Polarization and dielectric behavior of ac-aged polyethylene

High field polarization and dielectric relaxation behavior have been used to study the aging mechanisms of crosslinked polyethylene (XLPE) and low density polyethylene (LDPE) aged at constant alternating electrical field stress in a humid environment at room temperature. For this study, the dielectric spectroscopy data in the frequency range of 10^-5 to 10⁶ Hz and their comparative analysis, have been used to provide electrical analog models of the aging. The dielectric data in the frequency range of 10^-5 to 5×10 ^-2 Hz were calculated from the desorption current, whereas bridge measurements provide the data in the frequency range of 10 Hz to 10⁶ Hz. In addition, thermal transient current and thermally stimulated discharge current measurements have been carried out in order to determine the polarization and space charge accumulation in ac aged polyethylene. A correlation has been observed in the behavior of polarization and dielectric relaxation in the present work. The dielectric behavior of the XLPE cable samples and of the LDPE flat samples show a presence of both the inter-cluster (low frequency) charge exchange and the intra-cluster (high frequency) charge motion, the former mechanism becoming more dominant as the aging progresses. For the XLPE cable samples ac aged in a humid environment at room temperature, relaxation peaks obeying the fractional power law have been observed in addition to a quasi-dc process     

Area and volume effects on breakdown strength in liquid nitrogen

We measured dc and ac breakdown voltages in liquid nitrogen (LN ₂) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN₂ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN₂ was verified quantitatively; the breakdown strength decreased when increasing the {SEA}₉₀ (90% stressed electrode area) and the {SLV}₉₀ (90% stressed liquid volume). By changing the experimental conditions, it was verified that both area and volume effects, having a mutual correlation, simultaneously lead to the degradation of the breakdown strength in LN₂. In order to examine the area and the volume effects for the larger {SEA}₉₀ and {SLV}₉₀, we also measured the breakdown voltage with a coaxial cylindrical electrode. It was concluded that the dc and ac breakdown strengths in LN₂ decreased as the {SEA}₉₀ and the {SLV}₉₀ increased varying widely from 10⁰ to 10⁵ mm² and from 10 ^-1 to 10⁵ mm³, respectively     

Optical investigation of high-field conduction and prebreakdown ina dielectric liquid

Light emission studies of the high-field conduction and prebreakdown phenomena in a mixture of mono and dibenzyl-toluene (M/DBT), used as polypropylene impregnant in the all-film capacitor technology, have been undertaken to gain insight into the underlying physical mechanisms responsible for these processes. The absorption spectrum of this fluid, in the UV region, shows the appearance of structured bands at 348, 366 and 386 nm indicating a luminescence process via anthracene and 9-methylanthracene impurities at a concentration of 3×10^-3 mol/l. Optical and electrical measurements were performed simultaneously on the electrically stressed fluid. Electroluminescence of the impurities contained in M/DBT has been investigated using a new uniform field electrode arrangement. The light inception stress was determined to be ~ 20 μm^-1 for ac excitation (50 Hz). Configurations with different interfacial situations were studied and we have evidenced a correlation between charge injection and electroluminescence activity. Spectral analysis of the light radiated by streamers in M/DBT in a needle-plane electrode geometry, under step voltage (1 to 50 μs, 30 kV) revealed, for both polarities, the presence of H₂ and C₂ characteristic bands emerging from a continuum corresponding to the dissociation and recombination of molecular fragments     

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.     

交流电压下聚合物绝缘材料电致发光特性

设计了绝缘材料电致发光现象研究所用的实验装置,以低密度聚乙烯(LDPE)、聚丙烯(PP)和聚四氟乙烯(PTFE)三种聚合物绝缘材料为样品,研究了交流电压下聚合物材料表面电致发光的时域和相域特性,同时探讨了不同电极材料(铜、铝)以及在材料表面溅射金膜电极对聚合物表面电致发光现象的影响.实验结果显示,不同聚合物材料的电致发光起始电压和发光强度各不相同,但有相似的时域和相域特性曲线,阶升/降电压下的发光曲线均表现出类似于磁滞回线的现象;同时发现不同电极材料下以及溅射金膜电极后的材料的电致发光也表现出不同的发光起始电压和强度.认为电极与材料的接触势垒和材料内部空间电荷影响着材料的电致发光特性.     

Streamer initiation in mineral oil. Part I: electrode surfaceeffect under impulse voltage

Presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 μm points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S^-0.17 (S: electrode surface area, cm²). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage     

Weibull statistical analysis of area and volume effects on thebreakdown strength in liquid nitrogen

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN₂), we measured dc and ac breakdown voltages in LN₂ with sphere to plane and coaxial cylindrical electrode configurations. We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV). The dc and ac breakdown strength in LN₂ decreased to 1/5 with increasing SSEA and SSLV over the wide range from 10⁰to 10⁵ mm² and from 10^-1 to 10⁵ mm³, respectively. The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN₂ saturated with increasing SSEA and SSLV. From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN₂. The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV). Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV     

Phase resolved electroluminescence measurements in thin films of low density polyethylene using a charge coupled device camera

Electroluminescence (EL) produced by a commercially available additive free low density polyethylene film has been investigated under a 50 Hz AC electrical stress. The spatial distribution, spectral characteristics and the phase angle relationship of EL with respect to the 50 Hz applied AC voltage were studied using a sensitive Peltier cooled charge-couple device (CCD) camera. The experimental results from several test samples show that the EL can be divided into two different groups according to the EL spatial distribution: (1) homogeneous EL and (2) inhomogeneous EL. At the same applied voltage, the spectra of the homogenous and inhomogeneous EL were found to be significantly different. Homogeneous EL had maximum emission intensity in the red and near infrared (>700 nm), whereas the inhomogeneous emission had maximum intensity at shorter wavelengths (550-600 nm). Phase resolved measurements showed that homogeneous EL always produced equal emission intensity in the positive and negative half-cycles (symmetric distribution), whereas, inhomogeneous EL produced an asymmetric intensity distribution. To explain these results, it is suggested that homogeneous emission is associated with charge injection and bipolar recombination in the surface states on both sides of the sample, whereas inhomogeneous emission is associated with field enhancement at defects on one of the two sides of the film which allow charge to be injected into states more representative of the bulk material.     

HT-PEMFC耐CO多层复合结构阳极研究

高温聚合物电解质膜燃料电池(HT-PEMFC)使用富氢重整气代替纯氢进料时,其中高浓度CO(可达3×10-2)会导致电极毒化,使电池性能降低.采用Pt/C和PtRu/C催化剂制备多层结构阳极,研究了不同阳极结构在重整气进料时对电池性能的影响,优化得到最佳阳极结构(内侧Pt/C、外侧PtRu/C复合阳极).与传统Pt/C...     

Sliding Discharge Optical Emission Characteristics

In this work, several optical studies in an atmospheric pressure sliding plasma sheet have been performed. This discharge is generated using two electrodes flush mounted on an insulating flat plate (upper electrodes), and a third electrode flush placed on the opposite side of the plate facing the upper inter electrode gap (lower electrode). A DC negative voltage is applied to one of the two upper electrodes and to the lower electrode, while the other upper electrode is biased with an AC voltage. In this configuration a sliding discharge is produced on the flat plate within the upper electrodes gap. The sliding discharge optical emission of the spectral bands corresponding to the 0-0 transition of the second positive system of N₂ (lambda = 337.1 nm) and the first negative system of N₂ ⁺ (lambda = 391.4 nm) have been measured. Also the light spatial distribution in the plasma sheet has been studied using a CCD camera coupled to interferential filters corresponding to the wavelengths investigated. The reduced electric field in the plasma sheet has been derived from the measurement of the intensity ratio of the nitrogen lines. This study has been realized varying the amplitude of the DC voltage and the amplitude and frequency of the AC voltage. The reduced electric field strength is found to be almost constant for all the experimental conditions, with a value of 500 plusmn 100 Td (1 Td = 1.10^-17 V cm²).     

Relation between electroluminescence and degradation in XLPE

In order to clarify the relationship between electroluminescence (EL) and degradation in crosslinked polyethylene (XLPE), the EL measurement was carried out using three kinds of samples, block samples with two different electrode systems, and film samples. The spectral measurements revealed that the UV component of EL is below the detectable level of ~0.1 c/s (count per second), in contrast to partial discharge (PD) light from microvoids or the tiny electrical tree which includes a UV component of ≳2 c/s. Secondly, aging tests using XLPE block samples with needle-plane electrodes revealed that electrical degradation can take place even when the applied voltage is below the EL detection voltage. The above experimental results suggest that EL is not the origin of electrical degradation, i.e. photodegradation by the UV component of EL is not the dominant mechanism of electrical degradation     

Electrical relaxation in iron-containing glasses

Glasses with composition 70 mol%(SiO₂, B₂O ₃, P₂O₅, TeO₂)-15 mol% Fe ₂O₃-15 mol%(BaO, CaO) were prepared by the conventional melt quenching technique. The electrical relaxation of these glasses has been studied in the frequency range 20 to 10⁵ Hz. The small polaron hopping between iron ions in a different valence state Fe²⁺ to Fe³⁺ is found to be the principal conduction mechanism. The ratio of Fe²⁺ ions to the total iron content, C=Fe²⁺/Fe_tot, is one of the factors determining the electrical conductivity. The glass former has a minor influence on dc conductivity, except of TeO₂ glass where conductivity is three order of magnitude higher than those of other glasses. The ac conductivity as a function frequency is divided into two domains, a dc plateau, followed by a power law in frequency. These two regions are well distinguished in the complex plot of electric modulus where all data points for different temperatures reside on the single plot. The results are discussed in the frame of the Hunt theory of dielectric relaxation in glasses containing mobile charge carriers     

Electroluminescence from surface layer of insulating polymer underac voltage application

Electroluminescence from a surface layer of polytetrafluoroethylene (PTFE) has been investigated using a photomultiplier tube and an intensified charge-coupled device (ICCD) camera under ac or half rectified ac (HRAC) electric fields tangential to the polymer surface. The electroluminescence (EL) characteristics from the polymer surface layer showed a similarity to those from PTFE and other polymer films subjected to ac or HRAC electric fields in the normal direction to the surface. ICCD imaging of EL indicated a large area of EL activity where the EL intensity was maximum at the electrode edge and EL was still observable 2 mm from the electrode. The injection and transport of electrons and holes via surface states in the surface layer and the subsequent electron-hole recombination give rise to EL, strongly depending on the spatial distribution of the surface states. The injection length of charges can be estimated to be at least 2 mm from the electrode in the tangential direction to the surface     

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     

Investigation of space-charge effect on the AC treeing mechanism in an LDPE film by tsc and TSSP measurement

When an ac voltage is applied to a low-density polyethylene (LDPE) film placed under a needle-plane electrode system, there is a possibility of the formation of the space-charge field in the vicinity of the needle electrode by electrons (or holes) injected from the needle electrode. In this paper we investigated the properties of the space charge, e.g., the total amount of trapped charge formed in the vicinity of the needle electrode by using electrical measurements of thermally stimulated current (TSC) and thermally stimulated surface potential (TSSP). From the investigation, it was concluded that a space-charge field is formed in an LDPE film by the application of the ac voltage, and that the total amount of trapped charge by the application of the ac voltage is less than that by the application of the dc voltage, etc. Finally, on the basis of the present experimental results, we studied the space-charge effect on the ac treeing mechanism.     

The influence of surface charge on lightning impulse breakdown ofspacers in SF6

We have investigated the influence of surface charges on the discharge development across particle contaminated spacer surfaces under SF₆ for lightning impulse (LI) voltages (1.2/50 μs). Surface charges may be generated by dc, ac or lightning impulse stress. The discharge characteristic shows a strong reduction of the insulation strength if the applied voltage and the surface charge have opposite polarities. The investigations were performed with a needle protrusion attached to the bottom electrode to give severe field distortion. The bottom electrode was biased positively. The influence on the discharge process is observed by measuring the surface charge distribution and predischarge currents. The results reveal changes in streamer onset voltage, streamer to leader transition, and leader development     

Influence of dc and dc/ac composite fields on the flow electrification characteristics of transformer oil

Flow electrification characteristics of transformer oil were investigated by applying dc and dc/ac composite fields to both bare and covered coaxial cylinder electrode systems. For the bare electrode system, flow electrification characteristics were almost the same between positive and negative dc voltage applications and negative charges were generated for both dc and dc/ac composite voltage applications at high dc fields. For the covered electrode system, the apparent flow electrification characteristics differed between positive and negative dc voltage applications. For the former, negative charges were generated at high dc fields for both dc and dc/ac composite voltage applications, and for the latter, positive charges were always generated for both voltage applications. However, negative charge generation for the positive voltage application was attributed to the influence of negative charge generation at the bare part of the covered electrode end; the fundamental charge generation characteristics of the covered electrode showed an increase of positive charge generation by dc voltage application, and positive charge generation was accelerated by a composite ac field. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 10‐17     

Electroluminescence measurements to detect accumulated charge atthe electrode-insulator interface

Electroluminescence (EL) spectrum analysis is proposed to detect charge accumulation in insulators. The objective is to find the nature of accumulated space charges at the electrode-insulator interface by using only EL measurements recorded under a moderate ac 50 Hz field. The differences between spectra obtained with virgin samples and treated samples are analyzed. Some experience obtained with 500 μm thick low-density polyethylene (LDPE) samples are described to illustrate the expected modification of the EL spectrum vs, interfacial space charges. Direct space charge measurements, using the laser induced pressure propagation (LIPP) method are used to verify our assumptions. Good agreement is found between EL analysis and experimental data of space charges     

Composition Optimization and Phase Transformation of Si-Nanocrystal-Doped $hbox{SiO}_{x}$ for Enhancing Luminescence From MOSLED

Near-infrared (NIR) photo- and electroluminescence (PL and EL) of Si nanocrystals buried in Si-rich SiO_x, film, and their correlation with the structural phase transformation and the varied oxygen composition of SiO_x, are investigated. By detuning the N₂O flowing ratio (Y_{N 2 O} = [N₂O/(N₂O + SiH₄)] times 100%) from 93% to 80% during plasma-enhanced chemical vapor deposition growth, the oxygen composition ratio of the Si-rich SiO_x, can be adjusted from 1.64 to 0.88. The grazing incident X-ray diffraction and X-ray photoelectron spectroscopy spectra indicate that the SiO_x, transforms its structural phase from Si + SiO₂ isomer to Si + SiO + SiO₂ isomer. With O/Si ratio >1.24, the SiO_x, matrix becomes SiO₂ isomer, whereas the SiO_x, structure approaches SiO phase at O/Si ratio that is nearly 1.0. The formation of SiO matrix in SiO_x, grown at Y_{N 2 O} below 85% reduces the precipitated Si nanocrystal density from 2.8 times 10¹⁸ to 7 times 10¹⁶ cm^-3, and monotonically attenuates the NIR PL by one order of magnitude. Such a structural phase transformation from SiO₂ to SiO in SiO_x with lower O/Si ratio causes the degradation in EL power conversion efficiency and external quantum efficiency (EQE). Maximum EL power of 0.5 muW and EQE of 0.06% are obtained from MOSLED made on SiO_x, with optimized O/Si ratio of 1.24.     

Methods for estimation of the vacuum status in vacuum circuitbreakers

In this paper several methods are described for the estimation of the pressure inside vacuum interrupters which have been in operation for many years. The methods are applicable without demounting the interrupters and use only electrical measuring techniques. According to these methods the vacuum circuit breakers are subjected to synthetic circuits, switch-off limited currents, either 10 A dc or 300 A ac. With the 10 A dc method the height of arc voltage spikes are measured and represented in a histogram. The shape of the histogram is discriminative for pressures below or above 10^-2 Pa. Switching tests with currents of 200 to 400 A ac in inductive and capacitive circuits also showed different behavior for internal pressures below or above 10^-1 Pa. A three phase 200 A test device with a 10 kV recovery voltage, fed from a standard 380 V supply, showed similar results