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_i in solid insulation is too small to explain the reduction in the short-time breakdown voltage V_st of solids at radio frequencies. When breakdown is caused by internal discharges or by surface discharges in air, V _st at 50 Hz is usually between 5 V_i, and 20 V_i, so the reduction in V_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_st/V_i is smaller than in air, so in this case a tenfold reduction in V_st at higher frequencies may be caused by a reduction in the electric strength of the oil