Dielectric strength of air insulation for coordination of minimum clearances of overhead lines: a review

A survey is given of available data on the sparkover of air under various geometrical electrode arrangements. As positive switching impulses have been found to be the more severe stresses for external insulation, only the positive polarity is dealt with in detail; however, some data concerning the negative polarity in particular gap configurations have been included together with data for alternating voltages. Parameters affecting the dielectric strength of air insulation, such as gap geometry and spacing, radius of the conductor and the presence of surface protrusions, waveshape, voltage polarity and the effects of humidity and density, are surveyed. Finally, minimum clearances and gap factors for various electrode arrangements encountered in practice have been determined and compared with a recent IEC assessment method based upon data using the reference rod-plane airgap