Functional temperature endurance tests on a silicone glass fiber insulated 15 kv class dry type transformer system

Temperature classification of insulating materials by functional tests is now a reality rather than an ideal. With the advent of many new binding and impregnating resins such as the epoxy, silicones and polytetra-fluoroethylene, the task becomes hopeless in establishing temperatures on an organic or inorganic basis as has been done in the pasto Hence, the reasons are clear for the recent actions by the A. I. E. E. and N. E. M. A. standardizing groups in suggesting evaluation of insulation systems for transformers established on a temperature basis. This paper describes tests and behavior of a composite silicone glass fiber insulation system throng}, 350°C temperature used in 15 KV class dry type transformers. Thirteen model transformers were subjected to load cycling by circulation of current in the windings to obtain hottest spot temperatures. Each cycle consisted of standard test voltages prescribed for 15 KV class insulation, followed by aging temperature 48 hours, by full test voltage by 90 percent humidity 24 hours, and by full test voltage within 30 minutes of humidification. Aging temperatures were 200°C, 275°C and 350°C. Thermocouple and power factor readings were obtained for all component parts of the insulation structure. These tests duplicate actual service conditions in much more severity. Such a procedure builds confidence in the performance of insulation from the users as well as the manufacturers point of view. It substantiates recent action by N. E. M. A. in establishing 180°C hottest spot rise temperature (220°C maximum total temperature) for such an insulating system.