Review of the Lightning Shielding Against Direct Lightning Strokes Based on Laboratory Long Air Gap Discharges

长间隙放电是研究地面物体雷电屏蔽问题的最有效手段之一。首先介绍了国内外在雷电击距、直击雷防护措施的屏蔽性能试验和雷电迎面先导过程研究3个方面所取得的进展，并结合最新开展的长达6m问隙尺度的放电试验观测，对现阶段上述3个方面研究所存在的问题进行了分析。认为基于雷电击距建立的电气几何模型（electricgeometrymodel，EGM）仅适用于小尺度目标物的雷电屏蔽性能分析，现有的雷电屏蔽模拟试验方法仅能近似模拟不存在雷电迎面先导时的雷击过程，无法完全证明以提前流注发射模型装置为代表的非传统防雷装置具有比传统措施更优越的屏蔽性能。大尺度目标物的雷电屏蔽问题应聚焦于雷电迎面先导过程的研究，建立并完善雷电迎面先导过程的模拟试验方法和物理仿真模型。

Resistance Characteristics of Arc in Long Air Gap

Arc resistance is an important parameter for characterizing long arcs in air, and its laboratory testing is of importance for accurate arc modeling of electromagnetic transient caused by short circuit fault. Therefore, we constructed an experimental system to study the cha- racteristics of long AC arc in air. Driven by currents of 10 kA or 40 kA (root mean square value), the system produces arcs with different initial lengths of 1 m, 2 m and 4 m, and the movement of the arcs are captured by a high-speed camera. After performing experiments using the system, we carried out analysis and comparisons of the arc resistance of arcs with different lengths and different currents, as well as a study of the relationship between the macro-morphology and the resistance of the arcs. Conclusions were drawn from the experimental re- sults: the arc voltage had obvious saturation characteristics; the arc resistance increased with the increase of arc length and the decrease of current; the arcs bended or extended significantly in time and the peak arc voltage within a single cycle increased correspondingly; the arcs had voltage and current in the same phase. In the end, a formula of arc resistance based on the experiment results is derived.

Experimental Study on Branch and Diffuse Type of Streamers in Leader Restrike of Long Air Gap Discharge

One of the main problems in the Ultra High Voltage （UHV） transmission project is to choose the external insulation distance, which requires a deep understanding of the long air gap discharge mechanism. The leader-streamer propagation is one of most important stages in long air gap discharge. In the conductor-tower lattice configuration, we have measured the voltage, the current on the high voltage side and the electric field in the gap. While the streamer in the leader-streamer system presented a conical or hyperboloid diffuse shape, the clear branch structure streamer in front of the leader was firstly observed by a high speed camera in： the experiment. Besides, it is found that the leader velocity, width and injected charge for the branch type streamer are greater than those of a diffuse type. We propose that the phenomenon results from the high humidity, which was 15.5-16.5 g/m3 in our experiment.

Experimental study of rotating wind turbine breakdown characteristics in large scale air gaps

When a wind turbine is struck by lightning,its blades are usually rotating.The effect of blade rotation on a turbine＇s ability to trigger a lightning strike is unclear.Therefore,an arching electrode was used in a wind turbine lightning discharge test to investigate the difference in lightning triggering ability when blades are rotating and stationary.A negative polarity switching waveform of 250/2500 μs was applied to the arching electrode and the up-and-down method was used to calculate the 50%discharge voltage.Lightning discharge tests of a 1：30 scale wind turbine model with 2,4,and 6 m air gaps were performed and the discharge process was observed.The experimental results demonstrated that when a 2 m air gap was used,the breakdown voltage increased as the blade speed was increased,but when the gap length was 4 m or longer,the trend was reversed and the breakdown voltage decreased.The analysis revealed that the rotation of the blades changes the charge distribution in the blade-tip region,promotes upward leader development on the blade tip,and decreases the breakdown voltage.Thus,the blade rotation of a wind turbine increases its ability to trigger lightning strikes.

Sensitivity Analysis of Air Gap Motion with Respect to Wind Load and Mooring System for Semi-Submersible Platform Design

A design of semi-submersible platform is mainly based on the extreme response analysis due to the forces experienced by the components during lifetime. The external loads can induce the extreme air gap response and potential deck impact to the semi-submersible platform. It is important to predict air gap response of platforms accurately in order to check the strength of local structures which withstand the wave slamming due to negative air gap. The wind load cannot be simulated easily by model test in towing tank whereas it can be simulated accurately in wind tunnel test. Furthermore, full scale simulation of the mooring system in model test is still a tuffwork especially the stiffness of the mooring system. Owing to the above mentioned problem, the model test results are not accurate enough for air gap evaluation. The aim of this paper is to present sensitivity analysis results of air gap motion with respect to the mooring system and wind load for the design of semi-submersible platform. Though the model test results are not suitable for the direct evaluation of air gap, they can be used as a good basis for tuning the radiation damping and viscous drag in numerical simulation. In the presented design example, a numerical model is tuned and validated by ANSYS AQWA based on the model test results with a simple 4 line symmetrical horizontal soft mooring system. According to the tuned numerical model, sensitivity analysis studies of air gap motion with respect to the mooring system and wind load are performed in time domain. Three mooring systems and five simulation cases about the presented platform are simulated based on the results of wind tunnel tests and sea-keeping tests. The sensitivity analysis results are valuable for the floating platform design.

Study on Axial Flux Hysteresis Motors Considering Airgap Variation

Axial flux hysteresis motor (AFHM) is self-starting synchronous motor that uses the hysteresis characteristics of magnetic materials. It is known that the magnetic characteristics of hysteresis motor could be easily affected by air gap and structure dimensions variation. Air gap length plays an important role in flux distribution in hysteresis ring and influences the output torque, terminal current, efficiency and even optimal value of other structural parameters of AFHM. Regarding this issue, in this study effect of air gap variation on performance characteristics of an axial flux hysteresis motor and effect of air gap length on hysteresis ring thickness and stator winding turns is investigated. Effect of air gap length on electrical circuit model is perused. Finally, simulation of AFHM in order to extract the output values of motor and sensitivity analysis on air gap variation is done using 3D-Finite Element Model. Hysteresis loop in the shape of an inclined ellipse is adopted. This study can help designers in design approach of such motors.

Effect of Air Gap under Fabric on Thermal Protective Performance Using an Improved Apparatus

The bench top test is one of the most important and effective methods to evaluate the total thermal protective performance(TPP) of firefighters' protective clothing,which is greatly influenced by the air gaps entrapped.In this paper,to investigate the effect of air gap width on TPP,a new improved apparatus with two height changeable buttons to hold the thermal sensor was developed to get a series of air gap sizes from 0 mm to 40 mm.The TPP of two types of flame-resistant outer fabrics was measured with TPP test apparatus respectively.Analysis of temperature rise with each air gap width was made to determine the effects of different air gaps on protective performance.It was indicated that air gap size had great effect on TPP of fabrics in the bench top test.An air gap width above 8 mm was suggested for the thermal protective clothing design.

Transmission properties of frequency selective structures with air gaps

The transmission properties of compound frequency selective structures with dielectric slab and air gaps were investigated by computation and experimentation. Mechanism analyses were also carried out. Results show that the air gaps have a distinct influence on the transmission properties. Resonant frequency of the structure would increase rapidly when the air gap appears. After the gap gets larger to a specific value, generally 1/5 wavelength corresponding to the resonant frequency, the transmission properties would change periodically with the gap thickness. The change of transmission properties in one period has a close relationship with the dielectric thickness. These results provide a new method for designing a bandpass radome of large incidence angle and low loss with the concept of stealth shield radome.

Air Gap Prediction for Floating Bodies Using a 3D Numerical Wave Tank Approach

Computations for air gap response of a semisubmersible platform based on a 3D numerical wave tank approach are presented.The developed method is in time domain and can consider nonlinearities associated with incident wave and hydrostatic forces exactly in determining the body response, but the interaction hydrodynamics of radiation and diffraction are based on simplified linearization assumptions. The incident wave can be defined by any suitable wave theory and here defined by a fully nonlinear numerical wave model. After verifying the present computations results in its degenerated linearized version against the usual linear 3D Green function–based frequency-domain results for air gap predictions, systematic comparative studies are undertaken between linear and the approximate nonlinear solutions. It is found that nonlinear computations can yield considerably conservative predictions as compared to fully linear calculations, amounting to a difference of up to 30%–40% in the minimum air gap in steep ambient incident waves at high and moderate frequencies.

Impact Analysis of Air Gap Motion with Respect to Parameters of Mooring System for Floating Platform

In this paper, the impact analysis of air gap concerning the parameters of mooring system for the semi-submersible platform is conducted. It is challenging to simulate the wave, current and wind loads of a platform based on a model test simultaneously. Furthermore, the dynamic equivalence between the truncated and full-depth mooring system is still a tuff work. However, the wind and current loads can be tested accurately in wind tunnel model. Furthermore, the wave can be simulated accurately in wave tank test. The full-scale mooring system and the all environment loads can be simulated accurately by using the numerical model based on the model tests simultaneously. In this paper, the air gap response of a floating platform is calculated based on the results of tunnel test and wave tank. Meanwhile, full-scale mooring system, the wind, wave and current load can be considered simultaneously. In addition, a numerical model of the platform is tuned and validated by ANSYS AQWA according to the model test results. With the support of the tuned numerical model, seventeen simulation cases about the presented platform are considered to study the wave, wind, and current loads simultaneously. Then, the impact analysis studies of air gap motion regarding the length, elasticity, and type of the mooring line are performed in the time domain under the beam wave, head wave, and oblique wave conditions.

不同修补技术在较大鼓膜穿孔患者中的治疗效果及对听阈、鼓膜愈合的影响

目的探讨不同修补技术在较大鼓膜穿孔(TMP)患者中的治疗效果及对听阈、鼓膜愈合的影响。方法选取2022年1月至2023年2月120例较大TMP,按照随机数字表法分为3组,各40例。A组采取筛状软骨-软骨膜技术修补,B组采取软骨膜-软骨岛修补,C组采取栅栏软骨-软骨膜技术修补,术后均使用碱性成纤维细胞生长因子凝胶治疗。比较3组鼓膜愈合情况、围术期指标、手术前后听力情况[500、1000、2000、4000 Hz纯音听阈(PTA)、气-骨导差(ABG)平均值]、耳闷及并发症发生情况。结果A组鼓膜愈合率高于B组、C组,术中出血量少于B组、C组,手术时间、耳屏愈合时间、住院时间、术后干耳时间短于B组、C组(P<0.05)。A组术后1、3、6个月PTA、ABG平均值低于B组、C组(P<0.05);A组术后6个月耳闷程度低于B组、C组(P<0.05);A组并发症总发生率较B组、C组降低(P<0.05)。B组与C组上述指标比较差异均无统计学意义(P>0.05)。结论筛状软骨-软骨膜技术治疗较大TMP,能缩短手术时间,减少术中出血量,促进鼓膜愈合,改善耳闷,提高听力,并降低并发症风险。

轴径向静态偏心故障下外转子永磁发电机电磁转矩特性分析

对外转子永磁发电机轴径向静态偏心故障下的电磁转矩特性进行了理论解析、仿真计算和实验验证。首先,在考虑齿槽效应影响条件下,引入磁导修正系数ε(θ),推导出轴径向静态偏心前后气隙磁密表达式,并考虑绕组分布情况,构建一种新型的永磁发电机轴径向静态偏心故障下相电流数学模型,在此基础上得到轴径向偏心下电磁转矩解析表达式。其次,通过有限元仿真计算得到外转子永磁发电机轴径向静态偏心下的电磁转矩波动特性。最后,在一台外转子永磁发电机上通过故障模拟实验实例印证了解析分析和仿真计算结果。结果表明:相较于正常情况,径向偏心时,电磁转矩以直流、二倍频、四倍频成分为主,随着偏心程度的加剧,电磁转矩的直流、二倍频、四倍频成分幅值将随着增大;轴向偏心时,电磁转矩谐波成分不变,随着偏心程度的加剧,各谐波成分幅值将随着减小。对外转子永磁发电机气隙偏心故障分析的一个重要补充,对此类问题的现场检测和鉴定具有参考价值。

温度对车载高压电缆终端局部放电特性的影响研究

电缆终端作为高速列车的一个重要设备,在接触网与列车之间的能量传递中起着重要作用。然而由电缆终端界面缺陷引起的局部放电,严重影响着电缆终端的绝缘性能与列车的行车安全。通过实验的方法,制作了含有不同长度气隙的电缆终端试验品,并在27.5 kV的电压条件下进行了实验,探究了不同缺陷长度的电缆终端在不同环境温度下的局部放电特性。结果表明,相同气隙长度的电缆终端在27.5 kV的电压条件下,其局部放电量随着环境温度呈现出先上升后下降的趋势。根据这一特性可以知道在一定的电压等级之下,电缆终端的放电量主要受温度影响,当超过某一温度后,由于温升导致的电缆终端内部压力变化成为影响局部放电量的主要因素,并且随着气隙长度的增加,同一温度下的局部放电起始电压呈现增加的趋势。

40例鼓室成形术临床结果分析

目的研究Ⅲ、Ⅳ型鼓室成形术后听力改善的情况。方法回顾性总结美国匹兹堡大学医学中心眼耳研究所从1992-2000年随访的40例鼓室成形术病人,其中Ⅲ型和Ⅳ型各20例。Ⅲ型中男6例,女14例。平均年龄43.55岁(范围11-79岁)。Ⅳ型中男3例,女17例,平均年龄45.11岁(范围12-74岁)。Ⅲ型中9例有胆脂瘤。Ⅳ型中7例有胆脂瘤。20例Ⅲ型病人中4例采用金伯格人工听骨,16例采用自体-异体砧骨。20例Ⅳ型病人中10例采用金伯格人工听骨,4例采用理查德人工听骨,6例采用鲁森斯基人工听骨。所有的病人都行Ⅰ期听骨链重建。结果所有的病人在术后3月、6月、12月随访。Ⅲ型术后气骨导差距≤20dB者15例,占75%。Ⅳ型术后气骨导差距≤20dB者12例,占60%。结论Ⅲ、Ⅳ型鼓室成形术都能得到较好的听力改善。

棒电极直径和端部形状对短空气间隙交流击穿电压的影响研究

空气间隙交流击穿特性试验常采用棒棒、棒板电极作为试验电极,目前棒电极直径、棒端部形状对空气间隙交流击穿电压的影响尚不清晰。为此,用直径2 cm、1 cm的圆棒和直径1 cm的尖棒搭建棒棒、棒板电极,开展5~40 cm短空气间隙下的交流击穿试验,研究棒电极直径和端部形状对空气间隙交流击穿电压的影响。结果表明:在试验棒电极直径和端部形状范围内,空气间隙距离5~20 cm时,各棒棒或棒板电极空气间隙击穿电压相对偏差在5.93%~33%之间,随着空气间隙距离增大,棒电极直径和端部形状对棒棒、棒板空气间隙击穿电压的影响逐渐减弱;空气间隙距离大于20 cm时,各棒棒或棒板电极空气间隙击穿电压相对偏差均小于5%。研究认为,空气间隙距离超过20 cm时,棒电极直径和端部形状对棒棒、棒板空气间隙击穿电压无影响。

金属矿小断面硬岩巷道爆破掘进装药结构优化研究

金属矿山小断面硬岩巷道爆破掘进中常面临炸药能量利用率低、单循环进尺低、块度大等问题。以蚕庄金矿-780 m四分层大巷为背景,采用爆破损伤、应力和振速变化作为评价指标,对无封堵、封堵+空气间隔装药、封堵+水介质孔口间隔装药、封堵+水介质两端间隔装药等4种装药结构进行了模拟分析,选取最优的装药结构参数用于现场试验。研究结果表明:添加封堵有效防止了爆破能量的流失,提高了能量利用率。孔口加封水袋延长了封堵时间,保持了孔内较高的爆轰压力和较长的作用时间,从而更有效地破碎岩石。孔底放置水袋代替部分炸药,既保持了爆破效果,又防止了超挖现象。综合各方面考虑,封堵+水介质两端间隔装药结构表现最优。现场试验结果显示,优化后的装药结构使炮孔利用率平均达到95%,比优化前提高了15%。

武钢4号高炉炉缸温度异常升高的机理分析

武钢4号高炉炉缸侧壁温度异常升高,主要原因是炉缸碳砖与炉壳之间存在气隙,煤气串入导致热传导变差,再加上铁口区域渣铁环流加剧,碳砖热面渣铁壳的脱落,使渣铁水直接接触碳砖,导致碳砖温度急剧升高。通过灌浆,治理了串煤气,渣铁凝结层逐渐形成,炉缸温度恢复正常。

Numerical and Experimental Modelling of Gas Flow and Heat Transfer in the Air Gap of an Electric Machine

This work deals with the cooling of high-speed electric machines, such as motors and generators, through an air gap. It consists of numerical and experimental modeling of gas flow and heat transfer in an annular channel. Velocity and temperature profiles are modeled in the air gap of a high-speed test machine. Friction and heat transfer coefficients are presented in a large velocity range. The goals are reached acceptably using numerical and experimental research. The velocity field by the numerical method does not match in every respect the estimated flow mode. The absence of secondary Taylor vortices is evident when using time averaged numerical simulation.