直流真空断路器换流频率对开断特性影响研究

直流断路器作为提高直流输电可靠性、灵活性的重要环节, 是一项极具研究意义的关键技术。为给机械式直流断路器的振荡回路参数设计提供指导, 有必要研究换流频率不同时的介质恢复过程以及开断特性。本文对真空断路器弧后介质恢复过程分阶段进行分析, 提出了鞘层预备阶段的概念, 给出了鞘层预备阶段持续时间的计算方法, 进一步发展了鞘层发展理论, 在此基础上通过理论分析和仿真来研究真空断路器换流频率对开断能力的影响, 得到换流频率对开断极限电流的影响规律。     

电流频率对真空断路器开断能力影响的仿真分析

真空开关中电弧形态是影响电极表面烧蚀情况的主要因素之一。聚集型电弧对电极表面作用的能量较为集中,扩散型电弧能量较为分散,不同电弧形态导致的电极表面烧蚀程度有所不同,而电极表面的烧蚀情况是关乎开关电器寿命的重要指标。文章基于可拆卸真空灭弧实验平台,在工频交流200 A等级下对无氧铜平板触头分别进行直拉开断实验与旋转开断实验,同时对电极烧蚀进行仿真分析,实验与仿真结果表明：在交流小电流下,旋转开断方式有利于电弧提前达到分散形态,减少了电弧向电极表面中心作用的时间,从而减少了电极烧蚀质量损失,并且随着电极旋转角度的增加,烧蚀质量损失越小。

     

SF6气体密度继电器低温测量误差的分析与验证

文章对国内外SF6电气设备密度继电器气体压力测量进行了分析,提出采用修正到20℃时的额定压力控制参数,来保证SF6电气设备的安全可靠运行。为了在温度变化很大的情况下使密度继电器监测额定压力的数据具备准确性,对目前因为压力受温度影响造成的监测误差,可对SF6气体密度继电器进行温度补偿,并用实验室数据曲线验证了密度继电器监测引入修正值的可行性。     

双断口真空开关的动态介质恢复特性分析

从双断口真空开关的等值模型出发,分析了双断口真空开关的动态介质恢复过程,说明只要恢复电压的峰值和上升速度低于某一极限值,整个双断口真空开关并不会因为一个灭弧室发生重击穿而导致开断失败。这是因为另一个真空灭弧室的介质强度仍可能高于此时的恢复电压,它还可以承受整个恢复电压一个比较短的时间,当重击穿的真空灭弧室的介质恢复以后,共同完成分断过程。仿真计算和试验结果表明,由于各真空灭弧室断口的恢复电压分布不均匀,使得各灭弧室弧后电荷鞘层的发展不同,进而影响了各自的实际介质恢复特性。这可以解释双断口及多断口真空开关与单断口真空开关相比开断能力有显著提高的机理。     

Anisotropic etching of silicon in SF6 plasma

The reactive ion etching of silicon in SF₆ plasma is considered. During the experiment, silicon substrates are etched in SF₆ plasma at different pressures and energies of incident ions. High etching anisotropy is achieved decreasing the pressure in the reactor and increasing the energy of the bombarding ions. The obtained experimental measurements are compared with theoretical calculations. It is determined that the temperature of the sidewalls decreases with the decrease of concentration of F atoms due to suppressed plasmochemical etching of silicon. The etching anisotropy increases with the decrease of concentration of F atoms due to decreased desorption of SiF₄ molecules.     

Impact of SF6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF₆ gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl₄ plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF₆ plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.     

Turbulent effect of SF6 thermal plasmas inside a puffer-assisted self-blast chamber

In this study, we calculated the whole arcing history of SF₆ thermal plasmas happened inside a puffer-assisted self-blast chamber during the alternative current interruption process with two different turbulence models. It is very difficult to correctly realize all phenomena happened inside the interrupter using the computational schemes; therefore, we have been trying to simplify the physics related in the problem. Because most flows encountered in the process are turbulent, the ability to predict turbulence for this application is invaluable for the engineer to design the chamber reliably. The objective of this paper is to model turbulence by averaging the unsteadiness of the turbulence, which are called Reynolds averaged Navier-Stokes models, and to compare the results with pressure and temperature field distributions during the whole arcing history. It was found that the two-equation turbulent model predicts bigger mixing of momentum, heat and species on the arc discharge of this interrupter than the zero-equation model does.     

Simulations of Si and SiO2 etching in SF6 + O2 plasma

Chemical etching of Si and SiO₂ in SF₆ + O₂ plasma is considered. The concentrations of plasma components are calculated using values extrapolated from experimental data. Resulting calculations of plasma components are used for the calculation of Si and SiO₂ etching rates. It is found that the reaction constants for reactions of F atoms with Si atoms and SiO₂ molecules are equal to (3.5 ± 0.1) × 10⁻² and (3.0 ± 0.1) × 10⁻⁴, respectively. The influence of O₂ addition to SF₆ plasma on the etching rate of Si is quantified.     

Separative recovery with lime of phosphate and fluoride from an acidic effluent containing H3PO4, HF and/or H2SiF6

Fluoride content and flow-rate of fertilizer plant wastewater from phosphoric acid and/or triple superphosphate (TSP) production lead to the discharge of several thousand tons of fluoride (F⁻) per year and even more for phosphate (PO₄³⁻). Since sustainability is an important environmental concern, the removal methods should allow phosphorus and fluoride to be recycled as a sustainable products for use as raw materials either in agricultural or industrial applications. In the present work, separative recovery with lime of these two target species was investigated. A preliminary speciation study, carried out on the crude effluent, showed that two forms of fluoride: HF and H₂SiF₆ are present in a highly acidic medium (pH  2). Evidence that fluoride is present under both free (HF) and combined (H₂SiF₆) forms, in the phosphate-containing effluent, was provided by comparing potentiometric titration curves of a crude wastewater sample and synthetic acid mixtures containing H₃PO₄, HF and H₂SiF₆. In a second step synthetic effluent containing mixtures of the following acids: HF, H₂SiF₆ and H₃PO₄, were treated with lime. The behaviour of these compounds under lime treatment was analysed. The data showed that fluoride has a beneficial effect on phosphate removal. Moreover, by acting on the precipitation pH, a “selective” recovery of fluoride and phosphate ions was possible either from phosphoric acid/hydrofluoric acid or phosphoric acid/hexafluorosilicic acid mixtures. Indeed, the first stage of the separative recovery, led to a fluoride removal efficiency of 97–98% from phosphoric acid/hydrofluoric acid mixture. It was of 93–95% from phosphoric acid/hexafluorosilicic acid mixture. During the second stage, the phosphate precipitation reached 99.8% from both acidic mixtures whereas it did not exceed 82% from a solution containing H₃PO₄ alone. The XRD and IR analyses showed that during lime treatment, a H₂SiF₆ hydrolysis occurred, instead of CaSiF₆ solid formation, leading to CaF₂ precipitate. Calcium fluoride and calcium phosphate based-by-products resulting from the two-step treatment process can be used as raw materials in several industrial sectors, such as ceramic and phosphate fertilizer industries.     

Self-consistent particle simulation of radio frequency SF6 discharges: Role of ion recombination

The role of positive−negative ion recombination in simulating radio frequency (rf) SF₆ discharges is investigated based on Nanbu and Denpoh's theory [J. Phys. Soc. Jpn. 1998; 67: 1288]. The model of ion recombination is combined with the Particle-in-Cell/Monte Carlo (PIC/MC) simulation. Results show that the ion recombination is a dominant factor in simulating a steady rf discharge of SF₆. A criterion determining whether a periodic steady state is reached is proposed based on the ion recombination rate. It is found that the ion recombination rapidly increases as the gas pressure increases. From 25 to 50 m Torr, the loss of positive ions caused by ion recombination becomes comparable with that lost from collisions with electrodes.     

Highly stable nonlinear behavior against impulse current of ZnO-Pr6O11-CoO-Cr2O3-Y2O3 varistors

The nonlinear electrical behavior against the impulse stress (400-1200 A) in the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃ varistors has been investigated with sintering time. The sintering time did have a significant effect on the clamp ratio, which exhibits a surge protection capability. The varistors sintered for 1 h exhibited the best clamp characteristics, in which the clamp voltage ratio was in the range of K = 1.61-1.92 at a lower impulse current region (5-50 A) and was in the range of K = 2.06-2.55 at a higher single impulse current region (400-1200 A). The best electrical stability against the multi-impulse aging stress of 1200 A was also obtained from the varistors sintered for 2 h, where %ΔE_{1 mA} =−1.6%, %Δα =1.3%, and %ΔJ_L =−4.9%.     

Effect of SF6 incorporation in the cyclic process on the low temperature deposition of carbon nanofilaments

To enhance the carbon nanofilaments (CNFs) formation density, SF₆ was incorporated in the source gases (C₂H₂/H₂) during the initial deposition stage. The source gases and SF₆ were manipulated as the cyclic on/off modulation of C₂H₂/H₂/SF₆ flow in a thermal chemical vapor deposition system. The characteristics of the CNFs formation on the substrate were investigated according to the different cyclic modulation processes and the substrate temperatures. By the incorporation of SF₆ in the cyclic process CNFs could be formed at the substrate temperature as low as 350 °C. Among the different processes, mutual alternating cyclic modulation process of C₂H₂ and SF₆ could most highly enhance the CNFs formation density. The cause for the enhancement in the characteristics of as-grown CNFs by the incorporation of SF₆ was discussed in association with the slightly enhanced etching ability by the incorporation of SF₆.