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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.

XLPE(Cross-Linked Polyethylene) power cable is expected to be applied to a long distance power transmission cable in HVDC systems. DC transmission system is effective for the long distance power transmission, and XLPE cable is not necessary to give a periodical maintenance procedure. However, when DC high voltage is applied to XLPE, it is said that on electrical breakdown may occur due to an effect of space charge accumulation. While the relationship between them in LDPE(Low density polyethylene) was revealed, it has not been clear in XLPE yet. In the case of XLPE, not only the space charge behavior, the conduction current seemed to be also important factor to understand the relationship. In this paper, the space charge distribution and an external circuit current measurement were carried out for the materials, which were immersed in the by-products. It was found that the reduction of breakdown strength and a rapid change of the conduction current in the case of LDPE were observed, and them were much obvious in LDPE immersed in cumylalcohol than those observed in LDPE immersed in acetophenone.

In this paper we investigated the space charge accumulation characteristic in single and multi-layered samples composed of EPR laminated on top of LDPE. We measured when the polarity of the applied voltage was reversed and moreover, we compared the results to these obtained when a short circuit was provided before the voltage reversal.

This paper investigates the space charge distribution and external current performance of Ethylene Propylene Diene Monomer (EPDM) film simultaneously by an improved integrated PEA system. Through switching the space charge and current measurement circuits, the space charge and external current were detected alternately during the electric field of 60 kV/mm and 80 kV/mm. Small amounts of homocharges accumulated inside the sample. While, an obvious electron injection occurred from cathode (Al electrode), and they moved inside the sample. This leaded to an electric field distortion near the anode. However, hole injection seemed difficult to occur from anode (semiconducting layer). Based on the measured current density and the space charge distribution, we calculated the conduction current density distribution inside the sample. The results showed that a large conduction current occurred at the beginning of measurement, which is derived from the rapid electron injection and movement. Nevertheless, it decreased with the increase of time, indicating a uniform distribution of conduction current density (less than 1×10−6 A/m2). In addition, the dissipation energy density caused by space charge and conduction current was also estimated during the field application. It indicated that a high power density appeared inside the sample at the beginning of measurement. However, it gradually decreased with the increase of time. This information reveals the energy loss dynamics caused by space charge during the electric stress, which is of great importance to evaluate the influential mechanisms of space charge on aging and insulation failure.

As a long distance DC power transmission cable, XLPE (cross-linked polyethylene) insulated cable is considered to be used. Since the cable is supposed to be used at high-temperature because a large current flows in inner conductor, it is necessary to estimate the performance of the insulating material at high temperature. Especially, characteristics of a space charge accumulation and a conduction current in the material under a high DC stress at high-temperature is important to estimate it. Therefore, it has been developed a measuring equipment which is enable to measure the space charge distribution and the external circuit current density simultaneously at high temperature. In this report, several typical measurement results, that are obtained using the developed system, will be introduced. As a result, it is found that the moving speed of the injected packet-like charge increases with increase of temperature. Furthermore, it is found that a current value significantly depends on the polarity of the applied voltage is generated.