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Investigation of space charge accumulation processes in a chemically cross-linked polyethylene for ac usage (XLPE) and a modified XLPE for dc usage (DC-XLPE) at voltage polarity reversal was carried out using pulsed electro-acoustic (PEA) measurement system. We found that a huge amount of so called packet-like charge, some of which partially increased the electric field in the material to more than twice of the average applied stress, generated in XLPE at the polarity reversal of DC stress. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. On the other hand, we also found that the packet-like charge was not observed in DC-XLPE under the same experimental condition.

We developed a compact system for evaluating the deterioration of insulating materials used in high-voltage dc power apparatuses. The developed system can be used for simultaneous determination of three basic parameters important for evaluating the deterioration of high-voltage insulation materials, i.e., conductivity, dielectric constant, and charge accumulation. Charge accumulation, which is detrimental to high-voltage dc electrical apparatuses, such as power devices, transformers, and power cables, can be evaluated using this system. With this system, the charges accumulated are measured by integrating the current through a capacitor inserted between the highvoltage power source and the high-voltage apparatus to be tested. The obtained information is transmitted to the unit placed at ground potential. The evaluation is carried out by comparing the amount of initial charges in the insulating material with that of charges accumulated at a certain time after application of a high voltage. When the ratio of the charges accumulated at time (t) to that of the initial charge is 1, it is assumed that there was no charge accumulation; thus, no deterioration of the insulating material is assumed. However, when the ratio is greater than 1, the insulating material is considered to have deteriorated. Using the evaluation system, we developed, we examined the deterioration of insulating materials used in coaxial cables by exposure to 1) γ-rays and 2) high-temperature conditions. From these two experiments using the developed system, the deterioration of insulating materials was found to have advanced substantially when the charge accumulation ratio exceeded approximately over 2.0.