Particle-Initiated Breakdown Characteristics and Reliability Improvement in SF6 Gas Insulation

Particle-initiated breakdown characteristics in SF6 gas are investigated. Breakdown voltages are greatly influenced by particle diameter especially for AC voltages, and for very fine metallic wire particles are much higher than those for moderately thick ones. The breakdown stress of a gas gap is obtained as a function of particle length and can be estimated fairly well by assuming a corona stabilizing zone. For a spacer, the electric stress which is parallel to its surface dominates surface breakdowns and the relation between the stress and particle length is experimentally determined. In order to increase breakdown voltages, the effects of both the dielectric coating on a high-voltage electrode and spacer shape are examined. Finally, the effectiveness of the dielectric coating on a grounded electrode is demonstrated to prevent particle levitation.     

The behavior of charged conducting particles in electric fields

The presence of metal particles within a gas-insulated switchgear (GIS) might initiate a sparkover and lead to fatal failure of an electric power transmission system. In order to investigate the behavior of a particle in a GIS, the model electrodes and the measuring system are constructed. The motion of a particle was investigated by using a CCD camera and a high-speed video camera. Different shapes of elongated particles were made from 0.1-mm and 0.25-mm diameter wires. The measured values of lift-off field strength of a particle lying on the electrode agreed quite well with the theoretical predictions. The initial raising up of a particle usually started at the sharper edge, as expected. There were two types of different modes of particle motions: one is the bouncing motion between two electrodes or one electrode, and the other is the standing motion on the electrode. An interesting thing is that the standing motion is on the upper electrode, when a negative voltage is applied to the upper electrode. This phenomenon could be explained from the difference of positive and negative corona currents starting from both edges of a standing particle     

Electrical breakdown triggered by a free conducting spherical particle in saturated liquid He I and He II under uniform dc field

This paper is concerned with the pre-breakdown phenomena and the breakdown voltage characteristics, in the presence of a free moving conducting spherical particle, of saturated normal liquid helium (He I) and saturated superfluid liquid helium (He II) under uniform dc field. Experiments show that the particle lifts off around the theoretical value of the lift-off electric field, and the particle oscillates between the electrodes at higher applied voltages. In that case, the microdischarge appears just before the charged particle collides with the oppositely charged electrode, and the bubble is generated at the moment of every collision of the particle with the electrode. It is confirmed theoretically as well as experimentally that the maximum bubble radius in He II is nearly proportional to the E/sub in//sup 1/3/, which is the released energy from the particle for the bubble generation. The insulation environment before the electrical breakdown suddenly changes at the /spl lambda/-point since the bubble behavior in He I and He II greatly differs. In the characteristics of the breakdown voltage vs. liquid pressure, a clear discontinuity appears at the /spl lambda/-point. Furthermore, the breakdown voltage in the parallel plane gap contaminated by a particle is lower than that in the rod-plane gap without a particle. It is found that the breakdown voltage characteristics are closely related to the trigger effect of the microdischarge and the bubble generation.     

Corona characteristics for free conducting particles in various SF6-gas mixtures

This paper reports on experimental investigations concerning the behavior of pulsative corona discharges from free conducting wire particles of different sizes in parallel plane and concave electrode systems, insulated with SF₆ and its mixtures with nitrogen N ₂, triethylamine (C₂H₅)₃N and perfluorocarbon C₈F₁₆O gases. Corona inception and particle lift-off voltages as well as corona charge levels are measured for different experimental conditions. The results show that corona and particle lift-off characteristics are affected by particle parameters, electrode shape and gas composition. Generally, addition of triethylamine and perfluorocarbon to SF₆ reduces the corona charge level     

DC corona discharge from a wire particle floated with a microgap in parallel plate electrodes

Conductive particles existing in a high-voltage system could deteriorate the system performance owing to an electric discharge via the particles. In case when the particles exist in a dc high-voltage electrode system, the location of the particle would inevitably affect the discharge aspect. A corona-onset and breakdown voltage from a metallic wire particle were measured in detail. The wire particle has a length ranging from 6 to 12 mm, and a diameter of 0.25 mm. It was fixed between a parallel plate electrode with a spacing of 20 or 30 mm. When the particle was close to the negative electrode, a corona discharge occurred stably, and the corona-onset voltage was increased as the gap length between the negative electrode, and the positive end of the particle increased up to 0.8 mm. In contrast, when the particle was close to the positive electrode, the breakdown always occurred without preceding the corona discharge. At the middle region between the electrodes, the corona onset occurred at almost constant voltage.     

Assessing the risk of failure due to particle contamination of GISusing the UHF technique

The ultra high frequency (UHF) technique has been used to study the partial discharges produced by free conducting particles in gas insulated substations (GIS) with a view to predicting the probability of particle-triggered breakdown. Statistical techniques have been developed to describe the motion of a single particle. It is possible to identify whether the particle is capable of crossing to the busbar, which is a necessary condition for particle-triggered breakdown. In addition, the technique can be used to assess the size of the particle, which is useful for making a qualitative assessment of the breakdown risk. The implications for the continuous discharge monitoring of GIS are discussed     

A study on effects of conducting particles in SF6 gasand test methods for GIS

The effects of conducting particles in gas-insulated switchgear (GIS) on insulation performance were investigated. The objective was to recognise the relationship between the length of conducting particles and AC impulse flashover voltages. The data showed that the minimum harmful length of conducting particles at various voltages applied to GIS in service is governed by lightning surge amplitude corresponding to the basic impulse insulation level (BIL). The size of minimum harmful conducting particles was also estimated for various GIS ratings. To further improve the reliability of GIS, factory and on-site test methods are suggested for preventing insulation failures caused by conducting particles     

Estimation of dc breakdown mechanisms in air gaps containingfloating metallic particles

The characteristics of breakdown voltages against the position of floating metallic particles in atmospheric air are investigated experimentally. The breakdown mechanism is estimated on the basis of electric field calculation and visual observation of discharges at different voltages and a set of flow charts for calculation of breakdown voltage is proposed. The studied gaps are needle-to-plane and sphere-to-sphere electrodes of 120 mm in length forming Poissonian and Laplacian fields respectively and the shape of tested floating objects is a needle or sphere. The theoretical breakdown voltages obtained by the flow charts agree reasonably with the experimental ones     

Neutron production rate and plasma characteristics of spherically convergent beam fusion

Experimental and simulation results of a spherical glow discharge for a portable neutron source are presented. The experimental device is a 45‐cm‐diameter, 31‐cm‐high stainless‐steel cylindrical chamber, in which a spherical mesh‐type anode 30 cm in diameter is installed. The spherical grid cathode consists of 2.0‐mm‐diameter stainless‐steel wire, which is made into an open spherical grid of 5‐cm diameter. The system is maintained at a constant pressure of 1 to 15 mTorr by feeding hydrogen or deuterium gas. The basic characteristics of breakdown voltages versus pressure and electrostatic potential profiles were measured for hydrogen discharge. Using deuterium, a steady‐state neutron production of 10⁴ s^–1 was observed at a discharge of 40 kV, 2 mA. Motions of ions and electrons in the device were simulated by using a particle code, which is one‐dimensional in coordinate system and two‐dimensional in velocity space. It was confirmed by both the measurement and simulation that a virtual anode is formed in the central part inside the grid cathode. © 2001 Scripta Technica, Electr Eng Jpn, 135(2): 1–8, 2001     

DC pre-breakdown phenomena and breakdown characteristics in thepresence of conducting particles in liquid nitrogen

DC pre-breakdown phenomena and breakdown characteristics in the presence of free conducting particles in liquid nitrogen are studied experimentally. The results show that a microdischarge occurs when a charged particle is approaching an oppositely charged electrode. An intense microdischarge can trigger a complete breakdown of the gap. The breakdown voltage of a uniform field gap with a free metallic particle of mm size might be reduced well below that of a point-to-plane gap without a particle in liquid nitrogen. Heavy contamination by a metallic powder produces a large reduction in the breakdown voltage with a horizontal spacer surface. However carbon powder is less hazardous compared to metallic powder     

长绝缘子串带电测零的判断与分析

针对某220 kV开关站于2005年6月8日带电测零时发现多个绝缘子串电压分布异常,以及部分绝缘子被误判断为零值的现象,从串长和导线两方面分析了造成误判断的原因,并提出了防止误判断的措施,包括对串长超过标准串长的长绝缘子串零值检测不能使用短路叉,对分布电压测量仪测量结果要综合分析、正确判断等。     

Electrical breakdown characteristics of nitrogen and air at cryogenic temperature in a quasi‐uniform electric field

The breakdown voltages of the longer‐gap configurations in gaseous nitrogen and air that are necessary in designing superconducting electrical power apparatuses are measured at temperatures of 293 and 93 K. The quasi‐uniform electric field made by a sphere‐to‐sphere electrode with a diameter of 150 mm and a gap length of about 10 to 100 mm is used in the measurement of the breakdown voltages. When 50‐Hz ac and dc voltages are applied to the sphere‐to‐sphere gap, the breakdown voltages in nitrogen and air obey Paschen's law even at cryogenic temperatures (93 K). When a 1.4/50‐μs lightning impulse voltage is applied to the gap, the 50% breakdown voltage of nitrogen also obeys Paschen's law under UV irradiation of the cathode electrode. However, the breakdown voltage in air at 93 K is higher in the case of lightning impulse voltage applications, and the delay from impulse voltage application to breakdown occurrence is apparently longer at 93 K than at 293 K. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 28–33, 2000     

Partial and complete electrical breakdown in simulated hightemperature superconducting coils

The partial breakdown (PBD) and complete breakdown (BD) phenomena in a composite insulation system of glass fiber reinforced plastic (GFRP) and liquid nitrogen are investigated to find the PBD and BD characteristics in high temperature superconducting (HTS) coils at quench. The electrode system used is made from a coaxial spiral coil-to-cylindrical electrode with an insulation barrier and spacers, and is immersed in liquid nitrogen. A heater is mounted inside the coil electrode to generate boiling which occurs on quenched superconducting coils. The experimental results show that (1) the polarity of PBD initiation under 60 Hz ac applied voltage depends on the width l_c of the cooling channel, (2) PBD appears through a gas bubble locked between the coil and concentric insulation barrier due to the electrical gradient and the spacer, (3) breakdown voltages are affected severely by the risetime of the applied voltage and the current amplitude associated with PBD, (4) two kinds of BD mechanisms are found depending on the shape of the spacer, length of cooling channel and heater power     

Influence of sand/dust contamination on the breakdown ofasymmetrical air gaps under switching impulses

This paper discusses the influence of airborne and surface-adhering sand/dust contamination on the breakdown characteristics of asymmetrical air gaps when subjected to standard switching impulses of both polarities. Extensive measurements of breakdown voltages and breakdown times are carried out under clean and polluted conditions using rod-plane and sphere-plane gaps. When positive switching impulses are applied to gaps ⩽1.8 m, the presence of sand/dust particles ⩽150 μm diameter and ⩽2 g/m³ concentrations have insignificant influence on the breakdown characteristics of the air gaps. However, the presence of such particles can greatly affect the breakdown characteristics for negative switching impulses. Under such conditions, the particles can decrease V₅₀ by ⩽40% for gaps with field factor f ⩽3.5. For gaps with 3.550 by ⩽80%. For more nonuniform field gaps, V₅₀ values for clean and contaminated conditions are similar. The particles can also significantly influence the breakdown time lag characteristics under negative switching impulses. The observed effects are primarily caused by sand/dust particles which settle on the cathode, while the particles present in the interelectrode gap or on the anode have very little influence     

Technical Development on Partial Discharge Measurement and Electrical Insulation Techniques for Low Voltage Motors Driven by Voltage Inverters

This paper reviews the up-to-date research activities on electrical insulation techniques under repetitive surge voltages with steep wavefront for low voltage inverter-fed motors. Partial discharge (PD) inception, propagation, ageing and breakdown characteristics of magnet wires, including recently developed PD-resistant wires such as nanocomposite enameled wires, are described. PD measurement techniques under the steep-front voltages and IEC activity toward their standardization are also introduced.     

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     

工频电压下片状自由金属微粒带电特性与运动规律研究

GIS/GIL因安装、运输等环节会不可避免地产生多样性的金属微粒,而片状金属微粒因其"潜伏性"较难检出,从而威胁GIS/GIL的绝缘性能。采用数值计算结合实验方式开展了工频电压下片状金属微粒起跳与运动规律研究。片状金属微粒采用旋转椭球体模型进行等效,并通过理论推导获得片状金属微粒的带电量与临界起跳场强。搭建工频电压下片状金属微粒运动观测平台。采用高速相机观测其起跳和运动行为,并与数值计算结果进行对比分析。结果表明:片状金属微粒起跳场强与厚度的1/2次方近似呈正比关系,而随微粒半径增大呈现出微弱的上升趋势。密度较大的片状金属钢微粒竖起后,存在原地旋转与沿地电极表面来回滚动两种运动模式,并在微粒底部与电极间隙伴有微弱的局部放电;密度较小的片状金属铝微粒竖立后,在电极间隙内发生剧烈地上下跳动。     

纳米SiO2/低密度聚乙烯复合介质的击穿特性

聚合物纳米复合介质的击穿强度与纳米填充颗粒的质量分数、粒径和表面处理密切相关。以不同表面处理的纳米SiO2颗粒为填料,制备了不同填充质量分数、粒径的纳米SiO2/低密度聚乙烯(low density polyethylene,LDPE)复合介质,测试了其在交流、直流正极性和直流负极性3种不同类型电场下的击穿场强。结果表明:在所研究范围内,填充纳米SiO2颗粒可以提高低密度聚乙烯的击穿场强,并且随着填充质量分数的增加,复合介质的击穿场强有升高的趋势。纳米颗粒填充质量分数相同时,在12~16 nm粒径下,复合介质的击穿场强有随纳米SiO2粒径的升高而降低的趋势;在7 nm小粒径下,可能由于纳米颗粒容易团聚,导致击穿场强较粒径大时要低。另外,纳米SiO2颗粒表面经疏水性处理后,能够有效提高复合介质的击穿场强。     

Particle-initiated breakdown in a quasi-uniform field intransformer oil

The breakdown voltage induced by single metallic cylinders of millimeter lengths in a quasi-uniform AC field are reported. A particular electrode arrangement, including forced oil circulation, has been designed in order to avoid particle ejection out of the gap. Breakdown occurred in the range 25 to 80 kV/cm depending on the particle dimensions, and always when the particle was in contact with the positive electrode. These breakdown levels are clearly lower than those measured with a fixed particle subjected to a positive step voltage. All these measurements were sensitive to the influence of carbon contaminants, especially under step impulse voltage. As concerns free particles, the breakdown is thought to be controlled in our experimental conditions by the initiation of positive streamers. Calculations showed that the initiation field at the particle tip is not constant but mainly depends on the particle diameter     

Motion of conducting particles causing inadvertent outages in GIS

Inadvertent failure of a HV gas-insulated system (GIS) is traced to a seemingly innocuous mechanism in particle-induced breakdown. Impending dangers from inconspicuous conducting particles in a large physical system are quite intriguing. The dynamic behavior of the particle due to electric field/particle/fluid medium interaction is a complex phenomenon. To understand this interaction a credible database has been generated for the field-induced motion of the particles on a physical prototype model. Induced surface charge acquired by the particle is calculated based on the proposed model (field calculations) and by considering it to be a prolate ellipsoid. Methods for deactivating potentially dynamic particles using dielectric coated active parts and grounded enclosures, and a novel technique called reverse polarity charging have been suggested. Performance of GIS in the presence of metallic particles has been studied with bare electrodes and with dielectric coatings of different conductivities with a view to obtain an optimal value of the conductivity