Surface Charge Dynamics on Polymeric Insulating Materials for High Voltage Applications
Licentiate thesis, 2014
To meet increasing demands in electric energy, it is essential to enhance production of electricity from renewable energy sources (solar, wind and hydro). Such generation sites, however, are usually separated from consumption sites by long distances. An efficient transportation of energy requires implementations of high voltage direct current (HVDC) transmission systems, which operate today at rated voltages up to ±800kV. To provide electric insulation for such voltage levels, polymeric insulators are preferable due to a number of advantages over traditionally used ones made of glass or porcelain. The use of polymers, however, leads to surface charging and charge dynamics on insulating elements, which are inherent phenomena in HVDC insulation systems. Thus, knowledge about these processes is essential for proper insulation design, testing and co-ordination. Therefore, the conducted research aimed at providing information about fundamental mechanisms of electric charge transport in HVDC insulation and focused on analyzing roles of gas phase and properties of solid materials on surface charge dynamics.
The study was conducted utilizing flat samples of several types of HTV silicon rubber and cross-linked polyethylene, which are widely used in different HVDC applications. The electrical conductivities and dielectric permittivities of the materials were measured in time and frequency domain, respectively. To study variations of surface charges, the samples were exposed to corona generated in air from nearby sharp electrode that yielded accumulation of electric charges on gas-solid interfaces. Surface potentials induced by the deposited charges were measured at different instants after charging that allowed for obtaining surface potential decay characteristics for the studied materials. The measurements were conducted for both polarities of pre-deposited surface charges at different pressures of ambient air that provided a possibility to control the intensity of neutralization of the deposited surface charges by free counter ions present in air and to evaluate relative contribution of this process to the charge/potential decay. It was found that a reduction of air pressure weakened the intensity of the background ionization in gas and led to diminishing amount of free ions. Under these conditions, the contribution of gas neutralization to the total charge decay was reduced and decay mechanisms were determined solely by the properties of solid materials. Effects imposed by bulk and surface conduction in the solid material on surface charge dynamics were studied by means of experimental measurements and computer simulations. The obtained results allowed for evaluating threshold values of the volume and surface conductivities at which these transport mechanisms become essential. It is demonstrated that bulk conduction becomes dominant mechanism of surface potential decay if volume conductivity of the material is above ~10-16 S/m. The results of the modeling agree well with the measured characteristics if materials’ field-dependent conductivities are taken into account. The performed parametric studies also demonstrate that surface conduction may influence the potential decay if the corresponding conductivity exceeded ~10-17 S.