Device and method for detecting defects of outer sheath of 35kV single-core power cable

Abstract

The invention relates to the technical field of power supply line fault detection, and particularly discloses a 35kV single-core power cable outer sheath defect detection device and a test method.The 35kV single-core power cable outer sheath defect detection device comprises a direct-current power source transmitter, an intelligent receiver and a current coupling clamp, and the direct-current power source transmitter is provided with a high-voltage output interface, a protection grounding interface and a working grounding interface; the protection grounding interface and the working grounding interface are connected with a transformer substation grounding copper bar, the high-voltage output interface is connected with one end of the cable metal shielding layer, and the other end of the cable metal shielding layer is a metal shielding layer throwing end. The intelligent receiver is provided with a signal input port and a display, the signal input port is linearly connected with a signal output port of the current coupling clamp, a fault detection waveform is displayed on the display, and the current coupling clamp is clamped outside a cable; the position of a fault point can be judged by comparing the strength of the signal, the test detection method is simple, the abnormal grounding condition of the metal shielding layer of the single-core cable can be found in time, and cable grounding even short-circuit accidents are avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of power supply line fault detection, and in particular to a 35kV single-core power cable outer sheath defect detection device and a test method. Background Art

[0002] 35kV single-core power cable has been widely used for its advantages of large current carrying capacity, small bending radius, higher voltage resistance and flexible laying path. However, the operation and maintenance of power cables face many new problems and challenges. For example, during the laying process, high-voltage single-core power cables are subject to complex construction environments, especially mechanical forces such as traction, side pressure and clamping force at the turning points and shaft openings of the laying paths. The outer sheath may be damaged by bumps, cracks, deformations and scratches. In addition, due to irregular construction operations, poor quality and aging of the cable's own insulation sheath, the insulation of the cable outer sheath will be damaged, the metal shielding layer of the cable will be grounded at two or more points, and the circulation generated by the cable metal shielding will increase, resulting in continuous heating of the metal shielding layer, accelerating the aging of the main insulation, and even burning and igniting the outer sheath and main insulation of the cable, resulting in cable breakdown and grounding, fire, arc overvoltage causing cable two-phase grounding, short circuit tripping and power loss accidents, and some cables can enter moisture through the damaged outer sheath, and the main insulation will produce water tree aging, which will eventually lead to cable failure. In the prior art, a preventive test method for cables is performed using a cable partial discharge tester. However, cable partial discharge is only suitable for detecting cable defects with serious main insulation damage, and cannot effectively detect minor outer sheath defects in advance. However, the potential risk is high. If left unchecked, it will cause damage to the main insulation of the cable, cable grounding, or even short circuit failure. Therefore, it is urgent to design a 35kV single-core power cable outer sheath defect detection device and test method to solve the problem that the existing cable tester cannot effectively detect the cable outer sheath damage defect in advance, resulting in a high potential operation risk of the cable. Summary of the invention

[0003] In view of the problems existing in the prior art, the purpose of the present invention is to provide a 35kV single-core power cable outer sheath defect detection device and testing method, which can promptly detect abnormal grounding of the metal shielding layer of the single-core cable and take timely treatment measures to avoid damage to the cable main insulation, cable grounding and even short circuit accidents.

[0004] The technical solution adopted by the present invention to solve the technical problem is: a 35kV single-core power cable outer sheath defect detection device, comprising a DC power transmitter, an intelligent receiver and a current coupling clamp, wherein the DC power transmitter is provided with a high-voltage output interface, a protective grounding interface and a working grounding interface, wherein the protective grounding interface and the working grounding interface are connected to a grounding copper busbar of a substation, the high-voltage output interface is connected to one end of a cable metal shielding layer, and the other end of the cable metal shielding layer is set as a metal shielding layer empty end;

[0005] The intelligent receiver is provided with a signal input port and a display. The signal input port is linearly connected to the signal output port of the current coupling clamp. The fault detection waveform is displayed on the display. The current coupling clamp is clamped outside the cable.

[0006] Specifically, the high voltage output interface, protective grounding interface and working grounding interface are all arranged on the transmitter panel of the DC power transmitter, and the transmitter panel is also provided with a main power switch, a display screen, a working mode switch, a start-stop switch and a buck-boost switch.

[0007] Specifically, the main power switch turns on or off the working power of the DC power transmitter; the working mode switch can select the DC gear, the withstand voltage gear and the pulse gear; the start-stop switch controls the on and off of the high-voltage output interface; the buck-boost switch controls the adjustment of boost or buck by toggling left or right.

[0008] Specifically, the intelligent receiver is also provided with a switch, a gain plus adjustment button, a gain minus adjustment button and a battery charging stand.

[0009] Specifically, the switch turns on or off the power of the smart receiver.

[0010] Specifically, the cable includes but is not limited to an internal cable core wire and an external cable outer sheath, a cable metal shielding layer is arranged between the cable core wire and the cable outer sheath, one end of the cable is a substation side cable terminal, the substation side cable terminal is connected to the substation feeder switch cabinet, the cable metal shielding layer of the substation side cable terminal is connected to the high-voltage output interface of the DC power transmitter, and the other end of the cable is a user side cable terminal.

[0011] A testing method for a 35kV power cable outer sheath defect detection device comprises the following steps:

[0012] S1. Change the substation feeder switch cabinet cable line from operation to maintenance;

[0013] S2. Disconnect the end of the metal shielding layer of the three-phase power cable on the feeder switch cabinet side of the substation that is directly grounded to a free state, and disconnect the other end of the metal shielding layer of the three-phase power cable on the user side that is grounded through a voltage protector to a free state, and keep a sufficient safety distance from the grounding body;

[0014] S3. Install a DC power transmitter for cable outer sheath fault testing;

[0015] S4. Perform insulation resistance test on the metal shielding layer of phase A cable;

[0016] S5. Perform a DC withstand voltage leakage test on the metal shielding layer of the A-phase cable. Boost the DC power supply voltage to 5000V for 1 minute to see if the leakage current value is normal. If the leakage current value is very small, it means that the cable outer sheath is normal. If it is higher than the leakage current setting value, it means that the cable outer sheath is damaged.

[0017] S6. After the A phase test is completed, first slowly reduce the DC voltage to 0V, disconnect the main power switch of the DC power transmitter, and fully ground and discharge the metal shielding layer of the A phase cable;

[0018] S7, using the above-mentioned test method for phase A to test the metal shielding layers of the cables of phases B and C respectively;

[0019] S8. Find the defect point of the cable outer sheath: Send a DC pulse voltage to the cable metal shielding layer of the outer sheath defect phase, hold a cable fault intelligent receiver and a current coupling clamp, and test along the cable power supply path from the substation side to the user side;

[0020] S9. If there is a normal pulse waveform on the cable fault intelligent receiver, the outer sheath is damaged and the defect point is somewhere between this point and the user-side cable. If the waveform at the test point is severely attenuated, or there is no waveform displayed, it means that the outer sheath is damaged and the defect point is somewhere between this point and the substation-side cable. Repeat the test and troubleshooting until the defect point of the cable outer sheath is found.

[0021] Specifically, the installation steps of the DC power transmitter in step S3 include: at the feeder switch cabinet of the substation, the high-voltage output interface of the DC power transmitter is connected to one end of the metal shielding layer of phase A of the power cable through a dedicated cable, the working grounding interface of the DC power transmitter is connected to the grounding copper bus through a dedicated cable, and the protective grounding interface of the DC power transmitter is connected to the grounding copper bus through a dedicated cable.

[0022] Specifically, the insulation resistance test in step S4 includes using a 1 kV or 500 V megohmmeter to test the insulation resistance value of the metal shielding layer of phase A of the power cable.

[0023] The present invention has the following beneficial effects:

[0024] The 35kV single-core power cable outer sheath defect detection device and test method designed by the present invention adopts a pulse signal of a DC power transmitter to be transmitted from one side of the metal shielding layer where voltage is applied to the metal shielding layer to the other side. The signal is strong before the fault point and the signal strength drops sharply after passing the fault point. The location of the fault point can be determined by comparing the strength of the signal on the cable fault intelligent receiver. The test detection method is simple and can promptly detect abnormal grounding of the metal shielding layer of the single-core cable to avoid cable grounding or even short circuit accidents. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The present invention is a schematic diagram of the wiring structure of the metal shielding layer of a 35kV single-core power cable in the prior art.

[0026] Figure 2 It is a schematic diagram of the wiring structure of the 35kV single-core power cable outer sheath defect detection device of the present invention.

[0027] Figure 3 It is a schematic diagram of the panel layout of the DC power transmitter of the present invention.

[0028] Figure 4 It is a schematic diagram of the panel layout of the intelligent receiver of the present invention.

[0029] Figure 5 It is a schematic structural diagram of the current coupling clamp of the present invention.

[0030] Figure 6 It is a flow chart of a testing method of a 35kV single-core power cable outer sheath defect detection device of the present invention.

[0031] Figure 7 It is a waveform diagram on the intelligent receiver before the fault point in the embodiment of the present invention.

[0032] Figure 8 It is a waveform diagram on the intelligent receiver after the fault point in the embodiment of the present invention.

[0033] Fig. 9 It is a waveform diagram of a faulty intelligent receiver according to an embodiment of the present invention.

[0034] Fig.10 This is a test diagram for finding fault points on site during DC withstand voltage testing of the outer sheath of a 35kV single-core power cable according to an embodiment of the present invention.

[0035] In the figure: 1- cable core; 2- cable outer sheath; 3- cable metal shielding layer; 4- substation side cable terminal; 5- user side cable terminal; 6- metal shielding layer grounding; 7- metal shielding layer overvoltage protector grounding;

[0036] 8-DC power transmitter, 8.1-transmitter panel, 8.2-main power switch, 8.3-display screen, 8.4-high voltage output interface, 8.5-protective grounding interface, 8.6-working grounding interface, 8.7-working mode switch, 8.8-start-stop switch, 8.9-boost-down switch;

[0037] 9-intelligent receiver, 9.1-signal input port, 9.2-switch, 9.3-gain plus adjustment button, 9.4-gain minus adjustment button, 9.5-battery charging stand, 9.6-display;

[0038] 10-current coupling clamp, 10.1-signal output port;

[0039] 11-the empty end of the metal shielding layer; 12-the grounding copper busbar of the substation. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely further describe the technical solutions in the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0041] like Figure 1 As shown, a schematic diagram of the wiring structure of a 35kV single-core power cable metal shielding layer in the prior art. Under normal circumstances, the cable metal shielding layer 3 can only be directly grounded at one end, and the other end is grounded through the cable metal shielding layer overvoltage protector or is not grounded. The function of the cable metal shielding layer 3 is to maintain the ground potential. During normal operation, the cable metal shielding layer 3 passes a small capacitive current. When a short circuit or grounding fault occurs in the system, the cable metal shielding layer 3 can serve as a channel for the short-circuit current and play a role in shielding the electric field. Figure 1 The cable metal shielding layer 3 shown is directly grounded 6 on the substation side and is grounded 7 on the user side via a cable metal shielding layer overvoltage protector.

[0042] like Figure 2-Figure 5 As shown, a 35kV single-core power cable outer sheath defect detection device comprises a DC power transmitter 8, an intelligent receiver 9 and a current coupling clamp 10. The DC power transmitter 8 is provided with a transmitter panel 8.1. The transmitter panel 8.1 is provided with a main power switch 8.2, a display screen 8.3, a high-voltage output interface 8.4, a protective grounding interface 8.5, a working grounding interface 8.6, a working mode switch 8.7, a start-stop switch 8.8 and a step-up and step-down switch 8.9. The protective grounding interface 8.5 and the working grounding interface 8.6 are connected to a transformer substation grounding copper bus 12. The high-voltage output interface 8.4 is connected to one end 6 of a cable metal shielding layer 3. The other end of the cable metal shielding layer 3 is set as a metal shielding layer free end 11.

[0043] The main power switch 8.2 is used to turn on or off the working power of the DC power transmitter 8; the start-stop switch 8.8 is a switch for controlling the high-voltage output. After the main power switch 8.2 is turned on, the switch is started and stopped to control the on and off of the high-voltage output interface 8.4; the step-up and step-down switch 8.9 can be adjusted by toggling the step-up and step-down switch 8.9 left and right, and the percentage at the lower left corner of the display screen 8.3 shows the amplitude of the step-up. When 100%, the high-voltage output of the high-voltage output interface 8.4 is the maximum value; the high-voltage output interface 8.4 provides stable and reliable high voltage for measuring faults and withstand voltage tests. During operation, a dedicated high-voltage line is used to reliably connect the high-voltage output interface 8.4 to one end 6 of the cable metal shielding layer 3; Before operating the instrument test, the lead-out end of the protective grounding interface 8.5 should be separately and reliably grounded with the substation grounding copper bus 12 to ensure the safety of personnel and equipment; before operating the instrument test, the working grounding interface is the current return end and should be reliably grounded with the substation grounding copper bus 12 to ensure the safety of personnel and equipment; the LCD display 8.3 displays the current operating status and parameters through the human-machine dialogue interface; the working mode switch 8.7 has three gears: DC, withstand voltage, and pulse. The working mode is selected before starting up, and a certain working state is entered according to the work needs. After entering a certain working state, the start-stop switch 8.8 is pressed, and the step-up and step-down switch 8.9 is turned to increase the voltage or current to the required value to perform the corresponding work.

[0044] The parameters of the DC power transmitter 8 are as follows: working AC power supply: AC220V±10%, 50Hz; input power: 1000W; display mode: 5.6-inch LCD screen; output voltage: DC 0~15kV negative polarity high voltage; continuously adjustable; working mode: DC, withstand voltage, pulse; output maximum current: 1A. Can work continuously in short circuit; withstand voltage gear: 00~15kV negative polarity high voltage; maximum current: 1000μA. Volume: 450mm×330mm×350mm; weight: 20kg.

[0045] Working mode selection:

[0046] DC output: Output DC, mainly used for initial fault point detection, generally the continuous output power is adjusted to between 200W-700W;

[0047] Pulse output: Output pulse DC, output 1 second pulse width DC every 3 seconds, mainly used for fault location, generally the output power is adjusted to between 200W-400W;

[0048] Withstand voltage test: switch the working mode selection switch to the DC position, enter the DC mode interface according to the prompts on the LCD touch screen, and increase the voltage to the preset withstand voltage value. The duration is consistent with the withstand voltage test time. If there is no high-voltage breakdown in the sheath, return to the withstand voltage mode interface and start the withstand voltage test. The withstand voltage test is mainly used for preventive detection of cables and can measure the leakage current of the cable. The current measurement accuracy is μA level. When the current is greater than 1000μA, the system current test automatically switches to mA.

[0049] The intelligent receiver 9 is provided with a signal input port 9.1, a switch 9.2, a gain plus adjustment button 9.3, a gain minus adjustment button 9.4, a battery charging stand 9.5 and a display 9.6. The signal transmission port 9.1 is linearly connected to a signal output port 10.1 of a current coupling clamp 10. The fault detection waveform is displayed on the display 9.6. The current coupling clamp 10 is clamped outside the cable.

[0050] The switch 9.2 is used to turn the power on or off; the signal input port 9.1 is the connection port for connecting the intelligent receiver 9 and the current coupling clamp 10; the liquid crystal display 9.6, the intelligent receiver 9 uses a color liquid crystal display to display the signal waveform, strength, gain, and battery power; the gain plus adjustment button 9.3: press this button, the gain amplitude increases by 1 grid, the maximum is 10 grids; the gain minus adjustment button 9.4: press this button, the gain amplitude decreases by 1 grid, the minimum is 0 grid. The adjustable range of the gain is 0 to 10 grids. During the test, when the signal amplitude is too large, the user will reduce the gain; the battery charging stand 9.5 (spare), when the instrument is loaded with a rechargeable battery, it can be charged through this port; when the battery power is low, please replace the battery in time, or charge the rechargeable battery.

[0051] The current coupling clamp 10 is open type, and the fault phase cable can be passed through the opening. There is a mark indicating the direction of the current arrow on the current coupling clamp 10, and the inflow direction of the pulse current of the single-core cable must be the same as the direction indicated by the current arrow.

[0052] The parameters configured for the intelligent receiver 9 are as follows: display mode: color LCD display; display range: 50-0-50 (wider display range, higher resolution); sensitivity: 0.10mV; power supply: 6×1.5V; volume: 120mm×120mm×230mm; weight: 1kg.

[0053] During the test, the signal output port 10.1 of the current coupling clamp 10 is connected to the signal input port 9.1 of the intelligent receiver 9 via a control cable.

[0054] The cable includes an internal cable core wire 1 and an external cable outer sheath 2, a cable metal shielding layer 3 is arranged between the cable core wire 1 and the cable outer sheath 2, one end of the cable is a substation side cable terminal 4, the substation side cable terminal 4 is connected to the substation feed switch cabinet, one end 6 of the cable metal shielding layer 3 of the substation side cable terminal 4 is connected to the high voltage output interface 8.4 of the DC power transmitter 8, and the other end of the cable is a user side cable terminal 5.

[0055] like Figure 6 As shown, a testing method for a 35kV single-core power cable outer sheath defect detection device comprises the following steps:

[0056] 1. Change the substation feeder switch cabinet cable line from operation to maintenance.

[0057] Disconnect the circuit breaker on the user side of the substation feeder switch cabinet line, disconnect the circuit breaker of the substation feeder switch cabinet line; open the isolating switches on both sides of the circuit breaker, open the isolating switches on both sides of the circuit breaker; close the grounding switch on the user's incoming power supply side, and close the grounding switch on the outgoing line side of the substation feeder switch cabinet.

[0058] 2. Disconnect one end of the metal shielding layer 3 of the three-phase power cable on the substation feeder switch cabinet side that is directly grounded 6 to put it into a free state, and disconnect the other end of the metal shielding layer of the three-phase power cable on the user side that is grounded 7 through a voltage protector to put it into a free state, and maintain a sufficient safety distance from the grounding body.

[0059] 3. Install a DC power transmitter 8 for cable outer sheath fault testing.

[0060] At the feeder switch cabinet of the substation, the high-voltage output interface 8.4 of the DC power transmitter 8 is connected to one end 6 of the cable metal shielding layer 3 of phase A of the power cable through a dedicated cable, the working grounding interface 8.6 of the DC power transmitter 8 is connected to the grounding copper bus through a dedicated cable, and the protective grounding interface 8.5 of the DC power transmitter 8 is connected to the grounding copper bus through a dedicated cable.

[0061] 4. Perform insulation resistance test on the metal shielding layer of phase A cable.

[0062] Use a 1kV or 500V megohmmeter to test the insulation resistance value of the metal shielding layer 3 of the power cable phase A. If it is 15MΩ, it means the insulation resistance is normal. The standard value should be greater than 0.5MΩ / km.

[0063] 5. Perform a DC withstand voltage leakage test on the metal shielding layer 3 of the phase A cable. Boost the DC power supply voltage to 5000V gradually for 1 minute to see if the leakage current value is normal. If the leakage current value is very small, it means that the cable outer sheath 2 is normal. If it is higher than the leakage current setting value, it means that the cable outer sheath 2 has a damage defect.

[0064] First, turn on the main power switch 8.2 of the DC power transmitter 8, rotate the working mode switch 8.7 to the withstand voltage position, start the rotating buck-boost switch 8.9 to the boost position, and the DC power supply begins to gradually boost to 30% of the rated voltage, that is, the set value (5000V), for a certain period of time (1 minute), and check whether the leakage current value is normal from the tester LCD screen 8.3. Normally, the leakage current value is very small, such as 2.5mA. If the discharge leakage current is large, reaching tens of milliamperes, such as 50mA or even 300mA, it means that the cable outer sheath 2 has defects, and the metal shielding layer discharges to the ground during DC withstand voltage. The larger the leakage current value, the more serious the outer sheath defect.

[0065] If it is determined that the discharge leakage current of the metal shielding layer 3 of the phase cable is higher than the set current value (such as 20mA), it means that the metal shielding layer 3 of the phase cable has an abnormal discharge to the ground or the bridge, indicating that the cable outer sheath 2 is damaged.

[0066] 6. After the A phase test is completed, first slowly reduce the DC voltage to 0V, disconnect the main power switch 8.2 of the DC power transmitter 8, and fully ground and discharge the metal shielding layer 3 of the A phase cable.

[0067] 7. Use the above-mentioned test method for phase A to test the cable metal shielding layer 3 of phase B and phase C respectively.

[0068] 8. Find the defect point of the cable outer sheath 2: Send a DC pulse voltage to the cable metal shielding layer 3 of the defective phase of the outer sheath, and use a handheld cable fault intelligent receiver 9 and a current coupling clamp 10 to test along the cable power supply path from the substation side to the user side.

[0069] Rotate the working mode switch 8.7 to the pulse position, start the rotary buck-boost switch 8.9 to the boost position, send a DC pulse voltage, and there will be a higher leakage current. At this time, hold the cable fault intelligent receiver 9 and the current coupling clamp 10, and test along the cable power supply path from the substation side to the user side.

[0070] 9. If there is a normal pulse waveform on the cable fault intelligent receiver, the damaged defect point of the outer sheath is somewhere between this point and the user-side cable. If the waveform at the test point is severely attenuated, or there is no waveform displayed, it means that the damaged defect point of the outer sheath is somewhere between this point and the substation-side cable. Repeat the test and troubleshooting until the defect point of the cable outer sheath is found.

[0071] Turn on the main power switch 8.2 of the DC power transmitter 8, rotate the working mode switch 8.7 to the pulse gear, start the rotating buck-boost switch 8.9 to the boost position, the DC power supply starts to boost, output pulse DC, and output 1 second pulse width DC once every 3 seconds. The pulse signal is transmitted to the terminal along the cable metal shielding layer 3. The signal is strong before the fault point and the signal strength drops sharply after passing the fault point. The location of the fault point can be determined by comparing the strength of the signal. If the voltage is boosted to 1700V, sometimes it cannot be boosted. At this time, the leakage current is 690mA. At the same time, the cable fault intelligent receiver 9 and the current coupling clamp 10 are held and tested along the cable power supply path from the substation side to the user side. To improve the search efficiency, the waveform on the cable fault intelligent receiver 9 can be viewed in the middle part of the fault phase cable. If there is a normal pulse waveform, the damaged defect point of the cable outer sheath 2 is somewhere between this place and the user cable. If the test waveform is severely attenuated or no waveform is displayed, it means that the damaged defect point of the cable outer sheath 2 is somewhere between this place and the substation test cable. Repeat the test and investigation until the defective part of the cable outer sheath is found.

[0072] The 35kV single-core power cable outer sheath defect testing method is simple and can promptly detect abnormal grounding of the metal shielding layer of the single-core cable, thus avoiding cable grounding or even short circuit accidents.

[0073] Typical case 1: When conducting a DC withstand voltage test on the outer sheath of a company's 35kV converter I line cable, the leakage current was found to be 260mA. After sending a DC pulse voltage, the on-site handheld cable fault intelligent receiver and current coupling clamp were tested section by section. The waveform on the cable fault intelligent receiver before the fault point was as follows: Figure 7 The waveform on the cable fault smart receiver after the fault point is as follows Figure 8 As shown. Fig. 9 The waveform shown is a 35kV single-core power cable fault intelligent receiver. The waveform amplitude is small, indicating that the cable leakage current is large, which means that the cable outer sheath is seriously damaged. Fig.10 Provide a test diagram for finding on-site fault points during the DC withstand voltage test of a 35kV single-core power cable outer sheath.

[0074] The present invention is not limited to the above-mentioned implementation modes, and anyone should be aware of the structural changes made under the enlightenment of the present invention, and all those having the same or similar technical solutions as the present invention fall within the protection scope of the present invention.

[0075] The techniques, shapes, and structural parts not described in detail in the present invention are all well-known techniques.

Claims

1. A 35kV single-core power cable outer sheath defect detection device, characterized in that: It includes a DC power transmitter, an intelligent receiver and a current coupling clamp. The DC power transmitter is provided with a high-voltage output interface, a protective grounding interface and a working grounding interface. The protective grounding interface and the working grounding interface are connected to the grounding copper bus of the substation. The high-voltage output interface is connected to one end of the metal shielding layer of the cable. The other end of the metal shielding layer of the cable is set as the metal shielding layer empty end. The intelligent receiver is provided with a signal input port and a display. The signal input port is linearly connected to the signal output port of the current coupling clamp. The fault detection waveform is displayed on the display. The current coupling clamp is clamped outside the cable.

2. The 35kV single-core power cable outer sheath defect detection device according to claim 1 is characterized in that: The high voltage output interface, protective grounding interface and working grounding interface are all arranged on the transmitter panel of the DC power transmitter. The transmitter panel is also provided with a main power switch, a display screen, a working mode switch, a start-stop switch and a buck-boost switch.

3. The 35kV single-core power cable outer sheath defect detection device according to claim 2 is characterized in that: The main power switch turns on or off the working power of the DC power transmitter; the working mode switch can select DC gear, withstand voltage gear and pulse gear; the start-stop switch controls the on and off of the high-voltage output interface; the buck-boost switch controls the adjustment of boost or buck by toggling left or right.

4. The 35kV single-core power cable outer sheath defect detection device according to claim 1 is characterized in that: The intelligent receiver is also provided with a switch, a gain plus adjustment button, a gain minus adjustment button and a battery charging seat.

5. The 35kV single-core power cable outer sheath defect detection device according to claim 4 is characterized in that: The switch turns the power of the smart receiver on or off.

6. The 35kV single-core power cable outer sheath defect detection device according to claim 1 is characterized in that: The cable includes but is not limited to an internal cable core wire and an external cable outer sheath, a cable metal shielding layer is arranged between the cable core wire and the cable outer sheath, one end of the cable is a substation side cable terminal, the substation side cable terminal is connected to the substation feeder switch cabinet, the cable metal shielding layer of the substation side cable terminal is connected to the high voltage output interface of the DC power transmitter, and the other end of the cable is a user side cable terminal.

7. The testing method of the 35kV single-core power cable outer sheath defect detection device according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1. Change the substation feeder switch cabinet cable line from operation to maintenance; S2. Disconnect the end of the metal shielding layer of the three-phase power cable on the feeder switch cabinet side of the substation that is directly grounded to a free state, and disconnect the other end of the metal shielding layer of the three-phase power cable on the user side that is grounded through a voltage protector to a free state, and keep a sufficient safety distance from the grounding body; S3. Install a DC power transmitter for cable outer sheath fault testing; S4. Perform insulation resistance test on the metal shielding layer of phase A cable; S5. Perform a DC withstand voltage leakage test on the metal shielding layer of the A-phase cable. Boost the DC power supply voltage to 5000V for 1 minute to see if the leakage current value is normal. If the leakage current value is very small, it means that the cable outer sheath is normal. If it is higher than the leakage current setting value, it means that the cable outer sheath is damaged. S6. After the A phase test is completed, first slowly reduce the DC voltage to 0V, disconnect the main power switch of the DC power transmitter, and fully ground and discharge the metal shielding layer of the A phase cable; S7, using the above-mentioned test method for phase A to test the metal shielding layers of the cables of phases B and C respectively; S8. Find the defect point of the cable outer sheath: Send a DC pulse voltage to the cable metal shielding layer of the outer sheath defect phase, hold a cable fault intelligent receiver and a current coupling clamp, and test along the cable power supply path from the substation side to the user side; S9. If there is a normal pulse waveform on the cable fault intelligent receiver, the outer sheath is damaged and the defect point is somewhere between this point and the user-side cable. If the waveform at the test point is severely attenuated, or there is no waveform displayed, it means that the outer sheath is damaged and the defect point is somewhere between this point and the substation-side cable. Repeat the test and troubleshooting until the defect point of the cable outer sheath is found.

8. The testing method of the 35kV single-core power cable outer sheath defect detection device according to claim 7 is characterized in that: The installation steps of the DC power transmitter in step S3 include: at the feeder switch cabinet of the substation, connecting the high-voltage output interface of the DC power transmitter to one end of the metal shielding layer of phase A of the power cable through a dedicated cable, connecting the working grounding interface of the DC power transmitter to the grounding copper bus through a dedicated cable, and connecting the protective grounding interface of the DC power transmitter to the grounding copper bus through a dedicated cable.

9. The testing method of the 35kV single-core power cable outer sheath defect detection device according to claim 7 is characterized in that: The step of the insulation resistance test in step S4 includes using a 1 kV or 500 V megohmmeter to test the insulation resistance value of the metal shielding layer of the A phase of the power cable.

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