Charge injection and distribution in polymeric insulation systems

The build up of space charge above a critical field strength, Fc, can be regarded as an important mechanism for electrical tree nucleation and ageing [1]. For voltage ramps of msec, a homogeneous space charge cloud grows instantaneously if Vc, the voltage at which F = Fc, is surpassed. As a consequence, F will never be exceeded: we deal with field limiting space charge (FLSC model of ref. 1+2). Within our model, the microscopic physical interpretation of Fc is the field at which charge carriers change from very low, trap-controlled mobility to a high mobility [1,2]. In order to test experimentally the concept of injection of mobile carriers and concomitant formation of a homogeneous space charge zone around a point of field enhancement, a needle-plate geometry was chosen. The basic ideas and the quantitative predictions of the FLSC model could be verified [2,3].