Safety monitoring, early warning and assessment methods for wind turbine concrete tower

Abstract

The present invention discloses a method for safety monitoring, early warning and evaluation of a concrete tower of a wind turbine, which belongs to the field of wind power and includes the following steps: based on the stress mechanism of the concrete tower, analyzing the different stress stages of the concrete tower; determining the main wind direction and the position of the lowest frequency cable as the monitoring position; arranging a lateral acceleration sensor and a strain gauge at the monitoring position; determining the early warning value of the monitored concrete tower; collecting acceleration and strain signals, calculating the first-order frequency of the cable and the actual strain of the concrete, and comparing them with the early warning value to achieve safety monitoring and early warning of the concrete tower of the wind turbine. The present invention combines the cable frequency and the concrete strain according to the stress characteristics of the concrete tower structure of the wind turbine, realizes accurate early warning and evaluation of the cable force loss and the ultimate strain of the concrete, and effectively ensures the safety of the concrete tower structure.

Description

Technical Field

[0001] The present invention relates to the field of wind power, and in particular to a method for safety monitoring, early warning and evaluation of a concrete tower of a wind turbine generator set. Background Art

[0002] With the development of wind power research in my country, the increase in single-unit capacity and blade size has led to a continuous increase in tower height, and the rigidity of traditional pure steel towers can no longer meet the requirements. The new concrete-steel hybrid tower can not only effectively increase the tower height and power generation, but also has significant technical advantages in reducing the cost per kilowatt-hour, meeting the needs of wind turbine power generation development. The upper part of the wind turbine concrete-steel hybrid tower adopts a pure steel structure, and the lower part is a post-tensioned external prestressed concrete structure. Prestress is applied to the concrete section through prestressed cables to offset or reduce the tensile stress generated by external loads, thereby giving full play to the compressive properties of concrete.

[0003] During the use of external cables, there will be a variety of prestress losses, mainly including the following five types: (1) anchorage loss caused by deformation of the tensioning end anchor and shrinkage of the prestressed tendons; (2) friction loss caused by friction between the prestressed tendons and the duct wall, the tensioning end anchor and the steering device; (3) elastic compression loss of concrete; (4) prestress loss caused by stress relaxation of the prestressed tendons; (5) prestress loss caused by shrinkage and creep of concrete. During the operation of wind turbines, long-term reciprocating wind loads will cause cable force loss. In addition, tensioning construction quality problems may also cause the cable prestress to fail to meet the design requirements. For a long time, due to limited detection methods, it has been difficult to accurately determine the actual prestress of the cable, resulting in the inability to correct the prestress loss in time, which in turn causes the concrete tower to crack prematurely. If not handled in time, it will endanger the safety of the concrete tower structure in the long run. Therefore, it is necessary to monitor the cable force and concrete strain of the concrete tower for a long time to detect and deal with problems early.

[0004] At present, the detection methods of prestressed cable force mainly include direct method and indirect method. The direct method is realized by installing anchor cable meter or using magnetic flux method. It has the advantages of being simple and intuitive and real-time tracking of cable force changes, but it is costly and not suitable for wind turbines that have been put into use. The indirect method usually uses the frequency method to derive the relationship between the prestressed cable and its natural frequency based on the string vibration theory. The frequency method is simple to test, can be performed once or repeatedly, has good economic efficiency, is suitable for long-term and large-scale use, and has been widely used in the field of bridges. However, unlike the long-span bridge structure that bears constant loads mainly due to gravity, the concrete tower of the wind turbine mainly bears live loads represented by wind loads, and the variation range of its cable force is large. The frequency method needs to collect a certain number of acceleration signals and obtain the frequency value through Fourier transform. During the signal acquisition period, the load change will cause the cable force to change. If the cable force changes greatly, it will inevitably affect the test accuracy of the frequency method.

[0005] In the prior art, the safety monitoring of concrete towers of wind turbines has the following problems: first, tower monitoring is not associated with load conditions, making it difficult to establish a complete early warning and assessment system; second, there is a general lack of monitoring and early warning of the prestressed cable force in concrete towers and their potential losses; finally, the concrete strain in the concrete tower is not effectively associated with the effective prestressed force of the cables. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a wind turbine concrete tower safety monitoring, early warning and evaluation method with simple algorithm, safety and reliability.

[0007] The technical solution of the present invention to solve the above technical problems is: a wind turbine concrete tower safety monitoring, early warning and evaluation method, comprising the following steps:

[0008] Step S1: Based on the stress mechanism of the concrete tower, analyze the concrete tower at different stress stages;

[0009] Step S2: by collecting data and conducting on-site inspections, determine the main wind direction and the position of the lowest frequency cable as the monitoring position;

[0010] Step S3: arranging a lateral acceleration sensor and a strain gauge at the monitoring position;

[0011] Step S4: Determine the warning value of the monitored concrete tower, including the lower limit of the cable frequency f min , the initial compressive strain of the concrete section -ε0 and the ultimate compressive strain of the concrete section ε cu ;

[0012] Step S5: The acceleration and strain signals are collected in real time by the lateral acceleration sensor and the strain gauge, and input into the vibration signal acquisition instrument and computer analysis to calculate the first-order frequency f1 of the cable and the actual strain of the concrete. , and compare it with the warning value to achieve safety monitoring and early warning of the concrete tower of the wind turbine;

[0013] Step S6: Determine the concrete preload loss strain ε', the annual minimum actual strain of concrete, the annual maximum actual strain of concrete, and the lower limit of the first-order frequency of the cable f according to the annual monitoring data 1，min And compare and analyze with the corresponding data of the previous year to achieve an annual assessment of the safe operation of wind turbine concrete towers.

[0014] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S1, the stress mechanism of the concrete tower is:

[0015] After the prestress is applied to the concrete tower, the normal compressive stress of the concrete in the cross section of the concrete tower ispc for:

[0016] (1)

[0017] (2)

[0018] (3)

[0019] In the formula, A n is the net cross-sectional area of ​​concrete; I n is the converted net section inertia moment; e pn The distance from the center of gravity of the converted net section to the prestressing point; y n is the distance from the centroid of the converted net section to the calculated position; σ p2 is the normal stress of the concrete section caused by the secondary internal force of the prestress. Considering that the concrete tower is a cantilever statically determinate structure, σ p2 =0; G is the total weight of the tower above the calculation section; N p is the cable pretension; con A is the stress control for prestressing tendons; s is the cross-sectional area of ​​all longitudinal ordinary steel bars; σ pe is the effective prestress of the prestressed tendons; A p is the cross-sectional area of ​​all longitudinal prestressed tendons; σ l is the total loss value of prestressed tendons; σ l5 is the prestress loss value caused by concrete shrinkage and creep at the resultant point of the prestressed tendons; It represents the eccentric bending moment caused by the pre-tension of the cable. The result of the bending moment on the cross section is that one side is compressed and the other side is tensile, and the tension and compression are expressed as positive and negative signs;

[0020] The prestressed tendons in the concrete tower are arranged symmetrically, so N p e pn =0, so formula (1) is simplified to:

[0021] (4)

[0022] The tensile edge stress σ of the concrete section with the first crack control level under the standard combination of normal operating load conditions of the wind turbine ck Comply with the following provisions:

[0023] (5)

[0024] where σ ck Calculate as follows:

[0025] (6)

[0026] Where r is the outer diameter of the tower at the calculated section, M k Calculate the cross-sectional bending moment of the concrete tower under normal operating load conditions, which is determined by the host manufacturer;

[0027] Substituting equations (4) and (6) into equation (5), we can obtain:

[0028] (7)

[0029] Therefore, under the first level of crack control, the cable pre-tension meets the following requirements:

[0030] (8)

[0031] Therefore, under normal operating load conditions, the lower limit of the cable pretension for the first crack control level is for:

[0032] (9)

[0033] The cable force measurement is obtained by the frequency method. The relationship between the cable force N and the cable frequency f is as follows:

[0034] (10)

[0035] Where m is the mass per unit length of the cable; l is the length from the tensioning end to the anchorage end; f n is the nth-order frequency of the cable; n is the modal order, and in actual engineering, the first-order frequency f1 of the cable is taken;

[0036] N p,min Substituting into formula (10), the lower limit of the first-order frequency f of the cable under normal operating load conditions is determined 1, min for:

[0037] (11).

[0038] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S1, the analysis of different stress stages of the concrete tower includes analyzing the cable force in the tensile zone of the concrete section, the cracking state of the tensile zone of the concrete section and the change of the cable force under different stress states. The process of analyzing the cable force in the tensile zone of the concrete section is as follows:

[0039] As the external load gradually increases, the edge pressure of the tensile zone of the concrete section will gradually drop to 0, entering the tension zone decompression state. At this time, the calculated section bending moment of the concrete tower reaches the edge decompression bending moment M. c0 , according to formula (7):

[0040] (12)

[0041] As can be seen from the above formula, if there is N p loss, does not meet the design requirements, then M c0 will decrease accordingly, making M c0 ≤M k , the edge of the tensile zone of the concrete tower will enter the decompression state or even the tensile state prematurely;

[0042] At this time, by monitoring the edge strain of the concrete tensile zone and the cable frequency, it is determined through calculation and analysis whether the edge of the concrete tensile zone is in a decompression or tension state under normal operation, and at the same time, combined with whether the measured cable frequency is less than f 1,min , it can be determined whether there is cable prestress loss.

[0043] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S1, the process of analyzing the cracking state of the tensile zone of the concrete cross section is:

[0044] When the wind turbine exceeds the normal operating condition, the external load continues to increase, and the tensile zone of the concrete section will enter a cracking state. When the edge of the tensile zone of the concrete section reaches the ultimate tensile strain, the cracking moment M is reached. cr ; Due to the plastic deformation of concrete under tension, the stress diagram of the tensile zone of the concrete section is distributed in a curve. In order to continue to apply the elastic calculation formula, according to the moment equilibrium condition, the stress diagram of the tensile zone of the concrete section is equivalent to a triangular distribution, and the equivalent tensile stress of the tensile bottom edge is taken as γf tk , at this time, the force on the concrete section is expressed as:

[0045] (13)

[0046] Where γ is the plastic influence coefficient of the section resistance moment of the concrete member, , h is the section height, r l is the inner diameter of the annular section, f tk is the standard value of concrete tensile strength;

[0047] Similar to the cable force, if there is N p loss, then M cr As it decreases, the edge of the tensile zone of the concrete tower will enter the tensile cracking state prematurely; considering that the strain value of concrete after cracking is quite different, the cracking state is not suitable as a monitoring state.

[0048] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S1, the process of analyzing the cable force changes under different stress states is as follows:

[0049] Within normal operating load conditions, the cable force changes linearly with the external load, meeting the accuracy requirements of the frequency method test; when the load continues to increase to the cracking state, the cable force will surge as the concrete in the tension zone stops working; after cracking, the cable force increases significantly with the increase in load until the limit state.

[0050] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S2, the windward and leeward directions of the main wind direction and the directions of the two cables with the lowest detection frequencies are determined by collecting data and conducting on-site inspections, a total of four directions as monitoring directions.

[0051] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S4, the process of determining the early warning value of the monitored concrete tower is:

[0052] S41: Based on the design data of the wind turbine, calculate the lower limit of the first-order frequency f of the cable in the concrete tower section under the decompression state. 1,min ; Comprehensively monitor the initial measured cable frequency f0 of the cable with the lowest measured cable frequency, and consider the cable force changes under the operating state to determine the frequency monitoring warning lower limit f min :

[0053] f min =min{0.95f0, 0.95f 1, min} (14)

[0054] S42: According to the location of the monitoring section of the concrete tower, calculate the initial compressive strain generated by the self-weight of the upper concrete tower and the external prestress at the monitoring section of the concrete tower, that is, the initial compressive strain of the concrete section -ε0;

[0055] S43: Calculate the ultimate compressive strain ε of the concrete section based on the tower concrete design strength cu .

[0056] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, the specific process of step S5 is as follows:

[0057] S51: Monitoring and early warning of cable frequency; collecting and analyzing the signals detected by all lateral acceleration sensors, and calculating the i-th order frequency f of the cable i and the frequency monitoring warning lower limit f min For comparison, f i Less than f min , then the alarm is triggered immediately;

[0058] S52: Real-time monitoring and early warning of concrete strain;

[0059] In real-time monitoring of concrete strain, the initial compressive strain of the concrete section -ε0 is considered, and the actual strain of the concrete The relationship with the real-time test strain value ε is as follows:

[0060] (15)

[0061] In the formula, the real-time test strain value ε is recorded according to the actual measurement result of the strain gauge, and the actual strain of concrete is A positive value indicates tensile stress, and a negative value indicates compressive stress;

[0062] By taking the actual strain of concrete The ultimate compressive strain of the concrete section Compare to achieve real-time monitoring and early warning of concrete strain;

[0063] When the fan is in a windless shutdown state, the concrete preload loss can be judged by checking whether the dynamic monitoring curve exceeds the warning value; if there is cable force loss, the dynamic monitoring curve of strain will shift upward significantly under windless shutdown conditions, and the upward shift is the concrete preload loss strain. ; To this end, the loss strain limit Set as warning value, Press preload N p 10% of the calculation is determined, specifically:

[0064] (16)

[0065] In the formula, E c To calculate the elastic modulus of concrete at the cross section;

[0066] Will and For comparison, if > Then an alarm is given to shut down the machine;

[0067] For fans in the wind-receiving operation state, ≈0, it indicates that the concrete tower is in a decompression state; > When , it indicates that the concrete tower is in the compressive limit state; when < When , it indicates that the concrete tower is in normal condition.

[0068] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S52, when ≈0, the edge of the tensile zone of the concrete section is in a decompression state, and the f i and with 0.95f 1, minFor comparison, if f i ≥0.95 f 1, min , then it is normal; if f i <0.95 f 1,min , then an early warning is issued.

[0069] In the above-mentioned wind turbine concrete tower safety monitoring, early warning and assessment method, in step S52, when > When the concrete tower is in the compressive limit state, the synchronous monitoring strain data is retrieved and the minimum value of the measured strain ε is taken. min , change -ε0+ε min With ε cu Compare, if -ε0+ε min Greater than ε cu Then an alarm will be issued and the machine will be shut down.

[0070] The beneficial effects of the present invention are as follows: based on the stress characteristics of the concrete tower structure of a wind turbine generator set, the present invention proposes an on-site monitoring, early warning and evaluation scheme combining the cable frequency and concrete strain of the concrete tower under operating conditions. The cable frequency and concrete strain are combined to achieve accurate early warning and evaluation of cable force loss and concrete ultimate strain, thereby effectively ensuring the safety of the concrete tower structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 It is the overall flow chart of the present invention.

[0072] Figure 2 Schematic diagram of stress analysis of concrete tower section under normal operating load conditions.

[0073] Figure 3 This is the stress state diagram of the compression relief and cracking in the tensile zone of the concrete section.

[0074] Figure 4 Schematic diagram of the change of prestressed cable stress with load.

[0075] Figure 5 Schematic diagram of the accelerometer and strain gauge layout.

[0076] Figure 6 for Figure 5 Top view of the .

[0077] Figure 7 This is a schematic diagram of frequency method monitoring.

[0078] Figure 8 Schematic diagram of concrete strain monitoring and early warning.

[0079] Fig. 9 Schematic diagram of concrete strain monitoring and analysis considering prestress loss. DETAILED DESCRIPTION

[0080] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0081] like Figure 1 As shown, a wind turbine concrete tower safety monitoring, early warning and assessment method comprises the following steps:

[0082] Step S1: Based on the stress mechanism of the concrete tower, analyze the concrete tower at different stress stages.

[0083] like Figure 2 As shown in the figure, the stress mechanism of the concrete tower is:

[0084] After the prestress is applied to the concrete tower, the normal compressive stress of the concrete in the cross section of the concrete tower is pc for:

[0085] (1)

[0086] (2)

[0087] (3)

[0088] In the formula, A n is the net cross-sectional area of ​​concrete; I n is the converted net section inertia moment; e pn The distance from the center of gravity of the converted net section to the prestressing point; y n is the distance from the centroid of the converted net section to the calculated position; σ p2 is the normal stress of the concrete section caused by the secondary internal force of the prestress. Considering that the concrete tower is a cantilever statically determinate structure, σ p2 =0; G is the total weight of the tower above the calculation section; N p is the cable pretension; con A is the stress control for prestressing tendons; s is the cross-sectional area of ​​all longitudinal ordinary steel bars; σ pe is the effective prestress of the prestressed tendons; A p is the cross-sectional area of ​​all longitudinal prestressed tendons; σ l is the total loss value of prestressed tendons; σ l5 is the prestress loss value caused by concrete shrinkage and creep at the resultant point of the prestressed tendons; It represents the eccentric bending moment caused by the pre-tension of the cable. The result of the bending moment on the cross section is that one side is compressed and the other side is tensile, and the tension and compression are expressed as positive and negative signs;

[0089] The prestressed tendons in the concrete tower are arranged symmetrically, so N p epn =0, so formula (1) is simplified to:

[0090] (4)

[0091] The tensile edge stress σ of the concrete section with the first crack control level under the standard combination of normal operating load conditions of the wind turbine ck Comply with the following provisions:

[0092] (5)

[0093] where σ ck Calculate as follows:

[0094] (6)

[0095] Where r is the outer diameter of the tower at the calculated section, M k Calculate the cross-sectional bending moment of the concrete tower under normal operating load conditions, which is determined by the host manufacturer;

[0096] Substituting equations (4) and (6) into equation (5), we can obtain:

[0097] (7)

[0098] Therefore, under the first level of crack control, the cable pre-tension meets the following requirements:

[0099] (8)

[0100] Therefore, under normal operating load conditions, the lower limit of the cable pretension for the first crack control level is for:

[0101] (9)

[0102] The cable force measurement is obtained by the frequency method. The relationship between the cable force N and the cable frequency f is as follows:

[0103] (10)

[0104] Where m is the mass per unit length of the cable; l is the length from the tensioning end to the anchorage end; f n is the nth-order frequency of the cable; n is the modal order, and in actual engineering, the first-order frequency f1 of the cable is taken;

[0105] N p,min Substituting into formula (10), the lower limit of the first-order frequency f of the cable under normal operating load conditions is determined 1, min for:

[0106] (11).

[0107] like Figure 3 As shown, the analysis of the concrete tower at different stress stages includes the analysis of the decompression state of the tensile zone of the concrete section, the cracking state of the tensile zone of the concrete section and the change of the cable force under different stress states.

[0108] The process of analyzing the decompression state of the tensile zone of the concrete section is as follows:

[0109] As the external load gradually increases, the edge pressure of the tensile zone of the concrete section will gradually drop to 0, entering the decompression state of the tensile zone. Figure 3 As shown in the middle left figure; at this time, the concrete tower calculation section bending moment reaches the edge pressure relief bending moment M c0 , according to formula (7):

[0110] (12)

[0111] As can be seen from the above formula, if there is N p loss, does not meet the design requirements, then M c0 will decrease accordingly, making M c0 ≤M k , the edge of the tensile zone of the concrete tower will enter the decompression state or even the tensile state prematurely.

[0112] At this time, by monitoring the edge strain of the concrete tensile zone and the cable frequency, it can be determined through calculation and analysis whether the edge of the concrete tensile zone is in a decompression or tension state under normal operation, and whether the measured cable frequency is less than f 1,min , it can be determined whether there is cable prestress loss.

[0113] The process of analyzing the cracking state of the tensile zone of the concrete section is as follows:

[0114] When the wind turbine exceeds the normal operating condition, the external load continues to increase, and the tensile zone of the concrete section will enter a cracking state. When the edge of the tensile zone of the concrete section reaches the ultimate tensile strain, the cracking moment M is reached. cr ; Due to the plastic deformation of concrete under tension, the stress diagram of the tensile zone of the concrete section is distributed in a curve. In order to continue to apply the elastic calculation formula, according to the moment equilibrium condition, the stress diagram of the tensile zone of the concrete section is equivalent to a triangular distribution, and the equivalent tensile stress of the tensile bottom edge is taken as γf tk , at this time, the force on the concrete section is expressed as:

[0115] (13)

[0116] Where γ is the plastic influence coefficient of the section resistance moment of the concrete member, , h is the section height (mm), when h<400, take h=400, when h>1600, take h=1600, r l is the inner diameter of the annular section, f tk is the standard value of concrete tensile strength;

[0117] Similar to the decompression state, if there is N p loss, then M cr As it decreases, the edge of the tensile zone of the concrete tower will enter the tensile cracking state prematurely; considering that the strain value of concrete after cracking is quite different, the cracking state is not suitable as a monitoring state.

[0118] Under the ultimate bearing capacity stress state, like conventional prestressed concrete components, the pretension value of the cable has little effect on the ultimate bearing capacity of the concrete components, so it will not be discussed in detail.

[0119] The process of analyzing the change of cable force under different stress states is as follows:

[0120] Within normal operating load conditions, the cable force changes linearly with the external load, meeting the accuracy requirements of the frequency method test; when the load continues to increase to the cracking state, the cable force will surge as the concrete in the tension zone stops working; after cracking, the cable force increases significantly with the increase in load until the limit state.

[0121] like Figure 4 As shown, point A represents the failure stress f of the prestressed cable itself. pt In the normal operating load condition CD, the prestressed cable force changes linearly with the external load, and the increase is only within 5%, which meets the test accuracy requirements of the frequency method; when it continues to increase to the cracking load point E (i.e. cracking state), as the concrete in the tension zone stops working, the cable force will suddenly change, causing point E to rise to point F; after cracking, the speed at which the cable force increases with the load increases significantly until the limit state point G. At this stage, because the cable stress changes with the external load to a large extent, it is not suitable for the frequency method.

[0122] If there is a large prestress loss in the cable, not only is the starting point C′ lower, but D′E′F′G′ also move forward significantly. In the cracking stage of the concrete tower, F′G′ is also longer than FG. The cable force changes significantly due to external loads, resulting in an increase in the test error of the single frequency method. It is necessary to combine the strain monitoring results of the concrete section edge for joint analysis.

[0123] Taking into account that the axial force of the cable changes due to external loads, and the frequency method is an indirect method, it has the disadvantage of not being able to respond to the axial force of the cable in real time. In actual engineering, the cable force variation must be within 5%. Therefore, it is suitable for cable force testing under normal operating conditions of wind turbines. However, it is not applicable to post-cracking and extreme conditions where the cable force variation is large. At this time, the strain monitoring results of the concrete compression zone can be combined to conduct real-time monitoring, early warning and alarm of the concrete tower safety.

[0124] Step S2: By collecting data and conducting on-site inspections, the main wind direction and the position of the lowest frequency cable are determined as the monitoring position.

[0125] By collecting data and conducting on-site tests, the windward and leeward directions of the main wind direction and the directions of the two cables with the lowest detection frequencies are determined, a total of four directions as monitoring directions. If there is any overlap in the range, the number of monitoring directions can be reduced accordingly.

[0126] Step S3: Arrange the lateral acceleration sensor and the strain gauge at the monitoring position.

[0127] like Figure 5 , Figure 6 As shown, 1 is the prefabricated cylinder wall, 2 is the lateral acceleration sensor, 3 is the strain gauge, 4 is the cable, and 5 is the horizontal joint. Figure 6 The middle arrow indicates the prevailing wind direction.

[0128] Step S4: Determine the warning value of the monitored concrete tower, including the lower limit of the cable frequency f min , the initial compressive strain of the concrete section -ε0 and the ultimate compressive strain of the concrete section ε cu .

[0129] The process of determining the early warning value of the monitored concrete tower is as follows:

[0130] S41: Based on the design data of the wind turbine, calculate the lower limit of the first-order frequency f of the cable in the concrete tower section under the decompression state. 1,min ; Comprehensively monitor the initial measured cable frequency f0 of the cable with the lowest measured cable frequency, and consider the cable force changes under the operating state to determine the frequency monitoring warning lower limit f min :

[0131] f min =min{0.95f0, 0.95f 1, min} (14)

[0132] The reason why f0 is involved in jointly determining f min , mainly because f 1, min is a theoretical value, which may have a large deviation from f0; secondly, in some projects, f 1, min Cannot be calculated.

[0133] S42: According to the location of the monitoring section of the concrete tower, calculate the initial compressive strain generated by the self-weight of the upper concrete tower and the external prestress at the monitoring section of the concrete tower, that is, the initial compressive strain of the concrete section -ε0;

[0134] S43: Calculate the ultimate compressive strain ε of the concrete section based on the tower concrete design strength cu .

[0135] Step S5: The acceleration and strain signals are collected in real time by the lateral acceleration sensor and the strain gauge, and input into the vibration signal acquisition instrument and computer analysis to calculate the first-order frequency f1 of the cable and the actual strain of the concrete. And compare it with the warning value to achieve safety monitoring and early warning of the concrete tower of the wind turbine.

[0136] The specific process of step S5 is as follows:

[0137] S51: Monitoring and early warning of cable frequency; collecting and analyzing the signals detected by all lateral acceleration sensors, and calculating the i-th order frequency f of the cable i and the frequency monitoring warning lower limit f min For comparison, f i Less than f min , then the alarm is triggered immediately, such as Figure 7 As shown, Figure 7 The middle Q is the safe zone, and the f in the safe zone i For safety i , Q' is the alarm area, and f in the alarm area i To alarm i .

[0138] S52: Real-time monitoring and early warning of concrete strain;

[0139] The initial compressive strain of the concrete section -ε0 and the actual strain of the concrete should be considered in real-time monitoring of concrete strain. The relationship with the real-time test strain value ε is as follows:

[0140] (15)

[0141] In the formula, the real-time test strain value ε is recorded according to the actual measurement result of the strain gauge, and the actual strain of concrete is A positive value indicates tensile stress, and a negative value indicates compressive stress.

[0142] By taking the actual strain of concrete The ultimate compressive strain of the concrete section Comparison is made to achieve real-time monitoring and early warning of concrete strain.

[0143] When the fan is in a windless shutdown state, the concrete preload loss can be judged by checking whether the dynamic monitoring curve exceeds the warning value; if there is cable force loss, the dynamic monitoring curve of strain will shift upward significantly under windless shutdown conditions, and the upward shift is the concrete preload loss strain. ; To this end, the loss strain limit Set as warning value, Press preload N p 10% of the calculation is determined, specifically:

[0144] (16)

[0145] In the formula, E c To calculate the elastic modulus of concrete at the cross section;

[0146] like Fig. 9 As shown, Fig. 9 The solid line in the middle represents the initial state, and the dotted line represents the state after the pre-pressure loss. and For comparison, if > Then an alarm will be given and the machine will be shut down.

[0147] For a fan in the wind-operated state (not static state), ≈0, it indicates that the concrete tower is in a decompression state; > When , it indicates that the concrete tower is in the compressive limit state; when < When , it indicates that the concrete tower is in normal condition.

[0148] when ≈0, the edge of the tensile zone of the concrete section is in a decompression state, and the f i and with 0.95f 1, min For comparison, if f i ≥0.95 f 1, min , then it is normal; if f i <0.95 f 1,min , an early warning is issued. > When the concrete tower is in the compressive limit state, the synchronous monitoring strain data is retrieved and the minimum value of the measured strain ε is taken. min , change -ε0+ε min With ε cu Compare, if -ε0+ε min Greater than ε cu If the alarm is turned off, Figure 8 shown.

[0149] Step S6: Determine the concrete preload loss strain ε', the annual minimum actual strain of concrete, the annual maximum actual strain of concrete, and the lower limit of the first-order frequency of the cable f according to the annual monitoring data 1，min And compare and analyze with the corresponding data of the previous year to achieve an annual assessment of the safe operation of wind turbine concrete towers.

Claims

1. A wind turbine concrete tower safety monitoring, early warning and assessment method, characterized in that: The following steps are involved: Step S1: Based on the stress mechanism of the concrete tower, analyze the concrete tower at different stress stages; In step S1, the stress mechanism of the concrete tower is: After the prestress is applied to the concrete tower, the normal compressive stress of the concrete in the cross section of the concrete tower is pc for: N p =s pe A p -s l5 A s (2) s pe =s con -s l (3) In the formula, A n is the net cross-sectional area of ​​concrete; I n is the converted net section inertia moment; e pn y is the distance from the center of gravity of the converted net section to the point of action of the pretension force; n is the distance from the centroid of the converted net section to the calculated position; σ p2 is the normal stress of the concrete section caused by the secondary internal force of the prestress. Considering that the concrete tower is a cantilever statically determinate structure, σ p2 =0; G is the total weight of the tower above the calculation section; N p is the cable pretension; con A is the stress control for prestressing tendons; s is the cross-sectional area of ​​all longitudinal ordinary steel bars; σ pe is the effective prestress of the prestressed tendons; A p is the cross-sectional area of ​​all longitudinal prestressed tendons; σ l is the total loss value of prestressed tendons; σ l5 is the prestress loss value caused by concrete shrinkage and creep at the resultant point of the prestressed tendons; It represents the eccentric bending moment caused by the pre-tension of the cable. The result of the bending moment on the cross section is that one side is compressed and the other side is tensile, and the tension and compression are expressed as positive and negative signs; The prestressed tendons in the concrete tower are arranged symmetrically, so N p e pn =0, so formula (1) is simplified to: The tensile edge stress σ of the concrete section with the first crack control level under the standard combination of normal operating load conditions of the wind turbine ck Comply with the following provisions: s ck -s pc ≤0 (5) where σ ck Calculate as follows: Where r is the outer diameter of the tower at the calculated section, M k Calculate the cross-sectional bending moment of the concrete tower under normal operating load conditions, which is determined by the host manufacturer; Substituting equations (4) and (6) into equation (5), we can obtain: Therefore, under the first level of crack control, the cable pre-tension meets the following requirements: Therefore, under normal operating load conditions, the lower limit of the cable pretension for the first crack control level is N p,min for: The cable force measurement is obtained by the frequency method. The relationship between the cable force N and the cable frequency f is as follows: Where m is the mass per unit length of the cable; l is the length from the tensioning end to the anchorage end; f n is the nth-order frequency of the cable; n is the modal order, and in actual engineering, the first-order frequency f1 of the cable is taken; N p,min Substituting into formula (10), the lower limit of the first-order frequency f of the cable under normal operating load conditions is determined 1,min for: In step S1, the analysis of the concrete tower at different stress stages includes analyzing the cable forces in the tensile zone of the concrete section, the cracking state of the tensile zone of the concrete section, and the changes in the cable forces under different stress states. The process of analyzing the cable forces in the tensile zone of the concrete section is as follows: As the external load gradually increases, the edge pressure of the tensile zone of the concrete section will gradually drop to 0, entering the tension zone decompression state. At this time, the calculated section bending moment of the concrete tower reaches the edge decompression bending moment M. c0 According to formula (7), we can get: As can be seen from the above formula, if there exists N p loss, does not meet the design requirements, then M c0 will decrease accordingly, making M c0 ≤M k , the edge of the tensile zone of the concrete tower will enter the decompression state or even the tensile state prematurely; At this time, by monitoring the edge strain of the concrete tensile zone and the cable frequency, it is determined through calculation and analysis whether the edge of the concrete tensile zone is in a decompression or tension state under normal operation, and at the same time, combined with whether the measured cable frequency is less than f 1,min , it can be determined whether there is cable prestress loss; Step S2: by collecting data and conducting on-site inspections, determine the main wind direction and the position of the lowest frequency cable as the monitoring position; Step S3: arranging a lateral acceleration sensor and a strain gauge at the monitoring position; Step S4: Determine the warning value of the monitored concrete tower, including the lower limit of the cable frequency f min , the initial compressive strain of the concrete section -ε0 and the ultimate compressive strain of the concrete section ε cu ; Step S5: The acceleration and strain signals are collected in real time by the lateral acceleration sensor and the strain gauge, and input into the vibration signal acquisition instrument and computer analysis to calculate the first-order frequency f1 of the cable and the actual strain ε of the concrete. 实际 , and compare it with the warning value to achieve safety monitoring and early warning of the concrete tower of the wind turbine; Step S6: Determine the concrete preload loss strain ε', the annual minimum actual strain of concrete, the annual maximum actual strain of concrete, and the lower limit of the first-order frequency of the cable f according to the annual monitoring data 1，min And compare and analyze with the corresponding data of the previous year to achieve an annual assessment of the safe operation of wind turbine concrete towers.

2. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 1 is characterized in that: In step S1, the process of analyzing the cracking state of the tensile zone of the concrete cross section is as follows: When the wind turbine exceeds the normal operating condition, the external load continues to increase, and the tensile zone of the concrete section will enter a cracking state. When the edge of the tensile zone of the concrete section reaches the ultimate tensile strain, the cracking moment M is reached. cr ; Due to the plastic deformation of concrete under tension, the stress diagram of the tensile zone of the concrete section is distributed in a curve. In order to continue to apply the elastic calculation formula, according to the moment equilibrium condition, the stress diagram of the tensile zone of the concrete section is equivalent to a triangular distribution, and the equivalent tensile stress of the tensile bottom edge is taken as γf tk , at this time, the force on the concrete section is expressed as: Where γ is the plastic influence coefficient of the section resistance moment of the concrete member, h is the section height, r l is the inner diameter of the annular section, f tk is the standard value of concrete tensile strength; Similar to the cable force, if there is N p loss, then M cr As it decreases, the edge of the tensile zone of the concrete tower will enter the tensile cracking state prematurely; considering that the strain value of concrete after cracking is quite different, the cracking state is not suitable as a monitoring state.

3. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 2 is characterized in that: In step S1, the process of analyzing the cable force changes under different stress states is as follows: In the normal operating load condition, the cable force changes linearly with the external load, meeting the accuracy requirements of the frequency method test; when the load continues to increase to the cracking state, the cable force will surge as the concrete in the tension zone stops working; After cracking, the cable force increases significantly with the increase of load until it reaches the limit state.

4. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 1 is characterized in that: In step S2, by collecting data and conducting on-site tests, the windward and leeward directions of the main wind direction and the directions of the two cables with the lowest detection frequencies are determined, a total of four directions as monitoring directions.

5. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 3 is characterized in that: In step S4, the process of determining the early warning value of the monitored concrete tower is as follows: S41: Based on the design data of the wind turbine, calculate the lower limit of the first-order frequency f of the cable in the concrete tower section under the decompression state. 1,min ; Comprehensively monitor the initial measured cable frequency f0 of the cable with the lowest measured cable frequency, and consider the cable force changes under the operating state to determine the frequency monitoring warning lower limit f min : f min =min{0.95f0, 0.95f 1,min } (14) S42: According to the location of the monitoring section of the concrete tower, calculate the initial compressive strain generated by the self-weight of the upper concrete tower and the external prestress at the monitoring section of the concrete tower, that is, the initial compressive strain of the concrete section -ε0; S43: Calculate the ultimate compressive strain ε of the concrete section based on the tower concrete design strength cu .

6. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 5 is characterized in that: The specific process of step S5 is as follows: S51: Monitoring and early warning of cable frequency; collecting and analyzing the signals detected by all lateral acceleration sensors, and calculating the i-th order frequency f of the cable i and the frequency monitoring warning lower limit f min For comparison, f i Less than f min , then the alarm is triggered immediately; S52: Real-time monitoring and early warning of concrete strain; In real-time monitoring of concrete strain, the initial compressive strain of the concrete section -ε0 and the actual strain of the concrete ε 实际 The relationship with the real-time test strain value ε is as follows: e 实际 =-ε0+ε (15) In the formula, the real-time test strain value ε is recorded according to the actual measurement result of the strain gauge, and the actual strain of concrete ε is 实际 A positive value indicates tensile stress, and a negative value indicates compressive stress; By taking the actual concrete strain ε 实际 and the ultimate compressive strain ε of the concrete section cu Compare and realize real-time monitoring and early warning of concrete strain; When the fan is in a windless and stationary state, the dynamic monitoring curve is retrieved to see if it exceeds the warning value to determine whether the concrete has preload loss; If there is a loss of cable force, the dynamic monitoring curve of strain will shift upward significantly under windless shutdown conditions. The upward shift is the concrete preload loss strain ε′. Therefore, the upper limit of the loss strain ε′ is set as max Set as the warning value, ε′ max Press N p The 10% calculation is as follows: In the formula, E c To calculate the elastic modulus of concrete at the cross section; Releasing ε′ and ε′ max Compare, if ε′>ε′ max Then an alarm is given to shut down the machine; For a fan in the wind-receiving operation state, when ε 实际 ≈0, it indicates that the concrete tower is in a decompression state; when ε 实际 >ε cu When ε 实际 <ε cu When , it indicates that the concrete tower is in normal condition.

7. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 6 is characterized in that: In step S52, when ε 实际 ≈0, the edge of the tensile zone of the concrete section is in a decompression state, and the f i and with 0.95f 1,min For comparison, if f i ≥0.95f 1,min , then it is normal; if f i <0.95f 1,min , then an early warning is issued.

8. The wind turbine concrete tower safety monitoring, early warning and assessment method according to claim 6 is characterized in that: In step S52, when ε 实际 >ε cu When the concrete tower is in the compressive limit state, Retrieve the synchronous monitoring strain data and take the minimum value of the measured strain ε min , change -ε0+ε min With ε cu Make a comparison, If -ε0+ε min Greater than ε cu Then an alarm will be given and the machine will be shut down.

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