Comparative study on breakdown characteristics of trigger gap and overvoltage gap in a gas pressurized closing switch

Gas pressurized closing switches are one of the most important elements in FLTD-based systems. Improving the trigger performance of gas switches is useful for optimizing the output parameters and the reliability of the FLTD. In this paper, the breakdown characteristics of the trigger gap and the overvoltage gap are studied experimentally. The reasons for the different breakdown performance of the two gaps are also investigated. The results show that the breakdown delay of the trigger gap is more influenced by the trigger voltage, while the breakdown delay of the overvoltage gap is more influenced by the working coefficient and always higher than that of the trigger gap. The jitter of the trigger gap is more influenced by the trigger voltage and accounts more than 60% of the total switch jitter, while the jitter of the overvoltage gap is hardly changed with the trigger voltage as well as the working coefficient and maintains less than 1.4 ns. It is proved that the discharging product from the trigger gap can effectively reduce the breakdown delay and jitter of the overvoltage gap. Based on that, the effect and improvement of pre-ionization on the two gaps are also studied. It is concluded that the jitter of the trigger gap reduces obviously when the pre-ionization is added, while the pre-ionization almost has no effect on the jitter of the overvoltage gap. The jitter of the overvoltage gap is about two times higher than the trigger gap in the pre-ionizing switch.

The influence of pore characteristics on rock fragmentation mechanism by high-voltage electric pulse

High-voltage electric pulse(HVEP)is an innovative low-energy and high-efficiency technique.However,the underlying physics of the electrical breakdown within the rock,and the coupling mechanism between the various physical fields involved in HVEP still need to be further understood.In this study,we establish a 2D numerical model of multi-physical field coupling of the electrical breakdown of porous rock with randomly distributed pores to investigate the effect of pore characteristics(porosity,pore media composition)on the partial electrical breakdown of rock(i.e.the generation of a plasma channel inside the rock).Our findings indicate that the generation of a plasma channel is directionally selective and extends in the direction of a weak electrical breakdown intensity.As the porosity of the rock increases,so does the intensity of the electric field in the‘electrical damage’region—the greater the porosity,the greater the effectiveness of rock-breaking.As the fraction of pore fluid(S_(water)/S_(air))gradually declines,the generation time of the plasma channel decreases,and the efficacy of rock-breaking by HVEP increases.In addition,in this study,we conducted an indoor experiment utilizing an electric pulse drill to break down the rock in order to recreate the growth mode of the plasma channel in the rock.Moreover,the experimental results are consistent with the simulation results.In addition,the development of this type of partial electrical breakdown is confirmed to be related to electrode polarity and pore characteristics via the experiment of the symmetrical needle-needle electrode arrangement,which further demonstrates the mechanism of partial electrical breakdown.This research is significant for comprehending the process of electric impulse rock-breaking and gives theoretical guidance and technological support for advancing electric impulse drilling technology.

Improvements in reverse breakdown characteristics of THz GaAs Schottky barrier varactor based on metal-brim structure

The excellent reverse breakdown characteristics of Schottky barrier varactor(SBV)are crucially required for the application of high power and high efficiency multipliers.The SBV with a novel Schottky structure named metal-brim is fabricated and systemically evaluated.Compared with normal structure,the reverse breakdown voltage of the new type SBV improves from-7.31 V to-8.75 V.The simulation of the Schottky metal-brim SBV is also proposed.Three factors,namely distribution of leakage current,the electric field,and the area of space charge region are mostly concerned to explain the physical mechanism.Schottky metal-brim structure is a promising approach to improve the reverse breakdown voltage and reduce leakage current by eliminating the accumulation of charge at Schottky electrode edge.

Improvement of breakdown characteristics of an A1GaN/GaN HEMT with a U-type gate foot for millimeter-wave power application

In this study, the physics-based device simulation tool Silvaco ATLAS is used to characterize the electrical properties of an AlGaN/GaN high electron mobility transistor （HEMT） with a U-type gate foot. The U-gate AlGaN/GaN HEMT mainly features a gradually changed sidewall angle, which effectively mitigates the electric field in the channel, thus obtaining enhanced off-state breakdown characteristics. At the same time, only a small additional gate capacitance and decreased gate resistance ensure excellent RF characteristics for the U-gate device. U-gate AlGaN/GaN HEMTs are feasible through adjusting the etching conditions of an inductively coupled plasma system, without introducing any extra process steps. The simulation results are confirmed by experimental measurements. These features indicate that U-gate A1GaN/GaN HEMTs might be promising candidates for use in millimeter-wave power applications.

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.

Consistency of intensity ratio between spectral lines with similar self-absorption characteristics during ungated laser induced breakdown spectroscopy measurements

This work reports that the intensity ratio of spectral lines having similar self-absorption characteristics during laser induced breakdown spectroscopy(LIBS) analysis can become nearly constant over a wide range of irradiation conditions if the intensities are integrated over a sufficiently long time. It is shown that the plasma temperature and intensity ratio of these spectral lines have temporal similarity. The spectral lines with similar self-absorption properties may be selected to improve the accuracy and consistency of LIBS analysis results under an environment with fluctuating measurement conditions.

Dynamic Characteristics in Subnanosecond Breakdown and Generation of Supershort Avalanche Electron Beam

In this paper,subnanosecond-pulse and one-nanosecond-pulse generators are used to study the breakdowns in highly overvolted gaps in atmospheric pressure air.With different cathodes,we measured the applied voltage and discharge current to investigate the dynamic characteristics in the subnanosecond breakdown during the generation of a supershort avalanche electron beam.Especially,characteristics of dynamic displacement current are presented in the current paper,which is detected between the ionization wave front and a plane anode.It is shown that during a subnanosecond voltage rise time,the amplitude of the dynamic displacement current can be higher than 4 kA.It is demonstrated that the breakdown in the air gap is assisted by ionization processes between the ionization wave front and a plane anode.

Spectral characteristics of underwater laser-induced breakdown spectroscopy under high-pressure conditions

Laser-induced breakdown spectroscopy(LIBS) has been proven to be an attractive technique for in situ oceanic applications.However,when applying LIBS into deep-sea,the pressure effect caused by different ocean depths is inescapable and could have great influence on the LIBS signals.In this work,spectral characteristics of underwater LIBS were investigated as a function of pressure in the range of 0.1-45 MPa.A high-pressure chamber built in the laboratory was used to simulate the high-pressure deep-sea environment.Optimal laser energy and detection delay were first determined under different pressure conditions and were shown to be independent of the external pressure.The increase in pressure has a significant impact both on the peak intensity and line broadening of the observed spectra.The peak intensity of Na,Li and K lines increases with the increasing pressure until a maximum intensity is reached at 12.5 MPa.Above this value,the peak intensity decreases gradually up to 45 MPa.For Ca line,the maximum intensity was observed at 30 MPa.The line broadening keeps constant at low pressures from 0.1-10 MPa,while it increases linearly at higher pressures,indicating a higher electron density caused by the compression effect of the high external pressure.We also compared the spectral data obtained from the high-pressure chamber and from the field sea trials,and the good consistency between the laboratory data and sea-trial data suggested the key role of pressure effect on underwater LIBS signals for practical deep-sea applications.

Study on the Mathematical Model of Dielectric Recovery Characteristics in High Voltage SF6 Circuit Breaker

According to the stream theory, this paper proposes a mathematical model of the dielectric recovery characteristic based on the two-temperature ionization equilibrium equation. Taking the dynamic variation of charged particle＇s ionization and attachment into account, this model can be used in collaboration with the Coulomb collision model, which gives the relationship of the heavy particle temperature and electron temperature to calculate the electron density and temperature under different pressure and electric field conditions, so as to deliver the breakdown electric field strength under different pressure conditions. Meanwhile an experiment loop of the circuit breaker has been built to measure the breakdown voltage. It is shown that calculated results are in conformity with experiment results on the whole while results based on the stream criterion are larger than experiment results. This indicates that the mathematical model proposed here is more accurate for calculating the dielectric recovery characteristic, it is derived from the stream model with some improvement and refinement and has great significance for increasing the simulation accuracy of circuit breaker＇s interruption characteristic.

Study on the Pulsed Flashover Characteristics of Solid-Solid Interface in Electrical Devices Poured by Epoxy Resin

In electrical devices poured by epoxy resin, there are a lot of interfaces between epoxy resin and other solid dielectrics, i.e. solid-solid interfaces. Experiments were carried out to study the flashover characteristics of two typical solid-solid interfaces （epoxy-ceramic and epoxy~ PMMA） under steep high-voltage impulse for different electrode systems （coaxial electrodes and finger electrodes） and different types of epoxy resin （neat epoxy resin, polyether modified epoxy resin and polyurethane modified epoxy resin）. Results showed that, the flashover of solid-solid interface is similar to the breakdown of solid dielectric, and there are unrecoverable carbonated tracks after flashover. Under the same distance of electrodes, the electric stress of coaxial electrodes is lower than that of finger electrodes; and after the flashover, there are more severe breakdown and larger enhanced surface conductivity at interface for coaxial electrodes, as compared with the case of finger electrode. The dielectric properties are also discussed.

冷热冲击下纳米粒子改性硅橡胶/环氧树脂绝缘修复涂料耐电特性

为解决现有绝缘修复材料在大范围温度波动时出现绝缘件剥落、二次开裂、电气失效等问题,研制了多种纳米粒子填充的硅橡胶/环氧树脂复合修复涂料,对比分析了冷热冲击循环前后修复涂料的粘合强度、电气性能和微观结构。结果表明:冷热冲击会造成修复涂料分子主链断裂,产生低分子产物和氧化游离基团,导致涂料相对介电常数和介质损耗增大,击穿场强减小;而功能纳米母粒可提高界面结合能,阻碍聚合物分子的拉伸和扭转,使涂料具有相对稳定的化学结构,可防止冷热冲击下基体内局部劣化通道的延伸,从而有效抑制修复涂料耐电性能的下降;当玻璃纤维、纳米ZnO、SiO_(2)共混,填充量(质量分数)为16%时,涂料电气性能良好,且硅橡胶/环氧树脂复合修复涂料固化时间短,无溶胀现象发生,附着力良好,为设备绝缘快速修复提供可能。

红外偏振辐射模型对铬和铜元素的优化识别

针对优化激光诱导击穿光谱技术识别铬和铜元素的问题,本文提出了一种结合红外偏振检测理论的方法。利用菲涅尔反射定律和基尔霍夫理论分析了红外偏振产生机理,根据能量守恒定律验证了目标红外自发热辐射的偏振特性;利用红外偏振辐射传输模型及菲涅尔反射穆勒矩阵,结合温度的麦克斯韦-玻尔兹曼分布特性,推导出了等离子体红外偏振辐射信号模型,用于识别各激发能量下的重金属元素;利用5种不同能量的脉冲激光验证了模型的精准性。结果表明:在相同的激光能量下,红外偏振辐射光谱可以收集到更多的特征信号峰;在低能量激励下,红外偏振辐射光谱的优势更为突出。红外偏振辐射模型的构建是科学有效的,可用于优化重金属元素的快速检测。

柔直换流阀用相变冷却工质击穿特性研究

相变冷却技术冷却效率高、安全可靠,有望攻克柔直换流阀散热瓶颈,实现其高压、大容量发展,在换流阀冷却领域具有良好的应用前景。为实现相变冷却换流阀绝缘系统可靠设计,该文设计研制了三相态相变冷却工质可调频绝缘特性测试专用平台,并以换流阀用相变冷却工质为研究对象,依据换流阀典型运行工况,利用专用测试平台开展了相变冷却工质三相态、50~300 Hz频率范围的绝缘击穿特性研究。得到了相变冷却工质击穿特性随工质相态及电压频率的变化规律,分析了相变冷却工质不同相态的击穿机理,揭示了工质相态和电压频率变化对冷却工质击穿特性的影响机制。研究工作不仅解决了高频下相变冷却工质的三相态耐压测试方法问题,且所得研究成果为相变冷却柔直换流阀的绝缘设计奠定了基础,同时可指导相变冷却技术在其他电力电子器件及装备上的应用。

SF_(6)/N_(2)介质条件下GIS隔离开关操作动态击穿特性研究及其操作过电压的计算

从工程应用角度来看,SF_(6)/N_(2)混合气体仍是替代纯SF_(6)气体最有效的方案。为给SF_(6)/N_(2)混合气体作为介质的电气设备绝缘配合提供参考,该文针对SF_(6)/N_(2)混合气体不同混合比和不同气压条件下的隔离开关操作过电压开展了研究。SF_(6)/N_(2)混合气体的击穿特性是过电压计算的基础。研究表明:SF_(6)/N_(2)混合气体临界击穿电压与其气体密度分布呈线性关系。为此,该文建立多物理场耦合模型,采用场计算的方法分析SF_(6)/N_(2)混合气体在隔离开关操作过程中的动态击穿特性。同时为模拟隔离开关的动态操作过程,采用了动网格划分技术。随后对不同条件下混合气体的击穿电压进行了数据拟合工作,并采用拟合优度和显著性检验对拟合函数进行评价,最终采用二次多项式函数进行了拟合。在上述研究基础上,利用混合气体动态击穿特性和重燃判据建立了SF_(6)/N_(2)混合气体放电模型,并根据混合气体动态击穿特性解决了混合气体单次放电模型中上升时间常数的计算问题。最后以典型的550kVGIS间隔回路为例对SF_(6)/N_(2)混合气体介质气体绝缘变电站(gas insulated substation,GIS)隔离开关操作过电压进行了计算。计算结果表明:绝缘强度相同的SF_(6)/N_(2)混合气体与纯SF_(6)在隔离开关操作过程中重复击穿产生的特快速暂态过电压VFTO(very fast transient overvoltage,VFTO)波形及幅值相似且击穿次数相同;0.6MPa气室压强条件下,SF_(6)/N_(2)混合气体中SF_(6)体积占比从20%增加至40%时,隔离开关击穿次数及开关过程中VFTO_(SF6)/N_(2)全过程持续时间均可减少50%左右;30%SF_(6)体积占比条件下,气室压强从0.6MPa增加到0.8MPa时,隔离开关击穿次数减少一半、开关过程中VFTO_(SF6)/N_(2)全过程持续时间缩短30%左右。

预电离间隙在不同气体介质中的正负极性击穿特性

针对自触发预电离开关在不同电压等级下应用不同气体介质的需要,以及在双边驱动电磁脉冲模拟器中加载正负极性脉冲的需求,研究了预电离间隙在N_(2)、N_(2)/SF_(6)混合气和SF_(6)中正负极性下的击穿特性。研究了不同介质中间隙的击穿电压和时延随气压的变化规律,对比了正负极性下的间隙击穿特性。研究结果表明,在本文实验条件下,预电离间隙在N_(2)中的击穿过程相对更为稳定,且N_(2)中击穿电压随气压变化线性度最佳。相比N_(2)和混合气,间隙击穿电压仅在SF_(6)中随气压增大呈现饱和趋势。在一定气压范围内,N_(2)在负极性下的击穿电压略高于正极性,混合气和SF_(6)中负极性下的击穿电压明显高于正极性,间隙的击穿过程具有一定的极性效应。相比混合气和N_(2),间隙在SF_(6)中的正负极性击穿电压绝对差值和击穿时延绝对差值都相对较高,为减小双边自触发预电离开关的同步时延差,应首选N_(2)为预电离间隙的绝缘介质。在工程应用中需考虑极性效应和不同气体介质对预电离间隙击穿特性的影响。

诱导触发型气体间隙开关快速绝缘恢复特性

气体间隙开关动作迅速、结构简单,在电力系统中应用前景良好,但对其绝缘恢复特性研究尚少。为此,文中采用双脉冲法研究开关间隙距离、触发介质气压及其种类对气体开关绝缘恢复特性的影响规律。结果表明:诱导触发型气体间隙开关绝缘恢复经历过渡期、快速恢复期和饱和期3个阶段,饱和期的持续时间远大于前两阶段之和,且在快速恢复期无“平台现象”出现。减小开关间隙距离,气体开关绝缘恢复速率渐进增大,其绝缘基本恢复时间(绝缘恢复系数R_(U)>90%)可降低50%。触发介质气压对气体开关绝缘恢复特性影响显著,对绝缘恢复过程的影响特性存在差异。增大触发介质气压会减缓气体开关的绝缘恢复过程,0.1~0.3 MPa压缩干燥空气中,气体间隙开关绝缘基本恢复时间对应为11~40 ms。强电负性SF_(6)对气体开关绝缘恢复具有明显增强作用,其绝缘恢复速率接近于空气中的4倍。研究结果可为气体间隙开关快速绝缘恢复技术提供理论指导。

典型高风险植被火条件下导线——板间隙击穿特性

为准确评估山火条件下架空输电线路的跳闸风险,提升电网安全运行水平,文中对南方电网典型高风险植被火条件下导线—板间隙击穿特性进行了研究。搭建模拟植被火燃烧特征试验平台和模拟山火间隙击穿试验平台,研究了云南松、水杉、速生桉、灌木和茅草5种植被的火焰燃烧特征参数和间隙击穿特性,对不同植被的火焰温度、火焰高度、灰分含量和可燃物热值与导线—板间隙击穿特性的关系进行研究。研究结果表明上述因素与间隙平均击穿电压梯度近似呈线性关系,并拟合得到其多元线性回归公式。使用熵权法得到火焰温度在植被燃烧特征中所占权重最高,上述5种植被中云南松的山火跳闸风险最高。文章研究结果可为架空输电线路山火跳闸风险评估提供参考。

SF_(6)断路器触头劣化过程合闸预击穿特性研究

电容器组投切断路器触头烧蚀劣化后会改变合闸预击穿时间,这不仅会影响选相合闸效果,同时也说明预击穿电弧持续时间可作为衡量触头烧蚀状态的指标之一。建立了126 kV SF_(6)断路器灭弧室内合闸预击穿过程电—流体耦合仿真模型,研究了触头劣化过程中合闸预击穿特性,并提出了预击穿时间带电检测方法。结果表明:合闸过程中场强最大点总是出现在静弧触头表面;灭弧室内SF_(6)气体密度几乎保持不变;110 kV电压等级相电压下,触头预击穿电弧持续时间随烧蚀程度变化的试验值与仿真值吻合良好。从电场畸变角度解释了随着触头的表面烧蚀劣化程度增大对应预击穿时间的变化规律,结果可为断路器选相合闸策略和触头状态评估提供理论指导。

热态C_(5)F_(10)O/CO_(2)混合气体电击穿特性研究

热态气体的临界击穿场强是评估高压断路器弧后电击穿特性的基础数据,文中研究了C_(5)F_(10)O/CO_(2)混合气体在C_(5)F_(10)O混合比例k=0~10%、压强0.1~2.0 MPa、温度300~4 000 K范围内的临界击穿场强。基于质量作用定律数学模型,得到不同温度下混合气体的平衡组分,采用两项近似方法求解玻尔兹曼方程,分析混合气体的电子能量分布函数、折合电离和吸附系数,获得混合气体的临界折合击穿场强和临界击穿场强。结果表明:0.6 MPa下,温度低于3 000 K时,随着温度降低,k=0~10%混合气体的临界击穿场逐渐低于SF_(6);在温度高于3 000 K时,混合气体的临界击穿场强为SF_(6)的1.2倍以上,具有较强的绝缘能力。研究结果可为C_(5)F_(10)O/CO_(2)混合气体高压断路器弧后电击穿特性研究提供参考。

激光诱导击穿光谱技术预测煤的黏温特性

快速准确测定煤的黏温特性对气化炉操作具有重要意义,通常采取将煤烧灰测定其临界黏度温度的方法来确定煤的黏温特性。激光诱导击穿光谱(LIBS)与化学计量方法结合在煤炭分析方面有许多独特优势。收集41个代表性煤样,基于DL/T 660—2007标准方法,测定相应煤灰的临界黏度温度,将其与煤样的LIBS谱图关联,利用化学计量学方法建立预测煤灰临界黏度温度的LIBS分析模型。模型交互验证结果表明,LIBS方法与实验室标准方法之间具有良好的一致性,基于偏最小二乘回归(PLSR)、人工神经网络(ANN)和支持向量机回归(SVR)建立的分析模型对临界黏度温度的预测标准偏差分别为63、57、56℃,单个煤样的测试时间不超过5 min,满足工业现场快速分析的需求。