Method for eliminating induced voltage of metal sheath of three-core submarine cable

Abstract

The application provides a three-core submarine cable metal sheath induced voltage elimination method, comprising the following steps: obtaining a first induced voltage threshold of a submarine cable sea area section and a second induced voltage threshold of a submarine cable land section; according to a submarine cable metal sheath distributed induced voltage calculation model, a first section length of the submarine cable generating the first induced voltage threshold is calculated under a wet state; and according to the submarine cable metal sheath distributed induced voltage calculation model, a second section length of the submarine cable generating the second induced voltage threshold is calculated under a dry state; and the three-phase metal sheaths of the submarine cable are mutually short-circuited by the first section length or the second section length. By mutually short-circuiting the three-phase metal sheaths of the submarine cable, the problem of the PE semi-conductive layer overheating and melting due to the temperature rising caused by the induced voltage rising of the three-phase metal sheaths can be prevented.

Description

Technical Field

[0001] This invention relates to the field of submarine cable manufacturing processes, and more particularly to a method for eliminating induced voltage in the metal sheath of a three-core submarine cable. Background Technology

[0002] With the transformation of the global energy structure and the rapid development of technologies such as offshore wind power and transoceanic power transmission, submarine cables, as key equipment for power transmission, are becoming increasingly important. However, the long-term operation of submarine cables in complex marine environments such as humid and corrosive environments has made the safety of the cables themselves and their terminals one of the main bottlenecks restricting the development of submarine cable technology.

[0003] In the long-distance grounding of submarine cables, the problems of excessively high induced voltage and excessive circulating current at the midpoint of the submarine cable's metal sheath cannot be ignored. When the core of the submarine cable carries three-phase alternating current, an alternating magnetic field is generated in the surrounding space, and the metal sheath of the submarine cable, as part of a closed loop, will generate an induced electromotive force. Currently, the grounding method for submarine cables is double-end grounding, which directly grounds the first and last ends of the metal sheath to form a loop with the earth, thereby reducing the induced voltage at the end of the sheath. However, the long-distance laying characteristics of submarine cables cause the induced voltage at the midpoint of the metal sheath to be too high, even exceeding the interphase withstand voltage threshold. Using PSCAD simulation software, the interphase induced voltage of the metal sheath of a 90km, 500kV submarine cable can reach 2000V, which is enough to break down the PE sheath. According to previous experimental studies, when the voltage of the two-phase metal sheath rises to 220V, the highest temperature of the PE sheath rises to 60.9℃, and there are melting and adhesion marks between the two semi-conductive PE sheaths. Furthermore, due to long-term operation in the complex marine environment, coupled with the circulating current loss and heat generation caused by excessively high induced voltage, the PE semiconducting layer material inside the submarine cable will be affected by corrosion and thermal ablation, further leading to structural deterioration and performance degradation.

[0004] Currently, the field of submarine cable transmission technology lacks improvement projects for grounding methods of long-distance submarine cables, making it impossible to limit the maximum induced voltage of the sheath, reducing the service life of submarine cables, and causing economic and resource losses. Therefore, it is necessary to develop a method for canceling the induced voltage of each phase's metal sheath in three-core submarine cables. Summary of the Invention

[0005] The purpose of this invention is to provide a method for eliminating induced voltage in the metal sheath of a three-core submarine cable, so as to solve the problem of melting of the PE semiconducting layer caused by excessive induced voltage in the field of long-distance submarine cable power transmission technology.

[0006] This invention is achieved through the following technical solution:

[0007] One embodiment of the present invention provides a method for eliminating induced voltage in the metal sheath of a three-core submarine cable, comprising the following steps:

[0008] S1: Obtain the first induced voltage threshold of the submarine cable's marine section and the second induced voltage threshold of the submarine cable's terrestrial section;

[0009] S2: Based on the distributed induced voltage calculation model of submarine cable metal sheath, calculate the first segment length of the submarine cable that generates the first induced voltage threshold under wet conditions; and based on the distributed induced voltage calculation model of submarine cable metal sheath, calculate the second segment length of the submarine cable that generates the second induced voltage threshold under dry conditions.

[0010] S3: For submarine cables in the sea area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the first segment length as the maximum length value; for submarine cables in the land area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value; for submarine cables that have both sea area and land areas, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value.

[0011] In one embodiment, the step of obtaining the second induced voltage threshold of the onshore section of the submarine cable includes:

[0012] S11: Cut a sample of a three-core submarine cable to a preset length, remove the cable armor and filler layer, and use a fastening device to press the three-phase cable into a triangular shape;

[0013] S12: Set one or more temperature measurement points on the surface of the three-core submarine cable sample, and place temperature sensors on the temperature measurement points;

[0014] S13: Apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1 at this time and the voltage value U2 of the previous voltage application, and stop applying the voltage.

[0015] S14: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2, and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... dry This is the second induced voltage threshold.

[0016] In one embodiment, the step of obtaining the first induced voltage threshold of the submarine cable segment includes:

[0017] S21: Establish an environment box to simulate the actual operation of a submarine cable, wherein the environment box is equipped with simulated seawater;

[0018] S22: Execute steps S11 and S12, apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1' at this time and the voltage value U2' of the previous voltage application, and stop applying the voltage.

[0019] S23: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2', and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... wet This is the first induced voltage threshold.

[0020] In one embodiment, the calculation model for the distributed induced voltage of the submarine cable's metal sheath is established in the following manner:

[0021] An electromagnetic theoretical model of the three metal conductors of a metal-sheathed, three-core submarine cable is established.

[0022] Calculate the total magnetic flux of the metal sleeve and the three metal conductors based on the electromagnetic theoretical model;

[0023] The induced electromotive force (EMF) of the metal sleeve is calculated based on the total magnetic flux through the metal sleeve and the three metal conductors; the formula for calculating the induced EMF is:

[0024] ;

[0025] ;

[0026] ;

[0027] Among them, E A E is the induced electromotive force of the metal sleeve surrounding the first phase metal conductor. B E is the induced electromotive force of the metal sleeve surrounding the second-phase metal conductor. C ω is the induced electromotive force of the metal sheath surrounding the third-phase metal conductor; ω is the angular frequency of the load current in the metal conductor; S is the first distance, which is the distance between the first-phase metal conductor and the second-phase metal conductor; n is the ratio of the third distance to the first distance, which is the distance between the first-phase metal conductor and the third-phase metal conductor; m is the ratio of the second distance to the first distance, where the second distance is the distance between the second-phase metal conductor and the third-phase metal conductor; GMR S φ is the average geometric radius of the metal sleeve. paφ is the total magnetic flux of the metal sleeve surrounding the first-phase metal conductor. pb φ is the total magnetic flux of the metal sleeve surrounding the first-phase metal conductor. pc I represents the total magnetic flux of the metal sleeve surrounding the first phase metal conductor; I represents the effective value of the three-phase current.

[0028] In one embodiment, reactive power compensation devices are installed at both ends of the submarine cable to compensate for the capacitive reactive power generated by the submarine cable. In this case, the effective value of the three-phase current is calculated in the following way:

[0029] ;

[0030] Among them, U cl I represents the voltage at a distance l from the end of the submarine cable. cl P is the current at a distance l from the end of the submarine cable; P is the active power at the end of the submarine cable; Q cl Q represents the reactive power at a distance l from the end of the submarine cable. 1C Q1 is the reactive power absorbed by the equivalent inductance of the submarine cable; Q2 is the reactive power output at the end of the submarine cable; U2 is the voltage output at the end of the submarine cable; f is the system frequency of the current; L2 is the inductance connected in parallel at the end of the submarine cable; C c is the equivalent capacitance per unit distance of the submarine cable; l is the distance l from the end of the submarine cable.

[0031] In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes the following steps:

[0032] The calculated load current value I cl Substituting into the formula for calculating the induced voltage, we obtain the induced electromotive force as a function of the submarine cable length:

[0033] ;

[0034] The induced voltage value is obtained as a function of the induced voltage and the length of the submarine cable:

[0035] ;

[0036] Substitute the first induced voltage threshold and the second induced voltage threshold into the induced voltage function to obtain the first segment length and the second segment length, respectively.

[0037] In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes a step of verifying the accuracy of the distributed induced voltage calculation model:

[0038] Apply a three-phase load current to the submarine cable sample;

[0039] After the temperature of the submarine cable stabilizes, measure the magnitude of the induced voltage in the submarine cable.

[0040] The measured induced voltage of the submarine cable is compared with the calculated induced voltage value to determine the accuracy of the distributed induced voltage calculation model.

[0041] In one embodiment, the step of short-circuiting the three-phase metal sheaths of the submarine cable includes:

[0042] S31: Divide the submarine cable into segments according to the set segment lengths. Remove the PE semi-conductive layer of the three three-core submarine cables for a preset length between the two segment lengths, and waterproof the cut of the segments.

[0043] S32: Short-circuit the three-phase metal sheaths so that the metal sheaths of the three single-core cables in the stripped cable area are connected to each other.

[0044] S33: Use semi-conductive tape to wrap around the stripped portion of the three single-core cables, ensuring that the thickness at that location after wrapping is the same as the thickness of the untreated cable.

[0045] In one embodiment, the three-phase metal sleeve is shorted by lead plating.

[0046] In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes a step of verifying the reliability of the submarine cable after the three-phase metal sheaths are short-circuited together:

[0047] S41: Use a resistance measuring device to measure the phase-to-phase resistance of the metal sheaths of the submarine cable after the metal sheaths are short-circuited, and determine whether the phase-to-phase resistance is close to 0.

[0048] S42: Immerse the submarine cable with the metal sleeves short-connected in simulated seawater for a preset time to check whether the short-connection point of the submarine cable is continuous and reliable without any breaks, and whether there are any signs of water intrusion inside the metal sleeve, thereby judging the reliability of the short-connection point.

[0049] S43: After shorting the metal sheaths together, the submarine cable is wound up and unwound to its initial state. Check the surface of the submarine cable for indentations and the armor layer for deformation. Then, apply a phase voltage several times to the submarine cable and check whether the test cable is broken down.

[0050] S44: After shorting the metal sheaths together, the submarine cable is wound and then wound in the reverse direction. At the same time, dynamic tension is applied to check whether there are indentations on the surface of the submarine cable or whether the armor layer is deformed. Then, several times the phase voltage is applied to the submarine cable to check whether the test cable is broken down.

[0051] The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to the present invention has the following beneficial effects: By using a distributed induced voltage calculation model for the metal sheath of the submarine cable, the first segment length of the submarine cable that generates a first induced voltage threshold under wet conditions and the second segment length of the submarine cable that generates a second induced voltage threshold under dry conditions are calculated. Then, the three-phase metal sheaths of the submarine cable are short-circuited to each other using either the first segment length or the second segment length. By short-circuiting the three-phase metal sheaths of the submarine cable, the induced voltages in the metal sheaths of the three-core submarine cable can be mutually canceled, preventing the PE semiconducting layer from overheating and melting. This solution provides a new approach to addressing the problem of overheating and melting of the PE semiconducting layer due to excessive induced voltage during the operation of long-distance submarine cables. Attached Figure Description

[0052] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0053] Figure 1 This is a 3D diagram of a three-core submarine cable.

[0054] Figure 2 for Figure 1 A schematic diagram of the cross-section of a three-core submarine cable;

[0055] Figure 3 A schematic flowchart illustrating the method for eliminating induced voltage in the metal sheath of a three-core submarine cable provided in this application embodiment;

[0056] Figure 4 This is a schematic diagram for calculating the induced voltage of a metal sleeve under a single-loop configuration.

[0057] Figure 5 Circuit diagram for calculating load current of submarine cable lines after installing reactive power compensation devices;

[0058] Figure 6 A schematic diagram of the process for obtaining the second induced voltage threshold of the land section of a submarine cable;

[0059] Figure 7 A schematic diagram of the cross-section of a submarine cable after its armor has been removed;

[0060] Figure 8 A schematic diagram of the structure for setting up temperature measuring points on a submarine cable after the cable armor has been removed;

[0061] Figure 9 A flowchart illustrating the process for obtaining the first induced voltage threshold of a submarine cable segment in the sea area;

[0062] Figure 10 This is a schematic diagram illustrating the process of short-circuiting the three-phase metal sheaths of a submarine cable.

[0063] Figure 11 A schematic diagram showing the interconnection of the metal sheaths of three single-core cables in the stripped cable area;

[0064] Figure 12 A schematic diagram showing the structure for wrapping three single-core cables at the stripped locations using semi-conductive tape;

[0065] Figure 13 A flowchart illustrating the process for verifying the reliability of a submarine cable after the three-phase metal sheaths are short-circuited. Detailed Implementation

[0066] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are merely some embodiments of the invention, and not all embodiments. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0067] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0068] To address the problem of PE semiconductive layer melting caused by excessive induced voltage in the field of long-distance submarine cable transmission technology, the overall framework of the technical solution adopted in this application is as follows:

[0069] 1. The length l of the submarine cable required to generate the induced voltage threshold during the operation of a three-core submarine cable.

[0070] 2. A method is proposed to short-circuit the metal sheaths of three-phase submarine cables during the manufacturing process.

[0071] 3. Verify the accuracy of the induced voltage calculation method and the feasibility of the short-circuit method.

[0072] Specifically, regarding the length *l* of the submarine cable required to generate the induced voltage threshold during the operation of the three-core submarine cable, this application obtains two thresholds through experiments using the induced voltage thresholds of the onshore and offshore sections of the submarine cable. Combined with the calculation method for the distributed induced voltage of the submarine cable's metal sheath, the segment lengths of the submarine cable required to generate this induced voltage threshold are calculated under both dry and wet conditions. For the offshore section, the voltage threshold obtained from the wet test is used as the maximum value of the three-phase metal sheath short-circuit segment length of the submarine cable. For the onshore section of the submarine cable, the three-phase metal sheath short-circuit segment length calculated using the threshold obtained from the dry test is the maximum value. This ensures the safe and reliable operation of the entire submarine cable system. For the length from the final short-circuit point in the offshore section to the onshore section, the threshold obtained from the dry test should be used for calculation (considering that, under the same conditions, the voltage threshold obtained from the wet test will be higher than that from the dry test).

[0073] In a three-core land cable, each phase core is surrounded by a copper shielding strip. The three phases are in close contact through this shielding strip, and the induced voltages in the three phases cancel each other out due to superposition, resulting in minimal current flow across the shielding strip. Unlike land cables, three-core submarine cables have an additional semi-conductive PE sheath outside the metal sheath. The resistivity of the semi-conductive PE sheath is significantly lower than that of the metal sheath (which is on the order of 10⁻⁶). -8 In terms of grade (likely referring to a specific grade or level), significant resistance exists under both dry and wet conditions, meaning effective electrical conduction between the three core submarine cables cannot be assumed. In this case, the arrangement of the three core submarine cables is equivalent to a three-phase single-core terrestrial cable laid in an equilateral triangle. When a three-phase power frequency current flows through the three core cables, an alternating magnetic field is generated around them, inducing a voltage on the cable's metal sheath. A calculation model for the induced voltage on the cable's metal sheath is constructed based on electromagnetic theory. Subsequently, considering the significant reactive power generated by long-distance AC submarine cables in practical engineering, reactive power compensation devices are typically installed to compensate for the capacitive reactive power generated by the cables. A simple power flow calculation method is used to calculate the load current for different cable lengths after compensation. Substituting the load current value into the theoretical calculation model for the induced voltage on the metal sheath allows for the calculation of the induced voltage on the metal sheath of submarine cables of different lengths.

[0074] The threshold voltage U that causes the PE semiconductive sheath to melt was determined by dry-state testing of submarine cables. dry The result measured by the experiment is U dry This serves as the threshold voltage for the onshore section of the submarine cable. The test primarily involves applying a three-phase voltage to the three-phase metal sheath of the submarine cable, creating a voltage difference between the three phases. To ensure the reliability of the test results, a temperature measuring instrument is used to observe the temperature change of the PE semiconducting layer during the test. The voltage can only be increased further after the temperature of the PE semiconducting layer has stabilized. Finally, the voltage value that melts the PE semiconducting layer of the submarine cable is measured.

[0075] The threshold voltage U that causes the PE semiconductive sheath to melt was determined by wet testing of submarine cables. wet The result measured by the experiment is U wet This serves as the threshold voltage for the submarine cable's sea-crossing section. The test primarily involves immersing the submarine cable in seawater (3.1±0.2% saline solution by mass) and applying a three-phase voltage to the three-phase metal sheath of the cable, creating a voltage difference between the three phases. To ensure the reliability of the test results, the temperature change of the PE semiconducting layer is observed using appropriate temperature measuring instruments. The voltage is only increased after the PE semiconducting layer temperature stabilizes, and the final measured voltage value is the one that melts the PE semiconducting layer of the submarine cable. Because the heat dissipation conditions are better in seawater than in air, the voltage value measured in the wet test will be higher than that in the dry test.

[0076] Regarding the test method for obtaining the threshold of induced voltage in the metal sheath of a three-core submarine cable and the short-circuiting method for offsetting the induced voltage in the three-phase metal sheath, this application completes the process by stripping a predetermined length of the PE semiconductive layer from the three three-core submarine cables and short-circuiting the three-phase metal sheath.

[0077] Regarding the methods for verifying the accuracy of the distributed induced voltage calculation method for submarine cables and the reliability of the short-circuiting method for submarine cable metal sheaths, this application employs the following approach: When verifying the reliability of the distributed induced voltage calculation method for submarine cables, a submarine cable of several lengths is used. An appropriate load current is applied to the cable conductor, and the induced voltage of the cable metal sheath is measured. The measured value is compared with the result calculated using the distributed induced voltage calculation method for submarine cables, and the accuracy of the theoretical calculation method is judged based on the magnitude of the error. When verifying the reliability of the short-circuiting method, the reliability can be determined by measuring the phase-to-phase resistance of the three-phase sheaths (verifying the reliability of the short-circuiting method under static conditions) and by conducting water penetration tests and mechanical performance tests (verifying the reliability of the short-circuiting method under dynamic conditions).

[0078] To provide a more detailed description of the technical solution of this application, a brief introduction to the structure of the three-core submarine cable will be given first.

[0079] Please see Figure 1 and Figure 2A three-core submarine cable typically includes an armor layer and three-phase conductors encased within it. The armor layer comprises, from the inside out, a wrapping tape 10, a PP inner padding layer 11, an armored steel wire + asphalt layer 12, and a PP outer sheath. Each phase conductor includes, from the inside out, a water-blocking copper conductor 1, a conductor shield 2, XLPE insulation 3, an insulation shield 4, a semi-conductive water-blocking tape 5, an alloy lead sheath 6, and a semi-conductive PE sheath 7. Depending on the requirements, the three-core submarine cable may also include an optical fiber unit 8. Depending on the requirements, the armor layer may also contain filler 9 to secure the three-phase conductors and the optical fiber unit 8.

[0080] Please see Figure 3 One embodiment of the present invention provides a method for eliminating induced voltage in the metal sheath of a three-core submarine cable. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable includes the following steps:

[0081] S1: Obtain the first induced voltage threshold of the submarine cable's marine section and the second induced voltage threshold of the submarine cable's terrestrial section;

[0082] S2: Based on the distributed induced voltage calculation model of submarine cable metal sheath, calculate the first segment length of the submarine cable that generates the first induced voltage threshold under wet conditions; and based on the distributed induced voltage calculation model of submarine cable metal sheath, calculate the second segment length of the submarine cable that generates the second induced voltage threshold under dry conditions.

[0083] S3: For submarine cables in the sea area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the first segment length as the maximum length value; for submarine cables in the land area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value; for submarine cables that have both sea area and land areas, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value.

[0084] In the above method for eliminating induced voltage in the metal sheath of three-core submarine cables, a distributed induced voltage calculation model for the submarine cable metal sheath is used to calculate the first segment length of the submarine cable that generates the first induced voltage threshold under wet conditions, and the second segment length that generates the second induced voltage threshold under dry conditions. Then, the three-phase metal sheaths of the submarine cable are short-circuited to each other using either the first or second segment length. By short-circuiting the three-phase metal sheaths of the submarine cable, the induced voltages in the three-core submarine cable metal sheaths can be canceled out, preventing the PE semiconducting layer from overheating and melting. This solution provides a new approach to addressing the problem of overheating and melting of the PE semiconducting layer due to excessive induced voltage during the operation of long-distance submarine cables.

[0085] In one embodiment, the calculation model for the distributed induced voltage of the submarine cable's metal sheath is established in the following manner:

[0086] An electromagnetic theoretical model of the three metal conductors of a metal-sheathed, three-core submarine cable is established.

[0087] Calculate the total magnetic flux of the metal sleeve and the three metal conductors based on the electromagnetic theoretical model;

[0088] The induced electromotive force (EMF) of the metal sleeve is calculated based on the total magnetic flux through the metal sleeve and the three metal conductors; the formula for calculating the induced EMF is:

[0089] ;

[0090] ;

[0091] ;

[0092] Among them, E A E is the induced electromotive force of the metal sleeve surrounding the first phase metal conductor. B E is the induced electromotive force of the metal sleeve surrounding the second-phase metal conductor. C ω is the induced electromotive force of the metal sheath surrounding the third-phase metal conductor; ω is the angular frequency of the load current in the metal conductor; S is the first distance, which is the distance between the first-phase metal conductor and the second-phase metal conductor; n is the ratio of the third distance to the first distance, which is the distance between the first-phase metal conductor and the third-phase metal conductor; m is the ratio of the second distance to the first distance, where the second distance is the distance between the second-phase metal conductor and the third-phase metal conductor; GMR S φ is the average geometric radius of the metal sleeve. pa φ is the total magnetic flux of the metal sleeve surrounding the first-phase metal conductor. pb φ is the total magnetic flux of the metal sleeve surrounding the first-phase metal conductor. pc I represents the total magnetic flux of the metal sleeve surrounding the first phase metal conductor; I represents the effective value of the three-phase current.

[0093] Specifically, in a three-core land cable, each phase core has a copper shielding strip outside its main insulation. The three phases are in close contact through the copper shielding strip, and the induced voltages of the three phases cancel each other out due to superposition, resulting in no large current flowing through the copper shielding strip. Unlike land cables, three-core submarine cables have a semi-conductive PE sheath outside the metal sheath. The resistivity of the semi-conductive PE sheath is relatively low compared to the metal sheath (on the order of 10). -8(Level), regardless of whether it is in a dry or wet state, there is a large resistance, so it cannot be assumed that the three-core submarine cables can effectively conduct electricity. Therefore, the three-phase induced voltages in the metal sheath cannot be completely canceled out. In this case, the arrangement of the three-core submarine cables is equivalent to a three-phase single-core terrestrial cable laid in an equilateral triangle.

[0094] When a three-core cable carries a three-phase power frequency current, an alternating magnetic field is generated around it, inducing a voltage on the cable's metal sheath. When the two ends of the sheath are grounded, forming a closed loop, an induced current is generated in the sheath. Figure 4 shows a schematic diagram for calculating the induced voltage in the metal sheath under a single loop. Based on electromagnetic theory, the induced voltage in the metal sheath is calculated as follows: Let A, B, C, and P be four parallel metal conductors, with distances PA=D, PB=βD, PC=γD, AB=S, BC=mS, and CA=nS between them. Let the currents flowing through conductors A, B, and C be I0, I0, and I0, respectively. A I B I C The magnetic flux of the hinge between metals P and A can be calculated using the following formula.

[0095] (1)

[0096] GMR S The average geometric radius of conductor P

[0097] Similarly, the magnetic fluxes of phases B and C are respectively

[0098] (2)

[0099] Therefore, the total magnetic flux of conductor P and the three phases A, B, and C is

[0100] (3)

[0101] Let metal P be the sheath of phase A cable, meaning that the centers of P and A coincide. Then PA = GMR S , PB=D1, PC=D3, at this time equation (3) can be transformed into equation (4).

[0102] (4)

[0103] Assuming the three-phase load currents are balanced, then we have

[0104] (5)

[0105] I is the effective value of the three-phase current, therefore we have

[0106] (6)

[0107] Where ω = 2πf, f is the frequency of the load current;

[0108] Similarly, the induced electromotive force of the metal sleeves in phases B and C can be obtained as follows:

[0109] (7)

[0110] (8)

[0111] The above calculations can yield the induced electromotive force value of the metal sheath of a submarine cable per kilometer.

[0112] In one embodiment, reactive power compensation devices are installed at both ends of the submarine cable to compensate for the capacitive reactive power generated by the submarine cable. In this case, the effective value of the three-phase current is calculated in the following way:

[0113] ;

[0114] Among them, U cl I represents the voltage at a distance l from the end of the submarine cable. cl P is the current at a distance l from the end of the submarine cable; P is the active power at the end of the submarine cable; Q cl Q represents the reactive power at a distance l from the end of the submarine cable. 1C Q1 is the reactive power absorbed by the equivalent inductance of the submarine cable; Q2 is the reactive power output at the end of the submarine cable; U2 is the voltage output at the end of the submarine cable; f is the system frequency of the current; L2 is the inductance connected in parallel at the end of the submarine cable; C c is the equivalent capacitance per unit distance of the submarine cable; l is the distance l from the end of the submarine cable.

[0115] In practical engineering, long-distance AC submarine cables generate significant reactive power during charging. Reactive power compensation devices are typically installed to compensate for this capacitive reactive power. A simple power flow calculation method can be used to correct the compensated load current. The corrected current value is then substituted back into the theoretical calculation model to recalculate the induced electromotive force in the metal sheath. Taking double-ended compensation as an example, the circuit diagram is shown in Figure 5. Wherein:

[0116] 1) The submarine cable line starts at node 1 and ends at node 2;

[0117] 2) The input active power of node 1 is P1, the input reactive power is Q1, and the voltage is U1; the output active power of node 2 is P2, the output reactive power is Q2, and the voltage is U2.

[0118] 3)U cl I cl These represent the voltage and current at a distance l from the end of the submarine cable, respectively.

[0119] 4)r c l c C cThese are the equivalent resistance, inductance, and capacitance per unit distance of the submarine cable;

[0120] 5) f is the system frequency, L is the total length of the submarine cable, L1 is the inductance of the parallel reactance at the beginning of the submarine cable, L2 is the inductance of the parallel reactance at the end of the submarine cable, and Q 1C The reactive power absorbed by the equivalent inductance of the submarine cable.

[0121] Neglecting active power losses and voltage deviations along the cable route, derive the power at the end of the submarine cable. Where P2 = P, Q2 is...

[0122] (9)

[0123] Among them: Q1 ' For reactive power at the beginning of the submarine cable; Q C Q represents the reactive power of the equivalent capacitance along the submarine cable route. X The reactive power of the equivalent reactance along the submarine cable; Q L2 The reactive power of the parallel reactor at the onshore grid connection point; therefore, the reactive power Q1 at node 1 of the submarine cable's starting point. ' :

[0124] (10)

[0125] Reactive power Q of the equivalent capacitance along the submarine cable C :

[0126] (11)

[0127] Reactive power Q of the equivalent reactance along the submarine cable X :

[0128] (12)

[0129] The reactive power Q of the parallel reactance at the end of the submarine cable L2 :

[0130] (13)

[0131] The expression for the voltage along the line is:

[0132] (14)

[0133] The reactive power absorbed by the equivalent inductance of the submarine cable is

[0134] (15)

[0135] The expression for the current along the line is:

[0136] (16)

[0137] I was obtained through calculation. cl The result is updated to the load current I in formula (6), which can realize the calculation of the induced electromotive force of the cable metal sheath at each location. As needed, in actual calculations, the above algorithm can also be programmed, and the calculation efficiency can be improved by using a computer program.

[0138] Please see also Figure 6 In one embodiment, the step of obtaining a second induced voltage threshold for the onshore section of the submarine cable includes:

[0139] S11: Cut a sample of a three-core submarine cable to a preset length, remove the cable armor and filler layer, and use a fastening device to press the three-phase cable into a triangular shape;

[0140] S12: Set one or more temperature measurement points on the surface of the three-core submarine cable sample, and place temperature sensors on the temperature measurement points;

[0141] S13: Apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1 at this time and the voltage value U2 of the previous voltage application, and stop applying the voltage.

[0142] S14: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2, and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... dry This is the second induced voltage threshold.

[0143] Specifically, experiments can be conducted to obtain the induced voltage threshold of the onshore section of the submarine cable. The induced voltage threshold U that causes the PE semiconductive sheath to melt can be measured through dry-state testing of the submarine cable. dry ;

[0144] The experiment includes the following steps:

[0145] 1) Due to the inconsistency in the semi-conductive PE materials used by different submarine cable manufacturers, the melting temperature of the semi-conductive PE sheath in submarine cables produced by different manufacturers varies. Therefore, the melting point of the semi-conductive PE material used by the manufacturer is tested to obtain the melting temperature T0 of the semi-conductive PE material.

[0146] 2) Select a 2.5m long three-core submarine cable sample, strip the cable armor and filler layer, and use a fastening device to press the three-phase cable into a triangular shape, such as... Figure 7 As shown;

[0147] 3) Place a temperature sensor. The type of temperature sensor includes, but is not limited to, a non-insulated, withstand voltage patch temperature sensor. Set two temperature measurement points on the surface of the three-core submarine cable sample. Figure 8 As shown, temperature sensors were placed at two temperature measurement points. The cable sample used in the experiment was 2.5m long. Since the conductivity of the metal sheath is relatively high, the voltage difference between the first and last ends of the 2.5m long metal sheath can be almost ignored. Considering the accuracy of the measurement data, temperature sensors were placed at temperature measurement points A and B on the cable surface every 30cm.

[0148] 4) Apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable using a voltage regulator. Start the voltage from 0V and increase it by 15V each time. Before each voltage increase, observe the temperature change of the temperature sensor. Continue to increase the voltage after the temperature stabilizes. When the temperature exceeds the melting point T0 of the PE semiconductor, record the voltage value U1 and the previous voltage value U2, and then stop applying the voltage.

[0149] 5) Replace the cable sample with a new one. Place a temperature sensor in the new cable sample and apply voltage to the three-phase metal sheath. Start with U2 and increase the voltage by 1V each time. After the temperature stabilizes, continue increasing the voltage for the next time. Observe the readings from the temperature sensor. When the temperature exceeds the melting point T0 of the PE semiconductor, record the voltage value U at this point. dry .

[0150] Please see also Figure 9 In one embodiment, the step of obtaining the first induced voltage threshold of the submarine cable segment includes:

[0151] S21: Establish an environment box to simulate the actual operation of a submarine cable, wherein the environment box is equipped with simulated seawater;

[0152] S22: Execute steps S11 and S12, apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1' at this time and the voltage value U2' of the previous voltage application, and stop applying the voltage.

[0153] S23: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2', and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... wet This is the first induced voltage threshold.

[0154] Specifically, experiments can be conducted to obtain the induced voltage threshold of submarine cable sections in the sea area. The induced voltage threshold U that causes the PE semiconductive sheath to melt when immersed in seawater can be measured through wet testing. wet ;

[0155] The wet test is similar to the dry test in procedure, but the main difference lies in the operating environment of the test cable. The wet test involves immersing the submarine cable in salt water (simulating seawater) with a mass fraction of (3.1±0.2)%, to simulate the actual operating conditions of the submarine cable. The main steps are as follows:

[0156] 1) Construct a simulated operating environment chamber for submarine cables, using (3.1±0.2)% saline solution instead of seawater;

[0157] 2) Place temperature sensors. The types of temperature sensors include, but are not limited to, non-insulated withstand voltage patch temperature sensors. Set two temperature measurement points on the surface of the three-core submarine cable sample and place temperature sensors at the two temperature measurement points respectively. The cable sample used in the test is 2.5m long, and temperature sensors are placed at temperature measurement points A' and B' on the cable surface every 30cm.

[0158] 3) Apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable using a voltage regulator. Start increasing the voltage from 0V. Considering that the heat dissipation effect of salt water is better than that of air, increase it by 25V each time. Before each voltage increase, observe the temperature change of the temperature sensor. After the temperature stabilizes, continue to increase the voltage and observe the measured value of the temperature sensor. When the temperature exceeds the melting point T0 of the PE semiconductor, record the voltage value U1' at this time and the voltage value U2' of the previous voltage application, and then stop applying the voltage.

[0159] 4) Replace the cable sample with another section. Place a temperature sensor in the new cable sample and apply voltage to the three-phase metal sheath. Start with U2 and increase the voltage by 1V each time. Before each voltage increase, observe the temperature change of the temperature sensor. After the temperature stabilizes, continue to increase the voltage and observe the measured value of the temperature sensor. When the temperature exceeds the melting point of the PE semiconductor, T0, record the voltage value U at this time. wet .

[0160] According to the first induced voltage threshold U wet Second induced voltage threshold U dry Calculate the length l of the submarine cable required to generate the corresponding induced voltage threshold.

[0161] In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes the following steps:

[0162] The calculated load current value I clSubstituting into the formula for calculating induced electromotive force, we obtain the function of induced electromotive force and submarine cable length:

[0163] ;

[0164] The induced voltage value is obtained as a function of the induced voltage and the length of the submarine cable:

[0165] ;

[0166] Substitute the first induced voltage threshold and the second induced voltage threshold into the induced voltage function to obtain the first segment length and the second segment length, respectively.

[0167] Specifically, two voltage thresholds U obtained from dry-state and wet-state tests are used. dry and U wet Using the corresponding calculation formulas, the required segment lengths of the submarine cable to generate the induced voltage threshold under both dry and wet conditions were calculated. Considering that, under the same conditions, the voltage threshold obtained from the wet test will be higher than that from the dry test, let the result U obtained from the dry test be... dry As the threshold voltage for the onshore section of the submarine cable, the result U obtained from the wet test... wet This serves as the threshold voltage for the sea section of the submarine cable. It is important to note that this calculation method applies to both the sea and onshore sections of the submarine cable. For submarine cables laid in areas other than the sea section, the calculation method for the onshore section is used. The specific steps are as follows:

[0168] 1) Calculate the load current value I cl Substituting I into the induced voltage calculation formula in the first step of the specific implementation method (taking phase A as an example), we obtain formula (18). Formula (18) can be considered as the induced electromotive force E i E is a function of the length l of the submarine cable i (l).

[0169] (17)

[0170] (18)

[0171] 2) The numerical value U obtained through experiments S13 and S14 dry and U wet Substituting the specific values ​​into U in formula (19), the corresponding cable length l can be calculated.

[0172] (19)

[0173] In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes a step of verifying the accuracy of the distributed induced voltage calculation model:

[0174] Apply a three-phase load current to the submarine cable sample;

[0175] After the temperature of the submarine cable stabilizes, measure the magnitude of the induced voltage in the submarine cable.

[0176] The measured induced voltage of the submarine cable is compared with the calculated induced voltage value to determine the accuracy of the distributed induced voltage calculation model.

[0177] Please see also Figure 10 In one embodiment, the step of short-circuiting the three-phase metal sheaths of the submarine cable includes:

[0178] S31: Divide the submarine cable into segments according to the set segment lengths. Remove the PE semi-conductive layer of the three three-core submarine cables for a preset length between the two segment lengths, and waterproof the cut of the segments.

[0179] S32: Short-circuit the three-phase metal sheaths so that the metal sheaths of the three single-core cables in the stripped cable area are connected to each other.

[0180] S33: Use semi-conductive tape to wrap around the stripped portion of the three single-core cables, ensuring that the thickness at that location after wrapping is the same as the thickness of the untreated cable.

[0181] As needed, the three-phase metal bushing is short-circuited by lead plating.

[0182] The specific steps are as follows:

[0183] 1) During the manufacturing process of the three-core cable, before the three cores are bundled together, according to the calculated length of the submarine cable segments, three PE semi-conductive layers of three three-core cables with a length of L1 are stripped between the two segments, and the cuts of the two segments are waterproofed.

[0184] 2) Short-circuit the three-phase metal sheaths, including but not limited to lead-lined short-circuiting methods, to interconnect the metal sheaths of the three single-core cables in this stripped cable section, such as... Figure 11 As shown. In this embodiment, the metal sheaths of the three single-core cables are interconnected by lead plating. A lead layer 14 formed by molten lead is disposed between the metal sheaths 7. The length of L1 can be set according to the actual required voltage difference U1. If the voltage across L1 is U1, the current flowing through the metal sheath is I. n The total resistance of the metal sleeve in segment L1 is R1, the resistivity of the metal sleeve is ρ, and S is the cross-sectional area of ​​the metal sleeve. Then U1 is directly proportional to L1, as shown in the following formula: ; ;

[0185] 3) Use semi-conductive tape 15 to wrap around the stripped sections of the three single-core cables, such as... Figure 12 As shown, the thickness at this location after winding is approximately the same as the thickness of the untreated cable.

[0186] Please see also Figure 13 In one embodiment, the method for eliminating induced voltage in the metal sheath of a three-core submarine cable further includes a step of verifying the reliability of the submarine cable after the three-phase metal sheaths are short-circuited:

[0187] S41: Use a resistance measuring device to measure the phase-to-phase resistance of the metal sheaths of the submarine cable after the metal sheaths are short-circuited, and determine whether the phase-to-phase resistance is close to 0.

[0188] S42: Immerse the submarine cable with the metal sleeves short-connected in simulated seawater for a preset time to check whether the short-connection point of the submarine cable is continuous and reliable without any breaks, and whether there are any signs of water intrusion inside the metal sleeve, thereby judging the reliability of the short-connection point.

[0189] S43: After shorting the metal sheaths together, the submarine cable is wound up and unwound to its initial state. Check the surface of the submarine cable for indentations and the armor layer for deformation. Then, apply a phase voltage several times to the submarine cable and check whether the test cable is broken down.

[0190] S44: After shorting the metal sheaths together, the submarine cable is wound and then wound in the reverse direction. At the same time, dynamic tension is applied to check whether there are indentations on the surface of the submarine cable or whether the armor layer is deformed. Then, several times the phase voltage is applied to the submarine cable to check whether the test cable is broken down.

[0191] Specifically, the method for verifying the accuracy of the distributed induced voltage calculation method and the feasibility of the cable metal sheath short-circuiting method includes the following steps.

[0192] The accuracy of the distributed induced voltage calculation model is verified through the following experiments.

[0193] Take a sample of submarine cable of length L3 (a conventional submarine cable without short-circuiting), apply a three-phase load current to the cable sample, and after the cable conductor temperature stabilizes, measure the magnitude of the induced voltage of the three-phase cable. Compare the measurement results with the theoretical calculation model results of the induced voltage in the first step. By judging the magnitude of the error between the theoretical calculation results and the measured results, the accuracy of the theoretical calculation model of the induced voltage can be determined.

[0194] The following tests were conducted on the cables produced after the short-circuiting treatment to test the reliability of the short-circuiting points.

[0195] a. Feasibility of the short-circuit method (static):

[0196] Take a sample of submarine cable with a length of L4 (produced according to the short-circuiting method of the present invention), and use a resistance measuring device (including but not limited to a multimeter) to measure the phase-to-phase resistance of the metal sheath of this cable, namely the phase-to-phase resistance between phase A and phase B, phase A and phase C, and phase B and phase C, and determine whether the phase-to-phase resistance is close to 0.

[0197] b. Water permeability test under metal casing:

[0198] The test used a 50-meter section of submarine cable that had undergone short-splitting treatment. The cable was immersed in a saline solution with a mass fraction of (3.1±0.2)%. The cable sample was then subjected to thermal cycling to induce radial expansion. The test pressure was set at 0.3 MPa, and the water temperature was maintained between 5℃ and 35℃ for 48 hours. After the test period, the sample was removed from the water, and the short-splitting points were inspected for continuity, reliability, and absence of breaks. The reliability of the short-splitting points was assessed by checking for signs of water intrusion within the metal sheath.

[0199] c. Winding test

[0200] 1. Take 50 meters of cable that has been shorted (this section of cable must contain at least 2 shorting points) for the test, remove the torsional stress of the cable by passing it through a torsion relief wheel, and fix it to the winding reel;

[0201] 2. Use a winding test device to wind the sample cable at a constant speed (≤15r / min), and record the strain data for each turn;

[0202] 3. After winding 4 times, unwind in the opposite direction to the initial state, and check the sample cable surface for indentations and armor layer deformation.

[0203] 4. Apply a power frequency voltage of 3.5U0 (U0 is the phase voltage) for 30 minutes and check whether the test cable is broken down.

[0204] d. Tension bending test

[0205] 1. The test uses 100 meters of cable that has undergone short-circuiting treatment (this cable section must contain at least 2 short-circuit points). Both ends of the sample are fixed to the test wheel, and the sample is wound around the wheel 3 times (1.5 times in each direction), maintaining a unidirectional bending direction;

[0206] 2. Calculate the initial tension according to the formula, apply it to the target value at a rate of 3kN / min, continuously apply dynamic tension (simulating the movement of the laying pulley), and monitor the strain fluctuation simultaneously.

[0207] 3. Complete 3 full winding cycles (forward + reverse), and check the surface condition of the cable after each cycle.

[0208] 4. Check that the surface of the test cable is free of indentations and deformation of the armor layer. Apply a power frequency voltage of 3.5U0 (U0 is the phase voltage) for 30 minutes and check whether the test cable is broken down.

[0209] The following simulation calculation example illustrates the solution proposed in this application:

[0210] A 220kV offshore wind farm transmission system transmits electricity to an onshore station via a long-distance submarine cable. The transmission cable frequency is 50Hz, the cable length is 73km, the transmission capacity is 280MW, and the power factor is 0.9. The threshold values ​​obtained from dry and wet tests of the submarine cable are 675V and 1200V, respectively. The geometric and electrical parameters of the submarine cable are shown in Tables 1 and 2. The induced voltage in the metal sheath of the submarine cable was calculated using ATP electromagnetic transient simulation software. The simulation model is shown below. Figure 13 As shown.

[0211] Table 1 Geometric Parameters of Submarine Cables

[0212]

[0213] Table 2 Electrical Parameters of Submarine Cables

[0214]

[0215] Table 3 Simulation Results

[0216]

[0217] In the simulation model, the voltage was set to 220kV, and the compensated load current was 817A. Table 3 shows the simulation results: when the cable length is 8km, the induced voltage in the metallic sheath of the submarine cable is 672.8V, close to the threshold of 675V obtained from the dry-state test. Therefore, a length of 13km can be used as the segment length for the onshore section of the submarine cable. When the cable length is 28km, the induced voltage in the metallic sheath of the submarine cable is 1193V, close to the threshold of 1200V obtained from the wet-state test. Therefore, a length of 28km can be used as the segment length for the offshore section of the submarine cable.

[0218] Those skilled in the art will readily understand that the above description is merely an 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 scope of protection of the present invention.

Claims

1. A method for eliminating induced voltage in the metal sheath of a three-core submarine cable, characterized in that, Includes the following steps: S1: Obtain the first induced voltage threshold of the submarine cable's marine section and the second induced voltage threshold of the submarine cable's terrestrial section; The steps for obtaining the second induced voltage threshold of the onshore section of the submarine cable include: S11: Cut a sample of a three-core submarine cable to a preset length, remove the cable armor and filler layer, and use a fastening device to press the three-phase cable into a triangular shape; S12: Set one or more temperature measurement points on the surface of the three-core submarine cable sample, and place temperature sensors on the temperature measurement points; S13: Apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1 at this time and the voltage value U2 of the previous voltage application, and stop applying the voltage. S14: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2, and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... dry This is the second induced voltage threshold; S2: Based on the distributed induced voltage calculation model of the submarine cable metal sheath, calculate the length of the first segment of the submarine cable that generates the first induced voltage threshold under wet conditions. And based on the distributed induced voltage calculation model of the submarine cable metal sheath, the second segment length of the submarine cable that generates the second induced voltage threshold under dry conditions is calculated. S3: For submarine cables in the sea area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the first segment length as the maximum length value; for submarine cables in the land area, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value; for submarine cables that have both sea area and land areas, the three-phase metal sheaths of the submarine cable are short-circuited to each other with the second segment length as the maximum length value.

2. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 1, characterized in that, The steps for obtaining the first induced voltage threshold of the submarine cable segment include: S21: Establish an environment box to simulate the actual operation of a submarine cable, wherein the environment box is equipped with simulated seawater; S22: Execute steps S11 and S12, apply three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is 0V, and the voltage increment interval of the three-phase AC voltage is the first voltage interval. When the temperature exceeds the melting point of the PE semiconductive layer of the submarine cable, record the voltage value U1' at this time and the voltage value U2' of the previous voltage application, and stop applying the voltage. S23: Repeat steps S11 and S12, applying a three-phase AC voltage to the three metal sheaths of the three-core submarine cable. The initial voltage of the three-phase AC voltage is U2', and the voltage increment interval is the second voltage interval. When the temperature exceeds the melting point of the PE semiconducting layer, the voltage value U... wet This is the first induced voltage threshold.

3. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 2, characterized in that, The calculation model for the distributed induced voltage of the metallic sheath of submarine cables is established in the following way: An electromagnetic theoretical model of the three metal conductors of a metal-sheathed, three-core submarine cable is established. Calculate the total magnetic flux of the metal sleeve and the three metal conductors based on the electromagnetic theoretical model; The induced electromotive force of the metal sleeve is calculated based on the total magnetic flux of the metal sleeve and the three metal conductors. The formula for calculating induced electromotive force is: ； ； ； Among them, E A E is the induced electromotive force of the metal sheath surrounding the first-phase metallic conductor, expressed in V / km. B E is the induced electromotive force of the metal sheath surrounding the second-phase metal conductor, expressed in V / km. C ω is the induced electromotive force of the outer metal sheath of the third-phase metal conductor, in V / km; ω is the angular frequency of the load current of the metal conductor; S is the first distance, which is the distance between the first-phase metal conductor and the second-phase metal conductor; n is the ratio of the third distance to the first distance, which is the distance between the first-phase metal conductor and the third-phase metal conductor; m is the ratio of the second distance to the first distance, where the second distance is the distance between the second-phase metal conductor and the third-phase metal conductor; GMR S φ is the average geometric radius of the metal sleeve. pa φ is the total magnetic flux of the metal sleeve surrounding the first-phase metal conductor. pb φ is the total magnetic flux of the metal sleeve surrounding the second-phase metal conductor. pc I represents the total magnetic flux of the metal sleeve surrounding the third-phase metal conductor; I represents the effective value of the three-phase current.

4. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 3, characterized in that, Both ends of the submarine cable are equipped with reactive power compensation devices to compensate for the capacitive reactive power generated by the submarine cable. In this case, the effective value of the three-phase current is calculated as follows: ； Among them, U cl I represents the voltage at a distance l from the end of the submarine cable. cl P is the current at a distance l from the end of the submarine cable; P is the active power at the end of the submarine cable; Q cl Q represents the reactive power at a distance l from the end of the submarine cable. 1C Q1 is the reactive power absorbed by the equivalent inductance of the submarine cable; Q2 is the reactive power output at the end of the submarine cable; U2 is the voltage output at the end of the submarine cable; f is the system frequency of the current; L2 is the inductance connected in parallel at the end of the submarine cable; C c is the equivalent capacitance per unit distance of the submarine cable; l is the distance l from the end of the submarine cable.

5. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 4, characterized in that, It also includes the following steps: The calculated load current value I cl Substituting into the formula for calculating the induced voltage, we obtain the induced electromotive force as a function of the submarine cable length: ； The induced voltage value is obtained as a function of the induced voltage and the length of the submarine cable: ； Substitute the first induced voltage threshold and the second induced voltage threshold into the induced voltage function to obtain the first segment length and the second segment length, respectively.

6. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 5, characterized in that, It also includes steps to verify the accuracy of the distributed induced voltage calculation model: Apply a three-phase load current to the submarine cable sample; After the temperature of the submarine cable stabilizes, measure the magnitude of the induced voltage in the submarine cable. The measured induced voltage of the submarine cable is compared with the calculated induced voltage value to determine the accuracy of the distributed induced voltage calculation model.

7. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 1, characterized in that, The steps for short-circuiting the three-phase metal sheaths of submarine cables include: S31: Divide the submarine cable into segments according to the set segment lengths, and strip the PE semi-conductive layer of three single-core cables of a preset length between two segment lengths, and waterproof the cut of the segments. S32: Short-circuit the three-phase metal sheaths so that the metal sheaths of the three single-core cables in the stripped cable area are connected to each other. S33: Use semi-conductive tape to wrap around the stripped portion of the three single-core cables, ensuring that the thickness at that location after wrapping is the same as the thickness of the untreated cable.

8. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 7, characterized in that, The three-phase metal bushings are short-circuited by lead-lined connection.

9. The method for eliminating induced voltage in the metal sheath of a three-core submarine cable according to claim 7, characterized in that, It also includes steps to verify the reliability of submarine cables after the three-phase metal sheaths are short-circuited together: S41: Use a resistance measuring device to measure the phase-to-phase resistance of the metal sheaths of the submarine cable after the metal sheaths are short-circuited, and determine whether the phase-to-phase resistance is close to 0. S42: Immerse the submarine cable with the metal sleeves short-connected in simulated seawater for a preset time to check whether the short-connection point of the submarine cable is continuous and reliable without any breaks, and whether there are any signs of water intrusion inside the metal sleeve, thereby judging the reliability of the short-connection point. S43: After shorting the metal sheaths together, the submarine cable is wound up and unwound to its initial state. Check the surface of the submarine cable for indentations and the armor layer for deformation. Then apply a phase voltage several times to the submarine cable and check whether the test cable is broken down. S44: After shorting the metal sheaths together, the submarine cable is wound and then wound in the reverse direction. At the same time, dynamic tension is applied to check whether there are indentations on the surface of the submarine cable or whether the armor layer is deformed. Then, several times the phase voltage is applied to the submarine cable to check whether the test cable is broken down.

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