Electric field control by permittivity functionally graded materials and their lightning impulse withstand voltages for surface breakdown

The authors have been probing the possibility of applying functionally graded materials to solid insulators in order to control the electric field profiles, and resultant improvement of insulation level. Investigated in the paper are the effects of the capacitive bulk-grading of the coaxial disc-type and post-type model spacers on the electric field along the insulator surface and on the corresponding surface breakdown voltage when the lightning impulse voltage is applied. It is obtained from the experimental work that the bulk-graded model spacers employed in the investigation stand for higher positive lightning impulse voltage by 10 to 25% than the ungraded model spacers, depending greatly on the manner of grading and model geometry.