Electrical tree initiation

This paper reviews the recent understanding of the initiation mechanisms of electrical trees and the phenomena occurring during the induction period in the discharge free condition. The importance of charge injection in electrical tree initiation is well established, not only under dc or impulse stresses, but also under ac stress. Electroluminescence can be used as a monitor for charge injection. Tree initiation from field enhancing locations under moderate stress proceeds through cumulative degradation reactions and cavity formation caused by injected charge. The dependence of tree initiation on temperature and the nature of dissolved gases, underline the importance of free volume processes, and therefore of polymer morphology. Electron avalanche type tree initiation is strongly influenced by field modification due to space charge and also by the size of the high field region     

Space charge measurement techniques and space charge inpolyethylene

Recently, several new techniques such as LIPP, PIPS, PEA and TP methods have been developed to measure directly the space charge distributions in insulating polymers. Many papers have been published on space charge in insulating materials. In this paper, the space charge measurement techniques and space charge in polyethylene are reviewed. The space charge distributions in polyethylene depend strongly upon additives (antioxidants, antistatic agents, etc.), oxidation products, byproducts from the crosslinking reaction (acetophenone, etc.). Some of them enhance electron (or hole) injection from the electrode and, as a result, homo space charge is formed. Ionic carriers are supplied from some impurities to form hetero space charge. Space charge distributions are also sensitive to the electrode material and the interface between different materials. From the space charge behavior observed, the space charge effects on the high-field conduction and breakdown phenomena have been discussed quantitatively and some of the high-field phenomena in polyethylene have been elucidated     

Space charge effect for streamer initiation and propagation in water subjected to reciprocal traveling wave voltage pulse

This paper deals with initiation and propagation processes of streamer discharges in water subjected to high-speed repetitive voltage pulse. The 50%-duty repetitive pulses were produced by the reciprocal traveling wave in a pulse forming cable which was connected with a point-plane electrode gap in a water-cell. Laue plot of time lag distributions for the streamer initiation indicated that the initiation probability during the second pulse application became much higher than that during the first pulse application. We concluded that the change in space charges distribution due to internal field during the rest-time after the first pulse enhanced the streamer initiation probability at the second pulse application. Temporal development in the positive streamers during the repetitive pulse application was observed using a gated image intensifies. When the pulse was relatively high, the streamer discharges started the propagation at the first pulse and then the discharge channel became long step by step at the subsequent pulse applications. It was also found that the periodical emission due to excitation of OH radicals was detected for a long period after the development of the streamer discharge in a spectroscopic measurement.     

氟塑料中空间电荷的测量

为研究三种氟塑料形成空间的差别的相应机理 ,用电声脉冲法 (PEA)测量3种可熔性氟塑料四氟乙烯 -全氟(烷基乙烯基)醚共聚物(PFA)、四氟乙烯 -六氟丙稀共聚物(FEP)及乙烯 -四氟乙烯共聚物(ETFE)中空间电荷的分布 ,并用红外光谱 (IR)分析它们的结构特征 ,以分析氟塑料中空间电荷的形成机理。结果表明 ,PFA和FEP比ETFE更适合用于生产电线电缆。     

Study of the influence of injected charge on streamingelectrification

In this paper, the influence of injected charge on streaming electrification of transformer oil has been investigated using a combined model. The experimental results reveal that there are relations between the injected charge, the charge density and the leakage current, and the polarity effect of injected charge exist in the influence on streaming electrification. The influence of injected charge is attributed to the interaction between the electric double layer and the injected charge. The effect of oil velocity on injected charge has been analyzed also     

Space charge distributions in EPDM compounds

The formation of space charge in EPDM (ethylene-propylene-diene terpolymer) filled with ATH (alumina trihydrate) or clay has been investigated by measuring charge distributions. It is found that the type and amount of space charge developed in crosslinked EPDM or its compounds is governed by the type of diene in EPDM and the composition of the semicon electrodes. The amount of charge in the EPDM compounds containing 1,4-hexadiene decreases with an increase of ATH content and increases with an increase of clay content. The heat treatment of EPDM compounds in a vacuum oven increases the amount of space charge in them. These features were explained by the effects of low molecular weight molecules present at the surface of fillers. It is found that the homocharge increases as the clay content increases, while EVA decreases the homocharge in the EPDM compounds. Possible reasons for these results are discussed     

Space charge behavior in low-density polyethylene

The space charge distribution in low-density polyethylene (LDPE) was measured with the pulsed electroacoustic (PEA) method. We used three types of LDPE: LDPE-L and LDPE-H were prepared by the high pressure process, and m-LDPE was polymerized with a metallocene catalyst. Space charge in LDPE strongly depended upon the electrode material. Semiconductive electrodes enhanced carrier injection into LDPE and, as a result, space charge. The density, polymerization process, applied field, temperature and so on also affected the space charge behavior. This space charge behavior was compared with the results of dc current measurements     

Space charge formation in water-treed insulation

Observations of the space charge distribution around water trees are reviewed, and the nature of water trees analyzed from those results is discussed. It was concluded that at least two mechanisms are responsible for the formation of space charges. One is interfacial polarization due to the high conductivity of a tree channel, and the other is carrier injection from the tree tip at high electric fields     

Model for electrical tree initiation in epoxy resin

A model for electrical tree initiation in epoxy resins is presented in which the process is driven by the generation of new charge traps as a result of energy transferred to the polymer via charge recombination processes. The electroluminescence intensity expected from the model is computed and shown to be in agreement with the experimental data. In particular it is shown how the initial emission due to recombination can change to an emission arising from impact excitation in a natural way when the trap density reaches a level sufficient for the high electric field of the tree initiation region to connect shallow traps to form conducting filamentary paths in the form of percolation clusters. This result allows the electroluminescence behavior to be correlated with the observed onset of filamentary damage. The model is also shown to be able to explain the decrease in emission intensity observed at the time of transition between the two mechanisms for luminescence.     

Space charge and electrical conduction in maleic anhydride-graftedpolyethylene

The space charge behavior such as charge distribution and electrical conduction of maleic anhydride grafted LDPE (LDPE-g-MAH) was investigated. Heterocharge was found in LDPE and it decreased with increasing MAH graft ratio in LDPE-g-MAH. Conduction currents also decreased with increasing MAH graft ratio. The relationship between the space charge behavior and the electrical conduction characteristics in LDPE-g-MAH is discussed     

Space charge and dielectric response in polar fluids

The assumption of "low signal" response is important in relation to the interpretation of the interaction of the electrical field (dc and ac) with mobile charge carriers. The existence of such a response is assumed in linear samples under the condition of absence of space charge. Experimental evidence as well as theoretical arguments of formation of hetero-charge in liquid-like materials have been reported. The materials, of course, are of low electrical conductivity. It has been demonstrated that the existence of space charge field is not always followed by sample nonlinearity. It is the case, where space charge field acts as a macrodipole with particular time or frequency response function. In principle, dc and ac sample responses should be the same because of the lack of nonlinearities. However, there is experimental evidence that this statement is not always true. The important problem seems to be the detection of space charge effects, which is particularly difficult in the case of ac measurements. There is a proposal to use dc combined with ac or dc measurements     

Space charge and anomalous discharge currents in crosslinkedpolyethylene

Transient currents in cross-linked polyethylene (XLPE) have been investigated by using different parameters: temperature T, electrical field E_p and hydrostatic pressure P. Anomalous discharging currents flowing in the same direction as charging currents were observed from T=30 to 70°C, P=0.1 to 30 MPa and E_p=13 to 33 kV/mm. The space charge was measured using the pressure wave propagation method (PWP). Theoretical calculation revealed that this anomalous behavior can result from charge migration under a partial blocking condition at the electrodes. We discuss theoretical results which are in reasonable agreement with our discharge current measurements     

Space charge formation at the interface between a charge transportlayer and a polyester film

The organic photoconductor structure used in a copying machine consists of electron-conductive and ion-conductive layers, a charge generation layer, and a charge transport layer. We applied the pulsed electroacoustic method, which has been used to evaluate HV insulation, to measure the space charge behavior of the charge transport layer coated on a polyester film. On the surface of the charge transport layer, various electrodes were evaporated. An internal space charge accumulated at the interface between the charge transport layer and the polyester film under a dc electric field. We show that the charge distribution is formed by the injection of holes from the anode and by the surface charge, and that the hole injection is influenced by the anode material     

Space charge behavior of acrylic monomer-grafted polyethylene

This report describes space charge behavior of polyethylene (PE) grafted with acrylic monomers such as acrylic acid (AA), methacrylate (MA), ethylacrylate (EA) and n-butyl acrylate (NBA). The specimens were 0.7 mm thick with semicon electrodes, and vacuum degassed. Heterocharge is found in ungrafted control PE. This heterocharge decreases with the increase of AA content when AA is grafted onto the PE chains. This was attributed to an enhanced charge trapping at carbonyl sites provided by AA. When other acrylic monomers are grafted, heterocharge of the control PE increases first at low graft ratios and then decreases at higher graft ratios. This change was attributed to the combined effects of alkoxy groups acting as electron donating species and carbonyls acting as charge trapping sites. It seems that charge trapping at the carbonyls is sacrificed in part by the alkoxy groups at low graft ratios. All grafted polyethylenes are found at low fields below 1×10⁵ V/cm to obey the space-charge limited conduction (SCLC) mechanism. Effective charge mobilities of PE-g-AA and PE-g-NBA decrease with the increase of graft ratio while those of others remain unchanged     

Space charge in polyethylene/ionomer blends

Measurements of thermally stimulated currents were made on ionomer/polyethylene blends. The objective of this work was to study space charge formation in these materials. Two ionomers were compared; both ionomers contained methacrylic acid, neutralized by zinc in one and by sodium in the other. A large current peak centered near 60°C was observed in all blends and associated with the motion of space charges injected into the blends during poling. All blends accumulated large negative space charge concentrations. The approach to the equilibrium space charge distribution (negative and spatially uniform in all blends) is much more rapid in the blends containing the sodium ionomer than in the blends containing the zinc ionomer     

表面氟化聚乙烯中的空间电荷分析

根据在高场强下聚合物中的空间电荷主要来源于电荷的注入,而电荷的注入决定于聚合物的界面特性,用氟气表面处理聚乙烯试样改变表面层化学结构,来研究表面层的极性对空间电荷形成和注入的影响。测量了在不同场强下加压一定时间后短路条件下聚乙烯和表面氟化聚乙烯中空间电荷的分布情况,发现表面氟化聚乙烯的电荷注入的起始场强比聚乙烯试样的低,而且在相同场强下界面和介质中的空间电荷比聚乙烯多,在80kV／mm的高场强下表面氟化聚乙烯产生了空穴的注入,并通过测量材料的红外谱图对以上现象给出了微观的物理解释。     

Space charge phenomenology in polymeric insulating materials

Topics related to space charge phenomenology, particularly the origin of space charge and its interaction with the host insulating polymer, have been widely discussed in recent years. Due to the development of reliable techniques for direct observation of space charge it is now possible to obtain, besides obvious indications on space charge and electric field magnitude and location, quantities that provide additional information on space charge accumulation and depletion rate, apparent mobility and trap depth distribution. Therefore, commonly debated topics such as the nature of charge carriers, the conduction mechanism, the charge dynamics at interfaces and the relation with nanostructure can be re-examined on the basis of space charge observation and this is, indeed, the purpose of this paper.     

Temperature dependence of superstructure and electrical tree initiation voltage of polyethylene

Below the melting point, polyethylene forms a structure in which the crystalline and the amorphous regions exist together. Therefore, polyethylene is not homogeneous. It is thought that generation of a crack by Maxwell stress is one of the tree initiation causes. The crack is generated easier in an unhomogeneous than in a homogeneous structure. Above the melting point, the homogeneity may increase because all crystalline regions melt into the amorphous region. To examine the homogeneity of polyethylene and the relationship to the generation of electrical trees, polyethylene was heated up to 180 °C. The relationship between the superstructure and the electrical tree initiation voltage at various temperatures was examined. © 1999 Scripta Technica, Electr Eng Jpn, 128(3): 24–30, 1999     

Effect of liquid impregnation on electrical tree initiation in XLPE

The tree initiation voltage of impregnated crosslinked polyethylene (XLPE) with a dielectric liquid is drastically improved compared to normal XLPE for both ac and impulse voltage application. It is considered that liquid fills the polymer free volume, and removes space for electrons to be accelerated to high energy. The experimental results support that collision impact of electrons accelerated in polymer free volume is the dominant cause of electrical tree initiation, and also show that liquid impregnation is a promising way to improve performance and prolong the life of polymeric insulating materials