Study on charge de-trapping and dipolar relaxation properties of epoxy resin from discharging current measurements

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.