Dielectric interface characterization by means of space charge measurements

In most insulation systems several dielectric interfaces are present, in particular in cable systems (cables and accessories). Generally, interfaces are the weakest part of an insulation system, since the tangential component of the electric stress an interface can withstand is much lower than the dielectric strength of the insulating material(s). For this reason, the understanding of the physical phenomena controlling the electric field at the interface is of great importance. Especially under DC stress, the electric field at interfaces is quite difficult to predict because of internal charge build-up. To contribute to this discussion, the authors investigated the interfacial space charge behavior by means of the Pulse Electro Acoustic (PEA) method. For this purpose, laminar samples were tested. An especially developed test set-up, able to apply a controllable pressure at the sample, was used. In this way, the effects of pressure and application of grease/oil on the space charge distribution were studied for different interface materials. Moreover, the polarization phenomena at interfaces were theoretically analyzed and a comparison was made between measurement and theory.