ABSTRACT
Insulation properties can be characterized as self-restoring and nonself-restoring. Self-restoring insulation has the ability to “heal” itself following a flashover, and such insulation media is usually associated with a gas—air, SF6. Nonself-restoring insulation is assumed to have permanently failed following a flashover, and repairs must be affected before the equipment can be put back into service. The performance level of self-restoring insulation is usually addressed and defined in terms of the probability of a flashover. Insulation usually has a lower withstand capability when exposed to positive polarity impulses than when exposed to negative impulses; designs are usually based on positive magnitude impulses. The art of correlating equipment electrical insulation strengths with expected overvoltage stresses so as to result in an acceptable risk of failure while considering economics and operating criteria. When arresters are used to protect a transformer for example, an insulation level, which has a significantly higher capability than the maximum surge allowed by the arrester, is selected.
