A multi-dimensional stress analysis method for a substation framework after icing

Abstract

The application discloses a kind of multi-dimensional stress analysis methods after substation framework icing, comprising the following steps: establishing the three-dimensional finite element model of substation framework, obtains the icing monitoring data and microclimate data of substation site, executes icing growth and frost heaving stage analysis, executes icing stable load stage analysis, executes ice-melting unloading transient stage analysis, based on the analysis result of above-mentioned step, the safety margin of overall framework and each component is comprehensively evaluated, and outputs graded early warning signal and corresponding reinforcement or deicing control strategy.The application introduces the equivalent tension boundary condition of conductor icing, decouples the icing process into three stages of growth frost heaving, stable load and ice-melting unloading, and uses the method of spatial distribution coefficient matrix based on CFD precalculation, so that the analysis boundary is more in line with engineering practice, the risk identification is more comprehensive, the load mapping method is scientific and reasonable, the ice-melting unloading dynamic analysis is close to physical reality, and the operable operation and maintenance decision can be output.

Description

Technical Field

[0001] This invention relates to the field of power engineering structural safety assessment technology, and in particular to a multi-dimensional stress analysis method for substation structures after icing. Background Technology

[0002] The substation frame is the core load-bearing structure supporting high-voltage electrical equipment such as busbars and disconnectors, and its safety directly affects the power supply reliability of the regional power grid. In recent years, affected by abnormal global climate, extreme weather such as freezing rain and blizzards have frequently occurred in southern and central-eastern my country, leading to a significant increase in substation frame icing accidents. For example, in early 2025, the measured icing thickness in Zhejiang Province reached 15-20 mm, far exceeding the original design standard of 10 mm; in Xilingol League, a late spring cold snap and blizzards with strong winds caused severe damage to the frames and equipment of many substations. Currently, the structural safety assessment of substation structures covered by ice mainly relies on static verification of icing conditions in standards such as the "Design Code for Disaster Prevention and Mitigation of Transmission and Transformation Engineering" (Q / GDW 12667.1-2025). The typical process of this method is as follows: apply the icing load and wind load to the structural finite element model according to the combination coefficients specified in the code, perform linear or nonlinear static analysis, and determine the structural safety based on the stress ratio. However, engineering practice and accident investigations have shown that the above-mentioned traditional static verification methods have the following three significant shortcomings: I. Completely ignoring the tension coupling effect of conductor icing on the substation structure. In actual operation, the substation structure bears significant horizontal tension from conductors and down conductors. When conductors become icy, their weight per unit length increases dramatically, and the low-temperature contraction effect causes a sharp rise in conductor tension. This additional tension is directly transmitted to the structure through the hanging points, and is one of the key factors causing excessive stress at the end nodes of the structure's crossbeams, or even overall instability of the crossbeams. Existing methods completely decouple the structure from the conductors, leading to severely distorted boundary conditions in the analysis. Even if the stress ratio check of the structure itself is "qualified," damage may still occur in actual operation due to unexpected conductor tension. Second, the mechanical behavior of the entire icing process is not fully characterized. Existing methods only focus on the static state when icing reaches its peak, failing to consider the gradual stiffness degradation of the structure during the accumulation of icing from scratch, nor the transient dynamic impact effects that may be triggered by the melting and detachment of ice. Accident statistics show that some structural damage does not occur when the ice is at its thickest, but rather during the melting stage when temperatures rise and the ice layer rapidly detaches—at which point the elastic strain energy stored in the structure is suddenly released, potentially producing a dynamic amplification effect far exceeding the results of static analysis. Third, there is a lack of targeted analysis of the local damage mechanisms at the node level. Substation structures mostly use semi-rigid nodes connected by bolts. During icing, meltwater seeps into the bolt gaps and freezes, causing frost heave that can lead to local plastic deformation of the node plate or a decrease in bolt preload. Existing static verification methods simplify nodes to rigid or hinged connections, completely failing to capture such local damage, resulting in many typical hidden dangers such as "node bulges" and "loose bolts" being missed in the assessment. In summary, there is an urgent need to propose a method for analyzing the icing stress of substation structures that can accurately reflect the actual stress boundaries of the structure, cover the multi-stage mechanical behavior of the entire icing process, and identify local damage at nodes. Summary of the Invention

[0003] To address the above problems, this invention provides a multi-dimensional stress analysis method for substation structures after icing, comprising the following steps: Step S1: Establish a three-dimensional finite element model of the substation frame. The model includes the frame body structure, node connection construction and foundation constraints. Apply static tension load boundary conditions equivalent to conductor icing at the frame hanging points to reflect the coupling effect of conductor icing on the frame. Step S2: Obtain icing monitoring data and micro-meteorological data at the substation site, and based on the pre-calculated computational fluid dynamics (CFD) analysis results, map the measured icing thickness at a single point to the overall icing thickness distribution of each member of the frame through the spatial distribution coefficient matrix. Step S3: Perform the analysis of ice growth and frost heave stages. Using the mapped full-field ice thickness as input, perform quasi-static progressive loading nonlinear solution on the finite element model to calculate the stress, displacement and stability of the frame under each load step, and simultaneously evaluate the local damage state of the bolted connection nodes under the action of water-ice phase change frost heave force. Step S4: Perform the icing stability and load-bearing stage analysis. Using the structural stress state and nodal frost heave damage state output in Step S3 as initial conditions, evaluate the overall stability margin of the frame, the degree of bolt preload attenuation, and the safety of the foundation under long-term eccentric load under the action of icing peak constant load. Step S5: Perform transient phase analysis of ice melting and de-icing. Using the structural state updated in step S4 as the initial condition, establish the load-time history curve of block-by-block progressive ice melting according to the preset ice melting mode, and use the dynamic finite element method to calculate the dynamic amplification factor, transient stress peak and residual displacement of the frame during the ice melting process. Step S6: Based on the analysis results of steps S3, S4 and S5, a comprehensive assessment of the safety margin of the overall structure and each component is conducted. When the safety margin of any stage is lower than the preset threshold, a graded early warning signal and the corresponding reinforcement or de-icing control strategy are output. Furthermore, the static tension load boundary conditions applied at the frame hanging points in step S1 are obtained through the following steps: Based on the conductor design parameters, measured values ​​of conductor icing thickness, and ambient temperature, the horizontal tension increment of the conductor under icing and low-temperature conditions is solved using the conductor state equation. The horizontal tension increment is applied to the nodes corresponding to the conductor suspension points in the finite element model of the framework in the form of equivalent nodal concentrated forces. The conductor state equation adopts the following form: In the formula, σ n σ m The horizontal stress of the conductor under the unknown and known operating conditions, γ, are respectively. n γ m The specific load of the conductor under the corresponding working condition is given by l, where l is the span, E is the elastic modulus of the conductor, α is the coefficient of thermal expansion of the conductor, and t is the conductor's specific load. n t m This refers to the corresponding operating temperature. Furthermore, the spatial distribution coefficient matrix pre-calculated based on CFD in step S2 is generated in the following way: During the substation design or evaluation preparation phase, CFD software is used to simulate the spatial distribution of supercooled water droplet collision efficiency under the prevailing wind direction and design wind speed, and to calculate the local collision coefficient β on the surface of each member of the frame. i The collision coefficients of all members are then normalized to obtain the spatial distribution coefficient k of each member. CFD,i During the real-time analysis phase, the measured ice thickness d at the monitoring point will be... meas As a baseline value, the calculated icing thickness of each member is determined by the following formula: Furthermore, the evaluation of the local damage state of the bolted connection node under the frost heave force of water-ice phase change in step S3 specifically includes: Based on the on-site bolt gap inspection results or empirical parameters, determine the characteristic width of the joint gap and calculate the frost heave pressure F generated by the water-ice phase change. frost : In the formula, E ice Let ε be the elastic modulus of ice. exp A is the volume expansion rate of water-ice phase transition. gap The area of ​​the gap under pressure; The frost heave pressure is equivalent to the additional surface pressure acting on the normal direction of the gusset plate, and is applied to the corresponding node region in the finite element model to calculate the local stress state of the gusset plate under the combined action of frost heave force and external ice load. When the local stress exceeds the yield strength of the node plate material, the node is determined to have suffered frost heave plastic damage, and the connection stiffness of the node is reduced accordingly in the state inheritance of step S4. Furthermore, the assessment of the bolt preload attenuation in step S4 is achieved through at least one of the following methods: Method 1: Install ultrasonic bolt axial force sensors at key bolt connection nodes of the frame to monitor the change curve of preload force over time during load holding in real time. When the measured preload force decays to a preset proportion of the design preload force, an early warning is triggered. Method 2: In the absence of real-time monitoring data, the stiffness of the node connection is corrected according to the duration of icing load and the temperature cycle amplitude, based on a preset empirical reduction factor η, wherein the reduction factor η ranges from 0.80 to 0.90. Furthermore, the preset de-icing mode in step S5 includes at least one of the following: Mode A: The leeward side detaches first, followed by the windward side, simulating the uneven unloading process of melting ice dominated by solar radiation; Mode B: Segmented detachment from top to bottom, simulating the unloading process of ice layer sliding down segment by segment under the action of gravity when the temperature rises above freezing point; Mode C: The windward side detaches first, followed by the leeward side, simulating the unbalanced unloading process of the wind blowing off the ice layer on the windward side under conditions of strong wind and rising temperature. Mode D: Synchronous shedding across the entire cross section, used as the most unfavorable envelope condition for verification in extremely conservative designs. Furthermore, the establishment of the load-time history curve for segmented gradual ice removal in step S5 specifically includes: The ice layer on the surface of each member of the frame is discretized into multiple independent ice body units along the length of the member, and each ice body unit is assigned an independent unit mass attribute. Based on the selected de-icing mode, assign corresponding "life and death time" parameters to each ice element. The ice element is active before the de-icing time and provides mass and load contribution. After the de-icing time, it is "killed" and removed from the model. The transient dynamic response of the de-icing process was determined using either the explicit central difference method or the implicit Hilber-Hughes-Taylor time integration method, with the steel structure damping ratio set to 0.02~0.04 during the solution process. Furthermore, the rule for setting the safety margin threshold in step S6 is as follows: A Level 1 warning is triggered when the stress ratio of any component exceeds 0.90 during the icing growth stage. A level two warning is triggered when the stress ratio of any component exceeds 0.95 during the ice growth stage, or when the bolt preload decreases by more than 25% of the design value during the load-bearing stage. A Level 3 warning is triggered when the stress ratio of any component at any stage exceeds 1.00, or the stability coefficient is lower than 1.00, or the dynamic amplification factor exceeds 2.0 and the transient stress exceeds the material yield strength during the ice-breaking transient stage. Level 1 warning outputs the "strengthen inspection" command, Level 2 warning outputs the "prepare de-icing equipment" command, and Level 3 warning outputs the "immediately carry out de-icing operations or temporarily suspend operation" command. Furthermore, in step S3, the quasi-static progressive loading nonlinear solution uses the arc length method to track the stiffness degradation path of the structure during the increasing ice load process; when the solution reaches the point where the structural stiffness matrix is ​​singular or the arc length method cannot converge, it is determined that the limit ice thickness state has been reached. Furthermore, in step S1, the refined three-dimensional finite element model is used to simulate the bolt connection node using spring elements that consider semi-rigid characteristics. The initial rotational stiffness of the spring element is calibrated by experimental or empirical formulas based on the node structure and bolt preload. In step S4, the stiffness parameters of the spring element are dynamically updated according to the degree of preload attenuation. Compared with the prior art, the beneficial effects of the present invention are: First, the analysis boundary is more in line with engineering practice. By introducing the equivalent tension boundary condition of conductor icing, the load distortion caused by the complete decoupling of the structure and conductor in existing methods is compensated for, and the accuracy of stress analysis is greatly improved. II. Comprehensive Coverage for More Thorough Risk Identification. The icing process is decoupled into three stages: frost heave growth, stable load holding, and thawing and shedding. This not only assesses the static state at the peak of icing but also captures the localized node damage caused by frost heave and the dynamic impact effects of transient shedding, filling the gap in existing methods for assessing risks during the shedding stage. Third, the load mapping method is scientific and reasonable. Based on the spatial distribution coefficient matrix pre-calculated by CFD, the scale mismatch between the ice thickness at the monitoring points and the ice thickness distribution of all members in the field is effectively solved, enabling real-time monitoring data to accurately drive fine finite element analysis. IV. The de-icing dynamic analysis closely reflects physical reality. A segmented, progressive de-icing model is adopted instead of the idealized assumption of instantaneous full-section unloading, and reasonable structural damping is introduced to avoid overestimation of the dynamic amplification factor. The output evaluation conclusions are more valuable for engineering reference. V. Outputting actionable operation and maintenance decisions. Based on a three-level early warning system with quantified safety margins and corresponding reinforcement and de-icing strategy recommendations, the system provides substation operation and maintenance personnel with clear and actionable operational guidelines, achieving an effective connection from "calculation conclusions" to "operation and maintenance actions". Detailed Implementation This invention is applicable to steel structure frames (including A-type columns, gable columns, portal frames, lattice frames, etc.) of substations with voltage levels of 110kV and above. The analysis scope is limited to the main structure of the frame and does not include detailed modeling of electrical equipment such as conductors and insulator strings. However, the tension effect of conductors after icing is included as a boundary condition of the load at the frame hanging point in the analysis. The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. In the description of the embodiments, unless otherwise explicitly specified and limited, the terms "set," "connect," etc., should be interpreted broadly. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or a connection through an intermediate medium, or it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. Example 1: Safety Assessment of A-frame Structure in 220kV Substation After Icing 1. Project Overview A 220kV outdoor substation uses a herringbone steel pipe structure for its outgoing line frame, with a beam span of 15m and a column height of 12m. The main components of the frame are made of Q235B steel, with a design icing thickness of 10mm and a design wind speed of 25m / s. Bolted connections use 10.9 grade M24 high-strength bolts. In mid-January 2025, the area where the substation is located experienced approximately 48 hours of freezing rain. The on-site microwave icing monitoring device measured an ice thickness of 16mm at the crossbeam, with the ambient temperature fluctuating between -5℃ and -2℃. Traditional static verification methods, calculated according to the standard icing condition combination (icing + 0.25 wind load), showed that the maximum stress ratio of each component was 0.88, less than 1.0, concluding that the structure was "structurally safe." However, subsequent manual inspection revealed visible bulging deformation in the node plate connecting the crossbeam and the herringbone column. 2. Analysis process using the method of this invention Step S1: Establish the model and apply conductor tension boundaries. A refined finite element model of the structure was established using ANSYS software. The columns and beams were simulated using Beam188 beam elements, the gusset plates were simulated using Shell181 shell elements, and the bolted connections were simulated using Combin7 three-dimensional spring elements to simulate the semi-rigid connection characteristics. Based on the conductor parameters of this outgoing section (LGJ-400 / 35 steel-cored aluminum stranded wire, span 50m), substitute them into the conductor state equation to solve. The design conditions are known to be 15℃, no ice, and the conductor specific load γ... m=0.035N / (m·mm²); Icing condition is -3℃, icing thickness is 12mm (the conductor icing thickness is taken as 0.75 times the framework icing thickness, i.e., 12mm), specific load γ n =0.112 N / (m·mm²). The solution shows that the horizontal tension increment of the conductor is 2.3 times the design value. This tension increment is applied as a concentrated force to the beam hanging point node. Step S2: Monitoring data acquisition and full-field thickness mapping. The on-site microwave icing monitoring device was installed on the windward side of the mid-span of the crossbeam, and the actual thickness d was measured. meas =16mm. Based on the preliminary CFD calculations, the spatial distribution coefficients of each member of the frame are: windward beam k=1.15, leeward beam k=0.65, windward diagonal brace k=1.10, leeward diagonal brace k=0.60, column windward side k=1.05, column leeward side k=0.55. (According to d...) i =16×k i Calculate the ice thickness of each rod. Step S3: Analysis of ice accumulation growth and frost heave stages. The process of increasing the ice thickness from 0 to 16 mm was discretized into 6 load steps. A quasi-static solution was performed using the arc-length method, and the results are as follows: When the ice thickness is 12mm, the maximum stress ratio at the mid-span of the beam is 0.72, and the stress ratio at the hanging point node plate is 0.81. When the ice thickness is 14mm, the stress ratio at the mid-span of the beam rises to 0.85, and the local stress ratio of the gusset plate reaches 0.93; When the ice thickness is 16mm, after taking into account the conductor tension, the stress ratio at the mid-span of the beam rises to 0.96, which is close to the critical state. Regarding the frost heave analysis of the node, this connection node adopts a double-layer node plate structure with a gap of approximately 0.3 mm between the plates. The frost heave pressure due to water-ice phase change is calculated as follows: F frost =3×10 9 ×0.09×(150×100×10 −6 =4050N. Applying this pressure to the normal direction of the gusset plate, the calculation results show that the Mises stress in the local area of ​​the gusset plate reaches 268 MPa, which exceeds the yield strength of Q235B steel (235 MPa) by about 14%, and is in good agreement with the location and degree of the bulging deformation actually observed. Step S4: Analysis of the icing stabilization and load-bearing stage. The peak icing duration was approximately 36 hours. Since the substation did not have bolt axial force sensors installed, a conservative empirical nodal stiffness reduction factor η = 0.85 was used. After updating the spring element stiffness, the solution was recalculated. The results showed that the nodal stiffness reduction had a relatively small impact on the overall displacement of the structure (the mid-span deflection of the crossbeam increased by approximately 3.5%), and the foundation overturning resistance factor was 2.12, meeting the specifications. Step S5: Analysis of the transient stage of ice melting and de-loading. After the icing period ended, the temperature rose rapidly to 5°C, and the ice layer on the structure surface began to melt and detach. According to on-site personnel, the ice mainly peeled off from the leeward side, while the windward side melted last due to weaker sunlight. Mode A (leeward side detaches first, windward side detaches later) was selected for dynamic analysis. The ice layer on the beam was discretized into 30 ice elements (one every 0.5m). The ice elements on the leeward side were killed in three batches within t=0.3 s, and the ice elements on the windward side were killed in three batches between t=0.8 s and 1.2 s. The structural damping ratio was set to 0.03, and an explicit dynamic solution was used. The calculation results show that: The vertical dynamic amplification factor (DAF) at mid-span of the crossbeam is 1.22. The peak value of the local transient stress in the gusset plate was 248 MPa, which was slightly higher than the static analysis result (235 MPa), but still within the range of yielding. The de-icing process did not cause any new plastic damage. Step S6: Comprehensive evaluation and decision output. Based on the results of the three-stage analysis, the evaluation conclusions are as follows: The overall load-bearing capacity of the structure was close to its limit when the ice layer reached 16 mm, but no instability occurred. The bulging deformation of the node plate is caused by frost heave force, which is a localized damage and does not affect the overall load-bearing capacity of the structure; It is recommended to replace the damaged node plate during subsequent maintenance and apply low-temperature resistant sealant to the node gaps to prevent further freeze-thaw damage. 3. Comparative Analysis Traditional static verification methods, failing to consider conductor tension, analyze node frost heave, or assess the dynamic effects of de-icing, only conclude that "stress ratio 0.88, structural safety," failing to explain actual node deformations on-site and providing no targeted guidance for operation and maintenance. The method of this invention accurately identifies the causes and extent of node frost heave damage and its impact on the overall structural safety, providing clear and actionable maintenance recommendations, significantly improving the accuracy and practicality of the assessment. Example 2: Preventive Safety Assessment and Reinforcement Decision for Portal Structures in Heavy Ice Zones 1. Project Overview A 500kV substation is located in a heavy icing area at an altitude of approximately 1100m in Central China. The outgoing line frame adopts a portal steel pipe frame with a beam span of 22m and a column height of 18m, and is equipped with a two-way diagonal bracing system. The design basic icing thickness is 20mm, and the design wind speed is 30m / s. The area where this substation is located has historically experienced an extreme weather event with a measured ice thickness of 28mm. The maintenance unit plans to conduct a comprehensive assessment of the structure's anti-icing capabilities before winter to determine if reinforcement measures are necessary. 2. Analysis process using the method of this invention Step S1: Establish the model and apply conductor tension boundaries. A refined model of the portal frame was established using SAP2000 software. The members were simulated using frame elements, and the node plates were simulated using shell elements. Based on the parameters of the 500kV outgoing conductor (four-split LGJ-630 / 45 conductor, span 65m), the conductor tension increments at icing thicknesses of 20mm, 25mm, and 28mm were calculated and applied to the suspension points at both ends of the crossbeam. Step S2: Mapping monitoring data to full-field thickness. Since this embodiment is a preventative assessment and lacks real-time monitoring data, the design icing thickness and assumed over-design conditions are used as the analysis inputs. The spatial distribution coefficient is taken from the CFD pre-calculation results of similar structures in the same region. Step S3: Analysis of ice accumulation growth and frost heave stages. Progressive loading analysis was performed on three conditions with icing thicknesses of 20mm (design value), 25mm, and 28mm. Key results are as follows: 20 0.78 1.38 0.72 25 0.89 1.08 0.85 28 0.94 0.96 0.91 Analysis results show that when the ice thickness reaches 28 mm, the stability coefficient of the windward side brace drops to 0.96, which is lower than the safety limit of 1.0, indicating a risk of buckling instability. Step S4: Analysis of the icing stabilization and load-bearing stage. Assuming the peak icing effect persists for 48 hours under extreme weather conditions, and considering the relaxation effect of node preload, the node connection stiffness is corrected using an empirical reduction factor of 0.85, and the brace stability coefficient is reassessed. After correction, the brace stability coefficient under the 28mm load condition further decreases to 0.91. Step S5: Analysis of the transient stage of ice melting and de-loading. Considering that the substation is located in a heavy icing area, manual de-icing measures may be used during winter operation and maintenance. Therefore, mode D (synchronous shedding of the entire cross section) is selected as the most unfavorable envelope condition, while mode B (segmented shedding from top to bottom) is used as the reference condition for natural de-icing. The ice covering the inclined support surface was discretized into 20 ice elements. In the full-section synchronous shedding mode, all ice elements were killed simultaneously within t=0.2s; in the segmented shedding mode, the ice elements were killed in batches within 1.0s from top to bottom. Explicit dynamic analysis results: Mode B (segmented shedding) 1.18 182 Stablize Mode D (Full-section Synchronous) 1.52 235 Critical instability In Mode D, the peak transient compressive stress of the brace reaches 235 MPa, which has reached the material's yield strength, and the corresponding axial pressure has exceeded the buckling critical load, posing a risk of sudden instability. Step S6: Comprehensive assessment and reinforcement decision output. Overall assessment conclusion: The safety margin of this portal frame is sufficient under the design ice thickness of 20mm, but under the condition of ice thickness exceeding 25mm, the stability of the windward side diagonal brace becomes a limiting factor; if 28m of ice accumulation occurs and improper manual de-icing operation (synchronous de-icing of the entire cross section) is carried out, the diagonal brace is at risk of dynamic instability. Based on the above conclusions, the following reinforcement and maintenance recommendations are provided: (1) Structural reinforcement scheme: The windward side diagonal bracing is reinforced with external angle steel, that is, four L75×8 angle steels are symmetrically welded around the original diagonal bracing steel pipe to form a composite section. After reinforcement, the stability coefficient of the diagonal bracing under the 28 m icing + full cross-section de-icing condition is increased to 1.82, which meets the safety requirements. (2) Joint sealing and protection: Waterproof sealing treatment is applied to the gaps of the beam-column connection joint plate, low-temperature resistant silicone sealant is applied and a waterproof cover is installed to prevent damage from freezing. (3) Intelligent monitoring configuration: Install fiber optic strain sensors at the middle of the windward side diagonal brace and at the crossbeam hanging point, and install a microwave icing monitoring device on the windward side of the crossbeam mid-span to establish a real-time monitoring system. (4) Graded de-icing plan: Formulate standardized de-icing operation procedures, clarify that when the ice thickness reaches 22mm, de-icing preparation should be started, and when it reaches 25mm, de-icing operation should be carried out; the de-icing sequence should strictly follow the principle of "first the leeward side, then the windward side, first the lower part, then the upper part", and the length of a single de-icing operation should not exceed 2m to avoid triggering synchronous de-icing dynamic impact. 3. Effect Verification During the first winter after the reinforcement measures were implemented, the substation experienced a 36-hour period of freezing rain with a maximum ice thickness of 23 mm. Monitoring data showed that the stress level of the diagonal braces was far below the warning threshold, the overall structure responded smoothly, and no abnormal deformation was found during on-site inspections. The method of this invention successfully guided the decision-making for preventative reinforcement, avoiding potential structural safety accidents. 4. Comparative Analysis If the traditional static verification method is used, the stability coefficient of the inclined brace under 28mm icing conditions is calculated to be 1.05 according to the standard formula (without considering the increase in conductor tension, the reduction in node stiffness, or the dynamic effect of de-icing), and the conclusion would be "basically safe and can continue to operate". However, the method of this invention identifies the actual dynamic instability risk and provides a quantitative basis for reinforcement, avoiding tower collapse accidents that may be caused by an unsafe assessment. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them; when the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope claimed by the present invention.

Claims

1. A multi-dimensional stress analysis method for substation structures after icing, characterized in that, Includes the following steps: Step S1: Establish a three-dimensional finite element model of the substation frame. The model includes the frame body structure, node connection construction and foundation constraints. Apply static tension load boundary conditions equivalent to conductor icing at the frame hanging points to reflect the coupling effect of conductor icing on the frame. Step S2: Obtain icing monitoring data and micro-meteorological data at the substation site, and based on the pre-calculated computational fluid dynamics (CFD) analysis results, map the measured icing thickness at a single point to the overall icing thickness distribution of each member of the frame through the spatial distribution coefficient matrix. Step S3: Perform the analysis of ice growth and frost heave stages. Using the mapped full-field ice thickness as input, perform quasi-static progressive loading nonlinear solution on the finite element model to calculate the stress, displacement and stability of the frame under each load step, and simultaneously evaluate the local damage state of the bolted connection nodes under the action of water-ice phase change frost heave force. Step S4: Perform the icing stability and load-bearing stage analysis. Using the structural stress state and nodal frost heave damage state output in Step S3 as initial conditions, evaluate the overall stability margin of the frame, the degree of bolt preload attenuation, and the safety of the foundation under long-term eccentric load under the action of icing peak constant load. Step S5: Perform transient phase analysis of ice melting and de-icing. Using the structural state updated in step S4 as the initial condition, establish the load-time history curve of block-by-block progressive ice melting according to the preset ice melting mode, and use the dynamic finite element method to calculate the dynamic amplification factor, transient stress peak and residual displacement of the frame during the ice melting process. Step S6: Based on the analysis results of steps S3, S4 and S5, a comprehensive assessment of the safety margin of the overall structure and each component is conducted. When the safety margin of any stage is lower than the preset threshold, a graded early warning signal and the corresponding reinforcement or de-icing control strategy are output.

2. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The static tension load boundary conditions applied at the frame hanging points in step S1 are obtained through the following steps: Based on the conductor design parameters, measured values ​​of conductor icing thickness, and ambient temperature, the horizontal tension increment of the conductor under icing and low-temperature conditions is solved using the conductor state equation. The horizontal tension increment is applied to the nodes corresponding to the conductor suspension points in the finite element model of the framework in the form of equivalent nodal concentrated forces. The conductor state equation adopts the following form:

3. In the formula, σ n σ m The horizontal stress of the conductor under the unknown and known operating conditions, γ, are respectively. n γ m The specific load of the conductor under the corresponding working condition is given by l, where l is the span, E is the elastic modulus of the conductor, α is the coefficient of thermal expansion of the conductor, and t is the conductor's specific load. n t m This refers to the corresponding operating temperature.

4. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The spatial distribution coefficient matrix pre-calculated based on CFD in step S2 is generated in the following way: During the substation design or evaluation preparation phase, CFD software is used to simulate the spatial distribution of supercooled water droplet collision efficiency under the prevailing wind direction and design wind speed, and to calculate the local collision coefficient β on the surface of each member of the frame. i The collision coefficients of all members are then normalized to obtain the spatial distribution coefficient k of each member. CFD,i During the real-time analysis phase, the measured ice thickness d at the monitoring point will be... meas As a baseline value, the calculated icing thickness of each member is determined by the following formula:

5. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The evaluation of the local damage state of the bolted connection node under the action of water-ice phase change frost heave force in step S3 specifically includes: Based on the on-site bolt gap inspection results or empirical parameters, determine the characteristic width of the joint gap and calculate the frost heave pressure F generated by the water-ice phase change. frost :

6. In the formula, E ice Let ε be the elastic modulus of ice. exp A is the volume expansion rate of water-ice phase transition. gap The area of ​​the gap under pressure; The frost heave pressure is equivalent to the additional surface pressure acting on the normal direction of the gusset plate, and is applied to the corresponding node region in the finite element model to calculate the local stress state of the gusset plate under the combined action of frost heave force and external ice load. When the local stress exceeds the yield strength of the node plate material, the node is determined to have suffered frost heave plastic damage, and the connection stiffness of the node is reduced accordingly in the state inheritance of step S4.

7. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The assessment of the bolt preload attenuation in step S4 is achieved through at least one of the following methods: Method 1: Install ultrasonic bolt axial force sensors at key bolt connection nodes of the frame to monitor the change curve of preload force over time during load holding in real time. When the measured preload force decays to a preset proportion of the design preload force, an early warning is triggered. Method 2: In the absence of real-time monitoring data, the stiffness of the node connection is corrected according to the duration of icing load and the temperature cycle amplitude, based on a preset empirical reduction factor η, wherein the reduction factor η ranges from 0.80 to 0.

90.

8. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The preset de-icing mode in step S5 includes at least one of the following: Mode A: The leeward side detaches first, followed by the windward side, simulating the uneven unloading process of melting ice dominated by solar radiation; Mode B: Segmented detachment from top to bottom, simulating the unloading process of ice layer sliding down segment by segment under the action of gravity when the temperature rises above freezing point; Mode C: The windward side detaches first, followed by the leeward side, simulating the unbalanced unloading process of the wind blowing off the ice layer on the windward side under conditions of strong wind and rising temperature. Mode D: Synchronous shedding across the entire cross section, used as the most unfavorable envelope condition for verification in extremely conservative designs.

9. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The step S5, which establishes the load-time history curves for segmented gradual de-icing, specifically includes: The ice layer on the surface of each member of the frame is discretized into multiple independent ice body units along the length of the member, and each ice body unit is assigned an independent unit mass attribute. Based on the selected de-icing mode, assign corresponding "life and death time" parameters to each ice element. The ice element is active before the de-icing time and provides mass and load contribution. After the de-icing time, it is "killed" and removed from the model. The transient dynamic response of the de-icing process was determined using either the explicit central difference method or the implicit Hilber-Hughes-Taylor time integration method, with the steel structure damping ratio set to 0.02~0.04 during the solution process.

10. The multi-dimensional stress analysis method for substation structures after icing according to claim 1, characterized in that: The rule for setting the safety margin threshold in step S6 is as follows: A Level 1 warning is triggered when the stress ratio of any component exceeds 0.90 during the icing growth stage. A level two warning is triggered when the stress ratio of any component exceeds 0.95 during the ice growth stage, or when the bolt preload decreases by more than 25% of the design value during the load-bearing stage. A Level 3 warning is triggered when the stress ratio of any component at any stage exceeds 1.00, or the stability coefficient is lower than 1.00, or the dynamic amplification factor exceeds 2.0 and the transient stress exceeds the material yield strength during the ice-breaking transient stage. Level 1 warning outputs the "Strengthen Inspection" command, Level 2 warning outputs the "Prepare De-icing Equipment" command, and Level 3 warning outputs the "Immediately Perform De-icing Operations or Temporarily Suspend Operations" command.

11. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: In step S3, the quasi-static progressive loading nonlinear solution uses the arc length method to track the stiffness degradation path of the structure during the increasing icing load process; when the solution reaches the point where the structural stiffness matrix is ​​singular or the arc length method cannot converge, it is determined that the limit icing thickness state has been reached.

12. The multi-dimensional stress analysis method for substation structures after icing as described in claim 1, characterized in that: The refined three-dimensional finite element model established in step S1 uses spring elements that consider semi-rigid characteristics to simulate the bolt connection node. The initial rotational stiffness of the spring element is calibrated by experimental or empirical formulas based on the node structure and bolt preload. In step S4, the stiffness parameters of the spring element are dynamically updated according to the degree of preload attenuation.

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