NM Cable Insulation Service Life Prediction Using Materials Degradation Kinetics

摘要

The effect of the weight loss of plasticizer and other additives on breakdown voltage on two commercially available, residential nonmetallic (NM) power cables was assessed to develop a better understanding of the expected service life in different temperature environments. Two types of NM cables were selected having different types of PVC compounds in the cable construction. One cable insulation had a phthalate plasticizer while the other had trimellitate plasticizer. The insulated cable conductors were exposed to elevated isothermal temperatures (e.g. between 110 and 150℃) to accelerate the aging process. The elevated temperatures provided a faster rate of plasticizer migration and/or loss but are below the PVC dehydrochlorination temperature. An empirical correlation was developed between insulation weight loss and the change in breakdown voltage for the two NM cables. Activation energies were determined by fitting the above empirical correlation equation to predict the insulation service life. The oven isothermal aging test approach was defined as the low temperatures (LT) method (below dehydrochlorination). While the LT method of aging the samples in lower oven temperatures (< 150℃) is adequate, it is very time consuming. An alternative method employing Thermal Gravimetric Analysis (TGA) was explored to study the degradation and decomposition process of the cable's insulation as functions of temperature and programmed heating rate. The degree of insulation weight loss (e.g. the weight loss determined by the empirical correlation equation and the loss point) was taken as a factor of two times the minimum breakdown voltage of 6 kV as specified in UL 1449 safety standard "Surge Protective Devices". Then, according to Flynn and Wall method, the activation energy for the polymer degradation with the certain degree of weight conversion or loss can be determined by the slope from the plot of logarithm of the heating rate versus the temperature. If the TGA method is properly correlated with the oven thermal aging data and adequate heating process, it may be an alternative method for quick estimate of the electrical insulation service life. The results of the above two approaches for predicting insulation service lives of the cables were compared, and the causes for the discrepancy were proposed and discussed.