Basic Switching Surge Insulation Levels: A Proposed Philosophy for EHV Insulation Coordination

Paralleling the BIL insulation structure for impulse evaluation, a basic switching surge insulation level (BSL) is proposed which is more adaptable where switching surge insulation coordination is concerned. It provides 12 harmonically spaced divisions per octave (doubling of the kilovolt values) instead of eight nonuniform divisions over the same range as offered in the BIL structure. The BSL structure proposed offers a single set of kilovolt values which bear essentially the same ratio to the crest value of the maximum design voltage, line to ground (pu), for all of our four present or proposed EHV systems. The proposed values include the majority of the switching surge ratios now in use and are also based upon the recommended procedures of Z17.1 -1958, "preferred numbers," in that a system of preferred numbers is used in its development. Sample applications are presented.