Accelerated Aging Tests on High Voltage Cable, and Their Correlation With Service Records

Experience in recent years with underground cable for the higher operating voltages has developed the fact that specifications for the insulation were incomplete and inadequate, because some of its properties which were of slight importance for the lower operating voltages became of vital importance in cable for the higher voltages. Tests on impregnated paper insulation of the ordinary type as made at the factory heretofore have determined the quality of the insulation at the time of making the tests, but gave little indication of the rate or extent of the deterioration that might occur in service. When the Commonwealth Edison Company, Chicago, Ill., adopted 66-kv cable for tie lines between generating stations so that the cable practically formed a high voltage bus, it became very important that the cable should be reasonably free from failures in service. Unfortunately the cable included in thefirst installation in 1926 fell below this requirement, and some of it had to be replaced within a few years. A careful comparison of test and service records of the four makes of cable installed in 1926 showed that the cable had a wide range in quality. A marked difference was noted in the behavior of practically identical cable on a line heavily loaded and on a line moderately loaded. Accordingly, a series of tests was undertaken with the object of developing in a short time the deterioration which had been found in the several cables in service.