Opto-electric composite lightning protection cable and preparation method thereof

Abstract

The application relates to a photoelectric composite lightning protection cable and a preparation method thereof, which comprises a fiber unit and a force bearing layer, a conductive layer and a protection layer which are sequentially wrapped outside the fiber unit from inside to outside; the fiber unit comprises at least one optical fiber and a loose sleeve, an aluminum-plastic composite belt, a corrugated steel belt and a first sheath layer which are sequentially wrapped outside the optical fiber from inside to outside, and the optical fiber is filled with optical fiber ointment between the optical fiber and the loose sleeve; the force bearing layer comprises a fiber woven layer woven by high molecular fiber and a second sheath layer wrapped outside the fiber woven layer; the conductive layer comprises an inner conductive layer and an outer conductive layer wrapped outside the inner conductive layer, the inner conductive layer and the outer conductive layer are both twisted by multiple copper conductors, the twisting directions of the inner conductive layer and the outer conductive layer are opposite, and the ratio of the torsional stiffness of the inner conductive layer to the torsional stiffness of the outer conductive layer is 0.9 to 1.1; and the protection layer comprises a third sheath layer.

Description

Technical Field

[0001] This invention relates to a photoelectric composite lightning protection cable and its preparation method, belonging to the field of cable technology. Background Technology

[0002] As a core component of wind turbines, wind turbine blades operate in complex outdoor environments for extended periods. They require lightning protection cables to safely conduct lightning current to the ground during lightning strikes, ensuring the safety of the blades and the entire turbine. During operation, the blades experience periodic vibrations and torsion under wind loads, causing repeated deformation of the embedded lightning protection cables. This can lead to fatigue failures such as sheath cracking and conductor breakage. Traditional lightning protection cables often use low-hardness materials to increase overall flexibility and adapt to the repeated torsional movements of the blades, thereby extending their service life. However, this approach, focusing solely on material flexibility, cannot fundamentally solve the problem of fatigue failure under dynamic loads. Furthermore, traditional lightning protection cables have a limited function, only capable of conducting lightning. Because they are embedded inside the blades, their operational status cannot be monitored in real time, resulting in a lack of effective early warning before cable failure and posing a significant lightning strike hazard. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a photoelectric composite lightning protection cable and its preparation method. From the perspective of conductor stress, through the synergistic effect of the torsional stiffness matching of the conductive layer and the buffering of the load-bearing layer, the fatigue accumulation damage of the conductor is effectively eliminated, significantly improving the service life and reliability of the cable.

[0004] The present invention also provides a method for monitoring the condition and providing early warning of faults of wind turbine blades based on photoelectric composite lightning protection cables. This method monitors the attenuation changes of optical fibers in real time and combines them with a pre-built optical fiber attenuation-blade stress spectrum library based on blade fatigue tests to achieve early warning of cable failure and blade condition.

[0005] According to a first aspect of the present invention, a photoelectric composite lightning protection cable is provided, the photoelectric composite lightning protection cable comprising: an optical fiber unit and a load-bearing layer, a conductive layer, and a protective layer sequentially wrapped around the outer periphery of the optical fiber unit from the inside out;

[0006] The optical fiber unit includes at least one optical fiber and, from the inside out, a loose tube, an aluminum-plastic composite tape, a corrugated steel tape, and a first sheath layer wrapped around the outer periphery of the optical fiber. Optical fiber grease is filled between the optical fiber and the loose tube.

[0007] The load-bearing layer includes a fiber braided layer made of polymer fibers and a second sheath layer wrapped around the outer periphery of the fiber braided layer;

[0008] The conductive layer includes an inner conductive layer and an outer conductive layer wrapped around the outer periphery of the inner conductive layer. Both the inner conductive layer and the outer conductive layer are made of multiple copper conductors twisted together, and the twisting directions of the inner conductive layer and the outer conductive layer are opposite. The ratio of the torsional stiffness of the inner conductive layer to the torsional stiffness of the outer conductive layer is 0.9 to 1.1.

[0009] The protective layer includes a third sheath layer.

[0010] According to one embodiment, the protective layer further includes a nylon protective layer wrapped around the periphery of the third sheath layer.

[0011] According to one embodiment, the optical fiber has an excess length of 2.5‰ to 4‰; the excess length is the ratio of the difference between the length of the optical fiber and the length of the loose tube to the length of the loose tube.

[0012] According to one embodiment, at least one end of the fiber braided layer extends out of the cable end and forms a pull ring structure for connection with an external fastener during installation.

[0013] According to one embodiment, the polymer fiber is at least one of aramid, polyester, or nylon.

[0014] According to one embodiment, the polymer fiber is aramid, the linear density of the aramid is 1220 dtex to 3220 dtex, the weaving angle is 30° to 45°, and the weaving density is not less than 80%.

[0015] According to one embodiment, the torsional stiffness is determined based on the cross-sectional area of ​​the copper conductor in the conductive layer, the elastic modulus, the square of the mean diameter, and the product of the square of the sine and the cosine of the twist angle.

[0016] According to one embodiment, the first sheath layer, the second sheath layer, and the third sheath layer are all formed of polyurethane.

[0017] According to a second aspect of the present invention, a method for preparing the photoelectric composite lightning protection cable described in the first aspect of the present invention is provided, the method comprising the following steps:

[0018] S1: Fabrication of an optical fiber unit, including: selecting at least one optical fiber, extruding a loose tube around the outer periphery of the optical fiber using an extrusion process and filling it with optical fiber grease, controlling the excess fiber length to be 2.5‰ to 4‰; longitudinally wrapping an aluminum-plastic composite tape around the outer periphery of the loose tube, controlling the longitudinal wrapping overlap rate to be 10% to 15%, and applying adhesive to seal the overlap edge; coating the outer periphery of the aluminum-plastic composite tape with an adhesive material of 0.3mm to 0.5mm thickness and then rolling it together with a steel strip to form a corrugated steel strip with a corrugation height of 2mm to 3mm and a corrugation spacing of 2mm to 3mm; and extruding a first sheath layer.

[0019] S2: Preparation of the load-bearing layer includes: using a braiding machine to braid aramid fibers with a linear density of 1220 dtex to 3220 dtex at a braiding angle of 30° to 45° and a braiding density of not less than 80% onto the outer periphery of the optical fiber unit to form a fiber braided layer; and then using a semi-extrusion tube process to extrude a second sheath layer on the outside of the fiber braided layer.

[0020] S3: Fabrication of a conductive layer, comprising: using a double-layer reverse stranding method, stranding multiple tin-plated copper conductors to form an inner conductive layer and an outer conductive layer wrapped around the outer periphery of the inner conductive layer, wherein the stranding directions of the inner conductive layer and the outer conductive layer are opposite; and wrapping high-temperature resistant tape around each strand after stranding; wherein the diameter, number of conductors, stranding pitch, and stranding angle of the copper conductors of the inner conductive layer and the outer conductive layer are configured such that the ratio of their torsional stiffness is 0.9 to 1.1;

[0021] S4: Prepare a protective layer, including: extruding a third sheath layer and a nylon protective layer wrapped around the outer periphery of the conductive layer, controlling the thickness of the third sheath layer to be 1.8 mm to 2.5 mm, and controlling the thickness of the nylon protective layer to be 0.15 mm to 0.5 mm.

[0022] According to a third aspect of the present invention, a method for monitoring the condition and providing early warning of faults in wind turbine blades based on photoelectric composite lightning protection cables is provided, the method comprising the following steps:

[0023] By conducting fatigue tests on test blades pre-embedded with the photoelectric composite lightning protection cable described in the first aspect of this invention, fiber optic attenuation data and blade stress parameters were collected simultaneously to establish a fiber optic attenuation-blade stress spectrum library.

[0024] The photoelectric composite lightning protection cable described in the first aspect of the present invention is pre-embedded in the actual wind turbine blade. The attenuation change along the optical fiber is collected in real time by an optical signal acquisition device. When the detected attenuation change exceeds a preset threshold, the attenuation change is compared with the spectrum library to deduce the load parameters currently experienced by the blade. The optical signal acquisition device includes at least one of a distributed acoustic sensing device, an optical time domain reflectometer, and an optical fiber strain dispersive spectrometer.

[0025] Based on the magnitude and duration of the attenuation change, a warning message of the corresponding level is output.

[0026] This invention addresses the engineering problem of cable fatigue failure caused by the long-term reciprocating torsion of wind turbine blades, starting from the fundamental stress on conductors. On one hand, by controlling the ratio of torsional stiffness of the inner to outer conductive layer within the range of 0.9 to 1.1, an inherent torque balance independent of load is achieved, preventing cable rotation and twisting during use and eliminating fatigue fracture caused by torque accumulation at its source. On the other hand, a fiber braided layer is introduced as a stress-bearing buffer structure, so that the conductive layer no longer directly bears alternating mechanical stresses such as tension and bending, significantly reducing fatigue-accumulated damage to the cable under cyclic bending conditions. Through these dual designs, the overall stress structure of the cable is substantially improved, significantly enhancing the product's service life and operational reliability.

[0027] This invention utilizes the attenuation changes in optical fibers caused by external forces. Optical signal acquisition equipment monitors and records fiber attenuation data in real time. The monitored data is compared with a pre-established spectral library, constructed through blade fatigue tests, which includes the mapping relationship between fiber attenuation and blade stress. Based on this, early warnings can be provided for cable failures and abnormal blade conditions. This method is beneficial for guiding wind turbine maintenance and provides data support for OEMs to optimize blade structure design.

[0028] This invention controls the excess fiber length in the fiber unit to be between 2.5‰ and 4‰. Combined with the torque balance design of the inner and outer conductive layers and the design of the fiber braided layer, this ensures that the fiber remains in a low-stress state during the long-term reciprocating bending of the cable with the wind turbine blades, significantly reducing additional attenuation and guaranteeing the continuity and reliability of blade condition monitoring data. The fiber braided layer achieves cable lightweighting while ensuring high strength and reducing the additional load on the blades. The inner and outer double-layer copper conductors form a low-resistance lightning current discharge path, effectively coping with the transient high-current impact when the blades are struck by lightning. The synergistic effect of these designs enables the lightning protection cable to possess excellent fatigue resistance, efficient lightning discharge capability, and stable signal transmission performance under long-term operation within the wind turbine blades. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the radial cross-sectional structure of a photoelectric composite lightning protection cable provided in an embodiment of the present invention.

[0030] Figure 2 This is a schematic diagram of the layered structure of a photoelectric composite lightning protection cable provided in an embodiment of the present invention.

[0031] Explanation of reference numerals in the attached drawings: 1-Fiber optic unit; 11-Fiber optic cable; 12-Fiber optic grease; 13-Loose tube; 14-Aluminum-plastic composite tape; 15-Corrugated steel tape; 16-First sheath layer; 2-Bearing layer; 21-Fiber braided layer; 22-Second sheath layer; 3-Conductive layer; 31-Inner conductive layer; 32-Outer conductive layer; 4-Protective layer; 41-Third sheath layer; 42-Nylon protective layer. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to specific embodiments.

[0033] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0034] This invention provides a photoelectric composite lightning protection cable, such as... Figure 1 , Figure 2 As shown, the structure includes an optical fiber unit 1 and a load-bearing layer 2, a conductive layer 3, and a protective layer 4 sequentially wrapped around the outer periphery of the optical fiber unit 1. The optical fiber unit 1 includes at least one optical fiber 11 and, from the inside out, a loose tube 13, an aluminum-plastic composite tape 14, a corrugated steel tape 15, and a first sheath layer 16 wrapped around the outer periphery of the optical fiber 11. Optical fiber grease 12 is filled between the optical fiber 11 and the loose tube 13. The load-bearing layer 2 includes a fiber braided layer 21 made of polymer fibers and a layer wrapped around the fiber braided layer 21. 1. A second sheath layer 22 on the outer periphery; a conductive layer 3 includes an inner conductive layer 31 and an outer conductive layer 32 wrapped around the outer periphery of the inner conductive layer 31. Both the inner conductive layer 31 and the outer conductive layer 32 are made of multiple copper conductors twisted together, and the twisting directions of the inner conductive layer 31 and the outer conductive layer 32 are opposite; the ratio of the torsional stiffness of the inner conductive layer 31 to the torsional stiffness of the outer conductive layer 32 is 0.9 to 1.1; a protective layer 4 includes a third sheath layer 41 and a nylon protective layer 42 wrapped around the outer periphery of the third sheath layer.

[0035] In some specific embodiments of the present invention, the optical fiber unit 1 includes two G.657.A2 bend-resistant optical fibers 11. The stress screening range of the optical fibers 11 is 100kpsi to 200kpsi. G.657.A2 is a bend-insensitive single-mode optical fiber model standardized by the International Telecommunication Union (ITU-T) and has excellent bend resistance. The optoelectronic composite lightning protection cable of the present invention is adapted for use in wind turbine blade installation and can work stably under the complex conditions of periodic vibration and torsion of the blade. The aforementioned optical fiber 11, as the core component of blade condition monitoring, is not only bend-resistant but also seamlessly compatible with the conventional G.652.D single-mode optical fiber in the communication field, enabling interoperability and adaptation of splicing, detection, and transmission equipment. In particular, the two optical fibers can simultaneously and independently collect monitoring data. Through data comparison and cross-verification, "double verification" of monitoring results is achieved, effectively reducing the risk of misjudgment and significantly improving the accuracy and reliability of blade condition monitoring, which can fully meet the monitoring application needs of wind turbine blades in complex operating environments.

[0036] The loose tube 13 provides a protective space for the optical fiber 11. In some examples, the loose tube 13 is made of polybutylene terephthalate (PBT) extruded onto the outside of the optical fiber 11. The gap between the loose tube 13 and the optical fiber 11 is filled with optical fiber grease 12. The excess fiber length is controlled between 2.5‰ and 4‰. The excess fiber length refers to the ratio of the difference between the length of the optical fiber 11 and the length of the loose tube 13 to the length of the loose tube, which is characterized as a relative percentage in this application. PBT material has good aging resistance and bending performance, and its surface is smooth, making it less likely to damage the optical fiber 11 inside the tube. At the same time, PBT material has good melt flow properties, is easy to manufacture and process, and has a small shrinkage after cooling. The excess fiber length can be stably controlled by the difference in shrinkage rate between PBT material and optical fiber 11. The optical fiber grease 12 filled inside the loose tube 13 can effectively prevent moisture from entering and contacting the optical fiber 11, avoiding moisture absorption and performance degradation of the optical fiber 11. At the same time, the optical fiber grease 12 can also provide mechanical buffer for the optical fiber 11, further reducing the impact of external stress and protecting the performance of the optical fiber 11. It should be noted that the fill rate of the fiber optic grease 12 needs to be controlled at ≥70%. If the fill rate is too low, the grease cannot form a continuous and complete water-blocking and buffering layer; while if the fill rate is too high, it will easily cause abnormal extrusion of the PBT loose tube 13, and will also interfere with the precise control of the fiber optic excess length and cause grease waste. The determination of the fiber optic excess length is closely related to the cable's design tensile force and the material and structure of the fiber braid layer 21. Specifically, when the cable is subjected to tensile force, it will produce axial tensile deformation (lengthening) and radial contraction deformation (thinning), and its mechanical relationship can be expressed as: F= × ×η, where F is the tension force. The elastic modulus of the fiber braided layer material. Let η be the stress-bearing area of ​​the fiber braided layer 21, and η be the strain generated by the fiber braided layer 21 under the tension. Based on the above mechanical principles, the set value of the fiber excess length must satisfy ≥η to ensure that when the fiber braided layer 21 experiences maximum working strain, the fiber 11 remains in a relaxed state and does not directly bear the tension. If the fiber excess length is insufficient, the fiber 11 will be directly stressed during cable tension, thereby reducing the fiber fatigue life; if the fiber excess length is too large, the fiber 11 will experience local compression, resulting in increased optical signal attenuation and interfering with the accuracy of blade condition monitoring data. Compared with the 1‰ to 1.5‰ excess length range of conventional terrestrial optical cables, this application controls the fiber excess length within a reasonable range of 2.5‰ to 4‰ for the periodic stress change conditions of wind turbine blades. This ensures sufficient stress buffer margin while avoiding local compression problems caused by excessive excess length, thereby ensuring that the fiber 11 is always in a relaxed state with no or low stress, and guaranteeing the stability of fiber optic sensing transmission performance.

[0037] The aluminum-plastic composite tape 14 is used to prevent moisture from penetrating into the cable and thus preventing the optical fiber 11 from getting damp. The tape is made of aluminum foil and polyester film, combining the water-blocking properties of aluminum with the mechanical strength of polyester film, effectively preventing moisture from penetrating longitudinally along the cable. In some embodiments, the aluminum-plastic composite tape 14 is wrapped longitudinally, which offers higher process efficiency and faster processing speed compared to wrapping. The edges of the longitudinally wrapped aluminum-plastic composite tape are treated with adhesive, and the overlap rate is controlled within the range of 10% to 15% to ensure the reliable and stable structure of the wrapped tape and avoid defects such as missing wrapping. The thickness of the aluminum-plastic composite tape is preferably between 0.025 mm and 0.040 mm, for example, 0.025 mm, 0.036 mm, or 0.040 mm.

[0038] The corrugated steel strip 15 is used to provide mechanical protection for the optical fiber unit 1. In some embodiments, the corrugation height of the corrugated steel strip 15 is controlled to be 2 mm to 3 mm, and the corrugation spacing is set to be 2 mm to 3 mm, thereby providing mechanical protection while maintaining good bending performance.

[0039] The first sheath layer 16, the second sheath layer 22, and the third sheath layer 41 provide mechanical protection for their respective enclosed internal structures. The sheath layer materials are selected from highly elastic, low-temperature resistant materials, such as polyurethane, modified elastomers, ethylene propylene rubber, and neoprene rubber. In some embodiments, all of the above sheath layers are formed of polyurethane. Polyurethane can be processed using conventional extruders at temperatures ranging from 170°C to 210°C, with a simple process and low material loss. Polyurethane also possesses excellent tear resistance and good temperature adaptability, allowing it to be used in a temperature range of -40°C to 100°C. Compared to traditional rubber materials, polyurethane has higher mechanical strength and is less prone to mold growth.

[0040] The fiber braided layer 21 is the most important load-bearing structure of the cable, used to bear the main external loads, optimize the stress distribution of the cable, and prevent the conductive layer 3 from being directly stressed and thus causing fatigue fracture. In some embodiments, the fiber braided layer 21 is woven from polymer fiber materials, and the polymer fibers can be selected from one or more of polyester, aramid, or nylon. The braided structure gives the cable excellent flexibility, facilitating bending and laying. Simultaneously, the high Young's modulus of the polymer fibers results in low strain under stress, helping to maintain the overall structural stability of the cable and extending its service life. The tensile strength of the polymer fibers can reach 1800 MPa to 2060 MPa, while the tensile strength of conventional galvanized steel wire is only 350 MPa to 450 MPa. To meet the same load-bearing requirements, the polymer fiber braided layer requires a smaller cross-sectional area and is lighter, which is beneficial for achieving lightweight and small-diameter cable designs. Furthermore, this type of polymer fiber material has a wide operating temperature range, stable physical and chemical properties, and as a non-metallic material, it has no risk of salt spray corrosion. It can adapt to complex environments such as high temperature and humidity, extreme cold, and large temperature differences, fully meeting the installation and usage requirements inside wind turbine blades.

[0041] In a preferred embodiment of this application, the fiber braided layer 21 is made of aramid material, braided using a 24-spindle or 36-spindle braiding machine, and aramid yarns of 1220 dtex to 3220 dtex are selected according to the load-bearing requirements. While meeting the load-bearing requirements, appropriately increasing the number of braiding spindles helps reduce the fiber count per spindle and increases the continuous length of fibers per spindle, thereby facilitating large-length production and reducing the number of joints. The braiding angle is preferably controlled between 30° and 45° to ensure that the transverse shear force on the fibers under stress is within a reasonable range, thereby extending fatigue life. The braiding density is preferably 80% or higher. The braiding density directly affects the load-bearing effect; the higher the density, the more fibers per unit area, and the stronger the load-bearing capacity. However, excessively high braiding density will reduce production speed and cause material waste.

[0042] In a preferred embodiment of this application, the fiber braided layer 21 extends beyond both ends of the cable and forms pull ring structures. These pull ring structures are used to connect with external fasteners during installation, allowing the periodic stress on the cable to be primarily transferred to the fasteners through the fiber braided layer 21, thereby reducing the stress directly acting on the conductive layer 3. Compared to the conventional method of connecting conductors by crimping with hardware in cables, this structure effectively avoids failure problems such as conductor tripping and insulation bulging caused by periodic stress changes in the wind turbine blades.

[0043] Currently, the industry design requirements for wind turbine blade cables stipulate a tensile strength of not less than 10kN. Furthermore, according to IEC 61400-24 "Wind Power Generation Systems - Part 24: Lightning Protection," the conductor cross-sectional area of ​​copper conductor lightning protection cables for wind turbines must be not less than 50mm². In engineering design, the upper limit of the elastic strain for long-term stable operation of copper conductors is 0.6‰. Once this limit is exceeded, the long-term reliability of the conductor will be significantly reduced. Based on the test data in Table 1, it can be seen that this application uses a fiber braided layer as the main load-bearing structure, which can effectively reduce the overall tensile strain of the cable to below the upper limit of the elastic strain of copper conductors (0.6‰), ensuring that the copper conductor always operates within the safe strain range. In contrast, traditional copper core lightning protection cables lack an independent load-bearing structure, and their tensile strain significantly exceeds the safe limit of the elastic strain of copper conductors. Under the cyclic reciprocating load of wind turbine blades, they are prone to fatigue damage, resulting in a short overall service life and poor operational stability.

[0044] Table 1 provides a comparison example between traditional lightning protection cables and those of this application.

[0045]

[0046] The conductive layer 3 consists of two layers of copper conductors: an inner conductive layer 31 and an outer conductive layer 32. It forms a low-resistance path to guide lightning current to the grounding system. Both the inner conductive layer 31 and the outer conductive layer 32 are composed of multiple strands of copper conductors twisted together, with conductors in the same layer having the same diameter. The twisting directions of the copper conductors in the inner conductive layer 31 and the outer conductive layer 32 are opposite, and the ratio of the torsional stiffness of the inner conductive layer 31 to the torsional stiffness of the outer conductive layer 32 is 0.9 to 1.1. The torsional stiffness of the inner conductive layer 31 and the outer conductive layer 32 can be calculated using the following formula:

[0047]

[0048] In the formula, For the first The torsional stiffness of the layer For the first The cross-sectional area of ​​the copper conductor layer, For the first The elastic modulus of the copper conductor. For the first The median diameter of the copper conductor. For the first The twist angle of the first layer of copper conductors refers to the first layer of the stranded conductor. The angle between the copper conductor and the cable axis;

[0049] Among them, the twist angle of the copper conductor It can be calculated using the following formula:

[0050]

[0051] In the formula, For the first The stranding pitch of the copper conductor;

[0052] Among them, the cross-sectional area of ​​the copper conductor It can be calculated using the following formula:

[0053]

[0054] In the formula, For the first Number of copper conductors in the layer For the first The diameter of the copper conductor;

[0055] Among them, the middle diameter of the copper conductor It can be calculated using the following formula:

[0056]

[0057] In the formula, The diameter of the cable core refers to the diameter of the first... The outer diameter of the inner side of the base cable core to be stranded before the copper conductor is stranded and covered.

[0058] Among them, the number of copper conductors It can be calculated using the following formula:

[0059]

[0060] The sum of the cross-sectional areas of the copper conductors in the inner and outer conductive layers constitutes the total cross-sectional area of ​​the copper conductors. This total cross-sectional area is a core performance benchmark parameter of the lightning protection cable, directly determining the cable's lightning discharge capacity, thermal stability, and mechanical fatigue resistance. Using a preset minimum total cross-sectional area of ​​the copper conductors as a design constraint, the diameter, number of conductors, pitch, and stranding angle of the inner and outer copper conductors are matched and calculated according to the aforementioned formula. This ensures that the ratio of the torsional stiffness of the inner and outer conductive layers approaches 1, thereby achieving inherent torque balance characteristics independent of load in the structure. Through this design, when the cable is subjected to the periodic torsion of the wind turbine blades, the torsional torque generated by the inner and outer conductive layers cancels each other out, preventing overall cable distortion or internal structural damage, and significantly extending the cable's service life under dynamic operating conditions.

[0061] In a preferred embodiment of this application, before the inner conductive layer 31 is stranded and covered, the diameter of the inner base cable core to be stranded is 10mm. Specifically, the copper conductor diameter of the inner conductive layer 31 is 1.0mm, the number of conductors is 32, the stranding pitch is 96mm, and the stranding angle is 19.80°; the copper conductor diameter of the outer conductive layer 32 is 0.8mm, the number of conductors is 47, the stranding pitch is 120mm, and the stranding angle is 18.54°. At this time, the total cross-sectional area of ​​the copper conductors is 51.6mm², and the torsional stiffness ratio of the inner and outer conductive layers reaches 0.975, which achieves a good torque balance effect while ensuring the conductivity of the cable for lightning conduction.

[0062] In an optional embodiment of this application, before the inner conductive layer 31 is stranded and covered, the diameter of the inner base cable core to be stranded is 10mm. Specifically, the copper conductor diameter of the inner conductive layer 31 is 1.0mm, the number of conductors is 34, the stranding pitch is 180mm, and the stranding angle is 10.87°; the copper conductor diameter of the outer conductive layer 32 is 0.8mm, the number of conductors is 49, the stranding pitch is 220mm, and the stranding angle is 10.36°. At this time, the total cross-sectional area of ​​the copper conductors is 51.4mm², and the torsional stiffness ratio of the inner and outer conductive layers is 0.948, which also achieves a good torque balance effect and fully ensures the conductivity of the cable for lightning conduction.

[0063] The protective layer 4 provides mechanical protection for the cable and includes a third sheath layer 41 and a nylon protective layer 42 surrounding the third sheath layer 41. In some examples, the third sheath layer 41 is formed of polyurethane material with a thickness of 1.8 mm to 2.5 mm. The polyurethane sheath has good elasticity and mechanical strength, effectively buffering external impacts and compression. The nylon protective layer 42 has a thickness of 0.15 mm to 0.5 mm. Nylon material is chemically stable, has a smooth surface, and is wear-resistant, facilitating the pre-embedding of the cable inside the wind turbine blades. It also provides waterproof and moisture-proof protection, assisting the third sheath layer 41 in forming double protection.

[0064] This invention also provides a method for preparing the above-mentioned optoelectronic composite lightning protection cable, comprising the following steps:

[0065] Step S1: Fabrication of the optical fiber unit, including: selecting at least one optical fiber, extruding a polybutylene terephthalate (PBT) loose tube around the optical fiber using an extrusion process and filling it with optical fiber grease, with the grease filling rate controlled at ≥70%, and utilizing the difference in shrinkage rates between the loose tube and the optical fiber to control the excess length of the optical fiber between 2.5‰ and 4‰; subsequently, longitudinally wrapping a 0.036mm thick aluminum-plastic composite tape around the loose tube, with the longitudinal overlap rate controlled at 10% to 15%, and applying adhesive to seal the overlap edges; after coating the aluminum-plastic composite tape with an adhesive layer of 0.3mm to 0.5mm thick, feeding it together with a steel strip into a rolling mill to form a corrugated steel strip with a corrugation height of 2mm to 3mm and a corrugation spacing of 2mm to 3mm; finally, extruding a polyurethane first sheath layer to complete the fabrication of the optical fiber unit.

[0066] Step S2: Prepare the load-bearing layer, including: using a 24-spindle or 36-spindle braiding machine, braid aramid fibers with a linear density of 1220 dtex to 3220 dtex at a braiding angle of 30° to 45° and a braiding density of not less than 80% on the outer periphery of the optical fiber unit to form a fiber braided layer; then using a semi-extrusion process to extrude a second polyurethane sheath layer on the outside of the fiber braided layer. The semi-extrusion process can avoid the difficulty of stripping the wire caused by the sheath material penetrating into the fiber braided layer in the extrusion process, and also avoid the problems of sheath loosening and easy delamination in the extrusion process.

[0067] Step S3: Preparation of the conductive layer, including: smelting, slag removal, continuous casting and rolling of copper ingots or plates to produce copper rods with a diameter of 8mm; drawing the copper rods to a diameter of 1.2mm to 1.8mm using a large wire drawing machine; further drawing them into copper conductors of the required specifications using a small wire drawing machine; and finally annealing and tin plating. A double-layer reverse stranding method is used to strand multiple tin-plated copper conductors to form an inner conductive layer and an outer conductive layer wrapped around the outer periphery of the inner conductive layer. The stranding directions of the inner and outer conductive layers are opposite. The diameter, number of conductors, stranding pitch, and stranding angle of the inner and outer conductive layers are configured such that the ratio of their torsional stiffness is 0.9 to 1.1. After stranding, the inner and outer conductive layers are wrapped with high-temperature resistant tape for fixation. Considering the thermal effect of lightning impulse current (instantaneous thousands to tens of thousands of amperes), high-temperature resistant tapes with good thermal stability, non-flammability, and no impact on the conductivity of the conductor are selected, such as aramid tapes and tertrol tapes (polyester tapes).

[0068] Step S4: Prepare the protective layer, including: extruding a polyurethane third sheath layer and a nylon protective layer on the outer periphery of the conductive layer using a tandem extrusion method, controlling the thickness of the third sheath layer to be 1.8 mm to 2.5 mm and the thickness of the nylon protective layer to be 0.15 mm to 0.5 mm. Tandem extrusion allows for one-time molding, and because the two materials are processed separately, different process parameters and tooling can be selected for different materials, thereby achieving precise control over the material thickness. In optional embodiments, the third sheath layer and the nylon protective layer can also be extruded using a double-layer co-extrusion or a secondary extrusion method.

[0069] This invention also provides a method for wind turbine blade condition monitoring and fault early warning based on the above-mentioned photoelectric composite lightning protection cable, comprising the following steps:

[0070] Steps for establishing the spectrum library: By conducting fatigue tests on test blades pre-embedded with the above-mentioned photoelectric composite lightning protection cables, fiber optic attenuation data and blade stress parameters are collected simultaneously to establish a fiber optic attenuation-blade stress spectrum library.

[0071] Online monitoring steps: The aforementioned photoelectric composite lightning protection cable is pre-embedded into the actual wind turbine blade. Attenuation data along the optical fiber is collected in real time using optical signal acquisition equipment. When the detected attenuation change exceeds a preset threshold, the attenuation change is compared with a spectral library to deduce the current load parameters on the blade. The signal acquisition equipment includes at least one of the following: distributed acoustic sensor (DAS), optical time domain reflectometer (OTDR), or fiber strain dispersive spectrometer (PE).

[0072] Warning output steps: Based on the magnitude and duration of the attenuation change, output warning information of the corresponding level.

[0073] The above embodiments mainly describe the differences between the various embodiments. As long as the different optimization features between the various embodiments are not contradictory, they can be combined to form a better embodiment. For the sake of brevity, they will not be elaborated here.

[0074] It should be understood that the above description is only an exemplary embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Modifications, substitutions or adaptive changes that can be easily conceived by those skilled in the art without departing from the principle and essence of the present invention should all be covered within the scope of protection of the present invention.

Claims

1. A photoelectric composite lightning protection cable, characterized in that: It includes an optical fiber unit and a load-bearing layer, a conductive layer, and a protective layer that are sequentially wrapped around the outer periphery of the optical fiber unit from the inside out. The optical fiber unit includes at least one optical fiber and, from the inside out, a loose tube, an aluminum-plastic composite tape, a corrugated steel tape, and a first sheath layer wrapped around the outer periphery of the optical fiber. Optical fiber grease is filled between the optical fiber and the loose tube. The optical fiber has a fiber excess length of 2.5‰ to 4‰; the fiber excess length is the ratio of the difference between the length of the optical fiber and the length of the loose tube to the length of the loose tube. The load-bearing layer includes a fiber braided layer made of polymer fibers and a second sheath layer wrapped around the outer periphery of the fiber braided layer; the conductive layer includes an inner conductive layer and an outer conductive layer wrapped around the outer periphery of the inner conductive layer. Both the inner conductive layer and the outer conductive layer are made of multiple copper conductors twisted together, and the twisting directions of the inner conductive layer and the outer conductive layer are opposite. The ratio of the torsional stiffness of the inner conductive layer to the torsional stiffness of the outer conductive layer is 0.9 to 1.

1. The protective layer includes a third sheath layer.

2. The photoelectric composite lightning protection cable according to claim 1, characterized in that, The protective layer also includes a nylon protective layer wrapped around the outer periphery of the third sheath layer.

3. The photoelectric composite lightning protection cable according to claim 1, characterized in that, At least one end of the fiber braided layer extends out of the cable end and forms a pull ring structure for connection with an external fastener during installation.

4. The photoelectric composite lightning protection cable according to claim 1, characterized in that, The polymer fiber is at least one of aramid, polyester, or nylon.

5. The photoelectric composite lightning protection cable according to claim 1, characterized in that, The polymer fiber is aramid, the linear density of which is 1220 dtex to 3220 dtex, the weaving angle is 30° to 45°, and the weaving density is not less than 80%.

6. The photoelectric composite lightning protection cable according to claim 1, characterized in that, The torsional stiffness is determined based on the cross-sectional area, elastic modulus, square of the mean diameter, and the product of the square of the sine and cosine of the twist angle of the copper conductor in the conductive layer.

7. The photoelectric composite lightning protection cable according to claim 1, characterized in that, The first sheath layer, the second sheath layer, and the third sheath layer are all formed of polyurethane.

8. A method for preparing a photoelectric composite lightning protection cable according to any one of claims 1-7, characterized in that, Includes the following steps: S1: Fabrication of an optical fiber unit, including: selecting at least one optical fiber, extruding a loose tube around the outer periphery of the optical fiber using an extrusion process and filling it with optical fiber grease, controlling the excess fiber length to be 2.5‰ to 4‰; longitudinally wrapping an aluminum-plastic composite tape around the outer periphery of the loose tube, controlling the longitudinal wrapping overlap rate to be 10% to 15%, and applying adhesive to seal the overlap edge; coating the outer periphery of the aluminum-plastic composite tape with an adhesive material of 0.3mm to 0.5mm thickness and then rolling it together with a steel strip to form a corrugated steel strip with a corrugation height of 2mm to 3mm and a corrugation spacing of 2mm to 3mm; and extruding a first sheath layer. S2: Preparation of the load-bearing layer includes: using a braiding machine to braid aramid fibers with a linear density of 1220 dtex to 3220 dtex at a braiding angle of 30° to 45° and a braiding density of not less than 80% onto the outer periphery of the optical fiber unit to form a fiber braided layer; and then using a semi-extrusion tube process to extrude a second sheath layer on the outside of the fiber braided layer. S3: Fabrication of a conductive layer, comprising: using a double-layer reverse stranding method, stranding multiple tin-plated copper conductors to form an inner conductive layer and an outer conductive layer wrapped around the outer periphery of the inner conductive layer, wherein the stranding directions of the inner conductive layer and the outer conductive layer are opposite; and wrapping high-temperature resistant tape around each strand after stranding; wherein the diameter, number of conductors, stranding pitch, and stranding angle of the copper conductors of the inner conductive layer and the outer conductive layer are configured such that the ratio of their torsional stiffness is 0.9 to 1.1; S4: Prepare a protective layer, including: extruding a third sheath layer and a nylon protective layer wrapped around the outer periphery of the conductive layer, controlling the thickness of the third sheath layer to be 1.8 mm to 2.5 mm, and controlling the thickness of the nylon protective layer to be 0.15 mm to 0.5 mm.

9. A method for wind turbine blade condition monitoring and fault early warning based on photoelectric composite lightning protection cable, characterized in that, Includes the following steps: By conducting fatigue tests on test blades pre-embedded with the photoelectric composite lightning protection cable as described in any one of claims 1 to 7, fiber optic attenuation data and blade stress parameters were collected simultaneously, and a fiber optic attenuation-blade stress spectrum library was established. The photoelectric composite lightning protection cable according to any one of claims 1 to 7 is pre-embedded in the actual wind turbine blade. The attenuation change along the optical fiber is collected in real time by an optical signal acquisition device. When the detected attenuation change exceeds a preset threshold, the attenuation change is compared with the spectrum library to deduce the load parameters currently experienced by the blade. The optical signal acquisition device includes at least one of a distributed acoustic sensing device, an optical time domain reflectometer, and an optical fiber strain dispersive spectrometer. Based on the magnitude and duration of the attenuation change, a warning message of the corresponding level is output.

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