Space charge measurement as a diagnostic tool to monitor electrical ageing in polymeric materials

Abstract

Space charge is a term used to describe the excessive charges in volume. Such excessive charges can be electrons, ions or mixture of both. In solid dielectrics space charge tends to build up at discontinuities, such as at the dielectric-electrode interface, at the grain boundaries in crystalline dielectrics, at the boundaries between crystalline and amorphous regions in semi-crystalline dielectrics, and at various molecular sites simply called charge traps. Under the influence of the electric field the trapped charges may begin to detrap and move. Space charge phenomena have long been observed and their importance has been recognized over many years. In fact, space charge is recognized as a major factor in dielectric behavior, such as leakage, high-voltage reliability, and dissipation factor. However, its role in dielectric ageing is still the subject of research. Researchers have shown a special interest in cross-linked polyethylene (XLPE) because of its widespread use as high voltage cable insulation in power transmission. Many efforts have been made to quantify charge in solid dielectrics with a various degree of success. Research on space charge in solid dielectrics has been intensified in last two decades due to new development in non-intrusive measurement techniques. In this paper, we report an investigation on charge dynamics in polymeric materials after aged under ac electric field using the pulsed electroacoustic (PEA) technique. The emphasis is placed on charge decay and it has been found that the charge decay rate in additive free low density polyethylene (LDPE) becomes much slower in the aged sample in comparison with the unaged sample. The charge dynamics of the aged samples under dc bias differ from the sample without ac ageing, indicating changes brought in by ac ageing. It is believed that a slow decay rate of charge in the ac aged sample is related to the formation of deep traps in the material. However, chemical analysis by infrared spectroscope (FTIR) and Raman microscope reveals no significant chemical changes taken place in the bulk of the material after ac ageing. Further experiments have revealed that the deep trap formation is related to the electrode material used during the ageing. Au electrodes have little effect on the charge decay rate, indicating that the cause of the deep traps is due to charge injection. The influence of electrode material on clearly suggests that the process of charge injection leads to the changes in trapping characteristics. The effect of ac ageing has also been carried out on XLPE and similar conclusions have been reached. The findings presented in this paper clearly indicate that space charge measurement can be used as a diagnostic tool to monitor electrical ageing in polymeric materials. Compared with the conventional methods, it seems that space charge technique is more sensitive to the change in either physical structure or chemical alternation due to electric stressing.

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