Changing of space charge related material properties due to electrical aging

In recent years there has been a search for new insulating materials for HVDC applications that could replace traditional materials, like for example traditional epoxy compounds. Polycarbonate (PC) with different fillers is one of these materials. Many details of changes of the threshold for electrical aging of this material are already known. In contrast to this, very little is known about the effect of electrical aging on the type of space charge (homo or hetero charge) which accumulates in PC. For this purpose PC (with and without filler) was electrically aged and space charge measurements were performed at different stages of the aging process. The space charge measurements were done by means of the pulsed electro acoustical (PEA) method. Homo charge was measured in new specimens but in aged specimens mostly hetero charge was measured. Hetero charge profiles are therefore associated with electrical aging of the materials. No charge packet formation was found at the test conditions used in this project.