A Multi-Physic Field Lifetime Evaluation Formula for Insulators Under Pulsed Field and Mechanical Stress

By conducting experiments on the effect of the electric field on lifetime (
$N_{L}$
) of polymer samples, it is found that
$N_{L}$
dependent on the average electric field (
$E_{\mathrm {av}}$
) in quasi-uniform fields conforms to a minus power relationship when
$E_{\mathrm {av}}\le E_{\mathrm {BD}}$
, where
$E_{\mathrm {BD}}$
is the one-pulse breakdown field. By re-analyzing the mechanical stress effect (
$S$
) on
$N_{L}$
, it is found that
$N_{L}$
dependent on
$S$
conforms to a minus linear relationship when
$S \le S_{0}$
, where
$S_{0}$
is the critical stress at which the insulators will rupture. With these two results, a combined lifetime formula to evaluate
$N_{L}$
of insulators exposed to multiple physical fields consisting of pulsed electric field and mechanical stress is put forward. This formula is applied to evaluate the lifetime of a conical winding in Tesla-type generators before and after improvements, which agrees with the experimental results. The evaluation result supports the correctness of this formula. The multi-physic field formula can be used to evaluate the lifetime of insulators used in both electric field and mechanical stress.