Abstract

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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