Coordination of Insulation

The general scheme of coordination is discussed. Average circuit insulation used in 1930 is shown. The relation that switching surges and arcing ground overvoltages bear on coordination is discussed giving the number and magnitude of such waves found in service. Levels of insulation in and around the station for outdoor and indoor apparatus are suggested. It is pointed out why a point gap offers the most reliable method of establishing the level for impulse testing of transformers. Also, reasons are given why bushings as made at present should not be used as the sole level for impulse testing. It is shown that on account of the similarity of the volt-time curves for insulations and sphere gaps for short impulse waves, the use of a sphere gap in parallel with a rod gap will give an added protection against direct strokes on or near the apparatus terminals. Also, various proposed schemes are shown to give protection to the transformer and against outages for both fast and slow waves. For potential and current transformers having voltage ratings differing from standard it is suggested that the coordination gaps be selected from the curve of gaps vs. rated circuit voltages. It is shown how an air gap of standardized construction can be used to advantage in expressing the impulse flashover of insulators and bushings. Curves are given showing the gap spacings that are the equivalent in flashover to suspension and pedestal insulators for the ??/5, 1/10 and 1 ??/40 positive and negative waves.