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The behavior of polar polymeric dielectrics at high electric fields is controlled by several factors. Among them, dipolar polarization and trapping and de-trapping of space charge are very important, as they are related to the microstructural properties of the material. From the point of insulation diagnostics, it will be highly beneficial if these two are properly identified from material characteristics, as they can reveal a lot about the aging condition of the material. This study aims to characterize the behavior of dipoles and space charge in a polar dielectric from discharge current measurements. Epoxy resin, a widely used dielectric in high voltage system is chosen as the material of interest. Thin epoxy resin samples are stressed at different electric fields for a fixed time period. Then they are discharged to ground and the discharging current is recorded upto a time period of 10000s. The applied electric field was varied from 2kV/mm to 20 kV/mm during electrical stressing of the samples. The discharging current measurements were performed at two temperatures, 25°C and 50°C on same set of specimens. In this paper an attempt has been made to separate the space charge by correlating frequency domain to time domain measurements using Hamon approximation. It was observed that the charge de-trapping phenomena considerably affects the discharging currents when the samples were previously stressed at high fields (10 kV/mm and 20 kV/mm). The distribution of trapped charge along trap depth was also investigated. It was found, that at high electric stress (20 kV/mm), the trapped charge distribution is exponential in nature at the investigated trap depth range. [ABSTRACT FROM AUTHOR]

Formation of space charge in polymeric insulation is a critical issue in the design of high voltage electric systems. Accumulation of space charge accelerates the material aging and may lead to premature breakdown of the insulation. Therefore, it becomes increasingly important to understand the charge injection, trapping and de-trapping processes in dielectric under application of high voltage. However analysis of charge trapping and de-trapping is very difficult due to material properties and also the fact that the measurement of space charge itself is a very complicated issue. Most of the trapping and de-trapping investigation in literature has been performed through indirect means, by measuring the space charge distribution in a dielectric specimen under charging and discharging conditions. The effect of temperature on the trapping and de-trapping properties of polymeric insulating materials has also received limited attention. In this paper, the charge trapping and de-trapping behavior of low density polyethylene (LDPE), a widely used polymeric material for electrical insulation purpose, have been investigated using a direct method based on the depolarization characteristics of a stressed sample. A model has been developed to obtain the trapped charge distribution according to their energy level in the sample directly from the depolarization current. The measurements were performed at different temperature to study the effect of temperature. It was observed that temperature plays a critical role in charge injection, trap formation, trapping and de-trapping. [ABSTRACT FROM PUBLISHER]

It is generally accepted that formation and accumulation of space charge in polymeric insulation is a critical issue in the design of high voltage electric systems, as it leads to insulation degradation and premature failure under high voltage surges and polarity reversals. Even at service stress level, accumulation of space charge may cause electrothermal aging. The insulation degradation and aging is reflected in the trapping and detrapping characteristics of the insulation, making it a useful aging marker. Space charge measurement methods are frequently used for trapping and de-trapping analysis. However, estimation of charge trapping and de-trapping behavior is very difficult due to the involvement of material properties and also partly due to the fact that proper measurement of space charge is itself a very complicated issue, requiring very sophisticated experimental arrangement and signal processing techniques. Moreover, application of these methods is limited to polymeric insulations of certain geometries (planar or radial). In this paper, the charge trapping and de-trapping behavior of low density polyethylene (LDPE), a widely used polymeric material for electrical insulation purpose, have been investigated employing a direct method based on the discharge characteristics obtained after the sample was stressed for short durations. A relationship is established between the de-trapped charge and the extracted charge measured from the discharge current. From this relationship a relative distribution of trapped charges across different trap depths was obtained. It was noticed that the trapped charge distribution is of exponential nature in the trap depth range of 0.93?1.11 eV. The presented method facilitates direct estimation of trap distribution independent of the insulation geometry. It was also observed that the trapped charge is highly dependent on the applied field. [ABSTRACT FROM AUTHOR]