Abridgment of rationalization of transmission insulation strength — II: Need for, present status of, and necessary developments for carrying through

This paper, prepared with the cooperation of the members of the Insulator and Lightning Subcommittee, points out the present need for rationalizing transmission system strength on the basis of lightning voltage. The higher grade of service demanded of transmission systems today requires fewer interruptions. It is pointed out that for a four-year period the line interruptions due to lightning on an extensive 132-kv. network average 75 per cent of all line outages. Apparatus failures due to lightning, while not numerically great, can be materially reduced if the system insulation is coordinated on the lightning basis. Over-insulation of lines has been tried in some cases, particularly on wood pole lines, with varying degrees of success in reducing line outages. But this method of attacking the lightning problem does not consider the protection of station equipment where the most costly apparatus is subject to damage, and where apparatus damage may result in long service outage. It is pointed out that additional knowledge is necessary on lightning strengths of insulation, and apparatus to rationalize system voltage strengths on a lightning basis. This information is gradually being secured by various groups working on the problem. To aid in solving the lightning problem, it is proposed that a set of standard lest waves be adopted, by which insulation and apparatus if possible may be tested. With this knowledge of the lightning insulation strength of apparatus, it will be possible to design transmission systems more intelligently on a lightning basis so far as insulation is concerned, in addition to the present 60-cycle basis. On the basis of field data secured last year on wave-shapes of natural lightning, three standard test waves are proposed, having voltage — time characteristics similar to those actually observed. It is pointed out that lightning voltage should be designated in units peculiar to lightning and not in terms of 60-cycle voltage values.