ABSTRACT
When applying insulation coordination, it is useful to classify the insulation of a power network according to location and dielectric performance. Insulation is classified, according to location, as external or internal. The coordination of self-restoring insulation, unlike non-self-restoring insulation, lends itself to probabilistic treatment where the evaluation of the insulation's risk of failure is economically justified. Power system insulation is subjected to four classes of dielectric stresses: power-frequency normal voltage, temporary overvoltages, switching overvoltages, and lightning overvoltages. A transmission system comprises a large number of insulation components "in parallel". In a substation, the number of line-to-ground insulation structures may be as high as 100. The procedure leading to well-coordinated system insulation begins by evaluating the stresses to which insulation is subjected. If external insulation is designed on the basis of switching surge and lightning requirements, the strength for power frequency voltages and clean insulators, even in wet conditions, is normally very high.
