Long term DC characteristics in insulating material for HVDC cable

In present paper, the long term DC characteristics of chemically modified high density polyethylene (modified HDPE) have been evaluated using sheet samples of 1 mm thickness and cable samples of 6 mm thickness. Long term tests have been carried out, monitoring changes in leakage current and in space charge, for 7500 hours for sheet samples and 6000 hours for cable samples. It was found that the change in leakage current took place in at least four steps; in the first step the current is decreasing quickly and largely until 1 hour; in the following step the current is slightly decreasing from 1-10 hours; in the third step the current gradually decreases for 10-1000 hours; the last step shows a tendency to saturate over 1000 hours. The current seems very stable for long term applied voltage. On the other hand, the amount of space charge did not change in a few thousand hours and became small in the vicinity of the cathode over 4500 hours. In the vicinity of the anode, space charge was reduced slightly.