Surface potential dynamics on insulating polymers for HVDC applications

Doktorsavhandling, 2016

The use of high voltage direct current (HVDC) technology in power transmission systems is continuously expanding. Nowadays, HVDC transmissions operate at voltages up to 800 kV and higher levels are being developed. To secure continuous and reliable transportation of electric energy in such systems, materials used for electrical insulation should satisfy stringent requirements related to their performance under high electrical stresses. These concern, in particular, charge accumulation and its dynamics on surfaces of insulating elements which affect distributions of electric fields and may even influence flashover performance. Thus, the conducted study aimed at increasing understanding of surface charge dynamics on insulating polymers that is essential for proper design, testing and co-ordination of HVDC insulation. The work was performed utilizing flat samples (thicknesses ~2 mm and ~300 µm) of several types of high temperature vulcanized silicon rubbers. The materials were first characterized by measuring their electrical conductivities and complex dielectric permittivities. A non-contact technique, based on application of Kelvin type electrostatic probe, was thereafter used to measure surface potentials and their decay characteristics on single- and double-layered samples of these materials. The samples were located on a grounded metallic base and their open surface was pre-charged by means of a corona source in air under atmospheric pressure. The dynamic behavior of surface potential was afterwards investigated at various air pressures (1 bar, 600 mbar and 300 mbar) and temperatures (from room temperature to 70 oC), which allowed for minimizing the influence of gas phase on the decay of the deposited charges and for examining solely the effect of solid material properties. Furthermore, a computer model describing the surface potential dynamics has been developed and utilized for analyzing the results of the experiments.  The performed study has demonstrated that deposition of charges generated by corona on the open material surface induce potential distribution decaying with time but continuously preserving its initial space distribution. The decay is found to be slower at reduced gas pressures. It also depends on material conductivity, being faster on more conductive materials as well as at increased temperatures, well responding to the thermal activation of conduction processes. These facts indicate that bulk conduction is the dominant mechanism of surface potential decay under conditions of the present study, which could also be confirmed by the computer simulations. It was in addition observed that the decay on double-layered structures could be faster as compared to that on single-layered ones, if a more conductive material was used for the base layer, which remained in contact with the grounded metallic electrode. A model of interfacial polarization was employed to analyze this effect. The analyses of surface potential decay also allowed for independently determining bulk conductivity of the investigated materials and its variation with electric field strength, yielding results comparable with those obtained by means of the conventional method. The determination of material conductivity based on surface potential decay provides a number of advantages, in particular, a reduced measuring time and a wider range of the analyzed electric field strength.