Experimental platform for evaluating influence of wood humidity on lightning stroke probability of historic building

Abstract

The invention discloses an experimental platform for evaluating the influence of wood humidity on the lightning stroke probability of historic buildings, which relates to the technical field of high-voltage experiments and is composed of an impulse voltage generator, a high-speed camera, an oscilloscope, a lightning rod and a control terminal. The impulse voltage generator is used for generating negative polarity impulse voltage so as to simulate natural negative ground lightning; a high-speed camera for capturing a moving image; the oscilloscope is used for recording lightning rod tip voltage waveforms; and the control terminal is used for remotely controlling the single discharge of the impulse voltage generator and setting the charging voltage. According to the method, the wooden pole is used for equivalently replacing a complex equal-proportion ancient building model, natural thunder and lightning are simulated through the Marx voltage generator, and the discharge process of wood with different humidity is safely and accurately obtained through the oscilloscope and the high-speed camera; the experiment development threshold is greatly reduced, and convenience is provided for subsequent modeling analysis.

Description

Technical Field

[0001] The present invention relates to the technical field of high - voltage experiments, and more specifically, to an experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings. Background Art

[0002] Wooden ancient buildings are prone to lightning disasters. The increase in wood moisture caused by thunderstorms is considered an important reason for lightning strikes on wooden ancient buildings. Therefore, it is necessary to conduct simulated lightning experiments to explore the influence of wood moisture on the lightning strike probability of ancient buildings.

[0003] Existing simulated lightning experiment platforms and methods are mainly applied to metal materials such as lightning rods. As unique and immovable items, wooden ancient buildings obviously cannot be directly applied with voltage. Therefore, existing technologies are not applicable to the experiments to be carried out. If a scaled - down model of an ancient building is used, problems such as delicate structure and high manufacturing cost will be faced. Even if a corresponding model can be made, its complex structure will bring great difficulties and uncertainties to the finite - element analysis of the lightning discharge process. Summary of the Invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide an experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings, aiming to build an indoor laboratory platform for exploring the influence of wood moisture on the lightning strike probability of ancient buildings.

[0005] To achieve the above technical purpose, the present application provides an experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings, which is composed of an impulse voltage generator, a high - speed camera, an oscilloscope, a lightning rod, and a control terminal;

[0006] The impulse voltage generator is used to generate a negative - polarity pulse voltage to simulate natural negative cloud - to - ground flashes;

[0007] The high - speed camera is used to capture moving images;

[0008] The oscilloscope is used to record the voltage waveform at the tip of the lightning rod;

[0009] The control terminal is used to remotely control the single - shot discharge of the impulse voltage generator and set the charging voltage.

[0010] Preferably, when simulating natural negative cloud - to - ground flashes, the impulse voltage generator adopts an upper - plate - lower - rod structure; the impulse voltage is applied to the plate electrode, and the test material is erected on the ground by a bracket for lightning receiving.

[0011] Preferably, the test material is a cylinder, with a radius of 1 / 20 of the solid building pillar and a length of 2 / 5 of the height of the flat - plate electrode.

[0012] Preferably, the experimental platform is also used to obtain the humidity at the 1 / 3 and 2 / 3 lengths of the test material and keep the humidity constant during the experiment.

[0013] Preferably, the experimental platform is also used to perform lightning strike probability prediction on the experimental data, including the following steps:

[0014] Analyze the recorded waveforms;

[0015] By performing frame-by-frame comparison on the discharge process, obtain the upward leader initiation time, streamer initiation time, and final jump time;

[0016] Synchronize the voltage waveform diagram and the high-speed camera images according to time to obtain a voltage value / image-time chart, and based on the leader initiation time and streamer initiation time, query the leader initiation voltage and streamer initiation voltage values corresponding to each discharge in the voltage waveform diagram; according to the final jump time, obtain the breakdown voltage;

[0017] By obtaining the relationships between the lightning attraction ability of wood and the breakdown voltage, leader initiation voltage, streamer initiation voltage, and leader development speed respectively, predict the lightning strike probability of the test material at any humidity.

[0018] Preferably, the experimental platform is also used to obtain the exposure timing when the discharge channel on the electrode narrows in the high-speed camera image as the leader initiation time;

[0019] It is also used to process the image into a pixel point gray value table and find the moment corresponding to the value jumping from zero to a non-zero value near the electrode end, which is the streamer initiation time;

[0020] It is also used to observe the rod-plate streamer connection timing of the high-speed camera frame by frame as the final jump time.

[0021] Preferably, the lightning attraction ability of wood is negatively correlated with the breakdown voltage, leader initiation voltage, and streamer initiation voltage respectively, and is positively correlated with the leader development speed.

[0022] Preferably, the experimental platform is also used to predict the lightning strike probability of the test material at any humidity by obtaining the critical humidity value, where, find and the values of, draw the four values into a radar chart, and when the area of the radar chart is approximately equal to 1.35, the humidity of the wood corresponding to the test material is the critical humidity value of this kind of wood.

[0023] Preferably, the experimental platform is also used to judge the lightning strike occurrence probability according to the critical humidity value, where when the wood humidity is less than the critical humidity value, the lightning strike occurrence probability is approximately 0; when the humidity is greater than the critical humidity, the lightning strike occurrence probability applies y = a(1 - e 0.1x ), and the parameter a is obtained by fitting the humidity and breakdown rate data of each group.

[0024] The present invention discloses the following technical effects:

[0025] (1) The entire experimental scheme is remotely operated in the control room, providing a standardized, easy-to-operate, and safe solution for conducting simulated lightning strike experiments on humid wooden structures indoors;

[0026] (2) Using wooden columns to equivalently replace the ancient building model greatly reduces the threshold for conducting experiments and provides convenience for subsequent modeling analysis;

[0027] (3) Compared with previous similar experiments, a high-speed camera with a higher frame rate and an oscilloscope with a higher sampling rate are used to synchronously record the discharge process and voltage waveform, improving the accuracy of data acquisition. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 is the overall connection diagram of the experimental device described in the present invention;

[0030] Figure 2 is the implementation scheme of laboratory simulated lightning strike described in the present invention;

[0031] Figure 3 is the schematic diagram of the experimental data processing flow and lightning strike probability prediction method described in the present invention;

[0032] Figure 4 is the leader initiation moment described in the present invention;

[0033] Figure 5 is the final jump moment described in the present invention;

[0034] Figure 6 is the physical diagram of the experimental device described in the present invention;

[0035] Figure 7 is the measurement of wood moisture described in the present invention;

[0036] Figure 8 is the control terminal described in the present invention;

[0037] Figure 9 is the on-site diagram of the discharge experiment described in the present invention;

[0038] Figure 10 is the typical discharge process and its voltage waveform of each group described in the present invention;

[0039] Figure 11 is the leading development speed described in the present invention;

[0040] Figure 12 is the leading development time and breakdown voltage described in the present invention. Specific embodiments

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Apparently, the described embodiments are only some of the embodiments of the present application, rather than all of them. Usually, the components of the embodiments of the present application described and illustrated herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the present application claimed, but merely represents selected embodiments of the present application. All other embodiments obtained by those skilled in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0042] Such as Figures 1-12 As shown, the present invention provides an experimental method for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings, including the selection method of the shape, size, and type of the test material, the experimental grouping and material processing method, the connection and operation method of the experimental platform, the experimental data processing method, the data analysis method for the influence of wood moisture on the lightning strike probability of ancient buildings, and the calculation method for the lightning strike probability of specific wood at a specific humidity. This method uses wooden columns to equivalently replace complex scaled ancient building models, simulates natural lightning through a Marx voltage generator, and uses an oscilloscope and a high-speed camera to safely and accurately obtain the discharge process of wood with different humidities. Specifically, it includes the following contents:

[0043] It is carried out in three stages: pre-experiment preparation, mid-experiment implementation, and post-experiment processing.

[0044] The specific operations in each stage are as follows

[0045] 1. Pre-experiment preparation stage: Selection and processing of materials

[0046] Investigate and select the material type: When lightning strikes an ancient building, the lightning current generally forms a discharge channel by entering the ground through the wooden pillars connected to the metal tower brake of the building. Therefore, the wood used for the pillars of ancient buildings is most representative of the lightning receiving ability of the building.

[0047] Determine the material size and shape: Select a cylinder for the shape, with the radius being 1 / 20 of the pillar of the solid building, and the length being 2 / 5 of the height of the flat electrode suspended on the experimental platform

[0048] Experimental grouping and material treatment: In the experiment, the wood moisture was used as the independent variable, and it was divided into four groups: a. Air-dried wood: Naturally air-dried for 30 days to keep its surface and core dry. ; b. Slightly wet wood: Sprayed with clear water for 1 - 2 minutes to make its surface wet and the core dry; c. Water-saturated wood: Immersed in water for 1 - 2 hours to make both the inside and outside wet; d. 304 stainless steel pipe: Kept dry as a whole. The sizes and shapes of the four groups of materials should be the same.

[0049] 2. Experimental implementation stage:

[0050] Installation of the experimental platform: The overall connection method of the experimental device is as Figure 1 shown. The experimental device consists of a 4.8 MV Marx impulse voltage generator, a high-speed camera, an oscilloscope, a lightning rod, and a control terminal. The function of the impulse voltage generator is to generate a negative-polarity pulse voltage of 250 μs - 2500 μs to simulate natural negative cloud-to-ground lightning. The experiment adopted an upper-plate - lower-rod structure. The impulse voltage was applied to the plate electrode; the high-speed camera could capture moving images at an extremely high frame rate. The time of a single cloud-to-ground lightning flash is generally in the millisecond range. In order to capture the movement process of cloud-to-ground lightning, the frame rate of the high-speed camera was set to 300000 fps; the oscilloscope was used to record the voltage waveform at the tip of the lightning rod; the test materials were erected on the ground with brackets for lightning reception; the control terminal could remotely control the single discharge of the Marx generator through a computer and set its charging voltage.

[0051] After installing the platform, a safety check was carried out. Before the experiment started, it was confirmed that the grounding rod of the impulse voltage generator was separated, the channel indicator light of the oscilloscope was on, and the rod - plate electrode was within the frame of the high-speed camera. The humidity at the 1 / 3 and 2 / 3 positions of the wood length was measured with a resistive humidity measuring instrument, and the current humidity reading was recorded.

[0052] Twenty discharge experiments were carried out on each group of test materials in turn. The method of simulating a single lightning strike and completely recording it is as Figure 2 shown. The voltage generator was controlled by a computer to charge and discharge. The oscilloscope was used as the trigger source to synchronously trigger the current measurement device and the high-speed camera. When the oscilloscope received the discharge voltage signal, it started to record. At the same time, the oscilloscope output two TTL signals. One was connected to the computer of the current measurement device as the trigger source for current acquisition, and the other was connected to the high-speed camera to trigger the high-speed camera. Therefore, the oscilloscope and the high-speed camera worked within the same time period. The voltage data and photos could be synchronized according to time. Each group of test materials was discharged 20 times. Before the experiment started, the humidity of the wood was measured with a resistive humidity measuring instrument and recorded. Considering the thermal effect of the lightning current, after each discharge, the humidity of the wood needed to be measured. If the humidity decreased, clear water needed to be sprayed to maintain the humidity before the experiment.

[0053] 3. Experimental post-processing:

[0054] The experimental data processing flow and the lightning strike probability prediction method are as follows Figure 3 shown

[0055] Operate according to the following steps:

[0056] (1) Analyze the recorded waveforms. First, export the waveform diagram recorded by the oscilloscope with the suffix.CSV file, and use matlab to analyze the data points to draw the voltage waveform diagram of the discharge process

[0057] (2) By comparing the discharge process captured by the high-speed camera frame by frame on the computer, obtain the starting time of the upward leader, the starting time of the streamer, and the end jump time. In the high-speed camera image, the exposure timing when the discharge channel on the electrode narrows is the starting time of the leader; process the image into a pixel point gray value table, and find the time corresponding to the jump from zero to a non-zero value near the end of the electrode, which is the starting time of the streamer. Observe the streamer connection timing of the rod-plate of the high-speed camera frame by frame as the end jump time (the process is as shown Figure 4 shown); use the background photo taken by the high-speed camera in a bright environment to measure the actual side length l of each pixel point of the high-speed camera, read the upward leader tip coordinates (x1, y1) (x2, y2) at the starting time t1 and the end jump time t2 of the leader, and the leader development length is Leader development speed c. Breakdown voltage. Read the voltage at the end jump time from the voltage waveform diagram, and this voltage is the breakdown voltage. d. Breakdown rate

[0058] (3) Synchronize the voltage waveform diagram and the high-speed camera image according to time to obtain a voltage value / image-time chart. According to the leader starting time and the streamer starting time obtained in (2), query the leader starting voltage and the streamer starting voltage values corresponding to each discharge in the voltage waveform diagram; according to the end jump time, obtain the breakdown voltage

[0059] (4) The lightning strike ability of wood is negatively correlated with the breakdown voltage, the leader starting voltage, and the streamer starting voltage, and positively correlated with the leader development speed. Take the average value of the breakdown data within the same group. Calculate the values of and , draw the four values into a radar chart. When the area of the radar chart is approximately equal to 1.35, the humidity of the wood in this group is the critical humidity value of this kind of wood. When the wood humidity is less than the critical humidity value, the lightning strike probability is approximately 0; when the humidity is greater than the critical humidity, the lightning strike probability applies to the formula y = a(1 - e 0.1x ). In the formula, x represents the wood humidity percentage, and the parameter a can be obtained by fitting the humidity and its breakdown rate data of each group . From this formula, the lightning strike probability at any humidity of this kind of wood can be predicted

[0060] To reduce the number of repeated experiments, the lightning strike probability - humidity curves of the following woods in the same group can be approximately considered the same: teak, walnut, rubber tree, mahogany, toona sinensis, huanghuali, agarwood;

[0061] Poplar, beech, catalpa, nanmu, cedar;

[0062] Sandalwood, Chinese fir, cypress, willow;

[0063] Linden, Korean pine, larch.

[0064] Implementation case: Research on the lightning attraction ability of woods used in wooden-structured Huizhou ancient architecture:

[0065] 1. Preparation stage before the experiment: Selection and treatment of materials

[0066] Research and select the types of materials: Taking the wooden pillars in the ancestral hall of Hongcun, Huangshan City, Anhui Province as an example, the material of the wooden pillars is evergreen Chinese fir

[0067] Determine the size and shape of the materials: Select the cylinder shape, with the radius being 1 / 20 of the pillar diameter of 560 mm - 28 mm, and the length being 2 / 5 of the height of the flat electrode suspended on the experimental platform, 2 m.

[0068] Experimental grouping and material treatment: The experiment takes the humidity of Chinese fir as the independent variable and is divided into four groups: a. Air-dried wood: Naturally air-dried for 30 days to keep its surface and the core dry. ; b. Slightly wet wood: Sprayed with clear water for 1 - 2 minutes to make its surface wet and the core dry; c. Water-saturated wood: Immersed in water for 1 - 2 hours to make it wet inside and outside; d. 304 stainless steel pipe: Kept dry as a whole. The size and shape of the four groups of materials should be the same

[0069] 2. Experiment implementation stage:

[0070] The experiment is carried out in the high-voltage laboratory of Anhui Electric Power Research Institute. The temperature in the laboratory is 9.9 °C and the relative humidity is 64.6%.

[0071] (1) Connect the experimental device platform, and the physical diagram of the platform is as Figure 5 shown

[0072] (2) Measure the humidity at 1 / 3 and 2 / 3 of the wood before the experiment starts, as Figure 6 shown; Check the control terminal, and the physical diagram of the terminal is as Figure 7 shown

[0073] (3) Conduct the discharge experiment, as Figure 8 shown

[0074] 3. Post-experiment processing:

[0075] (1) Export the waveforms and synchronize them to obtain the typical discharge processes and their waveform diagrams of several groups of materials as Figure 9 shown.

[0076] (2) Calculate indicators such as the leader development speed, leader development time, breakdown voltage, etc., and the analysis results are as Figure 10 、 11 shown.

[0077] Conclusion: Under a strong background electric field, it is very difficult for a positive upward lightning leader to initiate on a dry wooden structure. However, when the surface of the wooden structure cannot remain dry, in a lightning environment, a lightning leader directly above can initiate at the tip of the wooden structure.

[0078] The present invention is described with reference to the flowcharts and / or block diagrams of methods, apparatuses (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, and the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing devices generate means for realizing the functions specified in one Figure 1 process or multiple processes and / or blocks Figure 1 block or multiple blocks.

[0079] In the description of the present invention, it should be understood that the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0080] Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims

1. An experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings, characterized in that, It consists of an impulse voltage generator, a high-speed camera, an oscilloscope, a lightning rod and a control terminal; The impulse voltage generator is used to generate a negative-polarity pulse voltage to simulate natural negative cloud-to-ground lightning; The high-speed camera is used to capture moving images; The oscilloscope is used to record the voltage waveform at the tip of the lightning rod; The control terminal is used to remotely control the single discharge of the impulse voltage generator and set the charging voltage.

2. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 1, characterized in that: The impulse voltage generator adopts an upper plate - lower rod structure when simulating natural negative cloud-to-ground lightning; the impulse voltage is applied to the plate electrode, and the test material is erected on the ground with a bracket for lightning reception.

3. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 2, characterized in that: The test material is a cylinder, with a radius of 1 / 20 of the solid building pillar and a length of 2 / 5 of the height of the flat plate electrode.

4. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 3, characterized in that: The experimental platform is also used to obtain the moisture at the 1 / 3 and 2 / 3 positions of the length of the test material and keep the moisture unchanged during the experiment.

5. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 4, characterized in that: The experimental platform is also used to process the experimental data and predict the lightning strike probability, including the following steps: Analyze the recorded waveform; By comparing each frame of the discharge process, obtain the upward leader initiation time, streamer initiation time and final jump time; Synchronize the voltage waveform diagram and the high-speed camera image according to time to obtain a voltage value / image - time chart, and based on the leader initiation time and streamer initiation time, query the leader initiation voltage and streamer initiation voltage values corresponding to each discharge in the voltage waveform diagram; obtain the breakdown voltage according to the final jump time; By obtaining the relationship between the lightning initiation ability of wood and the breakdown voltage, leader initiation voltage, streamer initiation voltage and leader development speed respectively, predict the lightning strike probability of the test material at any moisture.

6. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 5, characterized in that: The experimental platform is also used to obtain the exposure timing when the discharge channel on the electrode becomes narrower in the high-speed camera image as the leader initiation time; It is also used to process the image into a pixel point gray value table and find the moment when the value near the end of the electrode jumps from zero to a non-zero value, which is the streamer initiation time; It is also used to observe the rod-plate streamer connection timing of the high-speed camera frame by frame as the final jump time.

7. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 6, characterized in that: The lightning initiation ability of the wood is negatively correlated with the breakdown voltage, leader initiation voltage and streamer initiation voltage respectively, and positively correlated with the leader development speed.

8. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 7, characterized in that: The experimental platform is also used to predict the lightning strike probability of the test material at any humidity by obtaining the critical humidity value, where, calculate and values, draw the four values into a radar chart, and when the area of the radar chart is approximately equal to 1.35, the humidity of the wood corresponding to the test material is the critical humidity value of this kind of wood.

9. The experimental platform for evaluating the influence of wood moisture on the lightning strike probability of ancient buildings according to claim 8, characterized in that: The experimental platform is also used to judge the lightning strike probability according to the critical humidity value. Among them, when the humidity of the wood is less than the critical humidity value, the lightning strike probability is approximately 0; when the humidity is greater than the critical humidity, the lightning strike probability applies to y = a(1 - e 0.1x ), and the parameter a is obtained by fitting the humidity and breakdown rate of each group data.

Images