameter circuit model and solving it using matrix equations. The accuracy of this model rests on a fundamental physical premise: the wavelength of the signal. It must be much larger than the length of the cable segment being tested. ,Right now .

[0046] Therefore, to ensure model accuracy, the injection frequency should not exceed several kilohertz. Considering attenuation characteristics and model validity, the upper frequency limit should be even lower. To satisfy both conditions simultaneously, a stricter upper limit can be chosen, such as 1. .

[0047] Criterion 2: Ensure sufficient coupling efficiency and signal-to-noise ratio (determine the lower frequency limit).

[0048] According to Faraday's law of electromagnetic induction, the amplitude of the coupled-injected equivalent voltage source is proportional to the angular frequency of the injected signal:

[0049] To ensure injection efficiency and signal-to-noise ratio, a lower frequency limit needs to be set. This lower limit is usually significantly higher than the DC and extremely low frequency noise regions, and is generally chosen above 100Hz.

[0050] Criterion 3: Avoid strong interference bands of power frequency and its harmonics.

[0051] The cable sheath circuit contains a 50Hz power frequency circulating current with extremely large amplitude and its integer harmonics (especially odd harmonics such as the 3rd, 5th, and 7th). These harmonics constitute strong narrowband interference.

[0052] The aforementioned power frequency refers to a relatively low frequency selection under the premise of ensuring complete separation from the power frequency and its harmonics, and does not mean that it can revert to 50Hz itself. Any injected signal that coincides with or is too close to the power frequency or its integer multiples of harmonic frequencies (bandwidth overlap) will be submerged in background interference with amplitudes several orders of magnitude higher, making effective measurement impossible.

[0053] Combining the above three criteria, considering 1 The wavelength of electromagnetic waves in the soil can still reach over 100 km, and the length of the cable crossover section is generally 1. For a 2km cable, considering the application of a lumped parameter model to analyze the cable sheath, the following requirements need to be met. Here, we choose 1. This serves as the upper limit for the signal injection frequency.

[0054] A simulation circuit model of cable cross-interconnection and grounding was constructed. This model calculates the impedance and admittance per unit length of the cable between each conductor and the ground by setting cable cross-section parameters, forming a frequency-varying parameter matrix. Initial port conditions are set to solve the equations simultaneously to obtain the loop voltage and current responses. Signals are coupled and injected into the three-phase sheath loops A, B, and C through coupling inductors, and the three-phase sheath current responses are measured, as shown in the figure.

[0055] In the low-frequency range, a three-phase sheath cross-interconnection grounding system for high-voltage cables can be equivalent to a linear, time-invariant grounding system consisting of a resistor ( ), self-awareness ( ) and mutual induction ( A lumped parameter circuit network consisting of .

[0056] Considering the electromagnetic coupling relationship between the cross-connected grounding loops, an equivalent cross-connection circuit including self-impedance and mutual impedance parameters is established using Faraday's law of electromagnetic induction, as shown in the figure.

[0057] The cable lengths of the first, second, and third cross-connection sections are respectively , , ,by As a length reference, a length coefficient is defined. , . , , These are the self-impedances of the first three-phase cable sheath circuit, and due to the uniform distribution of parameters... , , Similarly, the self-impedance parameter of the three-phase cable sheath in the second cross-connection section can be obtained. , , The self-impedance parameters of the three-phase cable sheath in the third cross-connection section. , , These respectively represent the three-phase cable sheath circuits. Two phases, Two phases and The mutual impedance parameters between the two phases and between each loop segment are also uniformly distributed, and are determined by the segment length of the sheath, the spacing between sheaths, and the soil properties. The induced electromotive force of each loop is the sum of the voltage vector obtained by multiplying the self-impedance parameter formed by the loop itself and the ground, the mutual impedance parameter formed by electromagnetic coupling between loops, and the corresponding current. Based on the equivalent circuit shown in the figure and Kirchhoff's laws, the loop voltage and current equations can be written as follows:

[0058] (2-1)

[0059] (2-2)

[0060] (2-3)

[0061] In equation (2-1) , , These represent the induced electromotive force generated in the three-phase sheath circuit by the coupled-injected signal; in equation (2-2) Earth resistance represents the loss caused by the current flowing through the earth in the protective layer. , , Each corresponds to a single sheath circuit Mutually, Related Mutual inductance between phases; in equation (2-3) , , Represent , , Three-phase sheath circuit resistance , , Represent , , Self-inductance of the three-phase sheath circuit.

[0062] Multi-point injection and full-phase response measurement are used. Specifically, "first inject into phase A and measure the three-phase current; then inject into phase B and measure the three-phase current again." A single injection (3 equations) is insufficient for solving the problem. Therefore, multiple injections must be performed to obtain enough independent equations. Combining equations (2-1), (2-2), and (2-3) from step 4 with the excitation and response signals obtained in step 3, a linear equation set of loop impedance can be established. The loop impedance parameters can be calculated, and by taking the real part of the impedance, the resistance of the entire loop can be determined. This allows for the determination of whether there are poor contact defects in each cross-interconnection segment.

[0063] The specific implementation is as follows: Set the resistance at the defect point of phase A to 1Ω, the contact resistance at both ends entering the ground to 0.5Ω each, the cable sheath resistance to 35mΩ / km, the cable burial depth to 1m, and the soil resistivity to 100Ω. The cross-connection grounding sections are 400m in the first section, 390m in the second section, and 410m in the third section. The cable distribution parameters are assumed to be uniformly distributed and do not change with length.

[0064] The system of equations to be solved is shown below:

[0065] (2-4)

[0066] The specific solution results are shown in the table below:

[0067] Table 1. Mutual impedance and self-impedance parameters of cable cross-interconnection grounding loops at a 50Hz injection frequency.

[0068]

[0069] Table 2. Mutual impedance and self-impedance parameters of cable cross-interconnection grounding loops at an injection frequency of 1 kHz.

[0070]

[0071] The loop defect situation can be evaluated and judged based on the loop self-impedance and mutual impedance parameters obtained from step 5. It can be seen that the real part of the loop self-impedance measured in the defective loop is significantly larger than the real part of the self-impedance of the non-defective phase, which indicates... The circuit containing the phase sheath has poor contact defects. Comparing the simulation results in Table 1 (50Hz) and Table 2 (1kHz), it can be seen that under ideal conditions where power frequency interference is completely ignored, lower frequencies are more suitable for the lumped parameter model, and their parameter solution accuracy is higher. However, in actual live-line testing environments, 50Hz and its harmonic components constitute a background interference with extremely strong amplitude, making it impossible to inject and extract signals of the same frequency. Therefore, the core technical solution of this invention is to select the lowest possible heterogeneous injection frequency (typically 100Hz~200Hz) while completely avoiding the 50Hz power frequency and its integer multiples of harmonics, so as to obtain a sufficient signal-to-noise ratio while ensuring model accuracy. If actual conditions permit, the 100Hz~120Hz frequency band can be directly used as the preferred operating frequency.

[0072] In step three, the phase-locked amplification technique uses a reference signal with the same frequency as the injected signal to perform phase-sensitive detection on the acquired response current. From the 50Hz power frequency and its harmonic interference background, which are several orders of magnitude higher than the amplitude of the heterofrequency response signal, the weak amplitude and phase information of the heterofrequency response current are extracted.

[0073] The defect diagnosis criteria in step six are as follows: compare the real part of the self-impedance of each phase sheath circuit measured in real time with the average value of the real part of the self-impedance of the other two phases in the same group. When the difference of a certain phase exceeds the preset threshold, it is determined that the phase has a poor contact defect; or compare the real-time resistance value of the circuit under test with the historical resistance value of the previous test cycle. When the resistance increment exceeds the preset threshold, it is determined that a defect has occurred or worsened.

[0074] The operating frequency range determined in step one is 100Hz to 1kHz, and this frequency does not coincide with any of the 50Hz power frequency and its 2nd to 20th integer multiples of harmonics.

[0075] The electromagnetic coupling method described in step two uses an openable magnetic core coupling clamp. This coupling clamp serves as both a signal injection and isolation device. One side of its winding is connected to a signal generator to inject a different frequency voltage, while the other side forms a single-turn coupling circuit with the grounding wire of the sheath of the cable under test.

[0076] The matrix solution method described in step five employs a least squares estimation algorithm, utilizing redundant measurement data to solve the overdetermined system of equations, thereby reducing the impact of random noise and measurement errors on the impedance parameter calculation results.

[0077] The injected excitation signal is a signal sequence whose frequency components are different from the power frequency and its harmonics. This signal sequence is one of the following: sine wave, square wave, swept frequency signal or pseudo-random binary sequence. When a square wave or pseudo-random binary sequence is used as the excitation signal, the response signal extraction method in step three adopts the cross power spectrum analysis method or the time-domain synchronous averaging method.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A new method for measuring the resistance of the high voltage cable sheath circuit based on the injection of a heterodyne signal, characterized in that, Includes the following steps: Step 1: Selection and determination of different frequency operating frequencies. Collect the structural parameters and soil environment parameters of the high-voltage cable under test, establish the electromagnetic wave propagation model of the sheath-ground loop, calculate the attenuation coefficient and wavelength of electromagnetic waves of different frequencies, select the upper limit of the frequency of the injected signal based on the effectiveness of the lumped parameter model and the signal-to-noise ratio requirements, and determine an operating frequency that is different from the power frequency and its integer multiples of harmonics. Step 2: Injection of different frequency excitation signals. Using electromagnetic coupling, the sinusoidal excitation voltage signal of the working frequency selected in Step 1 is sequentially injected into the three-phase sheath grounding circuit of the high-voltage cable under test without interrupting the power supply to the cable line. Step 3: Response current acquisition and phase-locked extraction. The response current signal generated in the three-phase sheath grounding circuit is acquired synchronously each time an excitation signal is injected, and the amplitude and phase information of each response current are extracted from the strong power frequency background noise using phase-locked amplification technology. Step 4: Lumped parameter equivalent circuit modeling and loop voltage equation establishment. The three-phase sheath cross-interconnected grounding system is equivalent to a linear time-invariant lumped parameter circuit network containing self-impedance and mutual impedance. For each injection, a loop voltage equation containing self-impedance parameters, mutual impedance parameters, injection excitation voltage phasor and response current phasor is established based on Kirchhoff's voltage law. Step 5: Multi-point injection matrix decoupling and impedance parameter solution. Perform at least three independent injections and measurements on different phase circuits to obtain a set of independent circuit voltage equations. Combine these equations to form a system of equations and solve the real and imaginary parts of the self-impedance of each phase sheath circuit and the interphase mutual impedance using the matrix solution method. Step 6: Compare the real parts of the self-impedance of the three-phase sheathing circuit obtained by the solution with each other and compare them with their historical values ​​or preset thresholds. When the real part of the self-impedance of a certain phase is significantly greater than that of other phases or exceeds the preset threshold, it is determined that there is a poor contact defect in the sheathing grounding circuit of that phase.

2. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The method for selecting the upper limit of the injected signal frequency in step one is as follows: Calculate the wavelength λ of the electromagnetic wave in the sheath-ground loop, ensuring that it is greater than the length L of the maximum cross-interconnection segment being measured, satisfying the applicable condition λ≥L for the lumped parameter model, while ensuring that the injected frequency avoids integer harmonics of the power frequency, and select a frequency range that makes the ground loop impedance calculation model valid. The ground loop impedance calculation formula is: Where K0 is the modified zeroth-order Bessel function; Let be the propagation constant of electromagnetic waves in soil; d represents the soil conductivity; d represents the burial depth of the cable. ω is the radius of the cable sheath; ω is the angular frequency. It is the permeability in vacuum; j is the imaginary unit.

3. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 2, characterized in that, In step four, the induced electromotive force generated in the three-phase sheath circuit is calculated in the equivalent lumped parameter circuit network of the cross-interconnect system by coupling the injected signal into the circuit.

4. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 3, characterized in that, The matrix solution method in step five includes: performing an independent coupling injection and full-phase response measurement on the A-phase, B-phase, and C-phase sheathing circuits respectively, obtaining multiple circuit voltage equations, which constitute a matrix equation set. This equation set uses self-impedance and mutual impedance as unknowns, and the injected electromotive force vector and response current vector as known coefficients. The numerical solution of the impedance parameters of each circuit is obtained by solving this equation set.

5. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The phase-locked amplification technique in step three uses a reference signal with the same frequency as the injected signal to perform phase-sensitive detection on the acquired response current, extracting the weak amplitude and phase information of the different frequency response current from the 50Hz power frequency and its harmonic interference background, which is several orders of magnitude higher than the amplitude of the different frequency response signal.

6. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The defect diagnosis criteria in step six are as follows: compare the real part of the self-impedance of each phase sheath circuit measured in real time with the average value of the real part of the self-impedance of the other two phases in the same group. When the difference of a certain phase exceeds a preset threshold, it is determined that the phase has a poor contact defect; or compare the real-time resistance value of the circuit under test with the historical resistance value of the previous detection cycle. When the resistance increment exceeds a preset threshold, it is determined that a defect has occurred or worsened.

7. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The operating frequency range determined in step one is 100Hz to 1kHz, and this frequency does not coincide with any of the 50Hz power frequency and its 2nd to 20th integer multiples of harmonics.

8. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The electromagnetic coupling method described in step two uses an openable magnetic core coupling clamp. This coupling clamp serves as both a signal injection and isolation device. One side of its winding is connected to a signal generator to inject a different frequency voltage, while the other side forms a single-turn coupling circuit with the grounding wire of the sheath of the cable under test.

9. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The matrix solution method described in step five employs a least squares estimation algorithm, utilizing redundant measurement data to solve the overdetermined system of equations, thereby reducing the impact of random noise and measurement errors on the impedance parameter calculation results.

10. The novel method for measuring the loop resistance of high-voltage cable sheaths based on heterogeneous frequency signal coupling injection according to claim 1, characterized in that, The injected excitation signal is a signal sequence whose frequency components are different from the power frequency and its harmonics. This signal sequence is one of the following: sine wave, square wave, swept frequency signal or pseudo-random binary sequence. When a square wave or pseudo-random binary sequence is used as the excitation signal, the response signal extraction method in step three adopts the cross power spectrum analysis method or the time-domain synchronous averaging method.

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