Estimation of the Size of Voids in Coil Insulation of Rotating Machine

Voids produced in the impregnation process exist in mica insulation system of rotating machine coil. Distribution of those voids´ size, gap spacing, and discharge area is estimated based on the experimental data on the repetition rate and the apparent charge of partial discharges of sample coils. The method is grounded on several assumptions such as Paschen´s law for discharge inception voltage of the voids, zero residual voltage after the discharge. These assumptions are verified for model voids. A computer program was developed to obtain the void size distribution rapidly and accurately. The size of voids detected in-practical model coils are in the range of gap spacing and discharge area of 0.01 mm~0.2 mm and 5 mm2~300 mm2, respectively. The results are in conjunction with the tip-ups of tangent delta and also compared with the results of a method developed by Terase [1].