A method for detecting the force of external prestressed cables in wind turbine hybrid tower

Abstract

The present invention discloses a method for detecting the force of prestressed cables outside a wind power hybrid tower, which belongs to the field of wind power and includes the following steps: collecting basic data of cables to be detected; designing a local rigid coupling device; installing the local rigid coupling device; deploying acceleration sensors: arranging two mutually perpendicular acceleration sensors on the local rigid coupling device; obtaining the cable frequency; verifying the local rigid coupling condition; and calculating the cable force. The present invention reasonably selects a coupling agent through the calculation result of the local rigid coupling condition, determines the elastic modulus of the coupling agent, the equivalent diameter of the steel strand bundle, and the equivalent gap width between the steel strand bundles, and calculates the pouring height of the coupling agent, thereby satisfying the local rigid coupling condition, eliminating the interference of the local frequency in the test, and obtaining a unified overall cable frequency, which can improve the detection accuracy of the cable force.

Description

Technical Field

[0001] The invention relates to the field of wind power, and in particular to a method for detecting the force of prestressed cables outside the body of a wind power hybrid tower. Background Art

[0002] In the field of wind energy development, technological breakthroughs continue to push wind turbines toward larger capacities, resulting in a continuous increase in blade length, which in turn places higher demands on tower height. Traditional steel structure towers face challenges in terms of load-bearing capacity and cannot meet new height requirements. Therefore, concrete-steel hybrid towers (referred to as wind power hybrid towers) have gradually become an industry solution. Hybrid towers combine the advantages of concrete and steel. They can not only support taller towers, thereby improving the efficiency of wind power generation, but also significantly reduce power generation costs, providing strong support for the sustainable development of the wind power industry. Therefore, the application of wind power hybrid towers has received increasing attention. Figure 1 As shown, the top of the wind power hybrid tower is a pure steel tower tube 2, and the bottom is a post-tensioned external prestressed concrete tower tube 5. Prestress is applied to the concrete section through the external prestressed cable 4 to offset or reduce the tensile stress generated by the external load and give full play to the compressive performance of the concrete.

[0003] In the design of wind power hybrid towers, external prestressed cables are key components for enhancing structural stiffness and stability, and their cable tension state directly affects the overall performance of wind power hybrid towers. Since the cables are subjected to a variety of complex stresses such as wind force, temperature changes, and vibrations for a long time, their cable tension may decay to varying degrees. If they are not discovered and adjusted in time, the structural performance of the wind power hybrid tower may be reduced, and even cause serious accidents such as collapse. Therefore, regular testing of the cable tension of the external prestressed cables of wind power hybrid towers is a necessary measure to ensure the long-term safety and stable operation of wind power hybrid towers.

[0004] At present, the cable force detection technologies for external prestressed cables mainly include hydraulic method, force sensor method, stress release method and frequency analysis method. The hydraulic method uses a pressure gauge to measure the hydraulic pressure of the jack cylinder at the tensioning end to calculate the cable force, but the pressure gauge is removed after the construction is completed, and its subsequent use is limited. The force sensor method installs a force sensor between the cable and the anchor head to directly measure the pressure between the anchor head and the anchor pad, but it needs to be arranged before the cable is tensioned, and it cannot be installed on the built wind power hybrid tower. The stress release method infers the cable force by detecting the strain data near the damaged part through local damage to the cable (such as stripping, piercing, drilling, blind hole, grooving, etc.). Although it can directly measure the cable force and apply it to the built wind power hybrid tower, it will damage the cable structure and pose potential risks. The frequency analysis method uses the vibration signal of the cable to extract the vibration mode information and calculate the cable force through an appropriate mechanical model. It is easy to operate and will not damage the cable structure. It is suitable for new and built wind power hybrid towers, and is therefore widely used.

[0005] like Figure 2As shown, the cable 4 of the wind power tower is composed of a plurality of steel strand bundles 6, and gaps 7 are left between each steel strand bundle 6; Figure 3 As shown, the steel strand bundle 6 includes steel strands 8 , grease 9 and a PE jacket 10 .

[0006] In the application of wind power hybrid tower, although the environmental conditions are more stable than bridge cables, due to cost considerations, outer sleeves are usually not used to tightly wrap the cables for protection. This weakens the interaction between the steel strands inside the cables, and even causes the steel strands to detach, which in turn causes deviations in the frequency data in different directions. Figure 4 This is the frequency domain diagram of the cable frequency test. It can be seen from the figure that due to the detachment between the steel strands, there are differences in the frequencies measured in different directions, making it difficult to measure a unified overall cable frequency, which seriously affects the accuracy of identifying the cable force. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a method for detecting the force of prestressed cables outside the wind turbine hybrid tower body, which is simple to operate, has high detection efficiency and high precision.

[0008] The technical solution of the present invention to solve the above technical problems is: a method for detecting the force of external prestressed cables of wind power hybrid tower, comprising the following steps:

[0009] S1: Collect basic data of the cables to be tested, including cable length, cable line density and cable support constraint distribution;

[0010] S2: Design of local rigid coupling device: Based on the local rigid coupling condition, the relationship between the cable force and the cable frequency is constructed, and the local rigid coupling device used for cable force detection is designed;

[0011] S3: Install the local rigid coupling device: Select a convenient location at the bottom of the wind turbine tower to install the local rigid coupling device;

[0012] S4: Deploy acceleration sensors: Arrange two acceleration sensors perpendicular to each other on the local rigid coupling device;

[0013] S5: Obtaining the cable frequency: Using the deployed acceleration sensor, the acceleration signal of the cable is collected, and then the collected acceleration signal is processed by discrete Fourier transform to extract the cable frequency;

[0014] S6: Verify the local rigid coupling condition: compare whether the cable frequencies obtained in two directions after being collected by the acceleration sensor and processed by discrete Fourier transform are consistent, so as to verify whether the local rigid coupling condition is met;

[0015] S7: Calculate the cable force: If the local rigid coupling condition is met, the obtained cable frequency is substituted into the constraint equation between the cable force and the cable frequency to calculate the cable force.

[0016] In the above-mentioned method for detecting the force of the external prestressed cables of the wind power hybrid tower, in the step S2, the relationship between the cable force and the cable frequency is constructed as follows:

[0017] If there are two strands in each cable, the relationship between the cable force and the cable frequency is:

[0018] (1)

[0019] Where: L1 is the length of the lower section of the coupling point, L2 is the length of the upper section of the coupling point, T1 is the cable force of the first steel strand bundle, T2 is the cable force of the second steel strand bundle, ρ1 is the linear density of the first steel strand bundle, ρ2 is the linear density of the second steel strand bundle, is the characteristic value of the first strand bundle, is the characteristic value of the second strand bundle, , , ω is the circular frequency of vibration of the cable, , f is the cable frequency, k is the coupling stiffness;

[0020] The type of coupling device is related to the coupling stiffness. When the coupling stiffness k is less than 5×10 6 N / m, the coupling device meets the local flexible coupling condition; when the coupling stiffness k is greater than or equal to 5×10 6 When N / m, the coupling device satisfies the local rigid coupling condition. At this time, equation (1) is simplified to the following equation:

[0021] (2)

[0022] If the number of steel strands in each cable is greater than 2, the relationship between the cable force and the cable frequency is:

[0023] (3)

[0024] Where: , , i , T i are the characteristic value, linear density and cable tension of the i-th strand bundle, , , j , T j are the characteristic value, linear density and cable tension of the j-th steel strand bundle, i≠j, n is the number of steel strand bundles, For the Characteristic values ​​of the strand bundle.

[0025] In the above-mentioned method for detecting the force of prestressed cables in the external structure of wind power tower, in step S2, the factors affecting the coupling stiffness k include: the elastic modulus of the coupling agent, the equivalent diameter of the steel strand bundle, the pouring height of the coupling agent, and the equivalent gap width between the steel strand bundles, wherein the equivalent diameter of the steel strand bundle and the equivalent gap width between the steel strand bundles are related to the on-site conditions of the cable and are difficult to adjust when installing the coupling device; the elastic modulus of the coupling agent is affected by the type and formula of the coupling agent; the pouring height of the coupling agent is adjusted by the amount of the coupling agent, and finally, the coupling stiffness is ensured to satisfy k≥5×10 6 N / m;

[0026] The calculation formula of coupling stiffness k is:

[0027] (4)

[0028] Where: h is the pouring height of the coupling agent, E is the elastic modulus of the coupling agent, s is the equivalent gap width between the steel strands, s=(d+2s min ) / 2,s min is the minimum gap width between two steel strand bundles, and d is the equivalent diameter of the steel strand bundle.

[0029] In the above-mentioned method for detecting the force of external prestressed cables of wind power hybrid towers, in step S2, the step of designing a local rigid coupling device for detecting the force of cables is as follows:

[0030] S21: Select a coupling agent based on the ambient temperature and humidity conditions at the wind power tower site and the equivalent gap width s between the steel strands, and determine the elastic modulus E of the coupling agent;

[0031] S22: According to the cable distribution of the wind turbine hybrid tower, determine the equivalent diameter d of the steel strand bundle and the equivalent gap width s between the steel strand bundles, substitute them into the calculation formula of the coupling stiffness k, calculate the coupling agent pouring height h, and determine the hoop size accordingly.

[0032] In the above-mentioned method for detecting the force of external prestressed cables of wind power hybrid tower, in step S3, the step of installing the local rigid coupling device is:

[0033] S31: Install the ferrule on the cable to ensure that the cable fits tightly with the ferrule, which will serve as a fixing point for the subsequent installation of the acceleration sensor.

[0034] S32: Use cotton or plasticine to seal the gap between the bottom of the hoop and the hoop to prevent leakage when pouring the coupling agent;

[0035] S33: Clean the gap between the cable and the ferrule, remove dust, grease and foreign matter, and ensure that the contact interface between the cable and the ferrule is clean;

[0036] S34: The coupling agent material is uniformly injected into the gap between the cable and the hoop, and is cured under natural temperature and humidity conditions to form a local rigid coupling device.

[0037] In the above-mentioned method for detecting the force of prestressed cables outside the wind power tower, in step S5, the frequency difference method is used to extract the cable frequency, and the frequency difference between adjacent resonance peaks in the spectrum diagram is used to take the average value of multiple groups of frequency differences as the cable frequency. The cable frequency f is calculated as follows:

[0038] (5)

[0039] Where: N is the number of frequency differences, m is the ordinal number of frequency differences, F m is the frequency difference of the mth group.

[0040] In the above-mentioned method for detecting the force of prestressed cables in the external part of a wind turbine hybrid tower, in step S6, if the cable frequencies obtained by the acceleration sensor and discrete Fourier transform processing in two directions are inconsistent, the cable frequencies are made consistent by increasing the pouring height of the coupling agent.

[0041] In the above-mentioned method for detecting the cable force of the external prestressed cable of the wind turbine hybrid tower, in the step S6, the condition for satisfying the local rigid coupling condition is that the cable frequencies measured at different orientations are consistent.

[0042] In the above-mentioned method for detecting the force of the external prestressed cables of the wind power hybrid tower, in step S7, the constraint equation between the cable force and the cable frequency is:

[0043] (6)

[0044] Where: f is the cable frequency, l is the cable length, T is the cable force, and ρ is the cable linear density.

[0045] The beneficial effects of the present invention are:

[0046] 1. The present invention constructs a relationship between cable force and cable frequency based on local rigid coupling conditions, and designs a local rigid coupling device for cable force detection. That is, through the calculation results of the local rigid coupling conditions, a coupling agent is reasonably selected, the elastic modulus of the coupling agent, the equivalent diameter of the steel strand bundle and the equivalent gap width between the steel strand bundles are determined, and the pouring height of the coupling agent is calculated, so as to meet the local rigid coupling conditions, eliminate the interference of local frequencies in the test, obtain a unified overall cable frequency, and improve the detection accuracy of the cable force.

[0047] 2. When installing the local rigid coupling device, the present invention first installs the hoop on the cable to ensure that the cable fits tightly with the hoop, which serves as a fixing point for the subsequent installation of the acceleration sensor; then, cotton or plasticine is used to seal the gap between the bottom of the hoop and the hoop to prevent leakage during the pouring of the coupling agent; then, the gap between the cable and the hoop is cleaned to remove foreign matter such as dust and grease to ensure that the contact interface between the cable and the hoop is clean and free of dirt; finally, the coupling agent material is evenly injected into the gap between the cable and the hoop, and solidified under natural temperature and humidity conditions to form a local rigid coupling device. The present invention couples the local independent steel strand bundles, and then measures the cable frequency and derives the cable force. The arrangement process of the local rigid coupling device is simple, which greatly improves the efficiency of cable force detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of wind turbine hybrid tower structure.

[0049] Figure 2 Schematic diagram of the cross section of the cable.

[0050] Figure 3 This is a schematic diagram of the structure of the steel strand harness.

[0051] Figure 4 This is the frequency domain diagram of the cable frequency test without local rigid coupling device.

[0052] Figure 5 It is a flow chart of the present invention.

[0053] Figure 6 Schematic diagram of the prestressed cable system model of the coupling device.

[0054] Figure 7 Schematic diagram of the cable arrangement of the local rigid coupling device.

[0055] Figure 8 Schematic diagram of the cross section of two steel strand bundles.

[0056] Fig. 9 Schematic diagram of the distance between two steel strands at different locations.

[0057] Fig.10 This is a schematic diagram showing the equivalent distance between two steel strands at different locations.

[0058] Fig.11 This is the frequency domain diagram of the cable frequency test after the local rigid coupling device is arranged.

[0059] In the figure, 1 is a cabin, 2 is a pure steel tower, 3 is a transition section, 4 is a cable, 5 is a concrete tower, 6 is a steel strand bundle, 7 is a gap, 8 is a steel strand, 9 is grease, 10 is a PE jacket, 11 is a first steel strand bundle, 12 is a second steel strand bundle, 13 is a coupling agent, and 14 is a ferrule. DETAILED DESCRIPTION

[0060] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0061] In bridge engineering, an outer sleeve is usually wrapped around the steel strand bundle. The outer sleeve has a good constraint effect, so that the prestressed cable bundles are fully coupled, and the frequencies obtained in different directions are basically the same. Therefore, the cable force can be calculated based on the measured frequency. However, the cables of wind power hybrid towers do not use thick outer sleeves to tightly wrap the steel strand bundles. It is difficult to obtain a unified overall cable frequency by arranging sensors in different directions. To this end, the present invention arranges a local rigid coupling device on the cable to reduce the frequency detection deviation caused by the weak mutual constraint between the steel strand bundles.

[0062] like Figure 5 As shown, a method for detecting the force of external prestressed cables of a wind power tower comprises the following steps:

[0063] S1: Collect basic data of the cables to be tested, including cable length, cable line density and cable support constraint distribution;

[0064] S2: Design of local rigid coupling device: Based on the local rigid coupling condition, the relationship between the cable force and the cable frequency is constructed, and the local rigid coupling device used for cable force detection is designed;

[0065] If there are two strands in each cable, Figure 6 As shown in the figure, the relationship between the cable force and the cable frequency is:

[0066] (1)

[0067] Wherein: L1 is the length of the lower section of the coupling point, L2 is the length of the upper section of the coupling point, T1 is the cable force of the first steel strand bundle 11, T2 is the cable force of the second steel strand bundle 12, ρ1 is the line density of the first steel strand bundle 11, ρ2 is the line density of the second steel strand bundle 12, is the characteristic value of the first steel strand bundle 11, is the characteristic value of the second steel strand bundle 12, , , ω is the circular frequency of vibration of the cable, , f is the cable frequency, k is the coupling stiffness.

[0068] The type of coupling device is related to the coupling stiffness. When the coupling stiffness k is less than 5×10 6 N / m, the coupling device meets the local flexible coupling condition; when the coupling stiffness k is greater than or equal to 5×10 6 When N / m, the coupling device satisfies the local rigid coupling condition. At this time, equation (1) is simplified to the following equation:

[0069] (2)

[0070] According to formula (1), when the coupling device is a flexible coupling, the relationship between the cable frequency and the cable force is affected by the coupling stiffness k; however, when the coupling device is a rigid coupling, the relationship between the cable frequency and the cable force is not affected by the coupling stiffness k. Therefore, in actual engineering, the coupling device and coupling agent need to be reasonably designed to meet the local rigid coupling conditions.

[0071] Usually, the number of steel strands in each cable bundle of a wind turbine hybrid tower is greater than 2, and the relationship between the cable force and the cable frequency is:

[0072] (3)

[0073] Where: , , i , T i are the characteristic value, linear density and cable tension of the i-th strand bundle, , , j , T j are the characteristic value, linear density and cable tension of the j-th steel strand bundle, i≠j, n is the number of steel strand bundles, For the Characteristic values ​​of the strand bundle.

[0074] The factors affecting the coupling stiffness k include: the elastic modulus of the coupling agent, the equivalent diameter of the steel strand bundle, the pouring height of the coupling agent, and the equivalent gap width between the steel strand bundles. The equivalent diameter of the steel strand bundle and the equivalent gap width between the steel strand bundles are related to the on-site conditions of the cable and are difficult to adjust when installing the coupling device. The elastic modulus of the coupling agent is affected by the type and formula of the coupling agent. The pouring height of the coupling agent is adjusted by the amount of the coupling agent to ensure that the coupling stiffness meets k≥5×10 6 N / m.

[0075] The calculation formula of coupling stiffness k is:

[0076] (4)

[0077] Where: h is the pouring height of the coupling agent, E is the elastic modulus of the coupling agent, s is the equivalent gap width between the steel strands, and d is the equivalent diameter of the steel strands. Figure 6 , Figure 7 shown.

[0078] Since the gap between two steel strands varies in different parts of the steel strands, the equivalent gap width between the steel strands is calculated as follows:

[0079]

[0080] In the formula: Figure 8 , Fig. 9 As shown, f(x) represents the distance between the two strands at different locations, and f(0)=s min is the minimum distance between two strands, f(d / 2)= f(-d / 2)=s min +d is the maximum distance between two strands.

[0081] For the convenience of calculation, the curve relationship of the distance between two strands of steel strands at different locations is equivalent to a trapezoidal relationship, such as Fig.10 As shown:

[0082]

[0083] After simplification, we can finally get s=(d+2s min ) / 2.

[0084] The steps for designing a local rigid coupling device for cable force detection are as follows:

[0085] S21: Select a coupling agent according to the ambient temperature and humidity conditions at the wind power tower site and the equivalent gap width s between the steel strands, and determine the elastic modulus E of the coupling agent;

[0086] S22: According to the cable distribution of the wind turbine hybrid tower, determine the equivalent diameter d of the steel strand bundle and the equivalent gap width s between the steel strand bundles, substitute them into the calculation formula of the coupling stiffness k, calculate the coupling agent pouring height h, and determine the hoop size accordingly.

[0087] S3: Install a local rigid coupling device: Select a convenient location at the bottom of the wind turbine tower to install a local rigid coupling device.

[0088] The steps for installing a local rigid coupling device are:

[0089] S31: Install the ferrule on the cable to ensure that the cable fits tightly with the ferrule, which will serve as a fixing point for the subsequent installation of the acceleration sensor.

[0090] S32: Use cotton or plasticine to seal the gap between the bottom of the hoop and the hoop to prevent leakage when pouring the coupling agent;

[0091] S33: Clean the gap between the cable and the hoop, remove dust, grease and other foreign matter, and ensure that the contact interface between the cable and the hoop is clean;

[0092] S34: The coupling agent material is uniformly injected into the gap between the cable and the hoop, and is cured under natural temperature and humidity conditions to form a local rigid coupling device.

[0093] S4: Deploy acceleration sensors: Arrange two acceleration sensors perpendicular to each other on the local rigid coupling device.

[0094] S5: Obtaining the cable frequency: Using the deployed acceleration sensor, the acceleration signal of the cable is collected, and then the collected acceleration signal is processed by discrete Fourier transform to extract the cable frequency.

[0095] The frequency difference method is used to extract the cable frequency. The frequency difference method can reduce the influence of environmental factors and improve the accuracy of cable frequency extraction. The frequency difference method uses the frequency difference of adjacent resonance peaks in the spectrum diagram and takes the average value of multiple sets of frequency differences as the cable frequency. The calculation formula of the cable frequency f is:

[0096] (5)

[0097] Where: N is the number of frequency differences, m is the ordinal number of frequency differences, F m is the frequency difference of the mth group.

[0098] S6: Verify the local rigid coupling condition: compare whether the cable frequencies obtained in two directions after being collected by the acceleration sensor and processed by discrete Fourier transform are consistent, so as to verify whether the local rigid coupling condition is met;

[0099] If the cable frequencies obtained by the acceleration sensor and discrete Fourier transform processing in the two directions are inconsistent, it is necessary to increase the pouring height of the coupling agent to achieve the consistency of the cable frequencies. The condition for the local rigid coupling condition to be satisfied is that the cable frequencies measured from different directions are consistent.

[0100] S7: Calculate the cable force: If the local rigid coupling condition is met, the obtained cable frequency is substituted into the constraint equation between the cable force and the cable frequency to calculate the cable force.

[0101] According to the literature "The Influence of Rigid Coupling on the Self-Vibration Characteristics of Multi-Strand Structures", when the number of steel strands in each cable is 5, if the tensioning unevenness is controlled within 10%, the deviation between the cable frequency obtained based on local rigid coupling and the cable frequency obtained by full coupling of the entire cable will not exceed 5%. In addition, under the condition of fixed cable force unevenness, increasing the number of cables can also effectively reduce the frequency deviation. Considering that the number of steel strands in the wind power hybrid tower exceeds 20, the local rigid coupling test method can greatly reduce the measurement error of the cable frequency to less than 1.2%. For this reason, the constraint equation between the cable force and the cable frequency is:

[0102] (6)

[0103] Where: f is the cable frequency, l is the cable length, T is the cable force, and ρ is the cable linear density.

[0104] Implementation and Verification

[0105] The test site was selected at a wind turbine hybrid tower in a wind farm in Gansu. Table 1 lists the main parameters of the cables to be tested. The cables of the test wind turbine hybrid tower contain 26 strands of steel strands, each with an equivalent diameter d of 15.7 mm, and the equivalent gap width between the strands is between 0.1 mm and 5 mm. Given the small equivalent gap width between the strands, the coupling agent must have good fluidity, bonding properties, and a high elastic modulus to ensure that the coupling agent fully enters the gap. After comprehensive evaluation, epoxy resin can meet the above requirements.

[0106] The elastic modulus of epoxy resin ranges from 1 GPa to 5 GPa, and the lower limit of 1 GPa is selected as the reference value. In order to meet the requirements of rigid coupling, that is, to ensure that k reaches 5×10 6 N / m, it is calculated that the epoxy resin pouring height should be between 2.5318 mm and 4.0923 mm. In order to facilitate on-site construction, the installation height of the coupling device is set to 100 mm to 150 mm.

[0107]

[0108] Arrange the acceleration sensors in two different directions and compare the frequency domain diagrams before and after the coupling device is installed. Before coupling, the frequencies corresponding to the amplitude peaks in the two orthogonal directions are inconsistent, such as Figure 4 and Fig.11As shown, it shows that the gap between the steel strands is large and fails to form a whole, resulting in local vibration. The frequencies of each order are shown in Table 2. There are differences in the frequencies of each order obtained at the two measuring points before coupling. The cable frequency is calculated by the frequency difference method, and then the cable force is calculated according to formula (6). It is found that the cable force obtained at the two measuring points differs by 21.3t. This shows that if the coupling device does not meet the local rigid coupling conditions, it will seriously affect the accuracy of the frequency test results.

[0109]

[0110] After coupling, the frequencies corresponding to the amplitude peaks in the two directions are consistent, such as Fig.11 As shown, it shows that the coupling device meets the rigid coupling condition. The frequencies of each order are shown in Table 3. After coupling, the frequencies of each order obtained at the two measuring points are consistent, and the cable forces are also the same. The change in cable force after coupling can be reflected by the change in frequency, and the cable force of the wind power hybrid tower can be detected by measuring the frequency.

[0111]

[0112] The measured cable forces of the four cables with anchor meters installed were read, and the theoretical cable frequency was calculated according to formula (6). The theoretical cable frequency was compared with the measured cable frequency, and the results are shown in Table 4. Frequency error = (theoretical cable frequency - measured cable frequency) / measured cable frequency × 100%, and the frequency error result was rounded to two decimal places. The calculation results in Table 4 show that under the condition of local rigid coupling, the maximum error between the measured cable frequency and the theoretical cable frequency is only 0.61%.

[0113]

[0114] The four cables with the anchor cable meter installed were selected to obtain the measured cable frequency. The theoretical cable force was calculated according to formula (6) and compared with the measured cable force of the anchor cable meter. As shown in Table 5, cable force error = (theoretical cable force - measured cable force) / measured cable force × 100%. The cable force error result was rounded to two decimal places.

[0115]

[0116] The calculation results in Table 5 show that after the local rigid coupling device is arranged, the maximum error between the theoretical cable force and the measured cable force is only -1.18%. This shows that the proposed method for detecting the cable force of the external prestressed cable of the wind power hybrid tower can effectively reflect the actual stress situation and can be used as a reference for actual engineering.

Claims

1. A method for detecting the force of prestressed cables outside the wind power tower, characterized in that: The following steps are involved: S1: Collect basic data of the cables to be tested, including cable length, cable line density and cable support constraint distribution; S2: Design of local rigid coupling device: Based on the local rigid coupling condition, the relationship between the cable force and the cable frequency is constructed, and the local rigid coupling device used for cable force detection is designed; In step S2, the factors affecting the coupling stiffness k include: the elastic modulus of the coupling agent, the equivalent diameter of the steel strand bundle, the pouring height of the coupling agent, and the equivalent gap width between the steel strand bundles. The equivalent diameter of the steel strand bundle and the equivalent gap width between the steel strand bundles are related to the on-site conditions of the cable and are difficult to adjust when installing the coupling device. The elastic modulus of the coupling agent is affected by the type and formula of the coupling agent. The pouring height of the coupling agent is adjusted by the amount of the coupling agent to ensure that the coupling stiffness satisfies k≥5×10 6 N / m; The calculation formula of coupling stiffness k is: Where: h is the pouring height of the coupling agent, E is the elastic modulus of the coupling agent, s is the equivalent gap width between the steel strands, s = (d + 2s min ) / 2,s min is the minimum gap width between two steel strand bundles, and d is the equivalent diameter of the steel strand bundle; S3: Install the local rigid coupling device: Select a convenient location at the bottom of the wind turbine tower to install the local rigid coupling device; S4: Deploy acceleration sensors: Arrange two acceleration sensors perpendicular to each other on the local rigid coupling device; S5: Obtaining the cable frequency: Using the deployed acceleration sensor, the acceleration signal of the cable is collected, and then the collected acceleration signal is processed by discrete Fourier transform to extract the cable frequency; S6: Verify the local rigid coupling condition: compare whether the cable frequencies obtained in two directions after being collected by the acceleration sensor and processed by discrete Fourier transform are consistent, so as to verify whether the local rigid coupling condition is met; S7: Calculate the cable force: If the local rigid coupling condition is met, the obtained cable frequency is substituted into the constraint equation between the cable force and the cable frequency to calculate the cable force.

2. The method for detecting the force of prestressed cables outside the wind power tower according to claim 1 is characterized in that: In step S2, the relationship between the cable force and the cable frequency is constructed as follows: If there are two strands in each cable, the relationship between the cable force and the cable frequency is: Where: L1 is the length of the lower section of the coupling point, L2 is the length of the upper section of the coupling point, T1 is the cable force of the first steel strand bundle, T2 is the cable force of the second steel strand bundle, ρ1 is the linear density of the first steel strand bundle, ρ2 is the linear density of the second steel strand bundle, β1 is the characteristic value of the first steel strand bundle, β2 is the characteristic value of the second steel strand bundle, ω is the circular frequency of vibration of the cable, ω = 2πf, f is the cable frequency, k is the coupling stiffness; The type of coupling device is related to the coupling stiffness. When the coupling stiffness k is less than 5×10 6 N / m, the coupling device meets the local flexible coupling condition; When the coupling stiffness k is greater than or equal to 5×10 6 When N / m, the coupling device satisfies the local rigid coupling condition. At this time, formula (1) is simplified to the following formula: β1T1 sin(β2L1)sin(β2L2)sin(β1L)+β2T2 sin(β1L2)sin(β1L1)sin(β2L)=0 (2) If the number of steel strands in each cable is greater than 2, the relationship between the cable force and the cable frequency is: Where: β i , i , T i are the characteristic value, linear density and cable tension of the i-th strand bundle, β j , j , T j are the characteristic value, linear density and cable tension of the j-th strand bundle, i≠j, n is the number of strand bundles, β k′ is the characteristic value of the k′th strand bundle.

3. The method for detecting the force of prestressed cables in the wind power tower according to claim 2 is characterized in that: In step S2, the steps of designing a local rigid coupling device for detecting cable force are as follows: S21: Select a coupling agent based on the ambient temperature and humidity conditions at the wind power tower site and the equivalent gap width s between the steel strands, and determine the elastic modulus E of the coupling agent; S22: According to the cable distribution of the wind turbine hybrid tower, determine the equivalent diameter d of the steel strand bundle and the equivalent gap width s between the steel strand bundles, substitute them into the calculation formula of the coupling stiffness k, calculate the coupling agent pouring height h, and determine the hoop size accordingly.

4. The method for detecting the force of prestressed cables in the wind power tower according to claim 1, characterized in that: In step S3, the steps of installing the local rigid coupling device are: S31: Install the ferrule on the cable to ensure that the cable fits tightly with the ferrule, which will serve as a fixing point for the subsequent installation of the acceleration sensor. S32: Use cotton or plasticine to seal the gap between the bottom of the hoop and the hoop to prevent leakage when pouring the coupling agent; S33: Clean the gap between the cable and the ferrule, remove dust, grease and foreign matter, and ensure that the contact interface between the cable and the ferrule is clean; S34: The coupling agent material is uniformly injected into the gap between the cable and the hoop, and is cured under natural temperature and humidity conditions to form a local rigid coupling device.

5. The method for detecting the force of prestressed cables outside the wind power tower according to claim 2 is characterized in that: In step S5, the cable frequency is extracted by using the frequency difference method, and the frequency difference between adjacent resonance peaks in the spectrum diagram is used to take the average value of multiple groups of frequency differences as the cable frequency. The cable frequency f is calculated as follows: Where: N is the number of frequency differences, m is the ordinal number of frequency differences, F m is the frequency difference of the mth group.

6. The method for detecting the force of external prestressed cables of wind power hybrid tower according to claim 1 is characterized in that: In the step S6, if the cable frequencies acquired by the acceleration sensor and the discrete Fourier transform processing in the two directions are inconsistent, the cable frequencies are made consistent by increasing the pouring height of the coupling agent.

7. The method for detecting the force of external prestressed cables of wind power hybrid tower according to claim 6 is characterized in that: In step S6, the condition for satisfying the local rigid coupling condition is that the cable frequencies measured at different orientations are consistent.

8. The method for detecting the force of prestressed cables outside the wind power tower according to claim 5, characterized in that: In step S7, the constraint equation between the cable force and the cable frequency is: T=4ρf 2 l 2 (6) Where: f is the cable frequency, l is the cable length, T is the cable force, and ρ is the cable linear density.

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