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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 63 Issue 1 Previous Issue    Next Issue
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    CARBON NEUTRALITY
    Structural representation and optimization path of energy policy under the carbon neutral vision
    TANG Yunni, YAN Ruxue, ZHOU Yanling
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 1-14.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.036
    Abstract   HTML   PDF (6248KB) ( 291 )
    China's carbon-neutral vision imposes stricter and more urgent requirements for the transformation of the energy structure. The transformation and optimization of the energy structure is a crucial condition and measure for achieving the carbon neutrality goal. Studying the policy structure logic, such as the characteristics of policy subjects, policy tool selection preferences, and policy themes, has practical implications for the optimization of energy structure and the realization of carbon-neutrality goals. Based on the theory of policy structure, this study develops a three-dimensional analytical framework of “policy subject-policy tool-policy focus”. It employs various methods, such as policy tool mining, to perform textual analysis on 122 energy policies within the carbon-neutral vision. Through layer-by-layer coding and sub-item analysis, we explore the formal characteristics of policy texts and policy tool selection preferences from the dimensions of a hierarchical system, department distribution, and time series and comprehend the current focus of energy structure transformation policy. The study found that policy subjects involved a wide range of departments and a large number of departments, showing a certain degree of synergy, but the cross-departmental coordination and cooperation of policy subjects had not been formed; changes in the timing and number of policy releases were closely related to the top-level strategic arrangements of the country; policy direction guidance was clear and rich in content, but policy content is suspended and fragmented; there are various types of policy tools, mainly using environmental policy tools, but the imbalance and isomorphism of the use of various types of policy tools are exposed; the policy focus includes three aspects: controlling the intensity and total amount of energy consumption, adjusting the energy consumption of key industries structure, deepen institutional reform and encourage scientific and technological innovation. The carbon-neutral vision proposal is of great value for China to demonstrate its responsibility as a major country to build a community with a shared future for humanity and to internalize the concept of ecological civilization. The optimization of the energy structure is one of the core components of realizing the carbon-neutral vision. At this stage, the current policy strongly demonstrates China's top-down policy determination for green and low-carbon development, but we also cannot ignore the fact that China's energy revolution is under intense pressure. The transformation of energy structures is a complex social system project. Under the current transformation of energy structures beset by intense pressure and difficulties, there is room for improvement in the energy policy of my country. Based on the current policy framework and the emerging policy structure, the optimization path of energy policy in the future should focus on the following characteristics: 1) improving the coordination and linking of policy entities and establishing a cross-departmental and regional energy structure optimization mechanism; 2) emphasizing the connection and refinement of policy content. All departments and local governments should further refine and develop action and work plans based on the central policy guidelines; 3) concentrate on combining and balancing policy tools, as well as conducting research on the proportion and structure of the three types of policy tools: supply, demand, and environment. Adjust and optimize to maximize the combined effect of multiple policy tools.
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    Carbon emission prediction model during the material production stage for cold zone residential buildings
    LIU Yiming, LIU Nianxiong, XU Peiqi
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 15-23.   DOI: 10.16511/j.cnki.qhdxxb.2022.22.044
    Abstract   HTML   PDF (11566KB) ( 166 )
    A carbon emission prediction model was developed for the material production stage of residential buildings in Chinese cold zones for achieving the national carbon neutrality goals in residential construction. This study first analyzed the compositions and distributions of building material carbon emissions during production of 17 residential buildings. Linear regressions and ridge regressions were then used to relate the building material carbon emissions to the residential building space and volume design parameters. 10 carbon emission prediction models were then developed for the design parameters. The results show that one ridge regression model based on the number of building stories, the floor plan and the primary room configurations most accurately predicts the building material carbon emissions.
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    Whole process simulation method of sulfuric acid decomposition in the iodine-sulfur cycle for hydrogen production
    GAO Qunxiang, SUN Qi, PENG Wei, ZHANG Ping, ZHAO Gang
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 24-32.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.037
    Abstract   HTML   PDF (4786KB) ( 148 )
    The high-temperature gas-cooled reactor is a typical fourth-generation nuclear reactor. It has a high core outlet temperature and great potential for process heat utilization. Thermochemical iodine-sulfur cycle hydrogen production is an essential method of process heat utilization of high-temperature gas-cooled reactors that can achieve large-scale, low-carbon hydrogen production and is consistent with the two-carbon strategic goal of China. Furthermore, the development and utilization of clean energy can effectively alleviate the global energy crisis, and hydrogen energy is considered the most promising source of energy in this century and is receiving continuous attention from the industry. The iodine-sulfur cycle includes three chemical reactions: Bunsen, sulfuric acid decomposition, and hydroiodic acid decomposition. Sulfuric acid decomposition is carried out under high temperature and strong corrosive environment, and involves multiple physical and chemical processes such as flow, heat transfer, phase transition and reaction. Therefore, obtaining the thermal and reaction details of this link is critical for improving the efficiency of the iodine-sulfur cycle. Furthermore, it is crucial to study the thermal and decomposition reaction laws of the fluid in the bayonet sulfuric acid decomposition heat exchanger to improve the decomposition rate of sulfuric acid. In this study, the classical Lee model was improved by analogy to the phase transition mass transfer equation and the component transport equation using the phase transition mass transfer rate constant instead of the chemical reaction rate constant, and a coupled model of the sulfuric acid phase transition and two-step decomposition reaction was established. The whole process of sulfuric acid decomposition was simulated, and the effect of the specific surface area of catalyst particles on the decomposition was analyzed. The results show that the temperature of the catalytic reaction zone inside the bayonet heat exchanger meets the requirements of the sulfuric acid core reaction. The phase transition process is relatively brief, yet it can effectively enhance the direct heat exchange between sulfuric acid and helium. As the sulfuric acid flow increases, the length of the two-phase section also increases. The first decomposition and phase transition of sulfuric acid occur almost simultaneously. The reaction is complete, the conversion rate is high, and the molar fraction of sulfur trioxide is up to 46%. The second-step decomposition of sulfuric acid permeates the entire zone of catalytic activity. The first half zone has a high conversion rate, and the sulfur dioxide molar fraction is up to 33%. Since the gas mixture in the inner tube continuously transfers heat to the sulfuric acid in the annulus, the temperature of the gas mixture at the outlet is lowered, and a small amount of sulfuric acid is produced. The research results also show that when the specific surface area of the catalyst particles is large, the overall rate of sulfuric acid decomposition is significantly improved. The highest rate of sulfuric acid decomposition under the design conditions is about 85%.
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    MECHANICAL ENGINEERING
    Modal parameter estimates for a magnetic levitation planar motor based on density clustering
    SUN Haobo, YANG Kaiming, ZHU Yu, LU Sen
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 33-43.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.027
    Abstract   HTML   PDF (14745KB) ( 118 )
    Lightweight designs are needed for high acceleration and deceleration rates of a magnetic levitation planar motor (MLPM), but lightweight designs also lead to unacceptable vibrations in the MLPM. Accurate estimates of the MLPM modal parameters are the key to suppressing the vibrations. This paper presents a modal parameter estimation method based on density clustering. The system parametric frequency response function is obtained using a two-step iterative identification algorithm. Then, the DBSCAN algorithm is used for the modal analysis to remove the unstable mathematical modes. The outliers of the physical modes are also removed based on a normal distribution to obtain the final modal parameters. Simulations and tests show that this method can accurately estimate the system modal parameters.
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    Joint surface model based on total reflection optical image
    LUO Zhijun, YAN Shaoze
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 44-51.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.030
    Abstract   HTML   PDF (8393KB) ( 84 )
    The joint contact characteristics of mechanical structures strongly influence the stiffness, damping and other dynamic characteristics. Many contact models have been developed, but they are based on different theoretical models with some even giving diametrically opposed physical interpretations. This study used the total reflection method and image processing to study the evolution of polymethyl methacrylate (PMMA) contact spots loaded with positive pressures. The observations show that the variation of contact spots under normal force has three stages: the first stage is the linear increasing region under light loads, the second and third stages are the nonlinear increasing region under heavy loads. With light loads, the number of contact spots and the contact area are consistent with those predicted by the Greenwood-Williamson (GW) model, but differ from those predicted by the Majumdar-Bhushan (MB) model. With heavy loads, the GW and MB models both cannot adequately predict the physical characteristics due to the bulk deformation with heavy loads leading to smaller actual contact areas. Thus, more accurate contact model would be obtained when the influence of the bulk deformation is considered.
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    Influence mechanism of the oil film contact surface error of the hydrostatic guide rail on the geometric error of the guide rail
    ZHANG Yun, CAO Cong, JIANG Zhong, WANG LiPing
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 52-61.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.035
    Abstract   HTML   PDF (8630KB) ( 95 )
    With the advancement of research and development of weapon systems in China's major scientific projects and defense fields, diamond turning is widely used in the processing of high-performance parts, such as the precision of physical experimental samples and optical parts. Hydrostatic guideways have become the core components of ultraprecision lathes due to their high precision and rigidity. Its geometric error is critical in determining the machining accuracy of ultraprecision lathes. The main factors influencing the geometric errors of hydrostatic guide rails are manufacturing and assembly errors, as well as deformation errors caused by oil pressure. In this research, the fluid-structure coupling simulation model and the bolt preloading mechanical model of the hydrostatic guideway are established to improve the geometric error and accuracy retention of the hydrostatic guideway and discuss the structural deformation and bolt preloading of the hydrostatic guideway caused by the pressure due to oil supply and other factors. The error in a mathematical model to characterize the oil film joint surface under the action of manufacturing, assembly, and oil pressure is proposed using the deformation law of tight structure, and the deformation error transfer model under the action of oil film homogenization is established. The influence mechanism of the oil film bonding surface error of the guide rail pressure plate on the geometric error of the worktable is analyzed. The research shows that the error in the homogenization effect of the pressure oil film of the hydrostatic guideway makes its geometric error much lower than the surface error of its parts, but the pressure oil film cannot completely homogenize the error on the oil film joint surface. The increase in amplitude, linear error PV value, and angle error PV value are proportional as the two types of errors have the same growth rate. The maximum linear and angular geometric errors are 0.22 μm and 1.58 μrad, respectively. This shows that as the surface shape error of the parts increases, the equalization effect of the oil film error gradually weakens. Furthermore, the straightness error of the hydrostatic guideway is more sensitive to the error amplitude of the oil film joint surface, whereas the angle error is more affected by the wavelength and phase difference of the joint surface error function. Finally, the method is used to design the tolerance of hydrostatic guide rail components. When the error amplitude of the oil film bonding surface is within 3 μm, the experimentally measured guide rail straightness error is 0.26 μm/140 mm, and the pitch angle error is 1.72 μrad. The prediction result of the error transfer model for the geometric error of the worktable is close to the experimentally measured result, implying that the model prediction is adequate and the angle error is relatively large. This method can effectively predict the geometric error of the worktable in the designing stage of the guideway. Further, the present research provides a valuable reference for the manufacture and equipment of the hydrostatic guideway of the ultraprecision lathe.
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    Control parameter optimization of underwater gliders for underwater fixed-point exploration missions
    WU Qingjian, WU Hongyu, JIANG Zhihong, YANG Yunqiang, YAN Shaoze, TAN Lijie
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 62-70.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.032
    Abstract   HTML   PDF (5374KB) ( 169 )
    As a novel ocean exploration robot, an underwater glider can achieve space motion by adjusting its net buoyancy and attitude. In some exploration missions, the underwater glider must reach a specific location for virtual mooring and perform a fixed-point exploration, including the health monitoring of underwater equipment. The typical research aim is for the glider to reach the target exploration area at the earliest by consuming the minimum possible energy. To achieve this goal, the optimal control parameter configuration of the underwater glider must be determined. Therefore, this paper proposes the control parameter optimization method of underwater gliders for fixed-point exploration missions based on the dynamic theory, surrogate model technology, and multi-objective optimization algorithm. First, considering a typical underwater glider as the research object, this paper establishes the whole glider dynamic model using the Newton-Euler method. This dynamic model contains eight degrees of freedom and considers the effects of seawater density variation and hull deformation on the glider's net buoyancy. Considering the energy consumption of buoyancy adjustment, attitude adjustment, control, and measurement systems, the energy consumption model of the glider diving motion is established. On this basis, the sample points are obtained using an optimal Latin hypercube experimental design and dynamic simulation, and subsequently, the surrogate models are established using a quartic polynomial to fit the obtained sample points. Here, the input parameters of the quartic polynomial are the amounts of glider net buoyancy adjustment and movable internal mass block translation, and the output parameters are the energy consumption, diving motion time, and horizontal displacement of the glider to reach the target depth. Next, a mathematical optimization model is proposed. Specifically, the glider control parameters are selected as the optimization design variables; the optimization objective is to minimize the glider energy consumption and the diving motion time, simultaneously, and the horizontal displacement is used to construct the constraint. The surrogate models are employed to participate in the optimization calculation, which can improve the calculation efficiency. Finally, the non-dominated sorting genetic algorithm II is used to solve the abovementioned optimization problem. A numerical example is provided to validate the proposed optimization method. After optimization calculation, the Pareto optimal set is obtained, consisting of 74 sets of non-dominated solutions of control parameter values. The analysis results illustrate that once the target depth has been determined, the glider horizontal displacement shows an obvious difference under different control parameter values, implying that the glider can employ different control parameter configurations to perform underwater fixed-point exploration missions. Under a specific target depth, the quartic polynomial can accurately describe the mapping relationship among the net buoyancy adjustment amount, movable internal mass block translation amount, glider energy consumption, diving motion time, and horizontal displacement. Besides, the functional relationship between the glider control and performance evaluation parameters shows obvious nonlinearity and nonmonotonicity. Optimization results of the control parameters demonstrate a contrasting relationship between the energy consumption and the diving motion time of the glider. For practical engineering missions, the selection rule of the optimal solution is listed, and the optimization results are verified via dynamic simulation. On the basis of the dynamic theory, surrogate model technology, and multiobjective optimization algorithm, the proposed optimization method exhibits high calculation efficiency and can be used for guiding the glider control parameter configuration in actual fixed-point exploration missions. Besides, this optimization method is versatile and can be used in various types of underwater gliders.
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    Fluid-structure interaction simulation of sealing shield main drive seal properties
    XIANG Chong, LONG Weiyang, GUO Fei, ZHANG Xinyi, JIANG Jie
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 71-77.   DOI: 10.16511/j.cnki.qhdxxb.2022.26.043
    Abstract   HTML   PDF (6647KB) ( 184 )
    The shield main drive seal is composed of a labyrinth seal and four lip seals. The use process of the first seal of the shield machine main drive includes multiple stages. The lip seal will be greatly deformed during assembly and pressurization. Therefore, the sealing leakage process is actually a two-way fluid-structure interaction (FSI) process. However, the traditional two-way FSI finite element method must have a continuous flow field because the current dynamic mesh technology cannot solve the problem of the topological filling of the discontinuous flow field. In the initial interference assembly condition of the shield main drive seal, the seal flow fields are independent of each other, which makes the leakage simulation calculation process extremely complicated. Furthermore, the deformation process of the main drive seal under the fluid pressure difference not only involves a flow field calculation under large deformation but also includes complex calculation conditions such as the hyperelasticity of the main drive seal, two-phase flow, and non-Newtonian fluids. To analyze the leakage characteristics of the shield main drive sealing system, an FSI calculation method is established. The method is based on Abaqus and Fluent, and it can solve complex calculation conditions such as the hyperelasticity of the main drive seal, two-phase flow, and non-Newtonian fluids. First, the method calculated the assembly process of the sealing ring by Abaqus and given the initial boundary pressure on both sides of the sealing ring to make the clearance slightly open, which is the precondition for calculating the flow field. Second, we extracted the deformed solid model and rebuild the model. Then, the deformed model was used to calculate the flow field in Fluent, and when the flow field reaches a steady state, the fluid pressure on both sides of the sealing ring was collected. Because the grease flows slowly, the dynamic pressure and static pressure differ by an order of magnitude, so the calculation used static pressure in this paper. Then, the flow field pressure was transferred to the solid model, and the deformation of the seal ring was recalculated by Abaqus. Finally, the above calculation process was repeated until the calculation results of the two models converge. The shield main drive sealing experimental system is built, and the effectiveness of the method is experimentally verified. Through the calculation and analysis, it is concluded that when the initial sealing gap is large, the backflow will not produce because of the large pressure drop of the grease in the second chamber. Under the design parameters, the flow gradually reaches a steady state, and the seal is no longer deformed. The grease in the first chamber tends to flow back to the second chamber, but finally, the backflow phenomenon is not produced. A complete simulation calculation method is proposes for the study field of shield main drive sealing. The FSI model and results provide a theoretical basis and reference for research on the sealing structure performance of the main drive seal.
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    ELECTRICAL ENGINEERING
    Multiple time scale characteristics of converter network and simulation application of its different fineness models
    ZHANG Shuqing, TANG Shaopu, YU Siqi, LU Xun, ZHANG Donghui
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 78-93.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.031
    Abstract   HTML   PDF (11210KB) ( 152 )
    Multiple converter grid connections, network operations and controls involve a wide range of time scales that lead to many types of dynamics and stability problems. Simulations are often used to research these problems. However, the simulation capabilities and adaptability depend on the model fineness. Therefore, simulations should consider the time scales covered or reflected the physical processes or phenomenon to be studied to select the appropriate fineness model. This paper summarizes the research, applications and development trends of power grid simulations with multiple converters. The common characteristics of converter networking and control are analyzed for one system with a conventional multi-level voltage source converter (VSC) and one system with a modular multi-level converter (MMC) as examples. The response time characteristics of the main circuit and the control links are analyzed to determine the multiple time scale dynamics and time scale divisions of the converter networking. This paper analyzes the matching between the converter model fineness, the simulation requirements and the time scale of the dominant physical process. Simulations show the adaptability of four common fineness converter models, their applications to power systems, and the accuracy of the converter networking time scale analysis. The results also show adaptability of various fineness models and simulations.
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    Loss analyses of multi-port power electronic transformers based on DSIM simulations
    FAN Zhiqiang, ZHAO Zhengming, SHI Bochen, YU Zhujun, ZHENG Jialin
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 94-103.   DOI: 10.16511/j.cnki.qhdxxb.2022.22.041
    Abstract   HTML   PDF (5016KB) ( 185 )
    Multi-port power electronic transformers (PET) are key parts of modern power grids, but their losses are difficult to model. DSIM simulations are useful for complex power electronic converters, especially PET simulations. This study analyzed the losses in a multi-port PET which showed the effectiveness of DSIM software for analyzing complex power electronic converter systems. This paper described the PET loss model which was then validated by simulations and experiments. The results show the optimum efficiency operating point of a single PET. When two PETs are available, one PET in operation is the most efficient for light loads and two PETs in operation are the most efficient for heavy loads. This loss model analysis provides a reference for optimizing the efficiency of PET operations.
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    NUCLEAR ENERGY AND NEW ENERGY
    An effective thermal conductivity model of composite plates with distributed inner heat sources
    LIU Ziping, SUN Jun
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 104-113.   DOI: 10.16511/j.cnki.qhdxxb.2022.26.042
    Abstract   HTML   PDF (1502KB) ( 108 )
    Composite materials are widely used in several fields, such as energy industry. Composite materials have complex geometric structures and are usually equivalent to homogeneous materials in the process of thermal design, and then use an effective thermal conductivity model to describe the heat transfer process. Traditional methods for analyzing the effective thermal conductivity of composite materials are typically based on Fourier's law by applying a constant heat flow at both sides of the material to conserve the heat flows of the real and equivalent uniform materials. For composite materials with internal heat sources, such as nuclear fuel, the value and direction of the internal heat flow are affected by the distribution of heat sources; therefore, the method based on Fourier's law will be unsuitable for analyzing the effective thermal conductivity of such materials. To explore the influence of distributed inner heat sources on the effective thermal conductivity of composite materials, this study considers an infinite two-phase composite plate as the research object. The difference between the effective thermal conductivities with and without the inner heat sources is analyzed, and the effective thermal conductivity model is established for predicting the average temperature of the composite plate with inner heat sources. The influence of the distribution of inner heat sources on the effective thermal conductivity is analyzed. Furthermore, the influence of randomly distributed inner heat sources on effective thermal conductivity is quantified. Results show that the effective thermal conductivity for predicting the average temperature of the composite plate with inner heat sources is not affected by the size of the heat sources but by the locations of each heat-generating filling plate, i.e., the distribution of the inner heat sources. Additionally, it is affected by the number and size of the filling plates. The deviation in the effective thermal conductivities when the filling plates are randomly and uniformly distributed can be approximated as a normal distribution, and that for uniformly distributed filling plates is approximately equal to the highest probability case for the randomly distributed plates. The deviation of the effective thermal conductivity for the randomly and uniformly distributed filling plates decreases with the increasing number of plates (i.e., the more dispersed heat source distribution). The effective thermal conductivity of the composite plate with inner heat sources depends on more factors than the plate with no inner heat source, including the number and size of the filling plates and the distribution of the inner heat sources. The effective thermal conductivity of composite flat plate with randomly distributed heat generating plates is approximately same as that when they are uniformly distributed. The ideas and methods present in this study lay the foundation for future research on the effective thermal conductivity of nuclear reactor fuel.
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    Influence mechanisms and optimization of pulsed breakdown characteristics of a megavolt class cascade self-triggered switch
    WANG Tianchi, XIE Linshen, LI Junna, YANG Youheng, HUANG Tao, CHEN Zhiqiang, GUO Fan, DU Yingchao, CHEN Wei
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 114-124.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.034
    Abstract   HTML   PDF (5579KB) ( 77 )
    Megavolt switches in large pulses typically use a cascade configuration to increase operating voltage and ensure electric field uniformity. We must reduce their jitter when we need multipulse superposition to obtain a higher voltage pulse. Moreover, in the high-altitude electromagnetic pulse simulator, the megavolt pulsed switch needs to adopt the self-triggering technique to simplify the structure and reduce weight for good mobility and hoisting requirements. However, it is difficult to carry out a large number of experiments and completely reveal the influence mechanisms of the characteristics of megavolt switches via experiments. Therefore, this paper derives a breakdown probability distribution model of a cascade switch based on the characteristic parameters of a single-stage switch, which can be obtained easier from experiments. The experiment of the cascade switch is then carried out to validate the model analysis. In the model, we assume that the rising rate of the pulse voltage is a constant and that the breakdown probability of a single-stage switch follows the Weibull distribution. It is common in cascade switches to allow one stage to break down in triggered mode while others are in self-breakdown mode due to overvoltage. When the cascade switch has n stages, and one stage is triggered, it is seen as three equivalent switches based on their breakdown sequence. This paper assumes that x self-breakdown stages first close, the triggered stage closes next, and finally, (n-x-1) self-breakdown stages close. The range of x is [0, n-1]. Then the breakdown probability distribution model of each equivalent switch can be derived using the existing calculation method. The influence mechanisms of the breakdown characteristics of a three-stage cascade switch are semiquantitatively analyzed based on the model. If the uniformity of breakdown characteristics of each triggered switch can be guaranteed and three stages are all triggered, the switch jitter is approximately equal to the jitter of a single-stage triggered switch because the latter two stages break down under high overvoltage. The mean breakdown voltage of each stage is lower than a single-stage triggered switch. When only one stage is triggered, most probably, the triggered stage will close first, followed by two self-breakdown stages under high overvoltage. The switch jitter is significantly lower than that under the self-breakdown mode, and the mean breakdown voltage of each stage is higher than that when all three stages are triggered. However, the possibility that one self-breakdown stage closes first results in the switch jitter being approximately twice when all three stages are triggered. The improvement path of the switch jitter characteristics can be inferred using the model analysis. The jitter of the triggered single-stage switch should be reduced, and it is preferable to trigger all stages if the uniformity of the breakdown characteristic of each single-stage switch can be guaranteed. In the experiment, the three-stage cascade switch operates on a pulsed voltage with a rise time of 300 ns and an operating voltage range of 0.8—2.0 MV. Self-triggered continuous preionization is adapted to eliminate the influence of trigger gap jitter on the switch jitter, ensuring the uniformity of breakdown characteristics of each single-stage switch. When one stage is triggered, the time delay jitter is 2.9—7.7 ns, and the breakdown voltage jitter is 0.51%—2.21%. When three stages are triggered, the time delay jitter is 2.3—3.6 ns, and the breakdown voltage jitter is 0.59%—0.91%. The preceding analysis is verified, and the switch characteristics are improved compared to the original one. Moreover, the self-breakdown characteristic is more stable when the gas pressure exceeds 0.3 MPa; thus, the switch jitter is approximately the same when we trigger one or three stages.
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    CIVIL ENGINEERING
    Robust comprehensive evaluation of guide vane closure law in hydraulic turbines in moderately-high head hydropower plants
    LI Huokun, WAN Zihao, HUANG Wei, ZENG Min, FANG Jing, XIE Jie
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 125-133.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.033
    Abstract   HTML   PDF (2433KB) ( 88 )
    The closing law of guide vane in hydropower station affects the resulting hydraulic transients, which should have good robust performance. A comprehensive evaluation method for the robustness of guide vane closing law is constructed by using a penalty function and a weight function to generate the evaluation indexes of the hydraulic turbine's large fluctuation transition process from maximum rise rate of spiral case pressure, maximum draft tube vacuum, and maximum rise rate of rotational speed. The effect of various inflection point times, inflection point opening degrees, and guide vane effective closing times for a moderately-high head hydropower station on these three regulating guarantee parameters were evaluated by the orthogonal experimental method. The results show that the effective closing time has a relatively small influence on these three regulating guarantee parameters, while the inflection point opening degree and the inflection point time significantly affect the balance between the water hammer pressure and the unit's speed. These evaluation results for the guide vane closing process not only lead to each regulating guarantee parameter having sufficient safety margin, but also to a robust control system. This robustness evaluation method provides a reference for reasonable selection of guide vane closing law of moderately-high head hydropower plants.
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    Automatic extraction of mountain river information from multiple Chinese high-resolution remote sensing satellite images
    XUE Yuan, QIN Chao, WU Baosheng, LI Dan, FU Xudong
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 134-145.   DOI: 10.16511/j.cnki.qhdxxb.2022.22.043
    Abstract   HTML   PDF (15407KB) ( 152 )
    High-resolution geomorphic information from remote sensing images is a key part of mountain river research. However, the complete information about narrow rivers is difficult to extract automatically and accurately from complex backgrounds, especially with mountain shadows. This research uses a random forest (RF) algorithm with an artificial neural network (ANN), RF-ANN, to analyze remote sensing images. This method supports parallel operations and reduces the scale of the infrared data for noise removal to achieve pixel-level extraction of the river surfaces. The RivWidthCloud (RWC) method is improved using Laplacian and edge algorithms for automatic extraction of the bankfull river widths. The improved RWC method is generalizable since it does not require setting the discriminant threshold manually. The method is then applied to the Huangfuchuan River Basin on the Loess Plateau, China using images from the Chinese GF-1 and ZY-3 satellites as the primary data source to extract the river surfaces and widths of the rivers above level 2. The results show that the RF-ANN method has a 94.7% accuracy for extracting river surfaces. The bankfull river width extraction error is 1.07 m (about 0.5 pixels) and the minimum river width extracted by these methods is 6.1 m (about 3 pixels). R2 is 0.93 and the root men square errors (RMSE) is 1.52 for fitting the extracted river widths and the test river widths. For small rivers narrower than 10 m, the extraction error is 18.5%, for widths from 10 to 30 m the error is 8.8%, for widths from 30 to 90 m the error is 2.0%, and for rivers wider than 90 m the error is 0.7%. These results provide accurate datasets for watershed topography research in mountainous and other complex topographic regions.
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    PUBLIC SAFETY
    Crowd counting model for evacuation scenarios based on a cascaded CNN
    DENG Qing, ZHANG Bo, LI Yihao, ZHOU Liang, ZHOU Zhengqing, JIANG Huiling, GAO Yang
    Journal of Tsinghua University(Science and Technology). 2023, 63 (1): 146-152.   DOI: 10.16511/j.cnki.qhdxxb.2022.21.029
    Abstract   HTML   PDF (8355KB) ( 214 )
    Accurate crowd counts during evacuations can support real-time optimization of evacuation routes and scheduling of emergency resources. This study estimates the number of occupants in an evacuation passageway by setting a classification level and personnel density together in a cascaded convolutional neural network (CNN) crowd counting model based on analyses of existing methods. The method avoids the loss of image information and over fitting in the convolution process. The model estimates the real-time crowd count in crowded situations by learning the relationship between the number and the position of occupants in the image and by changing the image features. The model was implemented on the PyTorch platform with an identification accuracy for the validation set (612 photos) of 84.2% and for the test set (182 photos) of 83.6%, which shows that this method can accurately predict the number of evacuees in a monitoring screen.
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