Comparison of SF6/N2 and SF6/CO2 gas mixtures as alternatives to SF6 gas

The interest in SF₆ gas mixtures has been re-ignited in recent years by the issue of the greenhouse effect of the SF₆ gas, and most research work is now focused on a SF₆/N₂ gas mixture, which is suitable for application in electrical apparatus with slightly non-uniform fields. This paper presents a comparison of SF₆/N₂ and SF₆/CO₂ gas mixtures with a viewpoint of their possible applications to gas-insulated transformers, where both highly non-uniform field problems and partial discharges in gas/film insulation are inevitable. It is shown that in this case the dielectric strength of SF₆/CO₂ is superior to that of SF₆/N ₂ with a minor disadvantage related to the gas decomposition in SF₆/CO₂. However, this may not be a problem for the SF₆/CO₂ gas mixture to be used in gas-insulated transformers, where internal breakdown is not allowed     

Breakdown behavior of SF6 gas-insulated systems at lowtemperature

The controversy surrounding low-temperature SF₆ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF₆ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24°C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43×10¹⁹ cm^-3 (equivalent to pressures of 105 and 624 kPa, respectively, at 24°C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF₆-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by ~10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30°C), take a constant value, and be fairly independent of density     

Seismic behavior of `candle' type SF6 outdoor circuitbreakers and associated SF6 insulated current transformers

The authors analyze the seismic behavior of live tank circuit breakers of the candle-type architecture, i.e. breakers that support only one chamber per column and have a closing resistor built in the interrupting chamber. This arrangement has a lower interrupting module weight and the resistor is in the axis of the column. In the case of T or V configurations with two chambers per column and with a closing resistor chamber in parallel to the main chamber, it results in some large masses off-centered on each side of the column. It is shown that the candle architecture makes possible a simple simulation and economical solution to the problem of seismic resistance, SF₆ current transformers associated with these breakers are also modeled by a simple computational model     

Recovery of SF6 from N2/SF6 gasmixtures by using a polymer membrane

The investigation of gas recovery from N₂/SF₆ gas mixtures using a polymer membrane has been reported. It has been shown that the purity of recovered gas, the recovery loss, and the gas handling speed depend on various parameters such as original gas mixing ratio, gas feeding pressure, gas flow rate, and temperature of the membrane. A gas recovering system with two membrane separators connected in cascade has proved to be very efficient in attaining high purity in a recovered gas, with negligibly small recovery loss and high handling speed for practical use. Furthermore, this system could be applied for CO₂/SF₆, He/SF ₆ and even for ternary mixtures such as N₂/CO₂/SF₆ without any modification in the system, where we expect much better performance in comparison with N ₂/SF₆     

基于一维电弧模型的SF6混合气体灭弧性能评价

SF₆混合气体是广受关注的SF₆替代气体方案之一。为定量评估SF₆混合气体的灭弧性能,文中采用一维衰减电弧模型和玻尔兹曼方程相结合的方法,将电弧熄灭过程划分为热恢复阶段、弧前介质恢复阶段和弧后介质恢复阶段,分别引入热恢复率、弧前介质恢复率和弧后介质恢复率作为各阶段的评价参数,并计算三者的调和平均数作为综合评价参数,以此来评估SF₆-N₂、SF₆-CO₂、SF₆-CF₄以及SF₆-Air混合气体的灭弧性能。基于上述方法,文中初步探讨SF₆含量、背景气体种类和压强大小对SF₆混合气体灭弧性能的影响。结果表明,随着SF₆含量的减少,混合气体的灭弧性能整体上呈现下降趋势;当SF₆含量为10%～50%时,4种混合气体中SF₆-N₂的灭弧性能最优,其次...     

SF6气体绝缘设备的放电特性研究

随着SF6气体绝缘设备在系统中的广泛应用,对其绝缘状态的监测、检验以及相应的缺陷类型识别,成为当前亟待解决的问题.通过建立缺陷模型,对SF6气体在不同状态下放电特性进行试验分析,得到了不同缺陷情况下SF6绝缘设备的放电类型特征参数,和特征气体组分.结果表明:典型特征气体可作为实际检测的比对标准,也可作为SF6气体绝缘设备故障诊断专家系统的数据资料.     

Partial discharge and breakdown mechanisms in ultra-dilute SF6/N2 gas mixtures

With the goal of reducing SF₆ gas usage, we investigated partial discharge (PD) and breakdown (BD) mechanisms in ultra-dilute (0 to 1%) SF₆/N₂ gas mixtures. The experimental results and discussions are given in this paper. In particular, we focused our research on the transition characteristics of the electrical insulation performance with an extremely small amount of SF₆ content, the PD behavior on the applied voltage, and the relation among prebreakdown streamer, return stroke and BD. Moreover, we discussed the PD and so mechanisms with reference of space charge behavior. From these results, the transition threshold at which the influence of SF₆ gas on the discharge characteristics began to appear, was clarified to be SF₆ content k=10 ppm for a total pressure of 0.1 MPa     

Very fast transient overvoltages in GIS with compressed SF6-N2 gas mixtures

This paper discusses the characteristics of very fast transient overvoltages (VFTO) in SF₆-N₂ gas mixtures at different percentages of SF₆. A comparison of the VFTO characteristics of pure SF₆ with those of pure N₂ is also presented. The investigations are performed using a laboratory model GIS bus duct having a test gap used for simulating a switching event leading to the generation of VFTO. A capacitive voltage sensor is used to measure the VFTO peak magnitude and temporal characteristics. Measurements were carried out at two different gap spacings (0.20 and 0.61 mm) over a pressure range of 100 to 500 kPa. VFTO characteristics for N₂, SF₆ and SF₆-N₂ mixtures obtained from the experiments show similar trends. The level of surge peak magnitude is <2.0 pu for all cases when the gap was 0.20 mm, but it reaches a maximum of 2.41 pu at 0.61 mm gap. At 0.20 mm gap, in SF₆-N₂ mixtures, the difference in peak magnitudes is not significant for 10% and 20% SF₆ mixtures (between 200 and 400 kPa) and also for pure SF₆ and 40% SF ₆ (between 200 and 300 kPa). The occurrence of corona stabilization during breakdown of the gap may be the cause for such a behavior. Unlike the above observations at 0.20 mm gap, at 0.61 mm gap, the peak magnitudes strictly increase with pressure for the pure gases and gas mixtures. At 0.20 mm gap, the time to breakdown of the gap is found to be almost constant in all cases. But at 0.61 mm gap, the time to breakdown is seen to be dependent on the mixture, pressure, and breakdown voltage, and this observation is in accordance with Toepler's spark law     

Calculation of the uniform breakdown field strength of SF6 gas

We calculated the uniform dielectric breakdown field strength of SF₆ gas over the temperature range of 300 to 3000 K. The local thermal equilibrium (ITE) composition of the dissociated gas is connected to the electron impact collision cross sections of the species SF₆, F₂, F and S. The critical reduced electric field strength of the composition is determined by a balancing electron generation and loss modeled by chemical reactions evaluated by the electron energy distribution function (EEDF) derived from the Boltzmann transport equation. At room temperature, pure SF₆ has a critical reduced electric field strength of 362 Td. With increasing temperature and decreasing density we found a small decrease of this value, whereas at temperatures higher than 1500 K, dissociation starts to decrease the dielectric strength of the composition. Furthermore, we found that generation of electrons by (associative) detachment from F ^- starts to play an important role at temperatures >2500 K, where the critical field strength still has a value of 118 Td. This value is found to decrease rapidly afterwards with increasing temperature to the value of 38 Td at 3000 K. The calculated results agree very well with independently predicted values and measured data     

Space charge behavior in SF6 gas and sequentialgeneration of PD pulses

We observed time-resolved partial discharge (PD) characteristics under ac and dc conditions in order to demonstrate the effects of space charge behavior and the corona stabilization effect in SF₆ gas. From the experimental results, we found that the charge magnitude of the first PD in the positive half cycle of applied ac voltage depended only on the instantaneous voltage. Under dc conditions we found that the magnitude of the PD charge related closely to the time interval between PD pulses. Space charge behavior and the PD generation mechanism are interpreted diagrammatically, and we describe how positive ions reduce the magnitude of PD charge and activate the corona stabilization effect and that the negative ions shorten the time interval between PD pulses     

Particle-initiated breakdown characteristics on a ribbed spacersurface for SF6 gas insulated switchgear

Cylindrical post spacers were used to investigate the fundamental effect of ribs on particle-initiated breakdown characteristics. Improved breakdown characteristics, obtained by attaching the ribs, changed with the distance between them; the shorter the distance, the higher the breakdown voltage. When the distance was less than 20-30% of the spacer length, the breakdown voltage increased by 20-60%. The rib effectiveness for SF₆ gas insulated switchgear spacers was demonstrated     

Anomalous creeping flashover characteristics in N2/SF6 mixed gas under single pulse voltage

Creeping flashover characteristics for the needle electrode-plane electrode filled with N₂/SF₆ gas mixtures have been investigated. In the case of the needle-plane configuration, the flashover voltage was enhanced by an admixture of SF₆ into N ₂. On the other hand, by the insertion of a solid dielectric barrier between the needle and the plane, only the negative creeping flashover voltage was reduced drastically by the admixture of SF₆ gas into N₂ gas, and a highly emissive region at the top of a creeping corona was found in N₂/SF₆ gas mixture. This behavior is strongly affected by the creeping corona extension process, depending on the creeping distance and the gas pressure     

UHF detection of leader discharges in SF6

The ultra-high frequency (UHF) technique of partial discharge detection has been used to study the partial discharges produced by electrode protrusions and surface contamination in a gas insulated substation (GIS). The paper describes the stages of discharge development from inception to breakdown. The work has shown that the UHF technique may be used to identify distinct phases of discharge activity corresponding to discharge inception and a prebreakdown condition. The prebreakdown activity is detected as an increase in the UHF signal magnitude which has been attributed to the formation of leader-type discharges. These discharges form an essential part of the breakdown process, and their detection can be used to identify when the system is close to breakdown. The paper describes a filtering technique that may be employed to optimize the sensitivity of UHF leader detection     

Dielectric strength of SF6 insulation in EHVdisconnectors

A new method for the calculation of parameters of SF₆ insulation for extra HV GIS disconnectors is given. This method is based on a new criterion for leader inception, which is connected with the value of the peripheral electric field near the boundary of the streamer zone. The presence of such a critical field is confirmed by analysis of experimental results and physical considerations     

基于SF6混合气体绝缘性能的设备补气策略研究

随着电力需求的增长和环境保护要求的提高,SF₆气体的使用逐渐受到限制。SF₆混合气体在一定程度上减少了SF₆气体用量,目前已经在电气设备中应用。文中针对SF₆混合气体在220 kV气体绝缘组合电器（GIS）中发生泄漏引起的绝缘变化展开研究,通过改变微量的气压值和混合比,探究混合气体的绝缘性能变化,分析气压、混合比因素对工频击穿电压的影响规律,获取各气压下各比例混合气体的绝缘强度曲线图,从而得到保证设备安全稳定运行的补气策略。研究发现,混合气体击穿电压的变化规律呈现出随着压强和混合比的提高,非线性程度增大的特点,并且得到了设备安全运行的混合比和气压的边界值。文中的研究可以为SF₆/N₂混合气体绝缘设备提供运维规程和技术标准,同时为制定混合气体的检测技术标准奠定基础。     

Temporal development of partial discharge and its application tobreakdown prediction in SF6 gas

The authors measured the temporal change of partial discharge (PD) characteristics leading to breakdown in SF₆ gas for AC voltage application. At the final stage close to the breakdown, positive PD pulses with relatively high magnitude began to take place in the phase region near the applied voltage peak. This event was interpreted in terms of the change of PD type in SF₆ gas from streamer to leader. Optical observation also revealed that PD type transition occurred. With the results considered, the mechanisms of PD were discussed. Moreover, they discussed the possibility of breakdown prediction in GIS. An attempt was made to find a breakdown prediction parameter which characterized the change of PD type: the ratio R_L of maximum charge to the average charge of PD pulses appearing in the phase region near the peak in positive half cycle. It was found that R_L allowed to predict the time to breakdown successfully within the error of 4~20%     

Variation of surface flashover voltage in SF6 withpressure

Surface flashover characteristics of solid spacers in a rod-plane configuration have been investigated in SF₆, at pressures to 400 kPa, for switching impulse voltages to determine the effect of spacer, spacer materials and polarity of applied impulses. The effect of spacer material on the flashover voltage is not significant. For negative polarity impulses, the influence of the spacer is also insignificant. But for positive polarity impulses, at pressures <200 kPa, the spacer efficiency becomes >1.0. On the other hand, at pressures >200 kPa, the presence of the spacer drastically reduces the flashover voltage of the system. At about atmospheric pressure also, the spacer efficiency in air has been found to be >1.0, with the same electrode geometry     

六氟化硫断路器分析及检修方法

通过对SF6断路器采用SF6气体作为灭孤介质的优缺点进行了分析,论述了SF6气体优良的灭孤性能和其分解副产物的腐蚀性能及解决措施,并对SF6断路器的检修方法进行了阐述.     

SF6/N2 mixtures: basic and HV insulationproperties

The widespread use of SF₆ by the electric power and other industries has led to increased concentrations of SF₆ in the atmosphere. This causes concern as to possible effects on global warming, because SF₆ is a potent greenhouse gas. This paper first touches on this issue and then documents the behavior of high pressure gases such as N₂ and SF₆/N₂ mixtures that can be realistically considered as acceptable intermediate or long-term replacements for pure SF₆ in some HV applications. The possible use of dilute SF₆/N₂ mixtures as an alternative to pure SF₆ for some of industry's insulation needs (albeit at higher pressure) is documented, and existing knowledge on these mixtures and on the individual components (N₂ and SF₆), both basic and applied, is compiled. A guide to existing literature is provided