This work is an investigation and study of the mechanism of internal discharges in micro cavities in solid dielectrics with application to power cables. The magnitude, repetition rate and energy, in micro cavities in solid dielectric samples and electrical cables were studied. The field distribution in these cavities was simulated, modeled and calculated for certain applied voltage. Alternating high voltage at power frequency at room temperature was applied to different materials.The cavity shape, cylindrical or spherical, and its dimensions were changed. The size of the cavity was much smaller than the sample thickness. The cavities location and w.r.t electrodes were changed. The study was done on samples of low density polyethylene (LDPE), polyvinylchloride (P.V.C) and ethylene polypropylene rubber (E.P.R) because of their important industrial applications.