Measurement of space charge distribution in epoxy resin

Internal space charge behavior of insulating materials has recently attracted attention of many researchers, and a large number of experimental studies were carried out by using the materials for dc cables, such as XLPE, LDPE, and HDPE. Epoxy resins are used for insulation under strong electric fields in power apparatus and in electronic devices, and we investigated the behavior of internal space charge using the pulsed electroacoustic method. Two types of epoxy resins were studied: insulation-grade and craft-grade. When dc electric fields were applied to the craft resins treated by immersing them into room-temperature water for 8 and 24 h, positive and negative charges accumulated near the anode and the cathode, respectively, and the charge distribution changed with the immersion time. On the other hand, no charge was observed in the insulation-grade epoxy resin. Next, we treated the samples by immersing them into 100 °C water for 8 h. When the sample was treated for 8 h, hetero charge distribution, which means positive charges near the cathode and negative charges near the anode, was observed. This result is consistent with a previous paper reporting that by chemical analysis, secondary decomposition had occurred. These results show that water and temperature influence the internal space charge behavior of epoxy resins. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 9–16, 1999     

气体放电表面电荷和空间电荷测量方法

气体放电是气体具有导体性质和作用的一种现象。目前广泛应用于机械加工、化工生产、材料制备和环境保护等领域。放电电荷量作为气体放电中的关键参数,其精确性关系到放电系统的控制和实际应用效果。从表面电荷和空间电荷两个方面阐述了气体放电系统中常用电荷测量方法的测量原理、探头结构。在此基础上分析了各种测量方法的测量准确性和实际应用情况。     

Measurement of space charge distributions in PLZT using Kerr effect

Kerr effect technique is an optical measurement technique used to determine space charge distributions of a dielectric liquid under applied electric stress. In other words, such measurement can be used to study the conduction mechanisms of a dielectric liquid, such as charge injection threshold, bulk electric field stress, and aging. In this study, an instrument was set up to measure the absolute value of the electric field distributions in lead–lanthanum–zirconium–titanium (PLZT). PLZT was selected for the study of the instrumentation because the material has a high Kerr constant. In the experiment, charge injection threshold and bulk field strength of PLZT were measured under low applied voltages. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Measurement of the space charge field in transformer oil using itsKerr effect

Nonuniform electric fields >80 kV/cm, have been measured via a relative optical intensity measurement by exploiting the Kerr effect in transformer oil. Transformer oil was used as a Kerr medium in these measurements. Neither a knowledge of the Kerr coefficient nor the actual value of maximum transmitted intensity are required to obtain the absolute value of the electric fields. Electric fields were measured with ac and dc excitations. Space charges resulting from charge injection have been found in transformer oil at room temperature under electric stresses >150 kV/cm with ac excitation and 90 kV/cm under dc excitation. The magnitudes of space charges in transformer oil with ac applied voltages varied from 2 to 50 nC/cm³. The magnitudes of space charges with dc excitation varied from 60 pC/cm³ to 10 nC/cm³. The results illustrate the limitations of the electro-optic technique for investigations of the interfacial electric fields (gas-solid) with transformer oil as the Kerr medium     

Injection of a minimal space charge as mechanism for the initialphase of electrical polymer degradation

In this paper we present a model for polymer degradation during high-field stresses in analogy to the charge-to-breakdown concept in SiO ₂. The model is based on space charge injection under high fields which leads to a cumulative effect as in radiation damage. A critical total injected charge density Q_c is introduced, which is needed prior to any permanent damage. The assumption of a critical injection level explains the time and field dependence of the tree initiation time. The model is applied to electrical aging experiments in epoxy resins. It turns out that the critical charge for tree initiation is ~10 C/cm² similar to the value for trap creation in SiO₂ gate oxides     

Correlation between space charge distribution and water-treelocation in aged XLPE cable

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables     

Non-destructive methods for space charge distribution measurements: what are the differences?

For more than 25 years, various techniques have been introduced for the nondestructive measurement of the space charge distribution in solid dielectrics. All these techniques can be classified into a set of three methods: (i) the thermal method, (ii) the pressure-wave-propagation method and (iii) the electro-acoustic method. Despite their apparent differences they are based on the same physical idea: the perturbation of the equilibrium between electrostatic and elastic forces in the material induces a response, either electrical or mechanical, which can be analyzed. In this paper, we demonstrate that the information on the material contained in the response is identical whatever the method used. As a consequence the differences between methods stand only in the nature of the perturbation and its physical properties.     

钛酸钡对聚乙烯试样中空间电荷的影响

在高场强下用电声脉冲法(PEA)测量经BaTiO3表面处理后的聚乙烯试样中空间电荷的分布,比较纯聚乙烯试样与经BaTiO3表面处理后的聚乙烯试样中的空间电荷分布,试验结果表明,用BaTiO3表面处理的聚乙烯可以有效的抑制电荷的注入和空间电荷的形成。     

Measurement of streamer charge distribution on an insulator in SF6 Gas

The charge density produced by streamers on an insulator surface in SF₆ has been investigated by using a probe method with a high-speed temporal resolution. Concentric circular probes, which also act as a plane electrode, are used in this probe method. Probe signals are observed oscilloscopically and converted into the charge densities through a numerical calculation. This method reveals the charge distribution before a disturbance caused by the “back discharge.” The charge density thus obtained ranges from several nC/cm² up to about 60 nC/cm². The density depends on the pressure, voltage height and the position of the streamer. The electric field on the insulator is analyzed numerically taking into account the surface charge. The internal electric field of the streamer is found to be 40 ～ 50 kV/cm · atm when the streamer ceases its propagation. However, it partly exceeds the critical one (89 kV/cm · atm) during the propagation.     

Measurement of the particle space charge in the outlet of anelectrostatic precipitator using an electric field mill

It is shown that for certain conditions the concentration of the escaping particles can be related to the space-charge field in the outlet duct. The theory of operation along with a discussion of the sensitivity of the method is presented. The design and performance characteristics of a self-purging field mill are described and preliminary measurements based on field testing are reported. The results of three preliminary tests carried out in an industrial environment confirm the usefulness of the device for monitoring instantaneous emission levels     

Measurement of space charge in XLPE insulation under 50 Hz acelectric stresses using the LIPP method

Space charge is believed to play an important role in ac electrical ageing of polymeric insulation, particularly where electrical treeing is the dominant cause of premature failure. The majority of work to date has been on the space charge characteristics under dc electric stresses, whereas work on the dynamics and the role of space charge on electrical breakdown under 50 Hz ac conditions have only received limited attention. Consequently, a full understanding of space charge trapping and dynamics under ac operating conditions is required if ever more reliable polymer insulated cable systems are to be developed. The present paper reports on aspects of an on-going investigation into the measurement of space charge in XLPE insulation under 50 Hz ac stress using the laser induced pressure propagation (LIPP) technique. Some important features of the method are described such as, point on wave control of the laser and use of correction factors to offset variations in the laser power and ablation of the target material. Emphasis has also been placed on establishing a simple method for analysing data, termed "X-plots". Some preliminary results of the space charge and electric stress distribution of un-degassed and degassed crosslinked polyethylene (XLPE) plaques aged electrically under ac stress (30 kV_peak/mm) for 24 h are also presented     

Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms

It has been observed that voltage waveforms generated by power electronic converters may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can provide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors.     

Estimation of conductivity and permittivity of water trees in PEfrom space charge distribution measurements

Using an improved pulsed electroacoustic system which is capable of measuring space charge distribution at minimum intervals of 25 μs, the charge distribution inside water-treed polyethylene (PE) was examined. From the results, the frequency dependencies of conductivity and permittivity were examined for water trees grown in PE. The conductivity of the water-treed region is found to be >10¹⁰× higher than that of non-treed regions, while the permittivity increase is only slight     

Power frequency dependence of space charge behavior and partial discharge characteristics in SF/sub 6/ gas

The partial discharge (PD) and breakdown (BD) characteristics in SF/sub 6/ gas under commercial and higher frequency (/spl sim/600 Hz) ac voltage applications were investigated using high-speed electrical and optical measuring techniques with phase gate control method. Experimental results revealed that 400 Hz BD voltage at a certain gas pressure range was higher than that for 60 Hz and PD characteristics especially at the positive PD inception phase were much influenced by the applied power frequency. From these results, we clarified the dependence of space charge behavior on the applied power frequency and discussed the physical mechanism of PD and BD in SF/sub 6/ gas with consideration of the space charge behavior generated by PD in the previous half cycle of ac voltage.     

A rational consideration of space charge

Space charge formation and its increase with field under DC conditions is not a mystical phenomenon. The increase in space charge with applied field is an obvious and inevitable result of the interaction of field-dependent current density with spatially inhomogeneous resistivity. For common polymeric dielectrics, the current density makes a transition from a linear function of field to an exponential function of field at around 10 kV/mm. This causes a similar transition in the sample space charge in the same field region. However, this transition has no obvious connection with aging, and if it does prove to have a connection, the driving force is the rapidly increasing current density and not the space charge, which is a result thereof. As to the issue of the accuracy of space charge measurements, it may be more important to focus on providing a physical mechanism for 1-eV deep traps at an average separation of 3 nm within a polymeric dielectric. This would bring about the ability to engineer important dielectric properties as well as improved understanding of the physical basis of aging and other important phenomena in dielectrics.     

Analysis and discussion on conduction current based on simultaneousmeasurement of TSC and space charge distribution

A new analysis of conduction current distributed in dielectrics based on simultaneous measurements of thermally stimulated current (TSC) and time dependent space charge distribution is proposed. A new system pulsed electro-acoustic (PEA) method has been developed to enable simultaneous measurement of the TSC and the dynamic space charge and electric field distributions as a function of temperature within insulators. With the new system, the relationship between the TSC and the time dependent electric field distribution in electron beam (e-beam) irradiated PMMA has been investigated. From the time dependent electric field, the displacement current in dielectrics is obtained. The TSC is a typical external current which is represented as an addition of the displacement current and a conduction current in dielectrics. This paper makes it clear that the conduction current as a function of position is determined by the simultaneous measurement of the external current and the dynamic space charge distribution