A Method to Estimate Minimum Partial Discharge Inception Electric Field of Metallic Particles with Various Shapes in Mineral Oil

Metallic particles and protrusions degrade partial discharge inception voltages (PDIV) in liquid insulation system. For controlling metallic particle contamination with a certain criterion, it is necessary to know the PDIV when the system is contaminated by metallic particles being different in size and shape. In this paper, we propose a universal curve to estimate minimum PDIV (or PDIE, PD inception electric field) of metallic particles in mineral oil. The PDIV of a needle-plate electrode immersed in mineral oil was measured using needles with various curvature radius of the tip. The PDIV decreased as the tip radius became smaller in the region of the radius from 100 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex> down to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10\mu \mathrm{m}$</tex> However, on the contrary to the expectation, below <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \mu \mathrm{m}$</tex> radius, the PDIV didn't decrease even though the tip radius became smaller. In the range of curvature radius less than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10\mu \mathrm{m}$</tex>, the PDIV was almost constant. This experimental result shows that the minimum PDIV of metallic particles with any shape is given by regarding the curvature radius of particle to be 10 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m}$</tex>. Based on these experiments, a formula to calculate minimum PDIE is derived as a function of length of particle. PDIEs of various kinds of metallic particle were measured and all of them were plotted higher than the calculated minimum PDIE curve. Thus, the proposed formula was shown to be appropriate as the one indicating minimum PDIE.