Flashover behaviour of insulators with inhomogeneous temperature distribution in gas insulated systems under DC voltage stress

Due to the increasing need for long-distance energy transmission and space saving installations, the focus is more and more shifted to DC operated gas insulated systems. Reaching the steady state of the electrical field distribution after energizing, the resistive field is controlled by conductivities of insulating materials and SF₆. The heating of the inner conductor due to operating currents causes an inhomogeneous temperature distribution within the insulating material, influencing local conductivity. Hence, the location of the highest electrical field under DC stress is shifted with respect to the situation under AC or impulse voltage stress. Consequently, the flashover behaviour of insulators depends on the kind of the electrical stress (AC or DC). In this paper, a CFD model was developed based on experiments to precalculate the temperature distribution at various electrode temperatures. Following up, time-dependent dielectric calculations show the changing electrical field distribution. After reaching the steady state, its inverted character with a shifted high field region can be seen. Experimental results of long-term DC voltage tests confirm this general behaviour.