Cable metal shielding layer fault live detection constant current source

Abstract

The invention discloses a cable metal shielding layer fault live detection constant current source, which belongs to the technical field of live line fault diagnosis, and comprises an output end used for injecting adjustable direct current into a live cable; the primary protection module is used for inhibiting interference signals generated by a metal shielding layer of the live cable; the sampling module is used for collecting the output current of the output end after the interference signal is suppressed by the primary protection module; the secondary protection module is used for filtering the residual interference signal of the output current of the output end after the interference signal is suppressed by the primary protection module; the microcontroller unit is used for controlling the starting state of the secondary protection module according to the amplitude of the residual interference signal; the duty ratio adjusting module is used for dynamically adjusting the duty ratio of the pulse width modulation signal according to the output current of the output end; and the DC-DC converter is used for controlling and adjusting the output current of the output end according to the pulse width modulation signal. The constant current source can realize cable metal shielding layer fault live-line detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of live line fault diagnosis, and particularly relates to a constant current source for live detection of cable metal sheath faults. Background Art

[0002] With the rapid development of the power industry, the use of power cables is increasing day by day. Since power cables work in a complex environment for a long time, various faults are likely to occur. To ensure the reliable operation of the power grid, it is crucial to detect cable faults in a timely manner.

[0003] There are various types of cable faults and corresponding diagnostic methods are relatively rich. Among them, the detection of cable metal sheath faults usually judges the fault state by measuring the resistance value of the metal sheath. The measurement method is to inject a direct current into the cable, measure the voltage at both ends of the cable, and then calculate to deduce the resistance value of the cable.

[0004] However, the traditional constant current source device for injecting current can only be used when the cable is de-energized. This is because when the cable is in operation, an induced interference signal will be generated on the cable metal sheath. This interference signal usually appears as a power frequency fundamental wave (50Hz / 60Hz) and its 2 - 3 times harmonic frequency voltage or current, with randomly varying amplitudes and occasionally containing an ultra - low frequency near - direct current component. Due to the complex spectral composition and uncertain amplitude of this interference signal, it is very easy to cause the constant current source device to malfunction or directly damage the device. Therefore, the traditional constant current source device cannot be used when the cable is live. Summary of the Invention

[0005] The purpose of the present invention is to provide a constant current source for live detection of cable metal sheath faults, which can realize live detection of cable metal sheath faults.

[0006] To achieve the above purpose, the present invention provides the following technical solutions: A constant current source for live detection of cable metal sheath faults, comprising: An output terminal for injecting an adjustable direct current into a live cable; A primary protection module for suppressing the interference signal generated by the metal sheath of the live cable; A sampling module for collecting the output current after the interference signal is suppressed by the primary protection module at the output terminal; A secondary protection module for filtering the residual interference signal of the output current after the interference signal is suppressed by the primary protection module at the output terminal; A microcontroller unit for controlling the enabling state of the secondary protection module according to the amplitude of the residual interference signal; and for dynamically adjusting the duty cycle of the pulse - width modulation signal according to the output current at the output terminal; A DC-DC converter is used to control and regulate the output current at the output terminal according to a pulse-width modulation signal.

[0007] In the present invention, a primary protection module suppresses AC-DC interference, and a secondary protection module smooths the residual interference, enabling the constant current source to operate stably in a live cable environment, breaking through the limitation of the traditional constant current source that requires power-off detection, and realizing live detection of cable metal shielding layer faults.

[0008] Furthermore, the output current range at the output terminal is 0A to 10A, the regulation accuracy is -0.5% to +0.5%, and it has an overcurrent protection function, with a maximum instantaneous current limit of 12A.

[0009] The adjustable current range (0A to 10A) can adapt to the detection requirements of different cable specifications, expand the applicable range of the constant current source, and improve the application flexibility of the constant current source. Ensure that the measurement error of the metal shielding layer resistance is <1%, and enhance the accuracy of fault diagnosis. The overcurrent protection prevents equipment damage caused by accidental short circuits or overloads.

[0010] Furthermore, the primary protection module includes a discharge tube, a varistor, a reactor, and a diode connected in parallel to the output terminal.

[0011] The wave-blocking reactor filters out high-frequency AC interference and reduces the amplitude; the diode blocks DC and low-frequency interference signals to prevent reverse current from damaging the equipment; the front-end protection circuit (discharge tube + varistor) absorbs transient high voltages (such as lightning surges) to protect the sensitive components at the back end. Through the cooperation of multiple devices, it avoids equipment damage caused by the failure of a single protection, and improves the reliability of the constant current source.

[0012] Furthermore, the reactor uses a double-wire co-wound inductor with a Permalloy magnetic core, the total inductance is greater than 5mH, the DC resistance is less than 0.1Ω, and a non-polar aluminum electrolytic capacitor is connected in parallel.

[0013] The high saturation magnetic flux density of the Permalloy magnetic core can avoid magnetic core saturation caused by large DC currents and maintain the stability of the inductance value. The double-wire winding improves the coupling coefficient, reduces the DC resistance, and reduces heat generation and energy loss; the parallel capacitor enhances low-frequency filtering and reduces the overall volume.

[0014] Furthermore, the forward conduction voltage drop of the diode is less than 0.7V, the reverse breakdown voltage is greater than 100V, the reverse recovery time is less than 50ns, and the forward conduction current is greater than 10A.

[0015] The low forward voltage drop reduces the conduction loss, the high reverse voltage withstand prevents breakdown, the fast recovery time avoids signal delay, and the large current capacity adapts to high-power scenarios. The strict component selection reduces the risk of diode damage due to overvoltage or overheating and extends the device life.

[0016] Further, the sampling module uses a sampling resistor with a temperature coefficient less than 50 ppm / °C, and improves the signal-to-noise ratio of the sampling signal through a differential amplification circuit.

[0017] Furthermore, the sampling resistor is a high-precision low-temperature-drift resistor.

[0018] The low-temperature-drift resistor reduces the measurement error caused by temperature changes, and the differential amplification suppresses the common-mode noise, improving the signal-to-noise ratio (SNR>60dB).

[0019] Further, the secondary protection module includes a signal processing circuit, a controllable access circuit, and a band-stop filter connected in parallel across the sampling module. The signal processing circuit is used to reduce the residual interference signal proportionally and send it to the microcontroller unit for logical judgment. If the amplitude of the residual interference signal is greater than the amplitude threshold, the microcontroller unit controls the controllable access circuit to connect the band-stop filter to filter out the residual interference signal, and dynamically adjusts the duty cycle of the pulse-width modulation signal according to the output current after the residual interference signal is filtered by the band-stop filter at the output end. Otherwise, the microcontroller unit controls the controllable access circuit to disconnect the band-stop filter, and dynamically adjusts the duty cycle of the pulse-width modulation signal according to the output current after the interference signal is suppressed by the primary protection module at the output end.

[0020] Dynamically enabling the filter (only working when the interference exceeds the threshold) can reduce the delay introduced by fixed filtering and improve it. Directly filtering out the main interference components (power frequency and its harmonics) of the cable shielding layer can avoid signal distortion caused by "broad-spectrum filtering". Starting the secondary filtering only when the interference exceeds the threshold can reduce the ineffective filtering loss and improve the energy efficiency.

[0021] Further, the band-stop filter is a 4th-order band-stop filter, with a stopband attenuation greater than or equal to -40dB, a passband bandwidth greater than 500kHz, and the stopband center frequency set in the power frequency and 2 - 3 times harmonic frequency range.

[0022] The 4th-order design achieves high attenuation (-40dB) for power frequency and harmonic interference (such as 50Hz, 100 - 150Hz), avoiding the influence of residual interference on the sampling accuracy.

[0023] Further, the microcontroller unit uses a DSP chip and dynamically adjusts the duty cycle of the pulse-width modulation signal using the proportional-integral-derivative control algorithm.

[0024] Based on the closed-loop proportional-integral-derivative control (PID) of the microcontroller unit (MCU), the present invention dynamically adjusts the duty cycle of the pulse-width modulation signal, and adaptively adjusts the PID parameters to cope with complex working conditions (such as sudden load changes and strong interference), which can ensure the current accuracy (-0.5%~+0.5%), avoid output fluctuations caused by load changes or interference, and enhance the output stability.

[0025] Furthermore, the integral time constant and the derivative time constant in the proportional-integral-derivative control algorithm can be programmed and adjusted online to meet the control requirements under different interference environments.

[0026] The DSP chip quickly processes the PID algorithm and real-time signals, which can ensure that the closed-loop response time is <1ms, improving the current stability. Dynamically judge the interference intensity, adaptively adjust the working state of the protection module, and balance anti-interference and efficiency. For different cable working conditions (such as interference intensity, load impedance), flexibly optimize the PID parameters to avoid control instability caused by fixed parameters.

[0027] Furthermore, it also includes: A wireless communication module, which is used to transmit detection data to an external terminal in real time and receive remote control instructions to adjust the output current parameters.

[0028] Transmit detection data to the cloud in real time, support remote monitoring and fault warning, reduce the cost of manual inspection, and achieve intelligent operation and maintenance.

[0029] Furthermore, the shell of the constant current source is made of lightweight aluminum alloy material, and the internal layout adopts a modular design.

[0030] The lightweight design is convenient for on-site carrying and operation, meeting the mobile requirements of power inspection. The aluminum alloy shell enhances the heat dissipation efficiency, and the modular design reduces the difficulty of maintenance.

[0031] Compared with the prior art, the beneficial effects of the present invention are: The constant current source for live detection of cable metal shielding layer faults provided by the present invention comprehensively solves the problems of poor anti-interference, bulky volume, and control lag of traditional constant current sources in live detection through hardware design optimization (such as two-stage protection, miniaturized reactor), intelligent control strategies (PID algorithm, dynamic filtering), and function expansion (wireless communication, high-precision sampling), realizing high-precision, high-reliability, and portable detection of cable shielding layer resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a schematic structural diagram of the constant current source for live detection of cable metal shielding layer faults provided by the embodiment of the present application; Figure 2 is a schematic structural diagram of the primary protection module provided by the embodiment of the present application; Figure 3 is a schematic structural diagram of the secondary protection module provided by the embodiment of the present application; Figure 4 is a software control flow chart based on the PID algorithm provided by the embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The technical solution of the present application will be further described in detail below in conjunction with specific embodiments.

[0034] The embodiments of the present application will be described in detail below. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. Without conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other.

[0035] The embodiment of the present application provides a constant current source for live detection of cable metal shielding layer faults, as Figure 1 shown. The constant current source includes: An output terminal for injecting an adjustable direct current into the live cable; A primary protection module for suppressing the interference signals generated by the metal shielding layer of the live cable; A sampling module for collecting the output current after the interference signals are suppressed by the primary protection module at the output terminal; A secondary protection module for filtering the residual interference signals of the output current after the interference signals are suppressed by the primary protection module at the output terminal; A microcontroller unit for controlling the enabling state of the secondary protection module according to the amplitude of the residual interference signals; and for dynamically adjusting the duty cycle of the pulse width modulation signal according to the output current at the output terminal; A DC-DC converter for controlling and adjusting the output current at the output terminal according to the pulse width modulation signal.

[0036] The constant current source for live detection of cable metal shielding layer faults provided by the embodiment of the present application suppresses AC and DC interference through the primary protection module and smooths the residual interference through the secondary protection module, enabling the constant current source to work stably in the live cable environment, breaking through the limitation of the traditional constant current source that requires power-off detection, and realizing the live detection of cable metal shielding layer faults.

[0037] In this embodiment, the range of the output current at the output terminal is 0A to 10A, the adjustment accuracy is -0.5% to +0.5%, and it has an overcurrent protection function, and the maximum instantaneous current limit is 12A.

[0038] The adjustable current range (0A to 10A) can adapt to the detection requirements of different cable specifications, expand the applicable range of the constant current source, and improve the application flexibility of the constant current source. Ensure that the measurement error of the metal shielding layer resistance is <1%, and improve the accuracy of fault diagnosis. The overcurrent protection prevents equipment damage caused by accidental short circuits or overloads.

[0039] In this embodiment, the primary protection module includes a discharge tube, a varistor, a reactor, and a diode connected in parallel to the output terminal.

[0040] Specifically, as Figure 2 shown, for the AC interference signal, the amplitude of the interference signal is reduced through the two-stage filtering process composed of reactors. To prevent the device from being damaged due to the too-high amplitude of the interference signal at the front end of the first-stage filtering, a discharge tube and a varistor are added at the front end as protection. Whether the interference signal after the front-stage filtering is in the positive half-wave or the negative half-wave, it cannot be injected into the signal source device due to the unidirectional conductivity of the diode at the rear stage. The suppression of the DC signal interference also cannot be injected into the signal source due to the presence of the diode. Considering that the device is a DC current source, to prevent the DC loss caused by the current, a filter composed of reactors is adopted. Among them, for the design of the reactor, two main points are considered. One is that the cut-off frequency is low enough to filter out the interference signal significantly. Therefore, a large enough inductance value and capacitance value are required. To reduce the device size and considering that the DC current will cause inductance saturation, a toroidal core made of a composite material with strong DC bias resistance, high saturation magnetic flux density, and high magnetic permeability is selected for the magnetic core. To balance the parameters of high saturation magnetic flux density and magnetic permeability, a permalloy magnetic core is finally selected. The capacitor is selected as multiple non-polar aluminum electrolytic capacitors with a large capacitance value connected in parallel. The second point is the DC loss problem of the inductor. To minimize the DC loss and not reduce the inductance, according to the mutual inductance parallel formula L = (L1 + L2 - M 2 ), where L1 and L2 are the self-inductances of the two coils, and M represents the mutual inductance coefficient between the two coils. M 2 reflects the weakening effect of mutual inductance on the total inductance (the larger the mutual inductance, the smaller the total inductance), and 2M reflects the direct influence of mutual inductance on the equivalent total inductance. The double-wire co-directional winding method is adopted to increase its coupling coefficient as much as possible. After winding, the two coils are connected in parallel to ensure that the total inductance is greater than 5 mH and its DC resistance is less than 0.1 Ω. For the selection of the diode, it should have the characteristics of low forward conduction voltage drop, high reverse breakdown voltage, short reverse recovery time, and large forward conduction current.

[0041] The wave-blocking reactor filters out high-frequency AC interference and reduces the amplitude; the diode blocks DC and low-frequency interference signals to prevent the reverse current from damaging the device; the front-end protection circuit (discharge tube + varistor) absorbs transient high voltage (such as lightning surge) to protect the sensitive components at the rear end. Through the cooperation of multiple devices, the device damage caused by the failure of a single protection is avoided, and the reliability of the constant current source is improved.

[0042] In a possible embodiment, the reactor adopts a double-wire co-directional winding inductor with a permalloy magnetic core, the total inductance is greater than 5 mH, the DC resistance is less than 0.1 Ω, and non-polar aluminum electrolytic capacitors are connected in parallel.

[0043] The high saturation magnetic flux density of the permalloy magnetic core can avoid the magnetic core saturation caused by large DC currents and maintain the stability of the inductance value. The double-wire winding improves the coupling coefficient, reduces the DC resistance, and decreases the heat generation and energy loss; the parallel capacitor enhances the low-frequency filtering and reduces the overall volume.

[0044] In a possible embodiment, the forward conduction voltage drop of the diode is less than 0.7V, the reverse breakdown voltage is greater than 100V, the reverse recovery time is less than 50ns, and the forward conduction current is greater than 10A.

[0045] The low forward voltage drop reduces the conduction loss, the high reverse voltage withstand prevents breakdown, the fast recovery time avoids signal delay, and the large current capacity adapts to high-power scenarios. The strict selection reduces the risk of diode damage due to overvoltage or overheating and extends the device life.

[0046] In this embodiment, the sampling module uses a sampling resistor with a temperature coefficient less than 50ppm / °C and improves the signal-to-noise ratio of the sampling signal through a differential amplifier circuit.

[0047] In a possible embodiment, the sampling resistor is a high-precision low-temperature drift resistor.

[0048] The low-temperature drift resistor reduces the measurement error caused by temperature changes, and the differential amplification suppresses the common-mode noise and improves the signal-to-noise ratio (SNR>60dB).

[0049] In this embodiment, the secondary protection module includes a signal processing circuit, a controllable access circuit, and a band-stop filter connected in parallel across the sampling module. The signal processing circuit is used to proportionally reduce the residual interference signal and send it to the microcontroller unit for logical judgment. If the amplitude of the residual interference signal is greater than the amplitude threshold, the microcontroller unit controls the controllable access circuit to connect the band-stop filter to filter out the residual interference signal, and dynamically adjusts the duty cycle of the pulse width modulation signal according to the output current after the residual interference signal is filtered by the band-stop filter at the output end. Otherwise, the microcontroller unit controls the controllable access circuit to disconnect the band-stop filter and dynamically adjusts the duty cycle of the pulse width modulation signal according to the output current after the interference signal is suppressed by the primary protection module at the output end.

[0050] Specifically, such as Figure 3As shown in the figure, considering the non-ideal characteristics of the primary protection module, such as the forward conduction voltage drop of the diode not being 0 and the existence of reverse recovery time, etc., in order to prevent the fluctuation amplitude of the interference signal generated on the sampling resistor from exceeding the maximum value that the subsequent digital circuit can withstand, causing damage to the subsequent circuit and loop control disorder, a secondary protection module is added. Its working principle is that the signal at both ends of the sampling resistor is scaled down by the signal processing module, and then undergoes logical judgment by the MCU. If the signal amplitude is within the controllable range, the signal directly participates in the subsequent loop control output. If it exceeds, it enters the MCU after being filtered by a band-stop filter. This working method can not only protect the subsequent digital circuit but also improve the loop response speed as much as possible. Using a band-stop filter can accurately filter out interference signals with a relatively single frequency. The designed filter is a 4th-order band-stop filter, and its stopband attenuation can reach more than -40 dB, and the passband bandwidth is designed to be greater than 500 k.

[0051] Dynamically enabling the filter (only working when the interference exceeds the threshold) can reduce the delay introduced by fixed filtering and improve. Directly filtering out the main interference components (power frequency and its harmonics) of the cable shield layer can avoid signal distortion caused by "broad-spectrum filtering". Only start the secondary filtering when the interference exceeds the threshold, reduce the ineffective filtering loss, and improve the energy efficiency.

[0052] In a possible embodiment, the band-stop filter is a 4th-order band-stop filter, the stopband attenuation is greater than or equal to -40 dB, the passband bandwidth is greater than 500 kHz, and the center frequency of the stopband is set in the power frequency and the frequency range of the 2nd to 3rd harmonics.

[0053] The 4th-order design achieves high attenuation (-40 dB) for power frequency and harmonic interference (such as 50 Hz, 100 - 150 Hz), avoiding the influence of residual interference on the sampling accuracy.

[0054] In a possible embodiment, the microcontroller unit uses a DSP chip and dynamically adjusts the duty cycle of the pulse width modulation signal using the proportional-integral-derivative control algorithm.

[0055] Specifically, as Figure 4 shown, to ensure reliable output, the software samples a relatively large number of points for the feedback signal, which requires a relatively fast response speed for the entire feedback loop. If the protection module at the back end of the sampling resistor has been participating in the work, it will inevitably affect the overall response speed. Therefore, using the high-speed characteristics of the chip, the feedback signal is sampled and compared in real time. If it exceeds the maximum value that the digital chip can input, the secondary protection module is controlled to work. If it does not exceed, the secondary protection module does not need to participate in the work. In this way, the output stability can be improved as much as possible.

[0056] Based on the closed-loop proportional-integral-derivative control (PID) of the microcontroller unit (MCU), dynamically adjust the duty cycle of the pulse-width modulation signal, adaptively adjust the PID parameters to cope with complex working conditions (such as sudden load and strong interference), ensure the current accuracy (-0.5% to +0.5%), avoid output fluctuations caused by load changes or interference, and enhance output stability.

[0057] In a possible embodiment, the integral time constant and the derivative time constant in the proportional-integral-derivative control algorithm can be programmed and adjusted online to meet the control requirements under different interference environments.

[0058] The DSP chip quickly processes the PID algorithm and real-time signals, ensures that the closed-loop response time <1ms, and improves current stability. Dynamically judge the interference intensity, adaptively adjust the working state of the protection module, and balance anti-interference and efficiency. For different cable working conditions (such as interference intensity and load impedance), flexibly optimize the PID parameters to avoid control instability caused by fixed parameters.

[0059] In a possible embodiment, the constant current source further includes: A wireless communication module for real-time transmission of detection data to an external terminal and receiving remote control instructions to adjust the output current parameters.

[0060] Real-time transmission of detection data to the cloud, supports remote monitoring and fault warning, reduces the cost of manual inspection, and realizes intelligent operation and maintenance.

[0061] In a possible embodiment, the housing of the constant current source is made of lightweight aluminum alloy material, and the internal layout adopts a modular design.

[0062] The lightweight design is convenient for on-site carrying and operation, meeting the mobile requirements of power inspection. The aluminum alloy housing enhances the heat dissipation efficiency, and the modular design reduces the difficulty of maintenance.

[0063] The embodiment of the present application provides an adjustable constant current source control method for live detection of the resistance of the metal shielding layer of a cable line, including the following steps: Step S1: Suppress the AC and DC interference signals in the cable line through a primary protection module, where the primary protection module filters out AC interference using a wave-blocking reactance device and suppresses DC and low-frequency interference through a diode; Step S2: Input the signal processed by the primary protection module into a secondary protection module, and dynamically enable a band-stop filter to smooth the residual interference by real-time monitoring the amplitude of the interference signal, where the band-stop filter is only connected to work when the interference amplitude exceeds a preset threshold; Step S3: Collect the current signal processed by the secondary protection module through a current sampling module and transmit the signal to the digital control chip; Step S4: Based on the closed-loop control of the PID algorithm, dynamically adjust the duty cycle of the PWM output to maintain the DC output current of the constant current source stable at the target value; Step S5: According to the deviation between the real-time detected current value and the target value, adaptively adjust the integral time constant and the differential time constant in the PID parameters to optimize the system response speed and stability.

[0064] The embodiment of the present application provides a live detection method for cable metal shielding layer faults. Using the constant current source for live detection of cable metal shielding layer faults provided by any embodiment of the present application, the method includes the following steps: Under the live state of the cable, inject a DC current into the metal shielding layer through the constant current source; Suppress power frequency, harmonic and ultra-low frequency interference signals through a two-stage protection module; Based on the sampled voltage, calculate the shielding layer resistance value and judge the cable fault state.

[0065] The above are only the preferred embodiments of the present application. It should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present application, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present application.

Claims

1. A constant current source for detecting faults in the metal shielding layer of a cable, characterized in that: include: An output terminal for injecting an adjustable DC current into the live cable; The primary protection module is used to suppress interference signals generated by the metal shielding layer of the live cable; A sampling module is used to collect the output current of the output end after the interference signal is suppressed by the first-level protection module; The secondary protection module is used to filter out the residual interference signal of the output current after the interference signal is suppressed by the primary protection module at the output end; A microcontroller unit, used for controlling the enabling state of the secondary protection module according to the amplitude of the residual interference signal; and for dynamically adjusting the duty cycle of the pulse width modulation signal according to the output current of the output terminal; The DC-DC converter is used to control and adjust the output current at the output end according to the pulse width modulation signal.

2. The cable metal shield fault live detection constant current source according to claim 1 is characterized in that: The output current range of the output end is 0A~10A, the adjustment accuracy is -0.5%~+0.5%, and it has over-current protection function, and the maximum instantaneous current limit is 12A.

3. The cable metal shield fault live detection constant current source according to claim 1, characterized in that: The primary protection module includes a discharge tube, a varistor, an inductor and a diode connected in parallel to the output end.

4. The cable metal shield fault live detection constant current source according to claim 3, characterized in that: The reactor adopts a double-wire co-directionally wound inductor with a Permalloy core, with a total inductance greater than 5mH and a DC resistance less than 0.1Ω, and is connected in parallel with a non-polar aluminum electrolytic capacitor.

5. The cable metal shield fault live detection constant current source according to claim 3, characterized in that: The forward voltage drop of the diode is less than 0.7V, the reverse breakdown voltage is greater than 100V, the reverse recovery time is less than 50ns, and the forward current is greater than 10A.

6. The cable metal shield fault live detection constant current source according to claim 1, characterized in that: The sampling module uses a sampling resistor with a temperature coefficient less than 50ppm / ℃ and improves the signal-to-noise ratio of the sampling signal through a differential amplifier circuit.

7. The cable metal shield fault live detection constant current source according to claim 1, characterized in that: The secondary protection module includes a signal processing circuit, a controllable access circuit and a band-stop filter connected in parallel at both ends of the sampling module. The signal processing circuit is used to proportionally reduce the residual interference signal and send it to the microcontroller unit for logical judgment. If the amplitude of the residual interference signal is greater than the amplitude threshold, the microcontroller unit controls the controllable access circuit to access the band-stop filter to filter out the residual interference signal, and dynamically adjusts the duty cycle of the pulse width modulation signal according to the output current after the residual interference signal is filtered out by the band-stop filter at the output end. Otherwise, the microcontroller unit controls the controllable access circuit to disconnect the band-stop filter, and dynamically adjusts the duty cycle of the pulse width modulation signal according to the output current after the interference signal is suppressed by the primary protection module at the output end.

8. The cable metal shield fault live detection constant current source according to claim 7, characterized in that: The band stop filter is a 4th order band stop filter, the stop band attenuation is greater than or equal to -40dB, the passband bandwidth is greater than 500kHz, and the stop band center frequency is set to the power frequency and the 2nd to 3rd harmonic frequency range.

9. The cable metal shield fault live detection constant current source according to claim 7, characterized in that: The microcontroller unit adopts DSP chip and uses proportional-integral-differential control algorithm to dynamically adjust the duty cycle of the pulse width modulation signal.

10. The cable metal shield fault live detection constant current source according to claim 1, characterized in that: Also includes: The wireless communication module is used to transmit detection data to an external terminal in real time and receive remote control instructions to adjust output current parameters.

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