High field dielectric properties of low density polyethylene with a very small dent on film surface under AC ramp electric field application

Latest power cables which insulating material is cross-linked polyethylene (XLPE) have taken measures to prevent water tree deterioration. Thus, not only its operating electric field, but also its stability and reliability are getting higher. However, due to operating for long duration, it is also considered that there is a risk of sudden breakdown from a very small defect or contaminant in the insulator. Therefore, it is very important to study a conduction mechanism of LDPE with a very small dent on film surface under AC high field. Some typical phenomena of LDPE under AC high field, such as charge injection from an electrode, space charge formation at vicinity of electrode and so on, were reported by the authors [2], [3]. To estimate these conduction mechanisms, a simultaneous observation of dissipation current and EL has been carried out. These conduction mechanisms are the phenomena having the time-dependent. Thus, it is very important to make the electric field application history equal in each sample. So, a simultaneous observation under an AC ramp electric field application was introduced to control the electric field application history. It makes easy to evaluate the time dominant phenomena under AC high field. There is another advantage. The time for data acquisition is only 10 s from low electric field to high electric field. In this paper, in order to estimate the effects of the mechanical damage on surface to the dielectric properties, three kinds of mechanically damaged LDPE films were prepared. By using these samples, the simultaneous observation of dissipation current waveform and EL has been carried out under AC ramp electric field application. As the results of experiments, the effects by the mechanical damage on surface to the dielectric properties in AC high field are discussed.