DC and impulse treeing characteristics in insulating material for HVDC cable

We have developed new insulating material, modified high density polyethylene (modified HDPE), for high voltage DC cable, and tested several electrical properties of the material (in the forms of sheet and cable), such as DC dielectric breakdown, conductivity, and space charge property. In practical use, defects in material, especially metallic impurities are serious matter for cables. In present paper, the breakdown characteristics of modified HDPE have tested, using treeing sample with needle electrode, for four kinds of voltage (DC, DC polarity reversal, impulse (Imp), and impulse superposed on to DC voltage). As a result, it was clarified that the treeing breakdown voltage largely depended on kind of voltage. Namely, the order of breakdown strength were DC>Imp>Imp superposed on to DC⩾DC polarity reversal, and the scattering of breakdown voltage become smaller in the above order. With respect to radius dependence of needle tip, breakdown voltage was almost same value from 1.5 μm to several tens μm, showing polarity effect for DC and Imp. These experimental results are thought to correspond to the space charge formation near the needle tip, dependent on the kind of voltage