Thermally stimulated current and electrokinetic investigations of HV cable models

Space charge in extruded model polymeric high voltage cable samples has been assessed under various conditions using the thermally stimulated current (TSC) technique. All of the cable samples under investigation consisted of a stranded aluminium core, covered by three concentric layers of polymer: (1) an inner semi-conducting carbon-loaded polymer screen electrode; (2) an XLPE insulation layer, and (3) an outer semicon screen electrode. Effects on the TSC due to the composition of the materials used in both the XLPE insulation and the semiconducting screens have been studied. The semicon-insulation combination was also seen to have an important influence on the TSC. Thermal cycling affected the TSC, and is conclusively attributed to morphological changes in some of the samples. The TSC measurements of other samples could not be interpreted so definitely, however, as a consequence of their complicated TSC curves, which arose from the novel composition of the cable materials. TSC curves at different poling voltages were interpreted as appearing due to the relaxation of different poled mechanisms. The TSC was measured in the 27 deg. C to 107 deg. C temperature range. Poling voltages of upto 45kV in magnitude and of either a positive or a negative polarity were used to pole each sample at temperatures between room temperature and 107 deg. C. The maximum poling voltage magnitude of 45kV corresponds to a geometric electric field at the inner semicon-insulation interface of ∼ 14kV.mm-1 and a field at the outer interface of ∼ 7.4kV.mm-1. Typically, low poling voltages produced TSC peaks which were correlated with the work of other authors, and which showed these TSC to be attributable to charge accumulation at crystalline-amorphous boundaries. Space charge injection appeared to be promoted when higher voltages (and hence, fields) were employed. The voltage threshold above which space charge injection occurred was influenced by the different magnitudes of electrical field at the two different interfaces. A novel method of investigation, called the Electro-Kinetic (EK) measurement technique, was also used to measure electrically-induced displacements at the surface of the cable samples to further characterise the electrical response of the model cables. A correlation was seen between the EK measurements, made at room temperature and with an a.c. stimulating voltage of upto 10kVrms, and the total space charge derived by integrating the TSC with respect to time. (author)