Investigation and analysis of electrical aging of XLPE insulation for medium voltage covered conductors manufactured in Brazil

Today the knowledge of the ageing status of an electrical installation and its components is of fundamental interest for correctly inferring on the network economics. In Brazil, covered conductors must withstand an ageing test combining the stresses of heat, voltage to ground and corona. The observed failures indicate the necessity of complementary tests. In this paper Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC), contact angle and scanning electron microscopy (SEM) measurements on artificially aged and unaged cross-linked polyethylene (XLPE) insulation of medium voltage covered conductors is reported. The ESR spectroscopy results show clear evidence of the presence of free radicals in all unaged and aged XLPE insulation cables suggesting inadequate manufacture and processing conditions. The FTIR showed the presence of carbonyl groups in both, unaged and aged XLPE insulation indicating that oxidation process occurred during the manufacturing process. The TGA analysis demonstrates that the decomposition temperature of XLPE insulation did not change significantly except for one of the manufactures. It was observed in DSC analysis that the ageing of the XLPE insulation causes a decrease in intensity and broadening of the peak corresponding to fusion heat, indicating the bond scission in the backbone of the XLPE. The contact angle measurements showed hydrophilic character for all unaged and aged XLPE insulation possibly due to some oxidation during cables manufacturing. After ageing the contact angle is about 23.1° and 52.4°, indicating a significant increasing in the hydrophilicity of the XLPE insulation. The increase in hydrophilicity of aged XLPE insulation appears to be due to the increase in oxygen functional groups and the results are consistent with the FTIR spectroscopy measurements. The defects observed in SEM analysis of unaged XLPE insulation indicate a nonuniform mechanical and thermal stresses during cables manufacturing.