Applications of thermal analysis to studies of field aged distributton cables

Studies of changes with aging of distribution cables involve determining the effects of electrical, thermal and environmental stresses on the entire cable system. Changes in the dielectric insulation material are subtle. The materials employed are polyethylene and EP rubber. Differential thermal analysis and differential scanning calorimetry have been employed extensively in studies of changes of the morphology of such cables. The degree of crystallinity has been shown to change with aging under stress. Although the reasons are not completely understood such changes are significant in that they markedly affect the solubility, particularly of water, which in turn is associated with the phenomena of water treeing. Other uses for DSC and DTA include estimation of the amount of remaining antioxidant. Additional analysis of the cable system, particularly the inner and outer conduction shields, is facilitated by use of DTA and DSC. Thermogravimetric analysis is regularly used to determine the amount of filler in filled cables as well as to give an estimate of the thermal stability of the polymeric dielectric. These permit comparison of various materials and the estimation of the effect of additives. Thermal analysis can also give insight into the amount of residual stress in the cable resulting from previous mechanical mishandling of the cable.