Inhomogeneous field breakdown in GIS-the prediction of breakdownprobabilities and voltages. I. Overview of a theory for inhomogeneousfield breakdown in SF6

Inhomogeneous field breakdown in SF₆ has been investigated, and a theory has been developed which predicts the breakdown voltage for fast-rising (rectangular) waveforms applied to arbitrary electrode geometries and gas pressures. This theory facilitates the analytical design aspects of many GIS components where previously, engineering judgement and development testing were necessary. It is suggested that extensions of the theory, will permit breakdown voltage prediction for an arbitrary surge waveform, which will facilitate the assessment of test waveform efficacy and the development of a more reliable field, factory, and type tests     

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     

SF6/N2流注放电仿真及协同效应研究

建立反映气体放电过程中粒子运动特性的二维流体模型,采用有限元和通量校正传输法对该模型进行数值求解,计算了50%SF₆+50%N₂在均匀电场下的放电规律,模拟了流注发展过程中粒子密度的分布情况,分析放电过程中带电粒子对均匀电场的影响。搭建气体放电实验平台,测量平板电极下绝缘间隙5 mm时SF₆/N₂混合气体的击穿电压,将SF₆/N₂击穿电压的实测值与折算值进行对比,研究不同混合比、气体压强对SF₆/N₂协同效应的影响。结果表明：随着流注向阳极运动,放电间隙内的电子数密度不断增大;在放电初始阶段,空间电荷对电场的影响很小,随着电荷数量不断增加,空间电场产生明显畸变现象。SF₆/N₂混合气体击穿电压的实验测量值大于折算值,且SF₆含量越高,实测值和折算值越接近。可以看出,SF₆/N₂的协同效应在含有少量SF₆时较明显,而当SF₆含量较高时,混合气体的协同效应减弱。     

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     

On the calculation of breakdown voltages for uniform electricfields in compressed air and SF6

A novel method is described for calculating the breakdown voltage of uniform field gaps in compressed air and SF₆ without the need for experiments. This method is based on the criterion of self-recurring single-electron avalanches developed in the gap. It is shown that the results computed by this method for pd values to the right of Paschen's minimum (up to 15 kPa·m in air and up to 5 kPa·M in SF₆) are in good agreement with those measured experimentally. It is also shown that the use of the streamer criterion overestimates the breakdown voltage when applied for pd values where Townsend's mechanism is valid. In addition, it is shown that the size of the avalanche (and hence the parameter K) at breakdown is not constant as adopted in the literature; it depends upon the gap length and gas pressure     

Dielectric breakdown characteristics of an insulator with a sharp edge in SF6 gas under lightning impulse voltage

This article discusses the dielectric breakdown characteristics of an insulator with a sharp edge that forms inhomogeneous gaps in SF₆ under a lightning impulse voltage. The high voltage electrode is a plane electrode, and the grounded electrode is a plane electrode with a column insulator, which has an inside electrode. The column insulator has a height of 50 mm and a diameter of 50 mm. The inside electrode in the column insulator has a height of 20 mm or 30 mm or 40 mm. The inside electrode changes the electric field on the surface of the column insulator. The SF₆ gas pressure is 0.225 MPa. Dielectric breakdown does not occur at the edge of the column insulator, but rather at the top plane of the column insulator. The dielectric breakdown voltage is defined by the electric field at the center of the column insulator     

Evaluation of breakdown characteristics of gas insulated switchgears for non-standard lightning impulse waveforms - analysis and generation circuit of non-standard lightning impulse waveforms in actual field

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In this paper, first, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. Next, high-voltage circuits that generate these non-standard lightning surge waveforms were designed and constituted using EMTP (electro magnetic transients program) based on a circuit with a gap, inductors, and resistors connected in series and resistors and capacitors connected in parallel. Further, circuits were actually constructed, to obtain voltage waveforms approximately equal to those designed. Finally, the dielectric breakdown voltage-time characteristics were measured under several different conditions for the quasi-uniform SF₆ gas gap that represents an insulation element of a GIS. As a result, it was found that, in the tested range, the dielectric breakdown values for non-standard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 6% to 32%     

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%     

Inhomogeneous field breakdown in GIS-the prediction of breakdownprobabilities and voltages. II. Ion density and statistical timelag

For pt.I see ibid., vol.3, no.3, p.923-30 (1988). An improved model for the calculation of statistical time lags in SF₆ insulated electrode arrangements for positive impulse voltage is presented. On the basis of physical considerations, the production rate of initial electrons is ascribed to the local density of negative SF₆ ions and to field dependent electron detachment. Values of negative ion densities are measured directly, while the detachment coefficient is determined by a fitting procedure to measurements of statistical time lags     

AC particle-triggered corona discharge in low pressure SF/sub 6/ gas

This paper deals with AC particle-triggered corona discharge as a follow-up to our previous research with DC voltage to clarify the particle-triggered corona discharge process in SF₆ gas. Corona current pulses, charges associated with a corona current pulse, and corona light pulses were observed with an aluminum ellipsoidal particle suspended in a parallel plane electrode system under a SF₆ gas pressure range of 30 kPalesPles50 kPa by changing the particle position. Corona mode, phase (Phi)-charge (q) characteristics as well as corona discharge processes were discussed and the following results were obtained. AC corona mode depended on the instantaneous applied voltage, voltage gradient as well as the particle position and then, the Phi-q characteristics were also affected by those parameters. Charges flow into the floating particle due to coronas on the both tips of particle and excite field fluctuations around the opposite side of particle in addition to the applied AC field. The field fluctuation in SF₆ gas by the corona charges was about 4% of the applied field and much lower than that in air gap which was about 70%. That is, the corona development was suppressed effectively by high electron affinity of SF₆ gas even in the case of floating particle. The less effective interference between coronas on the both side of particle in SF₆ gas results in an obscure local minimum in the breakdown voltage characteristics as the particle is in the vicinity of electrode as contrasted with a drastic fall in the breakdown voltage by the particle in air     

Mechanism of electrical breakdown left of Paschen minimum

The breakdown mechanism of a gas at small PD values is considered. Comparing experimental results and theoretical analysis, the limits of different breakdown mechanisms are established. For SF₆ and Ar gases the conditions for streamer and those for the Townsend breakdown mechanism are found, as well as those for vacuum breakdown. The pressure values characteristic for avalanche and emission mechanisms of vacuum breakdown are obtained. Left from the Paschen minimum in the vicinity of the minimum point, edge-type breakdown is dominating, which currently is misinterpreted as an anomalous Paschen effect     

Evaluation of Breakdown Characteristics of Gas Insulated Switchgears for Non-Standard Lightning Impulse Waveforms - Breakdown Characteristics for Non-Standard Lightning Impulse Waveforms Associated with Lightning Surges

To lower the insulation specifications (specifically, the lightning impulse withstand voltage) of a gas insulated switchgear (GIS) and thus cut the equipment cost while maintaining the high reliability of its insulation performance, it is necessary to define in an organized way the insulation characteristics for non-standard lightning impulse voltage waveforms that represent actual surge waveforms in the field and compare them with the characteristics for the standard lightning impulse waveform quantitatively. In the preceding paper, lightning surge waveforms and disconnector switching surge waveforms at UHV, 500 kV, and 275 kV substations were analyzed and five to six non-standard lightning surge waveforms with basic frequencies of 0.6 to 5.0 MHz were identified. In this paper, the dielectric breakdown voltage - time characteristics were measured under several different conditions mainly for the quasi-uniform SF₆ gas gaps that represent an insulation element of a GIS toward four kinds of non-standard lightning impulse waveforms associated with lightning surges. As a result, in the tested range, the dielectric breakdown values for nonstandard lightning impulse waveforms were higher than for the standard lightning impulse waveform by 3% to 32%.     

Applicability of simple expressions for electrical breakdownprobability increase in vacuum and gas

Simple analytical expressions for the law of breakdown probability increase are suggested. They are tested by comparing with experimental data obtained by impulse and DC breakdown. The pressure 10^-4 Pa and interelectrode gaps d<0.1 mm are used for vacuum, and the pressure 1 bar-5 bars and the interelectrode gaps 1 mm-50 mm are used for gas breakdown. The insulation gas was a SF₆, N₂ and gas-mixture: SF₆-N₂. It is concluded that such a simplified approach is not appropriate for vacuum. It is also concluded that, for gas, the BPI (breakdown probability increase) law, in the form of a simple expression, is valid in the considered range of relevant experimental parameters     

Fundamental insulation characteristic of high-pressure CO/sub 2/ gas under actual equipment conditions

SF₆ gas has been widely used in electrical power equipment such as circuit breakers and transformers due to its superior insulation and interruption characteristics. However since 1997, SF₆ gas has been designated a greenhouse gas subject to emission restrictions at COP3 (The 3rd session of the Conference Of the Parties to the United Nations Framework Convention on Climate Change) so a new insulating gas is needed as a substitute for SF₆ gas. This research considers the use of high-pressure CO₂ gas as an insulator while stressing the environment aspects. Fundamental insulation data for the insulating gas acquired supposing gas insulated switchgears (GIS) consists of; (1) insulation breakdown characteristics under clean conditions and, (2) insulation breakdown characteristics with metallic particle contamination. The parameters in this case were assumed from an actual apparatus viewpoint, to be a high gas pressure up to 2.0 MPa, an electrode size capable of determining the surface area effect, the electrode surface roughness, and metallic particle length, etc. at the base electrode of the 72 kV GIS. As a result, experiments using these parameters revealed insulation characteristics for high-pressure CO₂ gas and that negative lightning impulse decided the insulation design, as well as the present SF₆ GIS. The need for taking measures to suppress PD under AC voltage and also the need for restricting metallic foreign particles around the central conductor and insulating spacer were recognized     

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     

Electrical breakdown of SF6 at small values of theproduct pd

The breakdown of SF₆ when static and impulse voltages are applied at small values (from 10^-4 bar-mm to 1 bar-mm) of the product pd was investigated. It was found that the prevailing breakdown mechanism up to 8×10^-3 bar-mm was the Townsend mechanism. Between 8×10^-3 and 5.5 bar-mm the Townsend and streamers mechanisms combined, whereas from 5.5 bar-mm on the streamers mechanism took over. The Paschen law was found to hold for the points lying to the right of the minimum when static voltage was applied, whereas edge-type breakdown occurred on the points lying to the left of the minimum. When impulse voltage was applied, the Paschen law was valid only for the points to the right of the 1 bar-mm point. The electrode material affected the static and impulse breakdowns through the values of its work function     

The influence of surface charge on lightning impulse breakdown ofspacers in SF6

We have investigated the influence of surface charges on the discharge development across particle contaminated spacer surfaces under SF₆ for lightning impulse (LI) voltages (1.2/50 μs). Surface charges may be generated by dc, ac or lightning impulse stress. The discharge characteristic shows a strong reduction of the insulation strength if the applied voltage and the surface charge have opposite polarities. The investigations were performed with a needle protrusion attached to the bottom electrode to give severe field distortion. The bottom electrode was biased positively. The influence on the discharge process is observed by measuring the surface charge distribution and predischarge currents. The results reveal changes in streamer onset voltage, streamer to leader transition, and leader development     

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     

Sulphur hexafluoride-A complex dielectric

Some of the factors that must be taken into account in designing with SF₆ are discussed. These factors include the effect of field inhomogeneities on the electrode which reduce the dielectric strength in SF₆ and the effect of SF₆ decomposition byproducts on the materials used in SF₆-insulated systems. Homogeneous and inhomogeneous field breakdown are discussed, and typical causes of failure in SF₆-insulated systems are examined. Many oversimplifications are used in the discussion, mainly to avoid having to define precisely the circumstances to which the assertions apply. In general, the system under consideration has been designed for a 30-year life, as is typical for a utility apparatus