Predictive Model of the Degradation of Cable Insulation Subject to Heat and Radiation
Degradation of cable insulation subject to heat and radiation is modeled by physics-based chemical, mechanical, and electrical approaches. Experimental data of cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR), and silicone rubber (SIR) are incorporated to validate the models.
Chemical approaches divide aging kinetics into reaction- and diffusion-controlled mechanisms rendering homogeneous and heterogeneous cross-sections, respectively. The degradation profile of a cross-section serves as the base of Dichotomy Model. The model dichotomizes one bulk material into virtually degraded and non-degraded parts. The quotient of the two parts is determined by a diffusion theory in diffusion-controlled cases or by an exponential distribution in reaction-controlled scenarios to predict the lifespan of the cable insulation. Besides the degradation of the insulation, the migration and decomposition of the antioxidant in the insulation are also modeled by a reaction-diffusion theory representing the uphill diffusion of the antioxidant. The uphill diffusion is driven by the unevenly-distributed activity coefficient linearly proportional to oxygen concentration with a negative slope.
Both mechanical and electrical approaches are derived from Dichotomy Model. The degradation of mechanical performance is focused on elongation at break (EAB) while that of electrical property is quantified by resistance. EAB and resistance as functions of time are deterministically developed. Incubation time is a parameter of the function representing the time period when the changes of the physical properties are negligible. Drop-off rate is the other parameter describing the trends of the properties. The values of the two parameters follow the trends of temperature and dose rates, which makes the functions predictable in different aging conditions. Unlike the state of the art, this model can accommodate the shape change of the curves of EAB and resistance along a time axis.
By Bayesian parameter estimation, the deterministic function has been converted into a probabilistic equation. This method can represent the thresholds of the EAB and resistance in field application with uncertainty denoting the reliability of cable insulation by expected lifespan in the form of probability. The approach is the first in the area.