Simultaneous space charge and conduction current measurements in solid dielectrics under high dc electric field

Abstract

The importance of space charge in solid dielectrics has been recognized for many years and various attempts have been made to map the distribution and assess its influence on the electrical performance of solid dielectrics. Significant development in non-destructive measurement techniques emerged two decades ago. Crosslinked polyethylene (XLPE) has been used for ac power cable insulation up to 500 kV for many years. There is a tendency to use XLPE for dc power cable. However, the easy formation of space charge under dc electric field within XLPE is a major concern for such an application. Space charge in insulation can distort electric field distribution, causing electric field increase in one region and reduction in another. The electric field enhancement could lead to degradation and result in premature failure. Electrical treeing in solid dielectrics is a typical example of local field enhancement due to space charge accumulation. In this report several popular non-destructive techniques are briefly reviewed. This is followed by detailed description of a modified pulsed electroacoustic (PEA) technique that allows simultaneous measurement of space charge and conduction current in a solid dielectric subjected to high dc electric fields. Finally, we report the relationship between space charge dynamics and electrical conduction current in XLPE using the modified PEA system. The effect of electrode material on both charge dynamics and current has been investigated using semiconducting material and aluminium. It has been found charge dynamics in the material depend on electrode configuration. More importantly, it has been noticed that the so called space charge limited transient current peaks are closely related to the meetings of negative and positive charge front in the bulk of the sample.

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