Electrical Breakdown and Space Charge Formation at High‐Temperature Region in Long‐Time Heating Treatment PVC

Polyvinyl chloride (PVC) is widely used as an insulating material in various electrical products. It is reported that an exothermic reaction reaching temperatures above 150 °C can be caused by overload currents or inferior electrical wire connections before the ignition of electrical products. The exothermic phenomenon may cause deterioration of insulating properties in PVC due to its chemical decomposition. It is necessary to clarify the degradation of insulating properties in PVC under thermal stress exceeding 150 °C for the safe use of electric products. In this investigation the space charge distribution and conduction current in the heat‐treated PVC sheet were measured in the range from room temperature to 200 °C in the presence of a dc electric field, using a high‐temperature PEA system. Positive charge injection and increasing conduction currents were observed before breakdown above 100 °C in 100 °C 300‐h heat‐treated samples and in non–heat‐treated samples. The results indicate the thermal breakdown process from the analysis of conduction currents and electric fields. In samples exposed to higher temperatures (150 °C 100 h), the breakdown strength deteriorated strongly in the range from room temperature to 90 °C. Increases in conduction current were observed in the entire temperature range before breakdown of the 150 °C 100‐h heat‐treated PVC. This indicates that heat treatment above 150 °C degrades the breakdown properties in the range from room temperature to 90 °C due to thermal decomposition accompanied by dehydrochlorination in PVC. The electric field is intensified near the cathode due to positive charge accumulation, and the breakdown strength begins to deteriorate only above 90 °C. This shows that thermal stress exceeding 150 °C causes deterioration of insulating properties and that the breakdown process is affected by space charge formation in PVC.     

Electrical Breakdown Properties and Space Charge Formation in High Temperature Region in Ultraviolet Ray Irradiated PVC

Polyvinyl chloride (PVC) is the most popular insulating material for electric wiring instruments. However, an exothermic reaction above 150 °C may cause deterioration of the insulating properties of PVC. Therefore, it is important to clarify the heat degradation in PVC, not only to investigate the ignition of electrical wiring products but also to use electrical products safely. It is known that ultraviolet (UV) irradiation causes chemical deterioration of PVC and an increase in its conductivity. Generally, it has been thought that the electrical breakdown properties, electrical conduction, and insulating performance are affected by space charge accumulation in an insulating material. A high temperature pulsed electroacoustic (PEA) system usable up to 250 °C has been developed, and the PEA system can measure the space charge distribution and conduction current in the high temperature range simultaneously. In this investigation, the space charge distribution and conduction current were measured up to electrical breakdown in a non‐UV irradiated sample (normal PVC) and in 353 nm and 253 nm UV‐irradiated PVC samples in the range from room temperature to 200 °C in a DC electric field. In the short wavelength UV irradiated PVC sample (253 nm, 300 h), a deterioration of breakdown strength at 90 °C to 150 °C and negative packet‐like charges were observed at 60 °C and 100 °C, a positive charge accumulated in front of both the anode and cathode above 90 °C, and a higher electric field near the cathode side because the positive charge of the cathode side was greater.     

High‐field electrical properties of new LDPE films prepared using metallocene catalyst

Now low‐density polyethylene (M‐LDPE) prepared using metallocene catalyst has narrower composition distribution and molecular weight distributions than linear low‐density polyethylene (LLDPE) using Ziegler catalyst. The authors compared the electrical insulating properties of M‐LDPE films and conventional LLDPE films. The high‐field current of M‐LDPE was found to be lower than that of LLDPE. The difference in current increased with the decrease of temperature. The high‐field current was much reduced in M‐LDPE with a low melting point. The impulse breakdown strength of M‐LDPE increased with the decrease of temperature. The insulating properties of M‐LDPE with a low melting point were improved at 30 °C. These results were explained by the fact that M‐LDPE with a low melting point includes more low‐molecular‐weight components. We also discuss the effects of antioxidant on the electrical properties of M‐LDPE. We compared the electrical conduction and breakdown strength of undoped M‐LDPE and antioxidant‐doped M‐LDPE. Differences in their electrical properties were minor. It was found that the excellent properties of M‐LDPE do not depend on the effects of antioxidant. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 1–9, 2000     

Electrical conduction in highly resistive polyolefin films modified by polar groups

Electrical conduction and space charge distribution in oxidized low-density polyethylene (samples A1–A6) and copolymers of ethylene and carbon monoxide (samples B1–B3) with different carbonyl contents have been studied. A V-shaped dependence of conduction current on carbonyl content was observed for samples A1–A6, while the current decreases monotonically with an increase in carbonyl content for samples B1–B3. The effect for polar groups on electrical conduction is discussed, focusing on their bonding forms in polymer molecules. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(3): 9–16, 1997     

Leakage current analysis of hydrothermal BaTiO3 thin films

Hydrothermal processing can deposit crystalline ferroelectric films at low temperatures of less than 150°C to achieve permittivities above 100. Such a process, hence, can be attractive in integrating thin film capacitors in organic, silicon or flex substrates. However, their poor insulation strength leading to high leakage current can prevent their wide acceptance. Lattice defects such as hydroxyl groups are attributed to their high leakage currents and lower Breakdown Voltages (BDVs). With appropriate thermal treatments, majority of the OH groups can be removed, leading to improved insulation characteristics. The leakage current behavior of as-synthesized and post-baked hydrothermal thin films are analyzed with various conduction models. The room temperature I-V characteristics are attributed to a combination of ionic and Space-Charge-Limited (SCLC) conduction models for films baked at 160°C while higher baking temperatures of 350°C agree well with Poole-Frenkel type conduction, with an activation energy of 0.57 eV for the defects. The defects, which are presumably OH groups or oxygen vacancies embedded in the barium titanate lattice, act as shallow traps and the trapping and detrapping results in easier conduction. A brief perspective is provided on the suitability of such a hydrothermal thin film capacitor approach for power supply applications.     

Electrical properties of silane crosslinked polyethylene incomparison with DCP crosslinked polyethylene

The electrical properties such as water tree length, electrical conduction, ac breakdown strength and space charge of silane crosslinked polyethylene (SXLPE) were investigated, with a purpose to compare this material with ordinary dicumyl peroxide (DCP) crosslinked polyethylene (XLPE). Experimental results show that SXLPE has a smaller water tree length, much lower conduction current density, a little bit higher ac breakdown strength than XLPE, and on SXLPE only a very small homocharge is seen. The better behaviour of SXLPE under electric stress is attributed to the absence of residual curing byproducts present in XLPE and the change of chemical structure by grafting and curing reactions     

A consideration of potentials of low‐density polyethylene using metallocene catalyst as insulator for power cable

We investigated the high‐field conduction, DC and impulse breakdown strength, space charge distribution, and tree inception voltage for three kinds of new low‐density polyethylene (LDPE) prepared using a metallocene catalyst (mLna, mLao, mLldao), linear LDPE prepared using a Ziegler catalyst (LLao), and LDPE prepared by a high‐pressure process (LDna). The dc and impulse breakdown strengths of LDPEs prepared using a metallocene catalyst were higher than those of LLao and LDna. The high‐field currents of LDPEs prepared using a metallocene catalyst were lower than those of LLao and LDna. A homo‐space charge was accumulated near the cathode in mLna. The tree inception voltage of mLna was higher than that of LDna. From these results, it is concluded that LDPE prepared using a metallocene catalyst has electrical insulating properties superior to the conventional LDPE and that the former has potential as a power cable insulator. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 17–25, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.1164     

Effects of manufacturing technology on electrical breakdown and morphology of thin films of low density polyethylene blended with polypropylene copolymer

Extruded films prepared from blends of low-density polyethylene (LDPE) and random copolymer of ethylene and propylene (EP) with the T-die method were studied with respect to electrical properties and morphology. Comparisons with data on blown films are made. These blends are of interest as improved LDPE for making XLPE for insulated power cable. In the high temperature region (90/spl deg/C), a specimen with a slightly higher EP content had higher impulse breakdown strength than that with a lower EP content, but no improvement of DC breakdown strength by blending could be found. The improvement of impulse breakdown strength (90/spl deg/C) is explained in terms of morphological changes by blending such as the orientation of chains in a film and the size of spherulites on the assumption of the thermal breakdown. In comparison, a T-die film had higher impulse breakdown strength than that of a blown film for the same composition. The impulse breakdown strength also increased with the use of the higher density LDPE. In the current versus electric field characteristics at 30/spl deg/C, the blend polymer with EP content of 5-10% showed a transition from LDPE behavior at low field region to EP behavior at high field region. However, no appreciable difference in current behavior among the specimens was observed at 90/spl deg/C, which suggests an incompatibility between the two materials that exists at 30/spl deg/C but not at 90/spl deg/C.     

Effect of tacticity on the dielectric properties of polystyrene

Dielectric properties of polystyrene (PS) were investigated by measuring electrical conduction current and frequency spectra of complex permittivity at temperatures from 20 to 199 °C. To clarify the effects of tacticity, the properties were compared among two kinds of syndiotactic PS (SPS) with different crystallinity values and one atactic PS (APS). Experimental results indicate that the two SPS samples show a high melting point of about 250 °C. The conductivity is lowest in SPS with the low crystallinity in the whole temperature range from 20 to 199 °C, while APS had a breakdown at 140 °C. Moreover, the low‐crystallinity SPS shows the smallest permittivity values and the smallest increase in dielectric loss factor toward low frequencies among the three samples. The high‐crystallinity SPS has α crystals. Since the density of α crystals is lower than the amorphous region, the charge transport and polarization are easier in the high‐crystallinity SPS than in the low‐crystallinity SPS. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

变频电机电磁线绝缘膜电树枝化击穿机理研究

阐述了调速牵引电机中电磁线绝缘破坏的问题及电树产生和发展的机理。以完好的和含有杂质的变频电机用电磁线为试样,在高压下老化直至击穿,对击穿点进行扫描电镜和能谱分析。试验结果表明,两试样的击穿均为树枝状击穿,金属杂质的存在降低了绝缘膜的电气强度。     

Preparation and electrical properties of sintered oxide composed of MnFeNiO4 with a cubic spinel structure

The preparation and electrical properties of a sintered body consisting of cubic spinel oxide MnFeNiO₄ were investigated. A sintered body with cubic spinel and NaCl-type oxides prepared at 1400 °C in Ar could be converted to one consisting of monophase cubic spinel oxide by oxidation at 1000 °C for more than 48 h in air. The electrical conductivity of a sintered body consisting of monophase cubic spinel oxide was confirmed to increase exponentially with increasing temperature, indicating that the oxide has intrinsic negative temperature coefficient (NTC) thermistor characteristics. The electrical conduction of the oxide was concluded to be controlled by the small polaron hopping mechanism. Changes in electrical conductivity, mobility, and charge carrier jump frequency in temperature dependence at 400 °C are assumed to be related to the variation in cation distribution accompanying the disproportionation of Mn ions.     

高压短脉冲作用下岩石击穿特性的实验研究

为了开发脉冲等离子体钻机,开展了高压短脉冲作用下岩石击穿的实验研究。以去离子水为工作介质,将不同种类和不同厚度的岩石放置于尖板电极间,通过加载不同大小的电压于岩石上,考察了岩石的电击穿概率与平均电场强度的关系。实验结果发现:7%空隙率的黄砂岩电击穿场强为70kV/cm;0.8%空隙率的白大理岩电击穿场强为160kV/cm。岩石孔隙率的大小对电击穿场强影响显著,孔隙率越大,电击穿场强越小。岩石厚度对电击穿场强也有一定影响,电击穿场强随厚度增加而减少。     

High-field conduction and breakdown in insulating polymers. Presentsituation and future prospects

We review the results of electrical conduction and breakdown research reported mainly after 1980 in Japan, and comment on the present situation and future prospect of research work in this field. Effects of introduction of polar groups into polymers on electric strength F_b are summarized. Influences of copolymerization and modification of morphology and chemical structure on F_b of polyethylene are also reviewed. Finally, we show currently used experimental techniques for direct observation of space charge, prebreakdown current and localized heat generation     

嵌入屏障的绝缘介质中电树生长模型

利用WZ模型将绝缘介质离散化,使用分形理论讨论了绝缘介质中电树发展的随机性和确定性。在原有WZ模型的基础上引入了分布耐电强度的概念,建立了新的模型。通过此二维模型仿真了在绝缘介质中嵌入了耐电强度高于原介质的屏障后的电树生长情况。在针板电极结构下用模型仿真了在绝缘介质中所嵌入屏障耐电强度的不同对电树发展造成的不同影响,并比较了不同情况下的平均击穿距离。通过比较得出,当嵌入耐电强度远大于原介质的屏障后,电树发展过程中需要绕过屏障进行生长,从而等效于增加了原有介质的厚度,使得整个介质的绝缘性能有一定程度的提高。     

Effects of inorganic fillers on withstanding short‐time breakdown and long‐time electrical aging of epoxy composites

Inorganic fillers were added to epoxy resin for preparing micro and nano filler/epoxy composite, and then the breakdown strengths, depths of erosion caused by partial discharge, and electrical tree experiments of composites were investigated. It was found that the loading of micro fillers usually decreases the short‐time breakdown strength (breakdown under a continuously rising voltage) of the composites. On the contrary, it increases the ability to withstand long‐time electrical aging (erosion or breakdown caused by electrical tree under a discontinuously rising voltage or constant voltage) of the composites. The effect of inorganic fillers in suppressing the electrical tree or erosion channel propagation in the composite is different during the process of withstanding short‐time breakdown and long‐time electrical aging. Defects are introduced during the mixing of fillers and epoxy resin, which play a role to decrease the short‐time breakdown strength and accelerate long‐time electrical aging. It is concluded that micro and nano inorganic fillers can suppress the electrical tree channel propagation and erosion caused by partial discharge, whereas defects play the opposite role in composites. Two opposite effects of inorganic fillers and defects exist simultaneously during the process of electrical aging and breakdown; whereas inorganic fillers play a primary role in long‐time electrical aging, defects play a primary role in short‐time breakdown.     

准均匀电场下C5F10O/干燥空气与C5F10O/N2的绝缘特性

SF_6因优异的电气特性广泛应用于电器绝缘设备中,但其产生的温室效应对大气环境具有极大损害。近年来,C_5F_(10)O作为一种环保型SF_6潜在替代气体受到国内外科研工作者的关注。为进一步探究C_5F_(10)O/干燥空气与C_5F_(10)O/N_2的绝缘特性,文中利用气体绝缘性能测试平台,对不同气压、不同C_5F_(10)O分压下的2种混合气体在准均匀电场下进行工频击穿试验。实验结果表明,C_5F_(10)O混合气体绝缘强度随气体压强的增大而增大;提高C_5F_(10)O分压亦可提高两类缓冲气体的绝缘强度,且对N_2绝缘强度提升相对值大于干燥空气。从绝缘强度考虑,适当增大C_5F_(10)O混合气体气压和C_5F_(10)O分压,C_5F_(10)O/干燥空气比C_5F_(10)O/N_2混合气体更具潜力替代室内中低压设备中的SF_6。     

Measurements of acoustic waves generated by electrical breakdown in a polypropylene film

Electrical breakdown of a polymer film generates a pressure wave that is believed to include information about the breakdown initiation point. We measured the breakdown pressure wave and the space charge distribution up to the electrical breakdown field by using the pulsed electro‐acoustic method in a 30 µm thick polypropylene film. We discuss electrical breakdown phenomena based on the breakdown pressure wave and the dependence of the space charge distribution on the applied field and temperature. At room temperature, the observed breakdown pressure wave had a pulse‐like shape with a width that depended on the polarity of the applied field. Positive space charge accumulation was observed near the cathode as a hetero space charge near the electrical breakdown field. At 60 °C, the width of the breakdown pressure wave showed no dependence on the applied field polarity and positive space charge accumulation was observed inside the film near the electrical breakdown field. These experimental results suggest that electrical breakdown phenomena are affected by hetero space charge accumulation and that the initiation point of electrical breakdown corresponds to the position of hetero space charge accumulation in 30 µm thick polypropylene film. ©1999 Scripta Technica, Electr Eng Jpn, 126(3): 1–8, 1999     

The effect of growth atmosphere and Ir contamination on electric properties of La<Subscript>3</Subscript>Ta<Subscript>0.5</Subscript>Ga<Subscript>5.5</Subscript>O<Subscript>14</Subscript> single crystal grown by the floating zone and Czochralski method

The influence of growth atmosphere and Ir contamination on electrical resistivity of langatate (La₃Ta_0.5Ga_5.5O₁₄; LTG) was studied. LTG single crystals were grown via the Czochralski method under different oxygen partial pressures with Ir contamination from the Ir crucible. In addition, LTG crystals were grown by the floating zone method and they were not contaminated by Ir. The electrical resistivity and ionic transport number of these crystals were measured in the temperature range 300–1000°C. The conduction mechanism of LTG changed at about 720°C. At T < 720°C, electronic conduction was dominant, and the resistivity was affected by growth atmosphere as well as Ir contamination. In contrast, at T > 720°C, ionic conduction was dominant, and the resistivity was affected only by Ir contamination. In both temperature regions, Ir contamination decreased the resistivity by an order of magnitude.     

High‐field electrical properties under AC voltage application in acetophenone‐coated LDPE film

This paper deals with the effects of acetophenone coating on dielectric properties such as tan δ and capacitance and on space charge formation up to electrical breakdown under AC voltage application in low‐density polyethylene (LDPE) film. The existence of acetophenone at the electrical interface enhances tan δ. tan δ in acetophenone‐coated specimens increased with electric field and decreased with the frequency in the high‐field and high‐temperature region. AC breakdown test revealed that the electric strength in noncoated specimens was higher than in acetophenone‐coated specimens, and that the electric strength also decreased with temperature. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(3): 1–7, 2003; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.10086     

Electrical treeing phenomena in two-layer silicone gel with different crosslinking degrees and its dielectric strength

Silicone gel is widely used to encapsulate power modules, and improvement of its dielectric strength has been required. So, the purpose of our research is to improve dielectric strength of silicone gel encapsulant, and we focus on crosslinking degree of silicone gel. Previous studies have shown that growth mechanism of electrical tree changes with crosslinking degree of silicone gel. This suggests the possibility that the presence of the interface by different crosslinking degrees inhibits the tree growth. In this paper, we have investigated the tree growth and breakdown characteristics in silicone gel—crosslinking degrees graded layer materials. The interfaces in our study are arrange as being vertical to the line of electric force. Consequently, it was clarified that barrier effect of interface by different crosslinking degrees and the relaxation of electric field in low crosslinking degree region retards on tree growth, which improves the dielectric strength.