insulation coordination[‚in·sə′lā·shən kō′ȯrd·ən‚ā·shən]
measures taken to coordi-nate the level of insulation in electrical equipment with the magnitudes of the overvoltages acting on it and the characteristics of the protection devices (protective gaps). The choice of an insulation level is a technical-economic problem: for each rated voltage of the electrical equipment there exists a technically achievable and economically most advantageous insulation level.
Before the advent of reliable arresters, insulation coordination was a method of grading insulation in which insulation breakdown’for example, in the equipment of an electric power substation’would be most probable at a place where the consequences of the breakdown are the least serious for service. Thus, the insulation of a power line is reduced as it approaches a substation, regarding the line as a unique arrester, and the strength of the internal insulation was made substantially greater than that of the external insulation: breakdowns of external insulation usually do not cause permanent damage. As methods of protection against overvoltages developed, the insulation level in electrical equipment came closer to the “natural level,” which for aerial power lines is defined as the breakdown voltage of the insulation under load and, for electrical machines and apparatus, as the rated service life of the insulation. Insulation chosen according to natural level must have a reliable system for protection from or limiting of overvoltages. Statistical methods, which were necessary because of the probabilistic nature of overvolt-ages, the process of aging of insulation, and other factors, came into general use in the 1960’s for solving the problems of the choice of insulation levels and the coordination of insulation.
D. V. RAZEVIG