Space charge accumulation and electric breakdown in XLPE under DC high electric field

We have observed space charge profiles using the PEA (pulsed electro-acoustic) system in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) under high electric dc field to study a relationship between space charge behavior and dielectric breakdown. XLPE, which is used as an insulating material of ac power cable, is also expected to be applied as the insulating layer of DC long distance power cable. To apply the polymeric materials for the insulating layer of a dc power cable, it is necessary to understand the "space charge" effect under high dc stress. In our previous research works, we have found that a large amount of, so called, packet-like charge generates in LDPE under dc high electric field of more than 100 kV/mm and it enhances the electric field locally in balk of the sample, and finally it leads a breakdown. On the other hand, such breakdown process has not been reported for XLPE yet. Because it was difficult to get a thinner XLPE sample as LDPE was. However, since we need to understand the relationship between the space charge and the breakdown in XLPE, we tried to measure them using specially arranged PEA system for high voltage application. In this report, we investigate the space charge formation and electric breakdown characteristics in XLPE. As the results, it is found that the relationship between the space charge and the breakdown in XLPE is completely different from that in LDPE. Since the XLPE was chemically made from LDPE using cross-linked agent at high temperature. It is assumed that the breakdown strength in XLPE is strongly affected by both of the residues of the cross-linking by products and the heat history.