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     

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     

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     

Partial discharge measurements in SF6 and air usingphase-resolved pulse-height analysis

Carries out a study on partial discharge (PD) phenomena in SF₆ gas and tries to construct a diagnostic system capable of separating noise from observed PD signals for assessing the condition of insulation performance in power apparatus. The authors have constructed a computer-aided PD measuring system which permits a time-sequential data acquisition and phase-resolved pulse height analysis. Using the PD measurement system, the authors measured 60 Hz AC PD in SF₆ gas and in air at atmospheric pressure for needle-plane electrodes with three different radii at the tip of the needle, 3, 30 and 500 μm. They proposed particular PD statistical parameters, unbalance ratios U _T and U_q of the pulse repetition rate and average of charge, respectively, between positive and negative PD pulses occurring per cycle; these parameters proved to be good indicators to distinguish PD occurring in SF₆ from that in air. The mechanism of PD inception was also discussed     

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     

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.     

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     

Simulation of limiting field behavior in electron swarms inSF6+N2 gas mixtures

Limiting field behavior of the electron swarms in SF₆+N ₂ mixtures is investigated by a Monte Carlo simulation technique. (E/N)_lim values directly obtained from the simulation are given for 5, 10, 20, 40, 60 and 80 SF₆ fractional component in the mixture together with the corresponding electron drift velocities and mean energies. Furthermore, in the E/N range of 243⩽E/N⩽606T_d, effective ionization coefficients and mean energies in SF₆+N₂ mixtures are also evaluated with fractional SF₆ contents of 0, 20, 40, 60, 80 and 100%. The simulation results obtained are compared with the available data in the literature     

Prediction of breakdown in SF6 under impulse conditions

Prediction of fast transient voltage-induced breakdown in quasi-homogeneous field geometries requires compounding the breakdown probability over time, while taking into account the field-dependent probability of electron detachment. In this paper, a breakdown probability model has been developed to predict the impulse breakdown under quasi-uniform fields, as this is the fundamental condition, the knowledge of which facilitates computation of breakdown probability under more complex conditions. The model will facilitate computation of breakdown probability under more complex conditions. It accounts for the effect of the streamer formation length on the critical volume and the probability of initial electron production by electron detachment from negative ions. The proposed model has been verified through comparison with the measured impulse breakdown probabilities. The predicted breakdown probabilities are in good agreement (±10%) with those measured     

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     

Statistical inference for breakdown voltage in SF6 GISfrom first breakdown data

The statistical distributions of first ac breakdown field data were shown to be represented by the three-parameter Weibull distribution. Many experimental data compiled from the literature in various gap configurations and in sphere-plane gap system in the present work, obtained in SF₆ at pressures of 0.2, 0.4 and 0.6 MPa were analyzed. By making use of the maximum likelihood method, the algorithm suggested provides parameter estimates with no computational difficulties. The compiled field data produces lower parameter estimates compared to those evaluated for individual data groups at the same pressure, and although the limiting field strengths tend to decrease as gas pressure increases, the slopes of the distributions are almost unaffected     

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%     

Partial discharge measurement in a long distance SF6 gasinsulated transmission line (GIL)

We evaluated the generation, radiation, and propagation characteristics of partial discharge (PD) signals in 275 kV full-scale SF₆ gas insulated transmission line (GIL) model with a length of 168 m. We analyzed the correlation between the PD current pulse and the radiating electromagnetic wave by comparing their frequency spectrums. In addition, simultaneous measurement was performed on the current pulse waveform of a single PD and the corresponding electromagnetic wave as propagated in GIL, with the phase gate control method. As a result, the PD signal attenuation rate of 50 MHz and 300 MHz for the propagation of 100 m was obtained as 2.5 dB and 8.7 dB, respectively. Furthermore, by selecting an appropriate measuring frequency, even a 1~2 pC PD could be detected at the measuring site 168 m away from the PD source in the GIL     

Simulation of corona discharge. Positive corona in SF6

Positive coronas in SF₆ under three voltage levels, 3.0, 3.5 and 4.0 kV, are studied for a 0.5 cm gap between a hyperboloid needle and a plane electrode at a gas number density N=2.12×10¹⁸ cm³. The current in the form of a burst of pulses is found to increase with increasing voltage. The initiation and development of successive avalanches are traced as function of time after voltage increases. The development of electron avalanches and streamers are due to ionization and photoionization and quenching of avalanches is due to space charge. The simulation provides a detailed structure of avalanches from which essential differences between positive and negative corona with regard to total field distribution, propagation of successive avalanches and ion distribution can be discerned     

Prediction of the breakdown voltage in SF6 under corona stabilization

Corona-stabilized breakdown in SF₆ can occur either through a streamer or a leader mechanism depending on the gas pressure. The paper derives an expression to predict the breakdown voltage in coaxial cylinders in the pressure range where the streamer mechanism is operative. The pre-sparkover current can also be determined. Excellent agreement is obtained with experimental results.     

Simulation of corona discharge. Negative corona in SF6

Negative coronas in SF₆ under three voltage levels are investigated by Monte Carlo simulation. For the purpose of simulation, the gap between a hyperboloidal needle and a plane is divided into two regions: region I where the electric field is very steep and most electrons and ions are accumulated, and region II the rest of the gap. In region I electron motion is simulated by dividing the region into a number of cells and a small cell size improves the accuracy. The magnitude of current pulse increases with increasing voltage and there appears to be more than one peak in each pulse as observed by experiment. The development of electron avalanches is due to ionization and photoionization in the high field region, while the quenching of the avalanches is due to the space charge field suppression. Also the accumulation of positive and negative ions are displayed in detail. The space charge field distortion is studied and found to increase with increasing applied voltage     

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...     

工频恒定电压下SF6气体中沿面放电发展过程研究

气体绝缘组合电器(GIS)中绝缘缺陷的存在会造成局部放电,导致设备劣化,进而危害到电力系统的稳定性。其中危害较严重的为绝缘子沿面放电缺陷,为探究其在工频恒定电压作用下的放电发展过程及规律,文中制作了“三结合”模型模拟SF 6中沿面放电缺陷并展开试验。保持工频电压在模型闪络电压的95%不变,每隔5 min记录一次局部放电信号,直至模型频繁发生闪络,分析各阶段放电统计参量的变化。结果表明沿面模型在闪络前的放电呈现放电重复率增加、放电量增加、放电间歇、放电再次增强的发展过程。放电间歇的产生与SF 6的强电负性、电场均匀化等均有一定关系。因此,需通过长时间放电检测以准确判定设备劣化阶段,并采取相应措施。