Effect of floating conducting objects on critical switching impulse breakdown of air insulation

The paper analyses the mechanism of breakdown of phase-to-ground and phase-to-phase air insulation in the presence of large conducting floating objects, under critical switching impulse stress. A new physical modelling approach is introduced which involves determination of the potential of the floating object by charge simulation technique, assessment of streamer breakdown and/or leader inception and propagation in the partial gaps and finally predicts the critical breakdown voltage of various configurations. As to phase-to-ground insulation, the investigation covers rod-plane, conductor-plane and conductor-tower leg configurations with different gap spacings as well as different shapes, dimensions and positions of the floating object. The phase-to-phase study additionally includes the effect of negative switching impulse content of the applied stress. The model is in excellent agreement with the experiment and provides a novel tool for assessment of the effect of floating objects on switching impulse breakdown of some basic air gap configurations relevant to live line work