Engineering: Fundamentals High Voltage 192
8.6 Insulation coordination
Insulation coordination is the correlation of insulation of the insulation ment with the characteristics of protective devices such that example, the s protected from excessive overvoltages. In a substation, for nsulation of transformers, circuit breakers, bus supports, etc., should have insulation strength in excess of the voltage provided by protective devices. Electric systems insulation designers have two options available to them choose insulation levels for components that would withstand all kinds of overvoltages, (ii) consider and devise protective devices that could be The the sensitive points in the system that would limit overvoltages there. unacceptable especially for e.h.v. and uah.v. operating levels because of the excessive insulation required. Hence, there has been great incen tive to develop and use protective devices. The actual relationship between the insulation levels and protective levels is a question of economics. Conventional methods of insulation coordination provide a margin of protection between ectrical stress and electrical strength based on predicted maximum over- ltage and minimum strength, the maximum strength being allowed by the protective devices.
8.6.1 Insulation level
Insulation level' is defined by the values of test voltages which the insulation of equipment under test must be able to withstand. In the earlier days of electric power, insulation levels commonly used were established on the basis of experience gained by utilities. As laboratory tech- niques improved, so that different laboratories were in closer agreement on t results, an international joint committee, the Nema-Nela Committee on Insulation Coordination, was formed and was charged with the task of estab- lishing insulation strength of all classes of equipment and to establish levels for various voltage classification. In 1941 a detailed document 13 was published giving basic insulation levels for all equipment in operation at that time. The presented tests included standard impulse voltages and one-minute power frequency tests In today's systems for voltages up to 245 kV the tests are still limited to lightning impulses and one-minute power frequency tests, see section 8.3. Above 300kV, in addition to lightning impulse and the one-minute power equency tests, tests include the use of switching impulse voltages. Tables 8.2 and 8.3 list the standardized test voltages for s245 kV and above 2300 kV respectively, suggested by IEC for testing equipment. These tables are based on a 1992 draft of the IEC document on insulation coordination.