Capacitive-resistive transition in gas insulated DC systems under the influence of particles on the insulator surface
Satisfying the requirements of recent energy transmission, like space saving installations, bridging vast distances between generation and consumption of energy as well as linking offshore wind farms, DC operated gas insulated systems are shifted into the focus. Compared to AC applications, the electrical DC field is, amongst others, determined by temperaturedependent conductivities of the insulating materials. After energising the system, the permittivity dominated electrostatic field is transformed into a quasi-stationary, conductivity dominated electrical field. Based on experiments, a CFD model was developed to pre-calculate the temperature distribution at various electrode temperatures. Following up, time dependent calculations show the changing electrical field distribution. Experimental results of long-term tests confirm its inverting character with a shifted high field region. In comparison to the electrical AC field, the higher absolute value of the electrical field strength under DC conditions decreases the insulation strength. The accumulation of space and surface charges is influencing the capacitiveresistive transition. Charge carriers can be produced by discharges due to several defects in GIS, especially by conducting particles, that cannot be excluded completely, in spite of high quality standards during manufacturing and assembling. With respect to the flashover behaviour, the effect of metallic particles on the insulator surface on the transition's time constant and on the absolute value of the changed electrical field strength is investigated.