Essential Factors in the Coordination of Line, Station and Apparatus Insulation

Generated, switching, and lightning surge voltages all influence the design and application of insulation. The first two might logically be taken as the bases for determining the minimum insulation levels at stations; also, for lines where lightning is not encountered. Since lightning voltages affect service primarily through their influence on lines, the insulation strengths suitable for the lines may bear little relation to the insulation strengths of the stations. Several lightning insulation levels are proposed for the coordination of line, station, and apparatus insulation; the basic or lowest level (determined by consideration of generated and switching surge voltages), being established by the line insulation or spillway gaps at the line station entrances, when the line is at a higher level; the busses and connections would constitute the next higher level; the apparatus bushings next; and finally, the apparatus internals. More complete data on the nature of transient voltages as they originate and are modified by the circuit, and the performance of insulation with these voltages applied, are required before insulation can be coordinated with assurance. During the interim it should be possible to accomplish substantial improvement to service by applying to specific cases as they arise, the knowledge and data already available and being gathered, along the general lines proposed in the paper.