Abridgment of essential factors in the coordination of line, station and apparatus insulation

Consideration of generated voltages and switching surges might logically be taken as the basis for determining the minimum amounts of insulation required for lines and stations. Lightning, when it is encountered, affects service primarily through its influence on lines. The lightning insulation strengths which may be suitable for the lines may bear little or no relation to the operating voltage or the basic insulation strengths found satisfactory at the stations. In order that service will be affected a minimum by the excessive lightning voltages which may be propagated to, or originate at the stations, it is desirable that they be discharged external to the station and Us apparatus. To accomplish this four lightning insulation levels are proposed; 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 entrances; the buses and connections constituting the next higher level; apparatus bushings next; and finally the apparatus internals. More complete data on the nature of transient voltages and the performance of insulation with these voltages applied, are required before insulation can be coordinated with assurance. During the interim period however 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. Since the entire project is distinctly in the development and research stage, the authors believe more progress should be accomplished before recognized standardization can be undertaken to advantage of such features as test waves, characteristics of insulation and insulation levels for lines, stations and apparatus.