Temperature effect on space charge dynamics in XLPE insulation

This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kV_dc/mm at four temperatures 25, 50, 70 and 90 degC for 24 h. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The DC threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown     

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.     

Space charge behavior in low density polyethylene at pre-breakdown

It has been known that the electrical breakdown of insulating materials is strongly affected by the formation of space charge in the bulk of the materials. Many researchers have attempted to study the relationship between the space charge and the breakdown; however, it has not been clarified yet. Although the pulsed electroacoustic (PEA) method has been widely used to observe space charge profiles, previous works have not shown clear evidence of the influence of the space charge on the breakdown. Therefore, we have developed a new PEA system with an interval of 0.5 ms to observe the space charge distribution continuously under the ultra-high electric field. Using this system, we observed the space charge dynamics in the low-density polyethylene (LDPE) at and around the breakdown. We also investigated the dependence of the applied electric field on space charge behavior. From the results, it is found that the injected charge packet moved faster and deeper under a relatively lower electric field rather than that under a higher electric field. Furthermore, we found that the maximum electric field in each specimen was almost the same when the breakdown happened in a specimen.     

DC conduction and electrical breakdown of MgO/LDPE nanocomposite

To understand basic electric properties of nano-sized magnesium oxide (MgO) / low-density polyethylene (LDPE) nanocomposite under DC voltage application, the volume resistivity, the space charge distribution and the breakdown strength were investigated. By the addition of nano-sized MgO filler, both the DC breakdown strength and the volume resistivity of LDPE increased. At the average DC electric field of about 85 kV/mm and more, a positive packet space charge was observed in LDPE without MgO nano-filler, whereas a little homogeneous space charge was observed in MgO/LDPE nanocomposite material at the front of electrode. From these results, it is confirmed that the addition of MgO nano-filler leads to the improvement of DC electrical insulating properties of LDPE.     

陷阱参数及其与交联聚乙烯电缆绝缘状态（英文）

Space charge issues have raised many attentions in recent years,especially in high voltage direct current(HVDC)application.Space charge accumulation in insulation system will give rise to acceleration of ageing and even cause premature failure of the material.However,from another angle,space charge might be also considered as a diagnostic tool of ageing for insulation materials.In this paper,a trapping-detrapping model has been developed to estimate trapping parameters of cross-linked polyethylene(XLPE)cable sections,which were taken from different HVAC operation conditions of 12 years and 8 years.The results reveal that,for both cable sections,samples from the inner location have the greatest trap density and the deepest trap depth.Additionally,breakdown strength tests and FTIR(Fourier-transform infrared)measurements on those samples have been carried out.From FTIR measurement results,the degree of oxidation among three layers could be found by the carbonyl index values.The oxidation degree of aged cable at the outer layer is higher than that at the other two layers probably because of the most sufficient contact with oxygen.Also,it has been noticed that the results from these measurements show some correlations with the estimated trapping parameters,especially for breakdown strength.     

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     

High repetition rate PEA system for in-situ space charge measurement during breakdown tests

In order to investigate transient space charge phenomena, it is essential that the space charge profile be observed at a high repetition rate. We have developed a new space charge measurement system using the pulsed electroacoustic (PEA) method, which can measure the space charge profiles every 10 /spl mu/s. It employs the most recent digitising oscilloscope model and a semiconductor switch. The effect of prestressing on impulse breakdown voltage of a low-density polyethylene sheet was investigated by using the new system. Experimental results suggest that positive charge injection was dominant immediately before the breakdown, and charge injection during the prestressing causes distortion of the electric field near the electrode, and enhances the subsequent charge injection due to the impulse voltage.     

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.     

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     

Self-controlled pre-breakdown discharges in planar symmetry

In measurements on Teflon FEP films charged in ⩽50 μm air gaps by microsecond impulse voltages, a uniform charge deposition on the films was observed. A regular increase of the film surface potential from a threshold value of the peak impulse voltage was found. However, for 300 μm air gap it was observed that abrupt charging occurs at a lower threshold voltage, indicating breakdown, and the charge deposition on the film becomes nonuniform. The behavior in air gaps <50 μm is explained here using Townsend's theory of pre-discharges. It is shown how and why the interposed insulating film acts to make the system self-controlled, thus avoiding breakdown in the air gap, despite the large values of the applied peak impulse voltage     

Interfacial properties of XLPE/EPDM laminates

Interfacial properties such as space charge accumulation and breakdown characteristics in crosslinked polyethylene (XLPE)/ethylene propylene diene terpolymer (EPDM) laminates were investigated. Homocharge is observed in EPDM containing 1,4-hexadiene while heterocharge is observed in EPDM containing 5-ethylene-2-norbornene. Interfacial charge develops when the EPDM is laminated with XLPE. The polarity of this interfacial charge reverses at less than a few tens of hours heat treatment at 80°C dc breakdown voltage shows a sequential change of an increase, a decrease and a slight increase as a function of heat treatment time, which holds true for both interfacial and volume breakdown voltages. A maximum breakdown voltage is observed at 20 to 24 h heat treatment. After long heat treatment, silicone grease used in the interface shows lower interfacial breakdown voltage than silicone oil for the dc case, which was attributed to the additives in the grease and the molecular weight of silicone molecules in oil and grease. Details of the results are given and their origins discussed     

Space charge characterization for the 21th century

Various methods of characterizing insulating materials by their ability to take up charge, retain it, and release it, are reviewed critically in search of measurable quantities that could be used to predict material behavior under stress up to failure conditions. Space charge characterization data on different types of materials from polymers to inorganic single crystals and ceramics are surveyed. The charging behavior is found to be influenced by many details such as surface condition and residual stresses. The traditional approach of linking dielectric breakdown to an intrinsic critical field for the material is tested against the newly emerging view that breakdown could be linked to space charge trapping at defect sites and to the attendant energetics of the mechanically strained lattice. The characterization process thus requires more care than was previously thought necessary, but after more research should become more predictive     

茂金属聚乙烯改善低密度聚乙烯的介电性能

针对高压、超高压直流塑料电缆中存在的空间电荷效应,大多数直流电力电缆常采用改性方法提高其介电性能,如空间电荷特性、体积电阻率和击穿强度等。常用的改性方法主要有添加剂、共混、接枝和二元共聚4种。共混技术较广泛用以改性聚乙烯电缆,提高其介电性能。因此用电声脉冲法(PEA)测量了MPE与LDPE共混试样中的空间电荷分布;用高阻计测量了共混物的体积电阻率,用阶梯电压测量了共混物的交流击穿场强。试验结果表明,1%MPE与LDPE共混能有效降低空间电荷效应,提高交流击穿场强7.9%,略降低体积电阻率。最后讨论了共混物的物质结构、电荷陷阱及介电性能间的关系。     

The breakdown mechanism of poly-p-xylylene film. Prestress effectson the breakdown strength

Impulse and dc breakdown strengths of 4 μm thick poly-p-xylylene (PPX) films were 5.9 and 4.1 MV/cm, respectively. They were independent of temperature in the temperature range from -60 to 60°C. dc prestressing for a long time (t_p=60 s) reduced impulse breakdown strength for both the same and the opposite polarity. But dc prestressing for a short time (t_p<1 s) increased impulse breakdown strength for the same polarity. These results were explained by positive space charge in PPX film. It was also concluded that positive charge carrier injected from the anode, spread in a 4 μm thick PPX film during a short time     

Interfacial charge in polyethylene/ethylene vinylacetate laminates

Space charge formation in polyethylene (PE)/ethylene vinylacetate copolymer (EVA) laminates has been investigated using a pulsed electroacoustic method. The PE shows heterocharge while the EVA shows a broad distribution of positive charge over the sample. The positive charge in EVA decreases with the increase of vinylacetate (VA) content in EVA. Interfacial charge is found in all laminates and remains unchanged by heat treatment at 100°C to 1 h and by coating chemicals such as silicone oil and trimethylolpropane trimethacrylate at the interfaces, This interfacial charge distorts the electric field distribution in the direction of increasing the electric field in the PE layers. The charge distribution in PE/EVA blends has been measured and is explained via the results obtained with the laminates     

Temporal development of partial discharge and its application tobreakdown prediction in SF6 gas

The authors measured the temporal change of partial discharge (PD) characteristics leading to breakdown in SF₆ gas for AC voltage application. At the final stage close to the breakdown, positive PD pulses with relatively high magnitude began to take place in the phase region near the applied voltage peak. This event was interpreted in terms of the change of PD type in SF₆ gas from streamer to leader. Optical observation also revealed that PD type transition occurred. With the results considered, the mechanisms of PD were discussed. Moreover, they discussed the possibility of breakdown prediction in GIS. An attempt was made to find a breakdown prediction parameter which characterized the change of PD type: the ratio R_L of maximum charge to the average charge of PD pulses appearing in the phase region near the peak in positive half cycle. It was found that R_L allowed to predict the time to breakdown successfully within the error of 4~20%     

空间电荷对油纸绝缘击穿和沿面闪络的影响

换流变压器是直流输电工程的核心设备之一,其结构和工作状况比传统交流变压器更为复杂,需要对其绝缘系统进行更深入的研究.本文针对换流变压器的特点,设计了相应的试验装置,对油纸绝缘中的体击穿和沿面闪络进行了试验研究.本文实现了变压器油纸绝缘空间电荷的测量,根据空间电荷的实测结果对油纸绝缘系统中极性反转现象进行了探讨,对直流电...     

Conductivity and space charge in LDPE containing nano- and micro-sized ZnO particles

DC conductivity and ac impedance measurements were made in air and in vacuum on samples of low density polyethylene to which nano-sized and micro-sized ZnO particles and a dispersant had been added. The samples were 150-200 mum thick. The temperature range was 30-70degC. The temperature dependence of the vacuum dc conductivity in samples containing the dispersant and 10% w/w nanosized ZnO followed an Arrhenius relationship closely, the conductivity being 1-2 orders of magnitude lower than that of a sample containing dispersant only. The addition of 10% w/w microsized ZnO had very little effect on the dc conductivity. The ac measurements were made in the frequency range 10 mHz-1 MHz. Addition of nanoparticles increased the ac conductivity at higher frequencies but decreased it at lower frequencies, the cross-over frequency increasing with increasing temperature. The real part of the relative permittivity of samples with nanoparticles was increased relative to that of samples containing dispersant only, at all temperatures, but the corresponding values in samples with microparticles were unchanged, within experimental error. Space charge profiles were obtained using the laser-intensity-modulation-method (LIMM). Space charge densities of order 300 Cm^-3 were measured in the bulk near the electrodes, several hours after poling at field strengths around 30 kV/mm.     

油纸绝缘介质的空间电荷积聚与消散特性

油纸材料的绝缘问题在换流变压器、直流套管、直流电缆等高压大型直流设备大量应用的情况下显得十分突出。为探讨高压直流设备绝缘的最主要问题—空间电荷效应,应用电声脉冲法(PEA)对油纸绝缘材料的空间电荷特性进行了研究。有关外加场强对油纸材料中空间电荷积聚情况的影响和在较高场强下油纸材料的击穿破坏与空间电荷关系的研究结果表明:①低场强下油纸材料中空间电荷以电离产生为主;而在较高场强下,先后在阴极和阳极产生了同极性载流子注入。②相对于聚乙烯而言,由于油纸材料的电导率较大,材料内的空间电荷在外加电场撤去后很快消散。③空间电荷的注入和运动会导致油纸材料的劣化和破坏。油纸材料中的空间电荷快速消散现象有利于直流设备在极性反转条件下的运行,为阐释油纸绝缘良好的长期性能提供了有重要意义的试验依据。