Impulse insulation characteristics of a composite insulation system having a wedge gap in SF6 gas

Impulse insulation characteristics were investigated in a composite insulation system having a wedge gap in SF₆ gas. The partial discharge inception voltages of wedge gaps with various types of film were measured and compared with the calculated breakdown voltages estimated from Paschen's curve of SF₆ gas. Also discussed is how the charge accumulated on the film surface due to a partial discharge had an effect on the creepage breakdown voltage. Partial discharge inception voltages in wedge gaps were higher with higher SF₆ gas pressures and with lower film permittivities. Creepage breakdown voltages depended little on gas pressures or on creepage distances. The dependency of breakdown voltages on gas pressures and the effect of polarity on the breakdown voltage differed with the types of film. This may be partly because the charge on the film due to partial discharge had an effect on the discharge propagation, and that charging of the film differed with the types of film.     

Comparisons of direct-current breakdown characteristics in dry air and SF6 gaps using a parallel-plane arrangement with variable height protrusion

Breakdown voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to nonuniform gap in dry air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge crossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the dc voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset. The experimental observations have been explained qualitatively on the basis of a streamer model and precise electric field calculations of gap.     

Insulation properties of a CO2/N2 50% SF6 gas mixture in a nonuniform field

This paper describes partial discharge (PD) inception and breakdown voltage characteristics of a CO₂/N₂/SF₆ gas mixture in a nonuniform field. These voltage characteristics were investigated with ac high voltage by changing the mixture rate of each gas of CO₂, N₂, and SF₆ gas and the gas pressure from 0.1 MPa to 0.6 MPa. It was found that adding a small amount of CO₂ gas into a N₂/SF₆ mixture causes a drastic increase in the breakdown voltage. For instance, when the mixture rate of SF₆ in N₂/SF₆ gas mixture is 50%, with the addition of 1% CO₂ the maximum breakdown voltage becomes 1.31 and 1.15 times higher than that of a 50% N₂/50% SF₆ gas mixture and pure SF₆ gas, respectively. Moreover, those voltage characteristics of a CO₂/N₂/SF₆ gas mixture were also investigated by changing the electric field utilization factor as well as by applying positive and negative standard lightning impulse voltages in order to discuss the corona stabilization effect, which seems to be one reason for the drastic increase in the breakdown voltage. These results and breakdown mechanism of the CO₂/N₂/SF₆ gas mixture are discussed on the basis of the corona stabilization effect and the dissociation energies of the component gases by observing PD light images, PD light intensities through a blue and red filter, and PD current waveforms. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(3): 34–43, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10019     

PD time‐sequential properties of SF6 and SF6 gas mixture including CO2

This paper deals with partial discharge (PD) time‐sequential properties of SF₆/N₂/CO₂ ternary gas mixture as well as SF₆ and SF₆/N₂ gas mixture under AC and positive DC voltage applications. The measurements were carried out by changing the gas pressure up to 0.6 MPa and applied voltage with the N‐shape characteristics of breakdown voltage versus gas pressure for each tested gas considered. We obtained experimental results of the gas pressure dependence of maximum peak value of PD current pulse as well as the relationship between the time interval of PD pulses and the peak value of PD pulse. We discuss the mechanism of increase in breakdown voltage by adding CO₂ into SF₆/N₂ gas mixtures in terms of change of PD type from streamer to leader discharge. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(3): 32–40, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20073     

SF6压力对GIS典型缺陷局部放电检测灵敏度的影响

气体绝缘开关设备(gas insulated switchgear,GIS)内部气压是影响局部放电检测灵敏度的重要因素。在局部放电实验平台中构建尖端、悬浮、气隙与沿面缺陷模型,基于特高频法、高频电流法与超声法开展气压0.2~0.5 MPa下局部放电信号特征的检测实验,并对比了高频电流法和超声法局部放电检测的灵敏度。实验结果表明：气压是影响尖端、悬浮与沿面缺陷放电的关键参数,3种缺陷放电起始电压与气压成正比,同一电压下的放电幅值和脉冲次数与气压成反比;气隙缺陷放电源于绝缘层内部气泡,因此设备气压对气隙缺陷放电影响不大。高频电流法可实现不同气压下的悬浮缺陷有效检测,但未测得气隙和尖端缺陷局部放电信号;SF₆压力降低,高频电流对沿面缺陷的检测灵敏度有所提升,但仍低于特高频法。超声法可实现不同气压下的悬浮和尖端缺陷有效检测,但未测得气隙和沿面缺陷局部放电信号;SF₆压力降低,超声法对尖端缺陷的检测灵敏度提高(当气压为0.2 MPa时,超声法对尖端缺陷的检测灵敏度与特高频法相当)。     

Effects of a barrier on creeping discharge characteristics in SF6 and N2 gases under pulse voltages

This paper describes the effect of a barrier on creeping discharge characteristics in SF₆, N₂, and their mixtures. The barrier height effect on the discharge voltage was investigated under a positive pulse voltage. The discharge voltage increased with increasing barrier height in SF₆ gas. On the other hand, the discharge voltage hardly changed with the barrier height in N₂ gas. In order to clarify the mechanism of the initial creeping corona, it was observed by optical techniques, including an ultra-high-speed electronic imaging system (IMACON 468). The creeping corona path revealed differences in images at various barrier heights. The accumulated charge induced on the barrier by the initial corona in SF₆ gas was able to suppress the development of the later creeping corona, in contrast with N₂ gas. © 1998 Scripta Technica, Electr Eng Jpn, 125(4): 1–8, 1998     

Creeping flashover characteristics in (N2/SF6) gas mixture under pulse voltage

The effect of a barrier between a needle electrode and a plane one in an (N₂/SF₆) gas mixture on creeping flashover was investigated using a microsecond pulse voltage. The SF₆ gas content was varied from 0% to 100%, and the gas pressure from 0.1 MPa to 0.3 MPa. The flashover voltage increased with increasing SF₆ gas content for a positive needle electrode. For a negative needle electrode, excepting the total pressure of 0.1 MPa, at which similar flashover characteristics were obtained to the positive case, a considerable decrease in flashover voltage was found in the case of a mixture of a few percent SF₆ in (N₂/SF₆) gas mixture at elevated total pressures. The corona behavior on the barrier in (N₂/SF₆) gas mixture was investigated by means of a high‐speed digital framing camera. © 2000 Scripta Technica, Electr Eng Jpn, 131(1): 1–9, 2000     

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。     

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。     

Impulse partial discharge and breakdown characteristics of rod–plane gaps in air and N2 gases

In response to growing environmental concerns, we attempted to develop switchgear without using SF₆ gas. In our research, we used compressed air and pure N₂ as an electrical insulation gas, because of their low global warming potential. In this paper, we examined the impulse breakdown and impulse partial discharge characteristics under various conditions related to nonuniformity of the electric field. The experimental results show that the breakdown voltage (BDV) of air is higher than that of pure N₂ gas under highly nonuniform field conditions in the rod–plane gap. On the other hand, the discharge inception voltage of air and N₂ were almost the same. Furthermore, first partial discharge (PD), leader discharge, and its transition to the breakdown were successfully observed through the measurement of discharge current and light emissions under impulse voltage application. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(3): 36–43, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10277     

添加气体对SF6低概率冲击击穿的影响

The low probability impulse breakdown voltage and the leader step time of SF₆ gas mixtures are measured. Based on the mechanism of transition from streamer to leader, the relationship between them is discussed, and the effect of gas additives on the low probability impulse breakdown is also interpreted.     

Voltage-current characteristics of high-current pulsed discharges in SF

A high-current pulsed discharge in SF₆ has been used as a closing switch for a pulsed power generator. The characteristics of a high-current pulsed discharge are not clear, since it is difficult to measure the time-dependent arc voltage accurately. In the present paper, the voltage-current characteristics and the time-dependent arc resistance of the high-current pulsed discharge in SF₆ are obtained accurately from the arc current and the breakdown voltage by solving the circuit equation. The electrode separation and the pressure are changed from 0.1 to 1 cm and from 200 to 760 torr, respectively. The current rise time is about 380 ns. The minimum arc resistance occurs at the maximum arc current, and is from 0.04 to 0.1 Ω under the experimental conditions. The voltage-current characteristics in SF₆, which are characterized by five phases, are similar to the characteristics in air but not in Ar.     

Control of SF6 discharge gap switch by XeCl excimer laser

Experimental studies have been carried out on triggering characteristics of the SF₆ discharge gap switch by use of XeCl excimer laser (wavelength = 308 nm). First, laser irradiation characteristics are studied on a pure SF₆ in the pressure range of p = 160 ～ 3,800 torr. Using a lens of f (focal length) = 133 mm, the laser is irradiated into the gas, where the energy absorption is studied. If the laser is injected into the gas with the energy above a certain threshold for the breakdown, the rate of energy absorption is found to be ～ 17 percent of the incident energy at p > 760 torr. Injecting the laser into the SF₆-filled gap switch (gap length = 7 mm, p = 760 torr), we have studied the triggering characteristics. Excellent triggering characteristics were obtained; delay time for the discharge ～ 20 ns, and the jitter ～ 260 ps when the gap voltage is operated at 99 percent of the self-breakdown voltage. In addition, the triggering characteristics are studied by changing the focusing point axially. It is found that both the delay time and the jitter decrease when the focusing point tends to approach the high-voltage electrodes.     

Investigation of partial discharge mechanism by simultaneous measurement of current waveform and light emission

We observed current pulse waveforms of partial discharge (PD) in SF₆ gas so as to investigate the PD mechanism. We also measured light intensity and light emission image of PD simultaneously under different conditions of applies voltage and SF₆ gas pressure. From these experiments, we found that the “double-peak current waveform” appeared at high pressure and high voltage conditions. We also analyzed the mutual correlation of waveforms between a single current and the light emission. Moreover, we obtained experimental evidence of filmentlike light image appearing at the PD tip under the same condition with double-peak current waveform. From the electric field analysis around the needle electrode tip, we believe that the filamentlike light image expands beyond the critical electric field of SF₆ gas. Thus, we concluded that these current waveforms with double peaks showed evidence of leader-type PD, leading to breakdown. Finally, we could point out that leader-type PD should be distinguished and measured for the diagnosis of GIS insulation performance. © 1999 Scripta Technica, Electr Eng Jpn, 129(4): 58–65, 1999     

Insulation characteristics of SF6 gas for nonstandard impulse voltage polarity reversal pulse waveforms

Evaluation of insulation strength for lightning surge that actually enters into substations is important in estimating insulation reliability of gas‐insulated equipment. The standard lightning impulse voltage (1.2/50 µs) is used for factory tests. However, the actual lightning surge waveforms in substations are complex and are usually superimposed with various oscillations. Insulation characteristics of SF₆ gas as a function of such complex voltages have not been sufficiently clarified. This paper deals with gap breakdown characteristics in SF₆ gas under submicrosecond pulses. Breakdown voltages are lower under a polarity reversal condition than under a monopolarity condition. The cause of this difference is discussed while observing discharge propagation using an image converter camera. The electrode size effect is also discussed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 146(4): 18–25, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10246     

A study on hybrid insulation composition joint using gas and solid insulator for gas‐insulated power equipment

SF₆ is used as the main insulation gas for gas‐insulated switchgear (GIS), but it has recently become a gas that must be restricted because of its greenhouse effects. To date, we have studied the insulation characteristics of compressed N₂ and CO₂ as possible alternatives for SF₆. We have reported that N₂ or CO₂ must be pressurized to 2.0 MPa when it is used as a substitute for SF₆ at 0.5 MPa. Therefore, we have proposed a hybrid installation composition that uses gas and solid insulators. Because the central conductor of GIS is covered by a solid insulator in this composition, a high‐pressure gas at 2.0 MPa is not needed. However, the joint of the solid insulator becomes a weak point for discharge development. In this paper, we describe an effective configuration for improvement of the withstand voltage based on experiments. The most effective connector was made of resin without an implant electrode and the most effective configuration was one without a solid–solid interface between the solid insulator of the central conductor and the resin connector. In this experiment, the improvement of breakdown electric field of the hybrid composition was 44% or more compared with the case of only gas insulation (conventional method). In addition, further improvement can be expected by optimizing the insulation creepage distance and configuration. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(1): 11–20, 2012; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21107     

Commercialization of an extra high-voltage nonflammable transformer

A perfluorocarbon (PFC)-immersed 275-kV transformer with compressed SF₆, gas has been developed. This paper describes the basic cooling and insulation characteristics of PFC, the ac partial discharge voltage, and the aging of PFC-immersed insulation. The results demonstrate that a prototype 275-kV 100-MVA three-phase transformer can be operated without any difficulties for an extended period of overvoltage. This prototype has an ac partial discharge initiation strength which is 1.5 times that of the ac test voltage and a lightning impulse breakdown strength which is 1.5 times that of the test voltage. A 275-kV 250-MVA three-phase transformer was built and is being operated at the Abe substation of Chubu Electric Power Co., Inc. The transformer has been operating satisfactorily.     

Barrier effect under nonuniform field in atmospheric pressure SF6 gas

Generally, it is known that insertion of a barrier under a nonuniform field in air increases the flashover voltage. This is called the barrier effect. However, there are many uncertainties that remain to be clarified about the barrier effect in SF₆ gas. Therefore, the influence of the barrier shapes and positions on the flashover voltage for the lightning impulse voltages and voltages (50 Hz) is examined by inserting a barrier between a hemispheric-end rod and a plate in SF₆ gas under near-atmospheric pressure. As a result, the following findings were obtained:. (1) Insertion of a disk-shaped barrier reduces the flashover voltage; a cup-shaped barrier whose surface curves almost along the equipotential line increased the flashover voltage by 30 to 40 percent;. (2) trapped charge on the barrier surface is of the same polarity as the applied voltage and the magnitude of positive charge was greater than that of negative charge. It was found also that there is a correlation between the amount of trapped charge and the flashover voltage; and. (3) application of a barrier under a nonuniform field in the atmospheric pressure SF₆ gas helps reduce the size of, for example, a cubic-type gas-insulated switchgear.     

Metallic particle motion and breakdown characteristics in GIS with 0.1‐Hz VLF (very low frequency) high voltage applied

VLF (Very Low Frequency) high voltage with frequency of 0.1 Hz will be utilized for an on‐site test of XLPE underground cables, instead of conventional dc high voltage test. Since XLPE cables are connected to GIS (Gas Insulated Switchgears) in substations, the influence of VLF voltage application to GIS insulation should be investigated. One of the most important characteristics for GIS insulation lies in the metallic particle contamination and its behavior, which may induce breakdown in GIS. From the above viewpoint, this paper discusses the metallic particle behavior and breakdown characteristics under VLF voltage application in GIS. Experimental results revealed that (1) Particle motion under VLF condition was similar to that under dc condition, while specific in the transient behavior at the polarity reversal. (2) Breakdown was induced by particles located in the vicinity of high voltage conductor at the instance of crossing the gap or in the firefly conditions. (3) Breakdown voltage in positive half cycle was higher than that in negative half cycle at the lower gas pressure, while lower at the higher gas pressure, which was attributed to the particle behavior and the breakdown mechanism of SF₆ gas. Consequently, metallic particles in GIS under VLF voltage application exhibited the specific behavior associated with the slow change of instantaneous voltage and polarity, and resulted in the complex pressure dependence of breakdown characteristics. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 33–40, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.1166     

Formation Characteristics of SF6 Decomposition under Partial Discharge Induced by Metal Protrusions with Varying Degrees of Severity

Due to its excellent insulating and arc-suppression properties, sulfur hexafluoride (SF₆) is widely used in gas-insulated switchgear. In the presence of partial discharge, SF₆ decomposes into various by-products according to discharge scenarios. These by-products are often used to detect and identify partial discharge. In this article, formation characteristics of SF₆ decomposition under partial discharge induced by metal protrusions with varying degrees of severity are investigated. The varying degrees of severity of partial discharge were assessed by partial discharge applied voltage and partial discharge inception voltage. Partial discharge applied voltage and partial discharge inception voltage are related to magnitude of partial discharge and varying degrees of the non-uniform electric field distribution distortion, respectively. The results show that the production rates of SO₂F₂ and SOF₂+SO₂ decrease with increasing partial discharge inception voltage, while increasing proportionally to partial discharge applied voltage. The ratio of SO₂F₂ to (SOF₂+SO₂) exhibits a similar trend. As partial discharge applied voltage increases, the production rates of CO₂ and CF₄ increase, but the ratio of CO₂ to CF₄ decreases. Based on the experiments, the formation mechanisms of SF₆ decomposition by-products under varying degrees of severity of partial discharge are analyzed.