Investigation of space charge injection, conduction and trapping mechanisms in polymeric HVDC mini-cables

Polymeric insulation materials have not been used in HVDC cable systems until recently. The main cause of this being the tendency of polymers to deplete accumulated charge very slowly. Research into the dynamics of space charge trapping, injection and conduction in polymers can reveal important information from which new materials can be designed. Solutions can be found in polymers other than the well-known XLPE or introducing additives into XLPE based materials. This thesis investigates the space charge accumulation dynamics of mini cable models consisting of several different XLPE based insulation materials and PE based semiconductive layers. The main difference between the materials being the type and concentration of additives. The influence of the semiconductor-insulation interfaces on the space charge accumulation threshold is investigated with the help of polarization characteristics obtained from space charge measurements conducted with the PEA method for cable geometry objects. Furthermore the apparent trap-controlled mobility of the charge carriers and the depths of the charge traps in the materials are evaluated using depolarization characteristics which are also obtained with the same PEA method. Extra measurements on thin plaque samples of both the semiconductive layers and the insulation materials, such as conduction current measurements and frequency domain dielectric spectroscopy, are used to obtain more information on the space charge accumulation dynamics of the mini cable samples.