Effects of frequency on the electric strength of polymers

The electric strength of air, in millimeter gaps, decreases at frequencies exceeding 100 kHz, but the effect on the discharge inception voltage V/sub i/ in solid insulation is too small to explain the reduction in the short-time breakdown voltage V/sub st/ of solids at radio frequencies. When breakdown is caused by internal discharges or by surface discharges in air, V/sub st/ at 50 Hz is usually between 5 V/sub i/, and 20 V/sub i/, so the reduction in V/sub st/ when the frequency is raised may be explained by the increased discharge repetition frequency causing more rapid degradation with a risk of cumulative heating at lower stresses. When tests are made in oil, V/sub st//V/sub i/ is smaller than in air, so in this case a tenfold reduction in V/sub st/ at higher frequencies may be caused by a reduction in the electric strength of the oil.>