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Repeated impulse voltage has long been considered to affect the insulation property of power equipment, which is called accumulative effect. This phenomenon is widely studied on oil paper insulation but rarely on oil polypropylene film insulation, which is widely used in power capacitors and other equipment. Therefore, the accumulative effect of repeated impulse on oil polypropylene insulation is studied in this work. First, a repeated impulse accumulating test platform was set up and the accumulation characteristics of oil polypropylene insulation with repeated impulses were tested and analyzed. Next, the residual breakdown voltage, surface morphology and gas composition dissolved in the oil in different aging levels were tested. The results indicated that the OPF insulation residual breakdown voltage decreased and the film surface roughness increased when subjected to the repeated impulses. The dissolved gas contents changed along with the impulse accumulation aging. Lastly, the possible mechanisms of the accumulative effect in oil polypropylene insulation are discussed based on the test results. [ABSTRACT FROM AUTHOR]

This document presents an experimental study on the propagation of first mode negative streamers along mineral oil-solid interfaces. Samples made of an oil impregnated kraft paper and a low-porosity paper made from cellulosic micro and nano fibrils, as well as different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)) are used as the solid. A comparison of the length, charge and velocity of streamers for all different mineral oil-solid interfaces is reported. It is shown that streamers propagate longer and faster along mineral oil-solid interfaces with low surface roughness, low porosity and higher electrical permittivity than mineral oil. Those streamers show a quasi-continuous injection of charge in the early stage of their propagation. This quasi-continuous charge injection consists of a sequence of small charge steps separated by few tens of nanoseconds in between. In comparison, the streamers that propagate along surfaces with similar permittivity to the mineral oil have lower injection of charge and higher stopping voltage conditions than streamers propagating free in the liquid without any solid barrier. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations are carried out to evaluate the enhancement of natural convection heat transfer in a dielectric liquid resulting from injection induced annular electro-convection. The liquid is confined between two concentric horizontal cylinders and subjected to the simultaneous actions of a direct current electric field and a thermal gradient. A unipolar injection from the inner cylinder introduces free space charges into the liquid. Numerical results are presented for the configuration of inner to outer diameter ratio of 0.5 and silicon oil as the working medium. It is found that the charge injection may induce a strong radial fluid motion, which consequently augments the heat transfer rate. Due to the linear stability nature of electro-convection, there exists a threshold value of the electric driving parameter, above which the electrical enhancement becomes manifest. In addition, when the electric driving parameter is sufficiently high, the flow is fully dominated by the Coulomb force, and thus heat transfer becomes independent of the thermal driving parameter. [ABSTRACT FROM AUTHOR]

The energy distribution of trapped charges and traps in low density polyethylene (LDPE) and high density polyethylene (HDPE) were investigated based on isothermal surface potential decay (ISPD) method in this paper. Corona charging measurement was conducted by a non-contact setup, which was specially designed to obtain the decay characteristics of thin polymer films with varied charging polarity implying the dynamics of electrons or holes. Considering space charge profile of polymers under high dc electric field and detrapping mechanism after removing the field, a model based on ISPD was proposed to obtain the trap distribution of polymers. By assuming different charge spatial distribution features, two kinds of trap energy distributions of LDPE and HDPE were achieved respectively, which reveal similar distribution characteristics. It is shown that there are mainly deep traps in LDPE and HDPE both for electron-type and hole-type traps. The mean trap depth of hole-type traps is lower than that of electron-type ones in LDPE, whereas the situation in HDPE is reversed. Additionally, more trapped charges are accumulated in HDPE with deeper trap depth than that of LDPE. It is indicated that the obtained results may imply essentially nature of hole/electron traps, i.e., the electron-type traps show an inter-chain character whereas the character of hole-type traps is intra-chain. Furthermore, the energy distribution of traps is further related to the unique aggregation structure of LDPE and HDPE. [ABSTRACT FROM AUTHOR]

Oil-polypropylene (PP) composite insulation has been widely used in power capacitors, and also being extended to other equipment such as specific transformers, etc. As discharge can readily creep along PP surface, the interface between oil and PP is a weak point in the insulation structure. The oil-PP insulation degrades under the impact of thermal and electrical stress during normal operation. In this study, the influence of thermal degradation on surface discharge characteristics for oil-PP insulation is investigated. Degraded samples are obtained by thermally accelerated aging of virgin oil-PP samples. The microscopic morphology of PP samples are observed with SEM, and the physical and electrical properties of degraded oil are measured. In addition, the isothermal surface potential decay (ISPD) measurement is employed to investigate the charge keeping capability of PP samples. After that, surface discharge detection experiments under AC voltage are performed for the virgin and degraded samples. The ISPD results indicate that charge decay rate of degraded PP sample is faster than virgin sample, which imply that space charge memory effect of virgin PP samples is more remarkable. For this reason, the PD inception phase of virgin sample is smaller than those of degraded samples. It is found that thermal degradation of oil-PP insulation enhances the surface discharge activity. It is considered that rough surface profile and higher bubble conservation capability of degraded PP sample, and impurities in degraded oil are responsible for the enhancement effect of thermal degradation on surface discharge. [ABSTRACT FROM AUTHOR]

We numerically investigated the annular electroconvection that takes place in a dielectric liquid lying between two concentric cylinder electrodes. A uniform injection of arbitrary strengths either from the inner or outer cylinder introduces free charge carriers into the system, and the resulting Coulomb force induces electroconvection. The problem is characterized by a linear instability that corresponds to the onset of flow motion. The linear stability criteria were determined from direct numerical results and by linear stability analysis, and the results obtained with the two approaches show an excellent agreement. We focused on the fully developed flow pattern in the finite amplitude regime. We observed very different flow motions that were highly dependent on the injection strength. [ABSTRACT FROM AUTHOR]

Polarization and depolarization current measurement is a non-destructive testing method to study the dielectric properties of insulating materials. Two types of mineral oils with different aging times have been studied using both time domain and frequency domain methods separately. In this paper, a novel approach to understanding the correlation between the electric conduction in time domain and frequency domain has been proposed. When the charge carriers drift to the electrodes, they may be blocked and accumulated near the electrode. The amount of charge in the vicinity of electrode has been estimated. In the depolarization period, these charge layers will return to the bulk resulting in a depolarization current. In this paper, an analytical solution to the time dependence of the depolarization current has been developed and used to fit the experimental results. The field and temperature dependence of the polarization and depolarization current have been used to study the characteristics of charge carriers in mineral oil. The calculated conductivity has been used to simulate the frequency response in mineral oil and the simulating results have been compared with the experimental data. [ABSTRACT FROM AUTHOR]

In this study, large-area dielectric breakdown performances of various bi-axially oriented polypropylene (BOPP)-silica nanocomposite films are studied by utilizing the self-healing multi-breakdown method presented in the Part I of this publication. In particular, the effects of silica filler content, pre-mixing method, co-stabilizer content and film processing on the large-area breakdown performance are analyzed. Nanostructural and film cross-sectional analyses are correlated to the breakdown responses. The optimum silica filler content is found to reside at the low fill fraction level (~1 wt-%) and automatic pre-mixing of the raw materials and the optimization of the orientation temperature are found to be preferable. The co-stabilizer Irgafos 168 is found to have a significant effect on the breakdown distribution homogeneity of the reference BOPP films. The breakdown response of the silica nanocomposites is found to be not only dependent on the active measurement area but also on the voltage ramp rate, indicating that the silica nanocomposites exhibit altered internal charge behavior under DC electric field. The area- and ramp-rate-dependence results exemplify the importance of careful breakdown strength evaluation of dielectric polymer nanocomposites. Above all, the results emphasize the fact that a thorough understanding and the optimization of the film processing parameters are crucial for achieving improved breakdown response in dielectric polymer nanocomposite films. [ABSTRACT FROM PUBLISHER]

In order to systematically investigate the modulating role of fluorination temperature and time on surface electrical properties of the epoxy resin insulator, epoxy resin sheets were surface fluorinated in a laboratory vessel using a F22/N22 mixture with 12.5% F22 by volume at 0.1 MPa and different temperatures of 25?95 ?C for different times of 5?120 min. Conductivity measurements have shown that the intrinsic surface conductivity of the epoxy resin insulator could be modulated from an even lower value to a nearly four orders of magnitude higher value than the value prior to fluorination by changing the fluorination temperature and time. Surface conductivity increased with increasing fluorination temperature at the same fluorination time, while decreased to some extent with increasing fluorination time at a given fluorination temperature. Surface potential decay measurements showed consistent results with the surface conductivity measurements. ATR-IR analyses revealed substantial changes in surface chemical composition and structure, depending on the fluorination temperature and time. SEM images clearly showed an increase of the fluorinated layer thickness with increasing fluorination temperature and time. Surface cracks appeared only at elevated fluorination temperatures and increased with fluorination temperature, while the surface became compact with the duration of the fluorination. These results, compared with the previous results on an epoxy resin insulator made of DGEBA epoxy resin with a lower epoxy value, also indicated a significant influence of the epoxy resin insulator itself or the epoxy resin raw material on surface electrical properties and surface physicochemical characteristics after fluorination. [ABSTRACT FROM AUTHOR]

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohm's law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm1.33 kV/mm), the obtained I-V2 characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well. [ABSTRACT FROM AUTHOR]

The effects of electron-attaching and electron-releasing additives in cyclohexane on the initiation and propagation of positive and negative streamers have been studied quantitatively. Fast impulses (<20 us, <40 kV) were applied on a 10 mm point-to-plane gap and studied by shadowgraphic imaging and a differential charge measurement technique. The properties of both positive and negative streamers depend on the specific electronic characteristic of the additives. Electron-attaching additives facilitate the propagation of negative streamers, whereas the most effective electron-releasing additives reduce initiation voltages and facilitate the propagation of positive streamers. Depending on the reactivity and concentration of the additives, streamer filaments become thinner and fewer while propagating faster and further. The Townsend-Meek theory for streamer inception in gases has been adapted to a solution and applied to analyze the voltage dependence of the positive streamer propagation. Results show a quantitative dependency on the ionization potential and concentration in agreement with experimental trends. [ABSTRACT FROM AUTHOR]

Polymers are widely used in indoor insulation. We analyze three epoxy samples with varying matrix and fillerproperties to determine the influence of water on their insulating properties. We observed a strong dependency of the leakage current and the sample temperature, under electrical stress, on the amount of absorbed water. Trihydrated alumina filler reduced the leakage current and temperature rise under electrical stress. [ABSTRACT FROM AUTHOR]

Conductivity and ionization mechanisms in liquid helium have been investigated by using high field conduction phenomena occurring near a sharp needle electrode. When the voltage Vs, applied between the needle and the plane is greater than some threshold voltage Vs, a corona-like discharge regime was observed. Charge carriers are generated in the high field region close to the point where an intense light emission is detected. Above Vs the mean current is space charge limited and the mobility of charge carriers has been deduced from analysis of current-voltage characteristics. Some parameters of the corona-like region have been deduced from a spectral analysis of the light emitted during the discharge. [ABSTRACT FROM AUTHOR]

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (~ MV/cm), and of propagation (requiring low electric field, ~ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data. [ABSTRACT FROM AUTHOR]

The chemistry of surface degradation by ultraviolet light (UV) of high temperature vulcanized dimethyl silicone rubber (HTV SIR) and ethylene propylene diene terpolymer (EPDM), has been studied by many researchers. Insulators fabricated from these materials are normally "qualified" for outdoor use by accelerated life tests that typically include salt-fog and high ac voltages but with insignificant levels of UV. The manufacturer usually counters this by subjecting samples of the material to UV in a weathering tester. However, the nature of the interaction of the UV with the other stress factors present in the accelerated life test remains unknown. This paper focuses on the effects of prolonged UV exposure (5000 h) on electrical and chemical surface properties. Periodic tests of contact angle, SEM, XPS, surface resistivity and surface voltage decay analyses were made. The contact angle of silicone rubber dropped slightly (5° after 1000 h of UV; 10° after 5000 h of UV) and little damage was indicated by SEM; but EPDM suffered a severe drop in contact angle (46° after 1000 h of UV; 68° after 5000 h of UV) accompanied by surface cracking and chalking. Surface resistivity (ρs) of HTV SIR increased by 50% to 2 × 1014 Ω/square after 5000 h of UV. This indicates unexpected lack of ionic character of the degraded surface, but may be partly due to UV generated SiO2 between the specimen and the electrode. Surface voltage decay after corona charging, a non-contacting technique with appeal because it is non-damaging and easy to apply to a variety of geometry. The accuracy of the Cresci theoretical equation for the geometry used was confirmed. We feel that these trends in surface resistivity correlate well with observations in the field where a tendency toward surface tracking in EPDM controls life while for HTV SIR life may depend more on the degree of contamination of the environment that can lead to surface erosion. The surface abundance trends of O - C = O and SiOx (x = 3 ∼ 4) with time indicate that O - C = O and SiOx have increased rapidly at short treatment times and then level off at about 10% and 50%, respectively, until the end of the test time (5000 h). We suspect that these trends arc important in the initiation of tracking in EPDM and of erosion rate increase in HTV SIR. It might also be indicative of the time when the SiOx functional groups start to degrade to SiO2 + Ox, which causes HTV SIR to undergo mainly a cross-linking reaction. [ABSTRACT FROM AUTHOR]

What has been the contribution of surface potential studies during the last four decades to our understanding of insulation physics? Which are the promising techniques emerging in this field? The present review covers the industrial context accounting for the development of surface potential measurements on insulating materials, the way they have been implemented, as well as the great variety of models produced to explain the charging and potential decay mechanisms. Several polarization or transport processes can be responsible for the decay of the potential. Though most of the models initially stemmed from electrostatics and semiconductor physics, around the notion of mobility, experiments on polymers required "thermodynamic" models, describing progressive charge detrapping. We also underline here that the three main different physical processes likely to be involved in the potential decay (dipolar relaxation, dispersive transport, slow detrapping) can lead in disordered materials to the same time response, the challenge being here is to design inventive new procedures to distinguish them. This comment also applies to most of electrical measurements on disordered materials. Kelvin probe microscopy, return voltage and thermostimulated potential measurements also illustrate the multiform development of surface potential measurement techniques, as their future prospects. [ABSTRACT FROM AUTHOR]

Elevated temperature is a key aging factor for metallized polymer film capacitors with the capacitor life expectancy halved with every Soc of temperature rise. For film capacitors in service, both application of a time-varying external voltage and the extinction of internal breakdown events can deposit significant heat onto the capacitor structure leading to an undesirable temperature rise. Often such heat generation is localized spatially and the resulting temperature gradient enhances the probability of subsequent breakdowns. To facilitate a thorough and generic understanding, a thermal model of metallized film capacitors is developed to simulate the dynamics of heat generation and transfer under the switching impact of an external voltage. Thermal processes in solids (polymer layers, electrode coating, and zinc-spray segment) are modeled with conduction mechanism, whereas convection mechanism is considered for silicone oil and air. Heat generation is induced either by ohmic losses of the externally applied electric field, or by internal break- down events within the metallized film capacitor structure. This allows a seamless interface between the heat transfer model and the equivalent circuit based field calculation model. Based this electrothermal model, numerical examples are used to study temperature distribution within film capacitors and then to examine the likelihood of localized temperature hot-spots that may lead to spatially preferential breakdowns. [ABSTRACT FROM AUTHOR]

New data of the electronic properties of liquid helium and liquid neon are discussed with respect to their relevance for the initiation of electric breakdown. It is concluded that under ideal conditions the cathode plays a decisive role in the initiation of the breakdown process. It is speculated that a Townsend type breakdown mechanism is absent in the liquid phase of these elements. [ABSTRACT FROM AUTHOR]

Two hitherto neglected mechanisms, which occur at the interface between insulating liquids and metal electrodes under high electrical fields, are considered and shown to be significant for the initiation of breakdown. One involves the Lippmann effect in which the electrical fields of the double layers at the electrodes reduce the interfacial tension and lead to the generation of low-density microcavities on the electrode surfaces. The other is the Auger effect in which non-radiative recombination of electrons and positive holes across the large energy gap between these states leads to secondary electrons of high energy. The coupling between these two mechanisms is expected to be highly conducive to streamer initiation at the electrodes. [ABSTRACT FROM AUTHOR]

Presents a numerical analysis of electrohydrodynamics in a round pipe based on the pressure-implicit with splitting of operators scheme under a periodic bound condition. Background on electrohydrodynamics; Analysis of the spatial distribution of pressure; Results and implications.

Presents a study on the development of the so-called streamers for the generation of electrical streamers in insulating liquids. What the model is based upon; Suggestion that the primary steps of streamer initiation arose from the mechanical nature of forces generated by an electrical field; Information on electro-mechanical forces; Details on the study.

Investigates the conduction of a nonpolar liquid filling in electricity. How it affects the electric field; Analysis of cases of increasing complexity; Characteristics of the current voltage; Results of the investigation.

Presents how the electrical characteristics of creeping discharges in transformer oil are compared with those of streamers developing in the liquid bulk. Main characteristics of creepage discharges in transformer oil; Comparison of bulk streamers with creepage discharges; Channel and charge distribution; Correlation between mean potential gradient along the channel and streamer capacitance; Conclusions.

Pre-breakdown phenomena in pure n-tridecane in liquid and solid state have been investigated experimentally in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Pre-inception currents in liquid and solid state were recorded and compared to finite element calculations to find plausible high-field conduction models for n-tridecane and cyclohexane. n-tridecane was chosen as a model system for polyethylene based on the known similarities in energy bands for polyethylene and alkanes. The results obtained in pure n-tridecane were compared to similar experiments on cyclohexane and polymer systems. High-field conduction in liquid n-tridecane was found to have a similar field dependence as previously reported for cross linked polyethylene (XLPE). Conduction currents and inception probabilities were found to be almost independent of polarity in frozen and liquid n-tridecane, which is also in line with results obtained for XLPE. Conduction currents were observed at lower voltages in n-tridecane than in cyclohexane, reflecting the lower space charge limited field (SCLF) in n-tridecane. Inception of streamers in liquid cyclohexane occurs at lower voltages than in liquid n-tridecane, and a stronger polarity dependence for all phenomena was observed in cyclohexane compared with n-tridecane. A possible explanation for the differences between n-tridecane and cyclohexane is given based on the field needed for electron avalanches and the SCLF in the two liquids. Inception voltages, light emission and charge injection were found to be similar in liquid and solid n-tridecane. This indicates that the same processes are responsible for inception and propagation of electrical trees in solids and streamers in liquids when the material is stressed by a fast transient. The similarity between the measured properties in solid and liquid phases leads to the conclusion that electrical treeing mainly takes place in the amorphous regions of the solid phase. [ABSTRACT FROM PUBLISHER]

Two electrically insulating fuels of mineral and biological origins have been tested for electrostatic charging in single and multiple blade-plane configurations. The aim is to investigate methods to dramatically increase the total injected current, with a view to developing an atomization system where the charge injection and atomization units are separate. In this study, the fuel is quiescent and only the injected current is measured. Current flux - electrode surface electric field (J-E) characteristics at different bladeelectrode gaps are presented. A current density of 6.5 A/m2 is obtained at a surface electric field of 100 MV/m using the 1-blade configuration. From the measured data, the relation between the blade-electrode gap (d) and the injected current flux is found as Jα d-m at constant voltage, where m is found to be 2 for flat charge collectors and as high as 4 for rounded collectors. The physics of the swirl motion caused by the Coulomb forces are discussed. The size of this induced flow is correlated to the performance of the multiple blade design. Charge injection, dielectric liquids, electrostatic charging, induced flow, multi-blade injector [ABSTRACT FROM AUTHOR]

A charge injection atomizer functions by introducing electric charge discharged from a high voltage electrode into a dielectric liquid, which subsequently atomizes the ejected liquid jet. Atomizer evolution thus far has proceeded through trial and error analysis of the experimentally measured electrical characteristics of the atomizer and of the quality of atomization. Within the atomizer, a coupled space charge and electric field exist, which can alter the internal flow pattern, thus creating electrohydrodynamic (EHD) instabilities that affect atomizer operation. Such a system has not been simulated in the past under forced flow conditions. In this work we simulate the internal flow of such a charge injection device in two dimensions; using experimental based boundary conditions. Initial results indicate that in the linear injection regime defined by the experimental data, the flow is only slightly unstable but in the transitional and highly non-linear regimes, the coupled space charge and electric field produce more instability in the liquid that must be investigated further. [ABSTRACT FROM AUTHOR]